1 //===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // Peephole optimize the CFG.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "llvm/ADT/APInt.h"
14 #include "llvm/ADT/ArrayRef.h"
15 #include "llvm/ADT/DenseMap.h"
16 #include "llvm/ADT/MapVector.h"
17 #include "llvm/ADT/Optional.h"
18 #include "llvm/ADT/STLExtras.h"
19 #include "llvm/ADT/ScopeExit.h"
20 #include "llvm/ADT/Sequence.h"
21 #include "llvm/ADT/SetOperations.h"
22 #include "llvm/ADT/SetVector.h"
23 #include "llvm/ADT/SmallPtrSet.h"
24 #include "llvm/ADT/SmallVector.h"
25 #include "llvm/ADT/Statistic.h"
26 #include "llvm/ADT/StringRef.h"
27 #include "llvm/Analysis/AssumptionCache.h"
28 #include "llvm/Analysis/ConstantFolding.h"
29 #include "llvm/Analysis/EHPersonalities.h"
30 #include "llvm/Analysis/GuardUtils.h"
31 #include "llvm/Analysis/InstructionSimplify.h"
32 #include "llvm/Analysis/MemorySSA.h"
33 #include "llvm/Analysis/MemorySSAUpdater.h"
34 #include "llvm/Analysis/TargetTransformInfo.h"
35 #include "llvm/Analysis/ValueTracking.h"
36 #include "llvm/IR/Attributes.h"
37 #include "llvm/IR/BasicBlock.h"
38 #include "llvm/IR/CFG.h"
39 #include "llvm/IR/Constant.h"
40 #include "llvm/IR/ConstantRange.h"
41 #include "llvm/IR/Constants.h"
42 #include "llvm/IR/DataLayout.h"
43 #include "llvm/IR/DerivedTypes.h"
44 #include "llvm/IR/Function.h"
45 #include "llvm/IR/GlobalValue.h"
46 #include "llvm/IR/GlobalVariable.h"
47 #include "llvm/IR/IRBuilder.h"
48 #include "llvm/IR/InstrTypes.h"
49 #include "llvm/IR/Instruction.h"
50 #include "llvm/IR/Instructions.h"
51 #include "llvm/IR/IntrinsicInst.h"
52 #include "llvm/IR/Intrinsics.h"
53 #include "llvm/IR/LLVMContext.h"
54 #include "llvm/IR/MDBuilder.h"
55 #include "llvm/IR/Metadata.h"
56 #include "llvm/IR/Module.h"
57 #include "llvm/IR/NoFolder.h"
58 #include "llvm/IR/Operator.h"
59 #include "llvm/IR/PatternMatch.h"
60 #include "llvm/IR/PseudoProbe.h"
61 #include "llvm/IR/Type.h"
62 #include "llvm/IR/Use.h"
63 #include "llvm/IR/User.h"
64 #include "llvm/IR/Value.h"
65 #include "llvm/IR/ValueHandle.h"
66 #include "llvm/Support/BranchProbability.h"
67 #include "llvm/Support/Casting.h"
68 #include "llvm/Support/CommandLine.h"
69 #include "llvm/Support/Debug.h"
70 #include "llvm/Support/ErrorHandling.h"
71 #include "llvm/Support/KnownBits.h"
72 #include "llvm/Support/MathExtras.h"
73 #include "llvm/Support/raw_ostream.h"
74 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
75 #include "llvm/Transforms/Utils/Local.h"
76 #include "llvm/Transforms/Utils/SSAUpdater.h"
77 #include "llvm/Transforms/Utils/ValueMapper.h"
78 #include <algorithm>
79 #include <cassert>
80 #include <climits>
81 #include <cstddef>
82 #include <cstdint>
83 #include <iterator>
84 #include <map>
85 #include <set>
86 #include <tuple>
87 #include <utility>
88 #include <vector>
89
90 using namespace llvm;
91 using namespace PatternMatch;
92
93 #define DEBUG_TYPE "simplifycfg"
94
95 cl::opt<bool> llvm::RequireAndPreserveDomTree(
96 "simplifycfg-require-and-preserve-domtree", cl::Hidden, cl::ZeroOrMore,
97 cl::init(false),
98 cl::desc("Temorary development switch used to gradually uplift SimplifyCFG "
99 "into preserving DomTree,"));
100
101 // Chosen as 2 so as to be cheap, but still to have enough power to fold
102 // a select, so the "clamp" idiom (of a min followed by a max) will be caught.
103 // To catch this, we need to fold a compare and a select, hence '2' being the
104 // minimum reasonable default.
105 static cl::opt<unsigned> PHINodeFoldingThreshold(
106 "phi-node-folding-threshold", cl::Hidden, cl::init(2),
107 cl::desc(
108 "Control the amount of phi node folding to perform (default = 2)"));
109
110 static cl::opt<unsigned> TwoEntryPHINodeFoldingThreshold(
111 "two-entry-phi-node-folding-threshold", cl::Hidden, cl::init(4),
112 cl::desc("Control the maximal total instruction cost that we are willing "
113 "to speculatively execute to fold a 2-entry PHI node into a "
114 "select (default = 4)"));
115
116 static cl::opt<bool> DupRet(
117 "simplifycfg-dup-ret", cl::Hidden, cl::init(false),
118 cl::desc("Duplicate return instructions into unconditional branches"));
119
120 static cl::opt<bool>
121 HoistCommon("simplifycfg-hoist-common", cl::Hidden, cl::init(true),
122 cl::desc("Hoist common instructions up to the parent block"));
123
124 static cl::opt<bool>
125 SinkCommon("simplifycfg-sink-common", cl::Hidden, cl::init(true),
126 cl::desc("Sink common instructions down to the end block"));
127
128 static cl::opt<bool> HoistCondStores(
129 "simplifycfg-hoist-cond-stores", cl::Hidden, cl::init(true),
130 cl::desc("Hoist conditional stores if an unconditional store precedes"));
131
132 static cl::opt<bool> MergeCondStores(
133 "simplifycfg-merge-cond-stores", cl::Hidden, cl::init(true),
134 cl::desc("Hoist conditional stores even if an unconditional store does not "
135 "precede - hoist multiple conditional stores into a single "
136 "predicated store"));
137
138 static cl::opt<bool> MergeCondStoresAggressively(
139 "simplifycfg-merge-cond-stores-aggressively", cl::Hidden, cl::init(false),
140 cl::desc("When merging conditional stores, do so even if the resultant "
141 "basic blocks are unlikely to be if-converted as a result"));
142
143 static cl::opt<bool> SpeculateOneExpensiveInst(
144 "speculate-one-expensive-inst", cl::Hidden, cl::init(true),
145 cl::desc("Allow exactly one expensive instruction to be speculatively "
146 "executed"));
147
148 static cl::opt<unsigned> MaxSpeculationDepth(
149 "max-speculation-depth", cl::Hidden, cl::init(10),
150 cl::desc("Limit maximum recursion depth when calculating costs of "
151 "speculatively executed instructions"));
152
153 static cl::opt<int>
154 MaxSmallBlockSize("simplifycfg-max-small-block-size", cl::Hidden,
155 cl::init(10),
156 cl::desc("Max size of a block which is still considered "
157 "small enough to thread through"));
158
159 // Two is chosen to allow one negation and a logical combine.
160 static cl::opt<unsigned>
161 BranchFoldThreshold("simplifycfg-branch-fold-threshold", cl::Hidden,
162 cl::init(2),
163 cl::desc("Maximum cost of combining conditions when "
164 "folding branches"));
165
166 STATISTIC(NumBitMaps, "Number of switch instructions turned into bitmaps");
167 STATISTIC(NumLinearMaps,
168 "Number of switch instructions turned into linear mapping");
169 STATISTIC(NumLookupTables,
170 "Number of switch instructions turned into lookup tables");
171 STATISTIC(
172 NumLookupTablesHoles,
173 "Number of switch instructions turned into lookup tables (holes checked)");
174 STATISTIC(NumTableCmpReuses, "Number of reused switch table lookup compares");
175 STATISTIC(NumFoldValueComparisonIntoPredecessors,
176 "Number of value comparisons folded into predecessor basic blocks");
177 STATISTIC(NumFoldBranchToCommonDest,
178 "Number of branches folded into predecessor basic block");
179 STATISTIC(
180 NumHoistCommonCode,
181 "Number of common instruction 'blocks' hoisted up to the begin block");
182 STATISTIC(NumHoistCommonInstrs,
183 "Number of common instructions hoisted up to the begin block");
184 STATISTIC(NumSinkCommonCode,
185 "Number of common instruction 'blocks' sunk down to the end block");
186 STATISTIC(NumSinkCommonInstrs,
187 "Number of common instructions sunk down to the end block");
188 STATISTIC(NumSpeculations, "Number of speculative executed instructions");
189 STATISTIC(NumInvokes,
190 "Number of invokes with empty resume blocks simplified into calls");
191
192 namespace {
193
194 // The first field contains the value that the switch produces when a certain
195 // case group is selected, and the second field is a vector containing the
196 // cases composing the case group.
197 using SwitchCaseResultVectorTy =
198 SmallVector<std::pair<Constant *, SmallVector<ConstantInt *, 4>>, 2>;
199
200 // The first field contains the phi node that generates a result of the switch
201 // and the second field contains the value generated for a certain case in the
202 // switch for that PHI.
203 using SwitchCaseResultsTy = SmallVector<std::pair<PHINode *, Constant *>, 4>;
204
205 /// ValueEqualityComparisonCase - Represents a case of a switch.
206 struct ValueEqualityComparisonCase {
207 ConstantInt *Value;
208 BasicBlock *Dest;
209
ValueEqualityComparisonCase__anon9ac0a6d60111::ValueEqualityComparisonCase210 ValueEqualityComparisonCase(ConstantInt *Value, BasicBlock *Dest)
211 : Value(Value), Dest(Dest) {}
212
operator <__anon9ac0a6d60111::ValueEqualityComparisonCase213 bool operator<(ValueEqualityComparisonCase RHS) const {
214 // Comparing pointers is ok as we only rely on the order for uniquing.
215 return Value < RHS.Value;
216 }
217
operator ==__anon9ac0a6d60111::ValueEqualityComparisonCase218 bool operator==(BasicBlock *RHSDest) const { return Dest == RHSDest; }
219 };
220
221 class SimplifyCFGOpt {
222 const TargetTransformInfo &TTI;
223 DomTreeUpdater *DTU;
224 const DataLayout &DL;
225 ArrayRef<WeakVH> LoopHeaders;
226 const SimplifyCFGOptions &Options;
227 bool Resimplify;
228
229 Value *isValueEqualityComparison(Instruction *TI);
230 BasicBlock *GetValueEqualityComparisonCases(
231 Instruction *TI, std::vector<ValueEqualityComparisonCase> &Cases);
232 bool SimplifyEqualityComparisonWithOnlyPredecessor(Instruction *TI,
233 BasicBlock *Pred,
234 IRBuilder<> &Builder);
235 bool PerformValueComparisonIntoPredecessorFolding(Instruction *TI, Value *&CV,
236 Instruction *PTI,
237 IRBuilder<> &Builder);
238 bool FoldValueComparisonIntoPredecessors(Instruction *TI,
239 IRBuilder<> &Builder);
240
241 bool simplifyReturn(ReturnInst *RI, IRBuilder<> &Builder);
242 bool simplifyResume(ResumeInst *RI, IRBuilder<> &Builder);
243 bool simplifySingleResume(ResumeInst *RI);
244 bool simplifyCommonResume(ResumeInst *RI);
245 bool simplifyCleanupReturn(CleanupReturnInst *RI);
246 bool simplifyUnreachable(UnreachableInst *UI);
247 bool simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder);
248 bool simplifyIndirectBr(IndirectBrInst *IBI);
249 bool simplifyBranch(BranchInst *Branch, IRBuilder<> &Builder);
250 bool simplifyUncondBranch(BranchInst *BI, IRBuilder<> &Builder);
251 bool simplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder);
252 bool SimplifyCondBranchToTwoReturns(BranchInst *BI, IRBuilder<> &Builder);
253
254 bool tryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI,
255 IRBuilder<> &Builder);
256
257 bool HoistThenElseCodeToIf(BranchInst *BI, const TargetTransformInfo &TTI,
258 bool EqTermsOnly);
259 bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
260 const TargetTransformInfo &TTI);
261 bool SimplifyTerminatorOnSelect(Instruction *OldTerm, Value *Cond,
262 BasicBlock *TrueBB, BasicBlock *FalseBB,
263 uint32_t TrueWeight, uint32_t FalseWeight);
264 bool SimplifyBranchOnICmpChain(BranchInst *BI, IRBuilder<> &Builder,
265 const DataLayout &DL);
266 bool SimplifySwitchOnSelect(SwitchInst *SI, SelectInst *Select);
267 bool SimplifyIndirectBrOnSelect(IndirectBrInst *IBI, SelectInst *SI);
268 bool TurnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder);
269
270 public:
SimplifyCFGOpt(const TargetTransformInfo & TTI,DomTreeUpdater * DTU,const DataLayout & DL,ArrayRef<WeakVH> LoopHeaders,const SimplifyCFGOptions & Opts)271 SimplifyCFGOpt(const TargetTransformInfo &TTI, DomTreeUpdater *DTU,
272 const DataLayout &DL, ArrayRef<WeakVH> LoopHeaders,
273 const SimplifyCFGOptions &Opts)
274 : TTI(TTI), DTU(DTU), DL(DL), LoopHeaders(LoopHeaders), Options(Opts) {
275 assert((!DTU || !DTU->hasPostDomTree()) &&
276 "SimplifyCFG is not yet capable of maintaining validity of a "
277 "PostDomTree, so don't ask for it.");
278 }
279
280 bool simplifyOnce(BasicBlock *BB);
281 bool simplifyOnceImpl(BasicBlock *BB);
282 bool run(BasicBlock *BB);
283
284 // Helper to set Resimplify and return change indication.
requestResimplify()285 bool requestResimplify() {
286 Resimplify = true;
287 return true;
288 }
289 };
290
291 } // end anonymous namespace
292
293 /// Return true if it is safe to merge these two
294 /// terminator instructions together.
295 static bool
SafeToMergeTerminators(Instruction * SI1,Instruction * SI2,SmallSetVector<BasicBlock *,4> * FailBlocks=nullptr)296 SafeToMergeTerminators(Instruction *SI1, Instruction *SI2,
297 SmallSetVector<BasicBlock *, 4> *FailBlocks = nullptr) {
298 if (SI1 == SI2)
299 return false; // Can't merge with self!
300
301 // It is not safe to merge these two switch instructions if they have a common
302 // successor, and if that successor has a PHI node, and if *that* PHI node has
303 // conflicting incoming values from the two switch blocks.
304 BasicBlock *SI1BB = SI1->getParent();
305 BasicBlock *SI2BB = SI2->getParent();
306
307 SmallPtrSet<BasicBlock *, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
308 bool Fail = false;
309 for (BasicBlock *Succ : successors(SI2BB))
310 if (SI1Succs.count(Succ))
311 for (BasicBlock::iterator BBI = Succ->begin(); isa<PHINode>(BBI); ++BBI) {
312 PHINode *PN = cast<PHINode>(BBI);
313 if (PN->getIncomingValueForBlock(SI1BB) !=
314 PN->getIncomingValueForBlock(SI2BB)) {
315 if (FailBlocks)
316 FailBlocks->insert(Succ);
317 Fail = true;
318 }
319 }
320
321 return !Fail;
322 }
323
324 /// Update PHI nodes in Succ to indicate that there will now be entries in it
325 /// from the 'NewPred' block. The values that will be flowing into the PHI nodes
326 /// will be the same as those coming in from ExistPred, an existing predecessor
327 /// of Succ.
AddPredecessorToBlock(BasicBlock * Succ,BasicBlock * NewPred,BasicBlock * ExistPred,MemorySSAUpdater * MSSAU=nullptr)328 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
329 BasicBlock *ExistPred,
330 MemorySSAUpdater *MSSAU = nullptr) {
331 for (PHINode &PN : Succ->phis())
332 PN.addIncoming(PN.getIncomingValueForBlock(ExistPred), NewPred);
333 if (MSSAU)
334 if (auto *MPhi = MSSAU->getMemorySSA()->getMemoryAccess(Succ))
335 MPhi->addIncoming(MPhi->getIncomingValueForBlock(ExistPred), NewPred);
336 }
337
338 /// Compute an abstract "cost" of speculating the given instruction,
339 /// which is assumed to be safe to speculate. TCC_Free means cheap,
340 /// TCC_Basic means less cheap, and TCC_Expensive means prohibitively
341 /// expensive.
computeSpeculationCost(const User * I,const TargetTransformInfo & TTI)342 static InstructionCost computeSpeculationCost(const User *I,
343 const TargetTransformInfo &TTI) {
344 assert(isSafeToSpeculativelyExecute(I) &&
345 "Instruction is not safe to speculatively execute!");
346 return TTI.getUserCost(I, TargetTransformInfo::TCK_SizeAndLatency);
347 }
348
349 /// If we have a merge point of an "if condition" as accepted above,
350 /// return true if the specified value dominates the block. We
351 /// don't handle the true generality of domination here, just a special case
352 /// which works well enough for us.
353 ///
354 /// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
355 /// see if V (which must be an instruction) and its recursive operands
356 /// that do not dominate BB have a combined cost lower than Budget and
357 /// are non-trapping. If both are true, the instruction is inserted into the
358 /// set and true is returned.
359 ///
360 /// The cost for most non-trapping instructions is defined as 1 except for
361 /// Select whose cost is 2.
362 ///
363 /// After this function returns, Cost is increased by the cost of
364 /// V plus its non-dominating operands. If that cost is greater than
365 /// Budget, false is returned and Cost is undefined.
dominatesMergePoint(Value * V,BasicBlock * BB,SmallPtrSetImpl<Instruction * > & AggressiveInsts,InstructionCost & Cost,InstructionCost Budget,const TargetTransformInfo & TTI,unsigned Depth=0)366 static bool dominatesMergePoint(Value *V, BasicBlock *BB,
367 SmallPtrSetImpl<Instruction *> &AggressiveInsts,
368 InstructionCost &Cost,
369 InstructionCost Budget,
370 const TargetTransformInfo &TTI,
371 unsigned Depth = 0) {
372 // It is possible to hit a zero-cost cycle (phi/gep instructions for example),
373 // so limit the recursion depth.
374 // TODO: While this recursion limit does prevent pathological behavior, it
375 // would be better to track visited instructions to avoid cycles.
376 if (Depth == MaxSpeculationDepth)
377 return false;
378
379 Instruction *I = dyn_cast<Instruction>(V);
380 if (!I) {
381 // Non-instructions all dominate instructions, but not all constantexprs
382 // can be executed unconditionally.
383 if (ConstantExpr *C = dyn_cast<ConstantExpr>(V))
384 if (C->canTrap())
385 return false;
386 return true;
387 }
388 BasicBlock *PBB = I->getParent();
389
390 // We don't want to allow weird loops that might have the "if condition" in
391 // the bottom of this block.
392 if (PBB == BB)
393 return false;
394
395 // If this instruction is defined in a block that contains an unconditional
396 // branch to BB, then it must be in the 'conditional' part of the "if
397 // statement". If not, it definitely dominates the region.
398 BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator());
399 if (!BI || BI->isConditional() || BI->getSuccessor(0) != BB)
400 return true;
401
402 // If we have seen this instruction before, don't count it again.
403 if (AggressiveInsts.count(I))
404 return true;
405
406 // Okay, it looks like the instruction IS in the "condition". Check to
407 // see if it's a cheap instruction to unconditionally compute, and if it
408 // only uses stuff defined outside of the condition. If so, hoist it out.
409 if (!isSafeToSpeculativelyExecute(I))
410 return false;
411
412 Cost += computeSpeculationCost(I, TTI);
413
414 // Allow exactly one instruction to be speculated regardless of its cost
415 // (as long as it is safe to do so).
416 // This is intended to flatten the CFG even if the instruction is a division
417 // or other expensive operation. The speculation of an expensive instruction
418 // is expected to be undone in CodeGenPrepare if the speculation has not
419 // enabled further IR optimizations.
420 if (Cost > Budget &&
421 (!SpeculateOneExpensiveInst || !AggressiveInsts.empty() || Depth > 0 ||
422 !Cost.isValid()))
423 return false;
424
425 // Okay, we can only really hoist these out if their operands do
426 // not take us over the cost threshold.
427 for (Use &Op : I->operands())
428 if (!dominatesMergePoint(Op, BB, AggressiveInsts, Cost, Budget, TTI,
429 Depth + 1))
430 return false;
431 // Okay, it's safe to do this! Remember this instruction.
432 AggressiveInsts.insert(I);
433 return true;
434 }
435
436 /// Extract ConstantInt from value, looking through IntToPtr
437 /// and PointerNullValue. Return NULL if value is not a constant int.
GetConstantInt(Value * V,const DataLayout & DL)438 static ConstantInt *GetConstantInt(Value *V, const DataLayout &DL) {
439 // Normal constant int.
440 ConstantInt *CI = dyn_cast<ConstantInt>(V);
441 if (CI || !isa<Constant>(V) || !V->getType()->isPointerTy())
442 return CI;
443
444 // This is some kind of pointer constant. Turn it into a pointer-sized
445 // ConstantInt if possible.
446 IntegerType *PtrTy = cast<IntegerType>(DL.getIntPtrType(V->getType()));
447
448 // Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*).
449 if (isa<ConstantPointerNull>(V))
450 return ConstantInt::get(PtrTy, 0);
451
452 // IntToPtr const int.
453 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
454 if (CE->getOpcode() == Instruction::IntToPtr)
455 if (ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(0))) {
456 // The constant is very likely to have the right type already.
457 if (CI->getType() == PtrTy)
458 return CI;
459 else
460 return cast<ConstantInt>(
461 ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false));
462 }
463 return nullptr;
464 }
465
466 namespace {
467
468 /// Given a chain of or (||) or and (&&) comparison of a value against a
469 /// constant, this will try to recover the information required for a switch
470 /// structure.
471 /// It will depth-first traverse the chain of comparison, seeking for patterns
472 /// like %a == 12 or %a < 4 and combine them to produce a set of integer
473 /// representing the different cases for the switch.
474 /// Note that if the chain is composed of '||' it will build the set of elements
475 /// that matches the comparisons (i.e. any of this value validate the chain)
476 /// while for a chain of '&&' it will build the set elements that make the test
477 /// fail.
478 struct ConstantComparesGatherer {
479 const DataLayout &DL;
480
481 /// Value found for the switch comparison
482 Value *CompValue = nullptr;
483
484 /// Extra clause to be checked before the switch
485 Value *Extra = nullptr;
486
487 /// Set of integers to match in switch
488 SmallVector<ConstantInt *, 8> Vals;
489
490 /// Number of comparisons matched in the and/or chain
491 unsigned UsedICmps = 0;
492
493 /// Construct and compute the result for the comparison instruction Cond
ConstantComparesGatherer__anon9ac0a6d60211::ConstantComparesGatherer494 ConstantComparesGatherer(Instruction *Cond, const DataLayout &DL) : DL(DL) {
495 gather(Cond);
496 }
497
498 ConstantComparesGatherer(const ConstantComparesGatherer &) = delete;
499 ConstantComparesGatherer &
500 operator=(const ConstantComparesGatherer &) = delete;
501
502 private:
503 /// Try to set the current value used for the comparison, it succeeds only if
504 /// it wasn't set before or if the new value is the same as the old one
setValueOnce__anon9ac0a6d60211::ConstantComparesGatherer505 bool setValueOnce(Value *NewVal) {
506 if (CompValue && CompValue != NewVal)
507 return false;
508 CompValue = NewVal;
509 return (CompValue != nullptr);
510 }
511
512 /// Try to match Instruction "I" as a comparison against a constant and
513 /// populates the array Vals with the set of values that match (or do not
514 /// match depending on isEQ).
515 /// Return false on failure. On success, the Value the comparison matched
516 /// against is placed in CompValue.
517 /// If CompValue is already set, the function is expected to fail if a match
518 /// is found but the value compared to is different.
matchInstruction__anon9ac0a6d60211::ConstantComparesGatherer519 bool matchInstruction(Instruction *I, bool isEQ) {
520 // If this is an icmp against a constant, handle this as one of the cases.
521 ICmpInst *ICI;
522 ConstantInt *C;
523 if (!((ICI = dyn_cast<ICmpInst>(I)) &&
524 (C = GetConstantInt(I->getOperand(1), DL)))) {
525 return false;
526 }
527
528 Value *RHSVal;
529 const APInt *RHSC;
530
531 // Pattern match a special case
532 // (x & ~2^z) == y --> x == y || x == y|2^z
533 // This undoes a transformation done by instcombine to fuse 2 compares.
534 if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE)) {
535 // It's a little bit hard to see why the following transformations are
536 // correct. Here is a CVC3 program to verify them for 64-bit values:
537
538 /*
539 ONE : BITVECTOR(64) = BVZEROEXTEND(0bin1, 63);
540 x : BITVECTOR(64);
541 y : BITVECTOR(64);
542 z : BITVECTOR(64);
543 mask : BITVECTOR(64) = BVSHL(ONE, z);
544 QUERY( (y & ~mask = y) =>
545 ((x & ~mask = y) <=> (x = y OR x = (y | mask)))
546 );
547 QUERY( (y | mask = y) =>
548 ((x | mask = y) <=> (x = y OR x = (y & ~mask)))
549 );
550 */
551
552 // Please note that each pattern must be a dual implication (<--> or
553 // iff). One directional implication can create spurious matches. If the
554 // implication is only one-way, an unsatisfiable condition on the left
555 // side can imply a satisfiable condition on the right side. Dual
556 // implication ensures that satisfiable conditions are transformed to
557 // other satisfiable conditions and unsatisfiable conditions are
558 // transformed to other unsatisfiable conditions.
559
560 // Here is a concrete example of a unsatisfiable condition on the left
561 // implying a satisfiable condition on the right:
562 //
563 // mask = (1 << z)
564 // (x & ~mask) == y --> (x == y || x == (y | mask))
565 //
566 // Substituting y = 3, z = 0 yields:
567 // (x & -2) == 3 --> (x == 3 || x == 2)
568
569 // Pattern match a special case:
570 /*
571 QUERY( (y & ~mask = y) =>
572 ((x & ~mask = y) <=> (x = y OR x = (y | mask)))
573 );
574 */
575 if (match(ICI->getOperand(0),
576 m_And(m_Value(RHSVal), m_APInt(RHSC)))) {
577 APInt Mask = ~*RHSC;
578 if (Mask.isPowerOf2() && (C->getValue() & ~Mask) == C->getValue()) {
579 // If we already have a value for the switch, it has to match!
580 if (!setValueOnce(RHSVal))
581 return false;
582
583 Vals.push_back(C);
584 Vals.push_back(
585 ConstantInt::get(C->getContext(),
586 C->getValue() | Mask));
587 UsedICmps++;
588 return true;
589 }
590 }
591
592 // Pattern match a special case:
593 /*
594 QUERY( (y | mask = y) =>
595 ((x | mask = y) <=> (x = y OR x = (y & ~mask)))
596 );
597 */
598 if (match(ICI->getOperand(0),
599 m_Or(m_Value(RHSVal), m_APInt(RHSC)))) {
600 APInt Mask = *RHSC;
601 if (Mask.isPowerOf2() && (C->getValue() | Mask) == C->getValue()) {
602 // If we already have a value for the switch, it has to match!
603 if (!setValueOnce(RHSVal))
604 return false;
605
606 Vals.push_back(C);
607 Vals.push_back(ConstantInt::get(C->getContext(),
608 C->getValue() & ~Mask));
609 UsedICmps++;
610 return true;
611 }
612 }
613
614 // If we already have a value for the switch, it has to match!
615 if (!setValueOnce(ICI->getOperand(0)))
616 return false;
617
618 UsedICmps++;
619 Vals.push_back(C);
620 return ICI->getOperand(0);
621 }
622
623 // If we have "x ult 3", for example, then we can add 0,1,2 to the set.
624 ConstantRange Span = ConstantRange::makeAllowedICmpRegion(
625 ICI->getPredicate(), C->getValue());
626
627 // Shift the range if the compare is fed by an add. This is the range
628 // compare idiom as emitted by instcombine.
629 Value *CandidateVal = I->getOperand(0);
630 if (match(I->getOperand(0), m_Add(m_Value(RHSVal), m_APInt(RHSC)))) {
631 Span = Span.subtract(*RHSC);
632 CandidateVal = RHSVal;
633 }
634
635 // If this is an and/!= check, then we are looking to build the set of
636 // value that *don't* pass the and chain. I.e. to turn "x ugt 2" into
637 // x != 0 && x != 1.
638 if (!isEQ)
639 Span = Span.inverse();
640
641 // If there are a ton of values, we don't want to make a ginormous switch.
642 if (Span.isSizeLargerThan(8) || Span.isEmptySet()) {
643 return false;
644 }
645
646 // If we already have a value for the switch, it has to match!
647 if (!setValueOnce(CandidateVal))
648 return false;
649
650 // Add all values from the range to the set
651 for (APInt Tmp = Span.getLower(); Tmp != Span.getUpper(); ++Tmp)
652 Vals.push_back(ConstantInt::get(I->getContext(), Tmp));
653
654 UsedICmps++;
655 return true;
656 }
657
658 /// Given a potentially 'or'd or 'and'd together collection of icmp
659 /// eq/ne/lt/gt instructions that compare a value against a constant, extract
660 /// the value being compared, and stick the list constants into the Vals
661 /// vector.
662 /// One "Extra" case is allowed to differ from the other.
gather__anon9ac0a6d60211::ConstantComparesGatherer663 void gather(Value *V) {
664 bool isEQ = match(V, m_LogicalOr(m_Value(), m_Value()));
665
666 // Keep a stack (SmallVector for efficiency) for depth-first traversal
667 SmallVector<Value *, 8> DFT;
668 SmallPtrSet<Value *, 8> Visited;
669
670 // Initialize
671 Visited.insert(V);
672 DFT.push_back(V);
673
674 while (!DFT.empty()) {
675 V = DFT.pop_back_val();
676
677 if (Instruction *I = dyn_cast<Instruction>(V)) {
678 // If it is a || (or && depending on isEQ), process the operands.
679 Value *Op0, *Op1;
680 if (isEQ ? match(I, m_LogicalOr(m_Value(Op0), m_Value(Op1)))
681 : match(I, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) {
682 if (Visited.insert(Op1).second)
683 DFT.push_back(Op1);
684 if (Visited.insert(Op0).second)
685 DFT.push_back(Op0);
686
687 continue;
688 }
689
690 // Try to match the current instruction
691 if (matchInstruction(I, isEQ))
692 // Match succeed, continue the loop
693 continue;
694 }
695
696 // One element of the sequence of || (or &&) could not be match as a
697 // comparison against the same value as the others.
698 // We allow only one "Extra" case to be checked before the switch
699 if (!Extra) {
700 Extra = V;
701 continue;
702 }
703 // Failed to parse a proper sequence, abort now
704 CompValue = nullptr;
705 break;
706 }
707 }
708 };
709
710 } // end anonymous namespace
711
EraseTerminatorAndDCECond(Instruction * TI,MemorySSAUpdater * MSSAU=nullptr)712 static void EraseTerminatorAndDCECond(Instruction *TI,
713 MemorySSAUpdater *MSSAU = nullptr) {
714 Instruction *Cond = nullptr;
715 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
716 Cond = dyn_cast<Instruction>(SI->getCondition());
717 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
718 if (BI->isConditional())
719 Cond = dyn_cast<Instruction>(BI->getCondition());
720 } else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(TI)) {
721 Cond = dyn_cast<Instruction>(IBI->getAddress());
722 }
723
724 TI->eraseFromParent();
725 if (Cond)
726 RecursivelyDeleteTriviallyDeadInstructions(Cond, nullptr, MSSAU);
727 }
728
729 /// Return true if the specified terminator checks
730 /// to see if a value is equal to constant integer value.
isValueEqualityComparison(Instruction * TI)731 Value *SimplifyCFGOpt::isValueEqualityComparison(Instruction *TI) {
732 Value *CV = nullptr;
733 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
734 // Do not permit merging of large switch instructions into their
735 // predecessors unless there is only one predecessor.
736 if (!SI->getParent()->hasNPredecessorsOrMore(128 / SI->getNumSuccessors()))
737 CV = SI->getCondition();
738 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
739 if (BI->isConditional() && BI->getCondition()->hasOneUse())
740 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
741 if (ICI->isEquality() && GetConstantInt(ICI->getOperand(1), DL))
742 CV = ICI->getOperand(0);
743 }
744
745 // Unwrap any lossless ptrtoint cast.
746 if (CV) {
747 if (PtrToIntInst *PTII = dyn_cast<PtrToIntInst>(CV)) {
748 Value *Ptr = PTII->getPointerOperand();
749 if (PTII->getType() == DL.getIntPtrType(Ptr->getType()))
750 CV = Ptr;
751 }
752 }
753 return CV;
754 }
755
756 /// Given a value comparison instruction,
757 /// decode all of the 'cases' that it represents and return the 'default' block.
GetValueEqualityComparisonCases(Instruction * TI,std::vector<ValueEqualityComparisonCase> & Cases)758 BasicBlock *SimplifyCFGOpt::GetValueEqualityComparisonCases(
759 Instruction *TI, std::vector<ValueEqualityComparisonCase> &Cases) {
760 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
761 Cases.reserve(SI->getNumCases());
762 for (auto Case : SI->cases())
763 Cases.push_back(ValueEqualityComparisonCase(Case.getCaseValue(),
764 Case.getCaseSuccessor()));
765 return SI->getDefaultDest();
766 }
767
768 BranchInst *BI = cast<BranchInst>(TI);
769 ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
770 BasicBlock *Succ = BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_NE);
771 Cases.push_back(ValueEqualityComparisonCase(
772 GetConstantInt(ICI->getOperand(1), DL), Succ));
773 return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ);
774 }
775
776 /// Given a vector of bb/value pairs, remove any entries
777 /// in the list that match the specified block.
778 static void
EliminateBlockCases(BasicBlock * BB,std::vector<ValueEqualityComparisonCase> & Cases)779 EliminateBlockCases(BasicBlock *BB,
780 std::vector<ValueEqualityComparisonCase> &Cases) {
781 llvm::erase_value(Cases, BB);
782 }
783
784 /// Return true if there are any keys in C1 that exist in C2 as well.
ValuesOverlap(std::vector<ValueEqualityComparisonCase> & C1,std::vector<ValueEqualityComparisonCase> & C2)785 static bool ValuesOverlap(std::vector<ValueEqualityComparisonCase> &C1,
786 std::vector<ValueEqualityComparisonCase> &C2) {
787 std::vector<ValueEqualityComparisonCase> *V1 = &C1, *V2 = &C2;
788
789 // Make V1 be smaller than V2.
790 if (V1->size() > V2->size())
791 std::swap(V1, V2);
792
793 if (V1->empty())
794 return false;
795 if (V1->size() == 1) {
796 // Just scan V2.
797 ConstantInt *TheVal = (*V1)[0].Value;
798 for (unsigned i = 0, e = V2->size(); i != e; ++i)
799 if (TheVal == (*V2)[i].Value)
800 return true;
801 }
802
803 // Otherwise, just sort both lists and compare element by element.
804 array_pod_sort(V1->begin(), V1->end());
805 array_pod_sort(V2->begin(), V2->end());
806 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
807 while (i1 != e1 && i2 != e2) {
808 if ((*V1)[i1].Value == (*V2)[i2].Value)
809 return true;
810 if ((*V1)[i1].Value < (*V2)[i2].Value)
811 ++i1;
812 else
813 ++i2;
814 }
815 return false;
816 }
817
818 // Set branch weights on SwitchInst. This sets the metadata if there is at
819 // least one non-zero weight.
setBranchWeights(SwitchInst * SI,ArrayRef<uint32_t> Weights)820 static void setBranchWeights(SwitchInst *SI, ArrayRef<uint32_t> Weights) {
821 // Check that there is at least one non-zero weight. Otherwise, pass
822 // nullptr to setMetadata which will erase the existing metadata.
823 MDNode *N = nullptr;
824 if (llvm::any_of(Weights, [](uint32_t W) { return W != 0; }))
825 N = MDBuilder(SI->getParent()->getContext()).createBranchWeights(Weights);
826 SI->setMetadata(LLVMContext::MD_prof, N);
827 }
828
829 // Similar to the above, but for branch and select instructions that take
830 // exactly 2 weights.
setBranchWeights(Instruction * I,uint32_t TrueWeight,uint32_t FalseWeight)831 static void setBranchWeights(Instruction *I, uint32_t TrueWeight,
832 uint32_t FalseWeight) {
833 assert(isa<BranchInst>(I) || isa<SelectInst>(I));
834 // Check that there is at least one non-zero weight. Otherwise, pass
835 // nullptr to setMetadata which will erase the existing metadata.
836 MDNode *N = nullptr;
837 if (TrueWeight || FalseWeight)
838 N = MDBuilder(I->getParent()->getContext())
839 .createBranchWeights(TrueWeight, FalseWeight);
840 I->setMetadata(LLVMContext::MD_prof, N);
841 }
842
843 /// If TI is known to be a terminator instruction and its block is known to
844 /// only have a single predecessor block, check to see if that predecessor is
845 /// also a value comparison with the same value, and if that comparison
846 /// determines the outcome of this comparison. If so, simplify TI. This does a
847 /// very limited form of jump threading.
SimplifyEqualityComparisonWithOnlyPredecessor(Instruction * TI,BasicBlock * Pred,IRBuilder<> & Builder)848 bool SimplifyCFGOpt::SimplifyEqualityComparisonWithOnlyPredecessor(
849 Instruction *TI, BasicBlock *Pred, IRBuilder<> &Builder) {
850 Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
851 if (!PredVal)
852 return false; // Not a value comparison in predecessor.
853
854 Value *ThisVal = isValueEqualityComparison(TI);
855 assert(ThisVal && "This isn't a value comparison!!");
856 if (ThisVal != PredVal)
857 return false; // Different predicates.
858
859 // TODO: Preserve branch weight metadata, similarly to how
860 // FoldValueComparisonIntoPredecessors preserves it.
861
862 // Find out information about when control will move from Pred to TI's block.
863 std::vector<ValueEqualityComparisonCase> PredCases;
864 BasicBlock *PredDef =
865 GetValueEqualityComparisonCases(Pred->getTerminator(), PredCases);
866 EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
867
868 // Find information about how control leaves this block.
869 std::vector<ValueEqualityComparisonCase> ThisCases;
870 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
871 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
872
873 // If TI's block is the default block from Pred's comparison, potentially
874 // simplify TI based on this knowledge.
875 if (PredDef == TI->getParent()) {
876 // If we are here, we know that the value is none of those cases listed in
877 // PredCases. If there are any cases in ThisCases that are in PredCases, we
878 // can simplify TI.
879 if (!ValuesOverlap(PredCases, ThisCases))
880 return false;
881
882 if (isa<BranchInst>(TI)) {
883 // Okay, one of the successors of this condbr is dead. Convert it to a
884 // uncond br.
885 assert(ThisCases.size() == 1 && "Branch can only have one case!");
886 // Insert the new branch.
887 Instruction *NI = Builder.CreateBr(ThisDef);
888 (void)NI;
889
890 // Remove PHI node entries for the dead edge.
891 ThisCases[0].Dest->removePredecessor(PredDef);
892
893 LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
894 << "Through successor TI: " << *TI << "Leaving: " << *NI
895 << "\n");
896
897 EraseTerminatorAndDCECond(TI);
898
899 if (DTU)
900 DTU->applyUpdates(
901 {{DominatorTree::Delete, PredDef, ThisCases[0].Dest}});
902
903 return true;
904 }
905
906 SwitchInstProfUpdateWrapper SI = *cast<SwitchInst>(TI);
907 // Okay, TI has cases that are statically dead, prune them away.
908 SmallPtrSet<Constant *, 16> DeadCases;
909 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
910 DeadCases.insert(PredCases[i].Value);
911
912 LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
913 << "Through successor TI: " << *TI);
914
915 SmallDenseMap<BasicBlock *, int, 8> NumPerSuccessorCases;
916 for (SwitchInst::CaseIt i = SI->case_end(), e = SI->case_begin(); i != e;) {
917 --i;
918 auto *Successor = i->getCaseSuccessor();
919 if (DTU)
920 ++NumPerSuccessorCases[Successor];
921 if (DeadCases.count(i->getCaseValue())) {
922 Successor->removePredecessor(PredDef);
923 SI.removeCase(i);
924 if (DTU)
925 --NumPerSuccessorCases[Successor];
926 }
927 }
928
929 if (DTU) {
930 std::vector<DominatorTree::UpdateType> Updates;
931 for (const std::pair<BasicBlock *, int> &I : NumPerSuccessorCases)
932 if (I.second == 0)
933 Updates.push_back({DominatorTree::Delete, PredDef, I.first});
934 DTU->applyUpdates(Updates);
935 }
936
937 LLVM_DEBUG(dbgs() << "Leaving: " << *TI << "\n");
938 return true;
939 }
940
941 // Otherwise, TI's block must correspond to some matched value. Find out
942 // which value (or set of values) this is.
943 ConstantInt *TIV = nullptr;
944 BasicBlock *TIBB = TI->getParent();
945 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
946 if (PredCases[i].Dest == TIBB) {
947 if (TIV)
948 return false; // Cannot handle multiple values coming to this block.
949 TIV = PredCases[i].Value;
950 }
951 assert(TIV && "No edge from pred to succ?");
952
953 // Okay, we found the one constant that our value can be if we get into TI's
954 // BB. Find out which successor will unconditionally be branched to.
955 BasicBlock *TheRealDest = nullptr;
956 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
957 if (ThisCases[i].Value == TIV) {
958 TheRealDest = ThisCases[i].Dest;
959 break;
960 }
961
962 // If not handled by any explicit cases, it is handled by the default case.
963 if (!TheRealDest)
964 TheRealDest = ThisDef;
965
966 SmallPtrSet<BasicBlock *, 2> RemovedSuccs;
967
968 // Remove PHI node entries for dead edges.
969 BasicBlock *CheckEdge = TheRealDest;
970 for (BasicBlock *Succ : successors(TIBB))
971 if (Succ != CheckEdge) {
972 if (Succ != TheRealDest)
973 RemovedSuccs.insert(Succ);
974 Succ->removePredecessor(TIBB);
975 } else
976 CheckEdge = nullptr;
977
978 // Insert the new branch.
979 Instruction *NI = Builder.CreateBr(TheRealDest);
980 (void)NI;
981
982 LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
983 << "Through successor TI: " << *TI << "Leaving: " << *NI
984 << "\n");
985
986 EraseTerminatorAndDCECond(TI);
987 if (DTU) {
988 SmallVector<DominatorTree::UpdateType, 2> Updates;
989 Updates.reserve(RemovedSuccs.size());
990 for (auto *RemovedSucc : RemovedSuccs)
991 Updates.push_back({DominatorTree::Delete, TIBB, RemovedSucc});
992 DTU->applyUpdates(Updates);
993 }
994 return true;
995 }
996
997 namespace {
998
999 /// This class implements a stable ordering of constant
1000 /// integers that does not depend on their address. This is important for
1001 /// applications that sort ConstantInt's to ensure uniqueness.
1002 struct ConstantIntOrdering {
operator ()__anon9ac0a6d60411::ConstantIntOrdering1003 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
1004 return LHS->getValue().ult(RHS->getValue());
1005 }
1006 };
1007
1008 } // end anonymous namespace
1009
ConstantIntSortPredicate(ConstantInt * const * P1,ConstantInt * const * P2)1010 static int ConstantIntSortPredicate(ConstantInt *const *P1,
1011 ConstantInt *const *P2) {
1012 const ConstantInt *LHS = *P1;
1013 const ConstantInt *RHS = *P2;
1014 if (LHS == RHS)
1015 return 0;
1016 return LHS->getValue().ult(RHS->getValue()) ? 1 : -1;
1017 }
1018
HasBranchWeights(const Instruction * I)1019 static inline bool HasBranchWeights(const Instruction *I) {
1020 MDNode *ProfMD = I->getMetadata(LLVMContext::MD_prof);
1021 if (ProfMD && ProfMD->getOperand(0))
1022 if (MDString *MDS = dyn_cast<MDString>(ProfMD->getOperand(0)))
1023 return MDS->getString().equals("branch_weights");
1024
1025 return false;
1026 }
1027
1028 /// Get Weights of a given terminator, the default weight is at the front
1029 /// of the vector. If TI is a conditional eq, we need to swap the branch-weight
1030 /// metadata.
GetBranchWeights(Instruction * TI,SmallVectorImpl<uint64_t> & Weights)1031 static void GetBranchWeights(Instruction *TI,
1032 SmallVectorImpl<uint64_t> &Weights) {
1033 MDNode *MD = TI->getMetadata(LLVMContext::MD_prof);
1034 assert(MD);
1035 for (unsigned i = 1, e = MD->getNumOperands(); i < e; ++i) {
1036 ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(i));
1037 Weights.push_back(CI->getValue().getZExtValue());
1038 }
1039
1040 // If TI is a conditional eq, the default case is the false case,
1041 // and the corresponding branch-weight data is at index 2. We swap the
1042 // default weight to be the first entry.
1043 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
1044 assert(Weights.size() == 2);
1045 ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
1046 if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
1047 std::swap(Weights.front(), Weights.back());
1048 }
1049 }
1050
1051 /// Keep halving the weights until all can fit in uint32_t.
FitWeights(MutableArrayRef<uint64_t> Weights)1052 static void FitWeights(MutableArrayRef<uint64_t> Weights) {
1053 uint64_t Max = *std::max_element(Weights.begin(), Weights.end());
1054 if (Max > UINT_MAX) {
1055 unsigned Offset = 32 - countLeadingZeros(Max);
1056 for (uint64_t &I : Weights)
1057 I >>= Offset;
1058 }
1059 }
1060
CloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(BasicBlock * BB,BasicBlock * PredBlock,ValueToValueMapTy & VMap)1061 static void CloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(
1062 BasicBlock *BB, BasicBlock *PredBlock, ValueToValueMapTy &VMap) {
1063 Instruction *PTI = PredBlock->getTerminator();
1064
1065 // If we have bonus instructions, clone them into the predecessor block.
1066 // Note that there may be multiple predecessor blocks, so we cannot move
1067 // bonus instructions to a predecessor block.
1068 for (Instruction &BonusInst : *BB) {
1069 if (isa<DbgInfoIntrinsic>(BonusInst) || BonusInst.isTerminator())
1070 continue;
1071
1072 Instruction *NewBonusInst = BonusInst.clone();
1073
1074 if (PTI->getDebugLoc() != NewBonusInst->getDebugLoc()) {
1075 // Unless the instruction has the same !dbg location as the original
1076 // branch, drop it. When we fold the bonus instructions we want to make
1077 // sure we reset their debug locations in order to avoid stepping on
1078 // dead code caused by folding dead branches.
1079 NewBonusInst->setDebugLoc(DebugLoc());
1080 }
1081
1082 RemapInstruction(NewBonusInst, VMap,
1083 RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
1084 VMap[&BonusInst] = NewBonusInst;
1085
1086 // If we moved a load, we cannot any longer claim any knowledge about
1087 // its potential value. The previous information might have been valid
1088 // only given the branch precondition.
1089 // For an analogous reason, we must also drop all the metadata whose
1090 // semantics we don't understand. We *can* preserve !annotation, because
1091 // it is tied to the instruction itself, not the value or position.
1092 NewBonusInst->dropUnknownNonDebugMetadata(LLVMContext::MD_annotation);
1093
1094 PredBlock->getInstList().insert(PTI->getIterator(), NewBonusInst);
1095 NewBonusInst->takeName(&BonusInst);
1096 BonusInst.setName(NewBonusInst->getName() + ".old");
1097
1098 // Update (liveout) uses of bonus instructions,
1099 // now that the bonus instruction has been cloned into predecessor.
1100 SSAUpdater SSAUpdate;
1101 SSAUpdate.Initialize(BonusInst.getType(),
1102 (NewBonusInst->getName() + ".merge").str());
1103 SSAUpdate.AddAvailableValue(BB, &BonusInst);
1104 SSAUpdate.AddAvailableValue(PredBlock, NewBonusInst);
1105 for (Use &U : make_early_inc_range(BonusInst.uses())) {
1106 auto *UI = cast<Instruction>(U.getUser());
1107 if (UI->getParent() != PredBlock)
1108 SSAUpdate.RewriteUseAfterInsertions(U);
1109 else // Use is in the same block as, and comes before, NewBonusInst.
1110 SSAUpdate.RewriteUse(U);
1111 }
1112 }
1113 }
1114
PerformValueComparisonIntoPredecessorFolding(Instruction * TI,Value * & CV,Instruction * PTI,IRBuilder<> & Builder)1115 bool SimplifyCFGOpt::PerformValueComparisonIntoPredecessorFolding(
1116 Instruction *TI, Value *&CV, Instruction *PTI, IRBuilder<> &Builder) {
1117 BasicBlock *BB = TI->getParent();
1118 BasicBlock *Pred = PTI->getParent();
1119
1120 SmallVector<DominatorTree::UpdateType, 32> Updates;
1121
1122 // Figure out which 'cases' to copy from SI to PSI.
1123 std::vector<ValueEqualityComparisonCase> BBCases;
1124 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
1125
1126 std::vector<ValueEqualityComparisonCase> PredCases;
1127 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
1128
1129 // Based on whether the default edge from PTI goes to BB or not, fill in
1130 // PredCases and PredDefault with the new switch cases we would like to
1131 // build.
1132 SmallMapVector<BasicBlock *, int, 8> NewSuccessors;
1133
1134 // Update the branch weight metadata along the way
1135 SmallVector<uint64_t, 8> Weights;
1136 bool PredHasWeights = HasBranchWeights(PTI);
1137 bool SuccHasWeights = HasBranchWeights(TI);
1138
1139 if (PredHasWeights) {
1140 GetBranchWeights(PTI, Weights);
1141 // branch-weight metadata is inconsistent here.
1142 if (Weights.size() != 1 + PredCases.size())
1143 PredHasWeights = SuccHasWeights = false;
1144 } else if (SuccHasWeights)
1145 // If there are no predecessor weights but there are successor weights,
1146 // populate Weights with 1, which will later be scaled to the sum of
1147 // successor's weights
1148 Weights.assign(1 + PredCases.size(), 1);
1149
1150 SmallVector<uint64_t, 8> SuccWeights;
1151 if (SuccHasWeights) {
1152 GetBranchWeights(TI, SuccWeights);
1153 // branch-weight metadata is inconsistent here.
1154 if (SuccWeights.size() != 1 + BBCases.size())
1155 PredHasWeights = SuccHasWeights = false;
1156 } else if (PredHasWeights)
1157 SuccWeights.assign(1 + BBCases.size(), 1);
1158
1159 if (PredDefault == BB) {
1160 // If this is the default destination from PTI, only the edges in TI
1161 // that don't occur in PTI, or that branch to BB will be activated.
1162 std::set<ConstantInt *, ConstantIntOrdering> PTIHandled;
1163 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
1164 if (PredCases[i].Dest != BB)
1165 PTIHandled.insert(PredCases[i].Value);
1166 else {
1167 // The default destination is BB, we don't need explicit targets.
1168 std::swap(PredCases[i], PredCases.back());
1169
1170 if (PredHasWeights || SuccHasWeights) {
1171 // Increase weight for the default case.
1172 Weights[0] += Weights[i + 1];
1173 std::swap(Weights[i + 1], Weights.back());
1174 Weights.pop_back();
1175 }
1176
1177 PredCases.pop_back();
1178 --i;
1179 --e;
1180 }
1181
1182 // Reconstruct the new switch statement we will be building.
1183 if (PredDefault != BBDefault) {
1184 PredDefault->removePredecessor(Pred);
1185 if (DTU && PredDefault != BB)
1186 Updates.push_back({DominatorTree::Delete, Pred, PredDefault});
1187 PredDefault = BBDefault;
1188 ++NewSuccessors[BBDefault];
1189 }
1190
1191 unsigned CasesFromPred = Weights.size();
1192 uint64_t ValidTotalSuccWeight = 0;
1193 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
1194 if (!PTIHandled.count(BBCases[i].Value) && BBCases[i].Dest != BBDefault) {
1195 PredCases.push_back(BBCases[i]);
1196 ++NewSuccessors[BBCases[i].Dest];
1197 if (SuccHasWeights || PredHasWeights) {
1198 // The default weight is at index 0, so weight for the ith case
1199 // should be at index i+1. Scale the cases from successor by
1200 // PredDefaultWeight (Weights[0]).
1201 Weights.push_back(Weights[0] * SuccWeights[i + 1]);
1202 ValidTotalSuccWeight += SuccWeights[i + 1];
1203 }
1204 }
1205
1206 if (SuccHasWeights || PredHasWeights) {
1207 ValidTotalSuccWeight += SuccWeights[0];
1208 // Scale the cases from predecessor by ValidTotalSuccWeight.
1209 for (unsigned i = 1; i < CasesFromPred; ++i)
1210 Weights[i] *= ValidTotalSuccWeight;
1211 // Scale the default weight by SuccDefaultWeight (SuccWeights[0]).
1212 Weights[0] *= SuccWeights[0];
1213 }
1214 } else {
1215 // If this is not the default destination from PSI, only the edges
1216 // in SI that occur in PSI with a destination of BB will be
1217 // activated.
1218 std::set<ConstantInt *, ConstantIntOrdering> PTIHandled;
1219 std::map<ConstantInt *, uint64_t> WeightsForHandled;
1220 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
1221 if (PredCases[i].Dest == BB) {
1222 PTIHandled.insert(PredCases[i].Value);
1223
1224 if (PredHasWeights || SuccHasWeights) {
1225 WeightsForHandled[PredCases[i].Value] = Weights[i + 1];
1226 std::swap(Weights[i + 1], Weights.back());
1227 Weights.pop_back();
1228 }
1229
1230 std::swap(PredCases[i], PredCases.back());
1231 PredCases.pop_back();
1232 --i;
1233 --e;
1234 }
1235
1236 // Okay, now we know which constants were sent to BB from the
1237 // predecessor. Figure out where they will all go now.
1238 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
1239 if (PTIHandled.count(BBCases[i].Value)) {
1240 // If this is one we are capable of getting...
1241 if (PredHasWeights || SuccHasWeights)
1242 Weights.push_back(WeightsForHandled[BBCases[i].Value]);
1243 PredCases.push_back(BBCases[i]);
1244 ++NewSuccessors[BBCases[i].Dest];
1245 PTIHandled.erase(BBCases[i].Value); // This constant is taken care of
1246 }
1247
1248 // If there are any constants vectored to BB that TI doesn't handle,
1249 // they must go to the default destination of TI.
1250 for (ConstantInt *I : PTIHandled) {
1251 if (PredHasWeights || SuccHasWeights)
1252 Weights.push_back(WeightsForHandled[I]);
1253 PredCases.push_back(ValueEqualityComparisonCase(I, BBDefault));
1254 ++NewSuccessors[BBDefault];
1255 }
1256 }
1257
1258 // Okay, at this point, we know which new successor Pred will get. Make
1259 // sure we update the number of entries in the PHI nodes for these
1260 // successors.
1261 SmallPtrSet<BasicBlock *, 2> SuccsOfPred;
1262 if (DTU) {
1263 SuccsOfPred = {succ_begin(Pred), succ_end(Pred)};
1264 Updates.reserve(Updates.size() + NewSuccessors.size());
1265 }
1266 for (const std::pair<BasicBlock *, int /*Num*/> &NewSuccessor :
1267 NewSuccessors) {
1268 for (auto I : seq(0, NewSuccessor.second)) {
1269 (void)I;
1270 AddPredecessorToBlock(NewSuccessor.first, Pred, BB);
1271 }
1272 if (DTU && !SuccsOfPred.contains(NewSuccessor.first))
1273 Updates.push_back({DominatorTree::Insert, Pred, NewSuccessor.first});
1274 }
1275
1276 Builder.SetInsertPoint(PTI);
1277 // Convert pointer to int before we switch.
1278 if (CV->getType()->isPointerTy()) {
1279 CV =
1280 Builder.CreatePtrToInt(CV, DL.getIntPtrType(CV->getType()), "magicptr");
1281 }
1282
1283 // Now that the successors are updated, create the new Switch instruction.
1284 SwitchInst *NewSI = Builder.CreateSwitch(CV, PredDefault, PredCases.size());
1285 NewSI->setDebugLoc(PTI->getDebugLoc());
1286 for (ValueEqualityComparisonCase &V : PredCases)
1287 NewSI->addCase(V.Value, V.Dest);
1288
1289 if (PredHasWeights || SuccHasWeights) {
1290 // Halve the weights if any of them cannot fit in an uint32_t
1291 FitWeights(Weights);
1292
1293 SmallVector<uint32_t, 8> MDWeights(Weights.begin(), Weights.end());
1294
1295 setBranchWeights(NewSI, MDWeights);
1296 }
1297
1298 EraseTerminatorAndDCECond(PTI);
1299
1300 // Okay, last check. If BB is still a successor of PSI, then we must
1301 // have an infinite loop case. If so, add an infinitely looping block
1302 // to handle the case to preserve the behavior of the code.
1303 BasicBlock *InfLoopBlock = nullptr;
1304 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
1305 if (NewSI->getSuccessor(i) == BB) {
1306 if (!InfLoopBlock) {
1307 // Insert it at the end of the function, because it's either code,
1308 // or it won't matter if it's hot. :)
1309 InfLoopBlock =
1310 BasicBlock::Create(BB->getContext(), "infloop", BB->getParent());
1311 BranchInst::Create(InfLoopBlock, InfLoopBlock);
1312 if (DTU)
1313 Updates.push_back(
1314 {DominatorTree::Insert, InfLoopBlock, InfLoopBlock});
1315 }
1316 NewSI->setSuccessor(i, InfLoopBlock);
1317 }
1318
1319 if (DTU) {
1320 if (InfLoopBlock)
1321 Updates.push_back({DominatorTree::Insert, Pred, InfLoopBlock});
1322
1323 Updates.push_back({DominatorTree::Delete, Pred, BB});
1324
1325 DTU->applyUpdates(Updates);
1326 }
1327
1328 // Here the BB is not a dead block but folded into its predecessors, so move
1329 // the probe and mark it as dangling.
1330 moveAndDanglePseudoProbes(BB, NewSI);
1331
1332 ++NumFoldValueComparisonIntoPredecessors;
1333 return true;
1334 }
1335
1336 /// The specified terminator is a value equality comparison instruction
1337 /// (either a switch or a branch on "X == c").
1338 /// See if any of the predecessors of the terminator block are value comparisons
1339 /// on the same value. If so, and if safe to do so, fold them together.
FoldValueComparisonIntoPredecessors(Instruction * TI,IRBuilder<> & Builder)1340 bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(Instruction *TI,
1341 IRBuilder<> &Builder) {
1342 BasicBlock *BB = TI->getParent();
1343 Value *CV = isValueEqualityComparison(TI); // CondVal
1344 assert(CV && "Not a comparison?");
1345
1346 bool Changed = false;
1347
1348 SmallSetVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
1349 while (!Preds.empty()) {
1350 BasicBlock *Pred = Preds.pop_back_val();
1351 Instruction *PTI = Pred->getTerminator();
1352
1353 // Don't try to fold into itself.
1354 if (Pred == BB)
1355 continue;
1356
1357 // See if the predecessor is a comparison with the same value.
1358 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
1359 if (PCV != CV)
1360 continue;
1361
1362 SmallSetVector<BasicBlock *, 4> FailBlocks;
1363 if (!SafeToMergeTerminators(TI, PTI, &FailBlocks)) {
1364 for (auto *Succ : FailBlocks) {
1365 if (!SplitBlockPredecessors(Succ, TI->getParent(), ".fold.split", DTU))
1366 return false;
1367 }
1368 }
1369
1370 PerformValueComparisonIntoPredecessorFolding(TI, CV, PTI, Builder);
1371 Changed = true;
1372 }
1373 return Changed;
1374 }
1375
1376 // If we would need to insert a select that uses the value of this invoke
1377 // (comments in HoistThenElseCodeToIf explain why we would need to do this), we
1378 // can't hoist the invoke, as there is nowhere to put the select in this case.
isSafeToHoistInvoke(BasicBlock * BB1,BasicBlock * BB2,Instruction * I1,Instruction * I2)1379 static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2,
1380 Instruction *I1, Instruction *I2) {
1381 for (BasicBlock *Succ : successors(BB1)) {
1382 for (const PHINode &PN : Succ->phis()) {
1383 Value *BB1V = PN.getIncomingValueForBlock(BB1);
1384 Value *BB2V = PN.getIncomingValueForBlock(BB2);
1385 if (BB1V != BB2V && (BB1V == I1 || BB2V == I2)) {
1386 return false;
1387 }
1388 }
1389 }
1390 return true;
1391 }
1392
1393 static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValueMayBeModified = false);
1394
1395 /// Given a conditional branch that goes to BB1 and BB2, hoist any common code
1396 /// in the two blocks up into the branch block. The caller of this function
1397 /// guarantees that BI's block dominates BB1 and BB2. If EqTermsOnly is given,
1398 /// only perform hoisting in case both blocks only contain a terminator. In that
1399 /// case, only the original BI will be replaced and selects for PHIs are added.
HoistThenElseCodeToIf(BranchInst * BI,const TargetTransformInfo & TTI,bool EqTermsOnly)1400 bool SimplifyCFGOpt::HoistThenElseCodeToIf(BranchInst *BI,
1401 const TargetTransformInfo &TTI,
1402 bool EqTermsOnly) {
1403 // This does very trivial matching, with limited scanning, to find identical
1404 // instructions in the two blocks. In particular, we don't want to get into
1405 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
1406 // such, we currently just scan for obviously identical instructions in an
1407 // identical order.
1408 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
1409 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
1410
1411 BasicBlock::iterator BB1_Itr = BB1->begin();
1412 BasicBlock::iterator BB2_Itr = BB2->begin();
1413
1414 Instruction *I1 = &*BB1_Itr++, *I2 = &*BB2_Itr++;
1415 // Skip debug info if it is not identical.
1416 DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
1417 DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
1418 if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
1419 while (isa<DbgInfoIntrinsic>(I1))
1420 I1 = &*BB1_Itr++;
1421 while (isa<DbgInfoIntrinsic>(I2))
1422 I2 = &*BB2_Itr++;
1423 }
1424 // FIXME: Can we define a safety predicate for CallBr?
1425 if (isa<PHINode>(I1) || !I1->isIdenticalToWhenDefined(I2) ||
1426 (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)) ||
1427 isa<CallBrInst>(I1))
1428 return false;
1429
1430 BasicBlock *BIParent = BI->getParent();
1431
1432 bool Changed = false;
1433
1434 auto _ = make_scope_exit([&]() {
1435 if (Changed)
1436 ++NumHoistCommonCode;
1437 });
1438
1439 // Check if only hoisting terminators is allowed. This does not add new
1440 // instructions to the hoist location.
1441 if (EqTermsOnly) {
1442 // Skip any debug intrinsics, as they are free to hoist.
1443 auto *I1NonDbg = &*skipDebugIntrinsics(I1->getIterator());
1444 auto *I2NonDbg = &*skipDebugIntrinsics(I2->getIterator());
1445 if (!I1NonDbg->isIdenticalToWhenDefined(I2NonDbg))
1446 return false;
1447 if (!I1NonDbg->isTerminator())
1448 return false;
1449 // Now we know that we only need to hoist debug instrinsics and the
1450 // terminator. Let the loop below handle those 2 cases.
1451 }
1452
1453 do {
1454 // If we are hoisting the terminator instruction, don't move one (making a
1455 // broken BB), instead clone it, and remove BI.
1456 if (I1->isTerminator())
1457 goto HoistTerminator;
1458
1459 // If we're going to hoist a call, make sure that the two instructions we're
1460 // commoning/hoisting are both marked with musttail, or neither of them is
1461 // marked as such. Otherwise, we might end up in a situation where we hoist
1462 // from a block where the terminator is a `ret` to a block where the terminator
1463 // is a `br`, and `musttail` calls expect to be followed by a return.
1464 auto *C1 = dyn_cast<CallInst>(I1);
1465 auto *C2 = dyn_cast<CallInst>(I2);
1466 if (C1 && C2)
1467 if (C1->isMustTailCall() != C2->isMustTailCall())
1468 return Changed;
1469
1470 if (!TTI.isProfitableToHoist(I1) || !TTI.isProfitableToHoist(I2))
1471 return Changed;
1472
1473 // If any of the two call sites has nomerge attribute, stop hoisting.
1474 if (const auto *CB1 = dyn_cast<CallBase>(I1))
1475 if (CB1->cannotMerge())
1476 return Changed;
1477 if (const auto *CB2 = dyn_cast<CallBase>(I2))
1478 if (CB2->cannotMerge())
1479 return Changed;
1480
1481 if (isa<DbgInfoIntrinsic>(I1) || isa<DbgInfoIntrinsic>(I2)) {
1482 assert (isa<DbgInfoIntrinsic>(I1) && isa<DbgInfoIntrinsic>(I2));
1483 // The debug location is an integral part of a debug info intrinsic
1484 // and can't be separated from it or replaced. Instead of attempting
1485 // to merge locations, simply hoist both copies of the intrinsic.
1486 BIParent->getInstList().splice(BI->getIterator(),
1487 BB1->getInstList(), I1);
1488 BIParent->getInstList().splice(BI->getIterator(),
1489 BB2->getInstList(), I2);
1490 Changed = true;
1491 } else {
1492 // For a normal instruction, we just move one to right before the branch,
1493 // then replace all uses of the other with the first. Finally, we remove
1494 // the now redundant second instruction.
1495 BIParent->getInstList().splice(BI->getIterator(),
1496 BB1->getInstList(), I1);
1497 if (!I2->use_empty())
1498 I2->replaceAllUsesWith(I1);
1499 I1->andIRFlags(I2);
1500 unsigned KnownIDs[] = {LLVMContext::MD_tbaa,
1501 LLVMContext::MD_range,
1502 LLVMContext::MD_fpmath,
1503 LLVMContext::MD_invariant_load,
1504 LLVMContext::MD_nonnull,
1505 LLVMContext::MD_invariant_group,
1506 LLVMContext::MD_align,
1507 LLVMContext::MD_dereferenceable,
1508 LLVMContext::MD_dereferenceable_or_null,
1509 LLVMContext::MD_mem_parallel_loop_access,
1510 LLVMContext::MD_access_group,
1511 LLVMContext::MD_preserve_access_index};
1512 combineMetadata(I1, I2, KnownIDs, true);
1513
1514 // I1 and I2 are being combined into a single instruction. Its debug
1515 // location is the merged locations of the original instructions.
1516 I1->applyMergedLocation(I1->getDebugLoc(), I2->getDebugLoc());
1517
1518 I2->eraseFromParent();
1519 Changed = true;
1520 }
1521 ++NumHoistCommonInstrs;
1522
1523 I1 = &*BB1_Itr++;
1524 I2 = &*BB2_Itr++;
1525 // Skip debug info if it is not identical.
1526 DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
1527 DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
1528 if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
1529 while (isa<DbgInfoIntrinsic>(I1))
1530 I1 = &*BB1_Itr++;
1531 while (isa<DbgInfoIntrinsic>(I2))
1532 I2 = &*BB2_Itr++;
1533 }
1534 } while (I1->isIdenticalToWhenDefined(I2));
1535
1536 return true;
1537
1538 HoistTerminator:
1539 // It may not be possible to hoist an invoke.
1540 // FIXME: Can we define a safety predicate for CallBr?
1541 if (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2))
1542 return Changed;
1543
1544 // TODO: callbr hoisting currently disabled pending further study.
1545 if (isa<CallBrInst>(I1))
1546 return Changed;
1547
1548 for (BasicBlock *Succ : successors(BB1)) {
1549 for (PHINode &PN : Succ->phis()) {
1550 Value *BB1V = PN.getIncomingValueForBlock(BB1);
1551 Value *BB2V = PN.getIncomingValueForBlock(BB2);
1552 if (BB1V == BB2V)
1553 continue;
1554
1555 // Check for passingValueIsAlwaysUndefined here because we would rather
1556 // eliminate undefined control flow then converting it to a select.
1557 if (passingValueIsAlwaysUndefined(BB1V, &PN) ||
1558 passingValueIsAlwaysUndefined(BB2V, &PN))
1559 return Changed;
1560
1561 if (isa<ConstantExpr>(BB1V) && !isSafeToSpeculativelyExecute(BB1V))
1562 return Changed;
1563 if (isa<ConstantExpr>(BB2V) && !isSafeToSpeculativelyExecute(BB2V))
1564 return Changed;
1565 }
1566 }
1567
1568 // Okay, it is safe to hoist the terminator.
1569 Instruction *NT = I1->clone();
1570 BIParent->getInstList().insert(BI->getIterator(), NT);
1571 if (!NT->getType()->isVoidTy()) {
1572 I1->replaceAllUsesWith(NT);
1573 I2->replaceAllUsesWith(NT);
1574 NT->takeName(I1);
1575 }
1576 Changed = true;
1577 ++NumHoistCommonInstrs;
1578
1579 // Ensure terminator gets a debug location, even an unknown one, in case
1580 // it involves inlinable calls.
1581 NT->applyMergedLocation(I1->getDebugLoc(), I2->getDebugLoc());
1582
1583 // PHIs created below will adopt NT's merged DebugLoc.
1584 IRBuilder<NoFolder> Builder(NT);
1585
1586 // Hoisting one of the terminators from our successor is a great thing.
1587 // Unfortunately, the successors of the if/else blocks may have PHI nodes in
1588 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
1589 // nodes, so we insert select instruction to compute the final result.
1590 std::map<std::pair<Value *, Value *>, SelectInst *> InsertedSelects;
1591 for (BasicBlock *Succ : successors(BB1)) {
1592 for (PHINode &PN : Succ->phis()) {
1593 Value *BB1V = PN.getIncomingValueForBlock(BB1);
1594 Value *BB2V = PN.getIncomingValueForBlock(BB2);
1595 if (BB1V == BB2V)
1596 continue;
1597
1598 // These values do not agree. Insert a select instruction before NT
1599 // that determines the right value.
1600 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
1601 if (!SI) {
1602 // Propagate fast-math-flags from phi node to its replacement select.
1603 IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
1604 if (isa<FPMathOperator>(PN))
1605 Builder.setFastMathFlags(PN.getFastMathFlags());
1606
1607 SI = cast<SelectInst>(
1608 Builder.CreateSelect(BI->getCondition(), BB1V, BB2V,
1609 BB1V->getName() + "." + BB2V->getName(), BI));
1610 }
1611
1612 // Make the PHI node use the select for all incoming values for BB1/BB2
1613 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
1614 if (PN.getIncomingBlock(i) == BB1 || PN.getIncomingBlock(i) == BB2)
1615 PN.setIncomingValue(i, SI);
1616 }
1617 }
1618
1619 SmallVector<DominatorTree::UpdateType, 4> Updates;
1620
1621 // Update any PHI nodes in our new successors.
1622 for (BasicBlock *Succ : successors(BB1)) {
1623 AddPredecessorToBlock(Succ, BIParent, BB1);
1624 if (DTU)
1625 Updates.push_back({DominatorTree::Insert, BIParent, Succ});
1626 }
1627
1628 if (DTU)
1629 for (BasicBlock *Succ : successors(BI))
1630 Updates.push_back({DominatorTree::Delete, BIParent, Succ});
1631
1632 EraseTerminatorAndDCECond(BI);
1633 if (DTU)
1634 DTU->applyUpdates(Updates);
1635 return Changed;
1636 }
1637
1638 // Check lifetime markers.
isLifeTimeMarker(const Instruction * I)1639 static bool isLifeTimeMarker(const Instruction *I) {
1640 if (auto II = dyn_cast<IntrinsicInst>(I)) {
1641 switch (II->getIntrinsicID()) {
1642 default:
1643 break;
1644 case Intrinsic::lifetime_start:
1645 case Intrinsic::lifetime_end:
1646 return true;
1647 }
1648 }
1649 return false;
1650 }
1651
1652 // TODO: Refine this. This should avoid cases like turning constant memcpy sizes
1653 // into variables.
replacingOperandWithVariableIsCheap(const Instruction * I,int OpIdx)1654 static bool replacingOperandWithVariableIsCheap(const Instruction *I,
1655 int OpIdx) {
1656 return !isa<IntrinsicInst>(I);
1657 }
1658
1659 // All instructions in Insts belong to different blocks that all unconditionally
1660 // branch to a common successor. Analyze each instruction and return true if it
1661 // would be possible to sink them into their successor, creating one common
1662 // instruction instead. For every value that would be required to be provided by
1663 // PHI node (because an operand varies in each input block), add to PHIOperands.
canSinkInstructions(ArrayRef<Instruction * > Insts,DenseMap<Instruction *,SmallVector<Value *,4>> & PHIOperands)1664 static bool canSinkInstructions(
1665 ArrayRef<Instruction *> Insts,
1666 DenseMap<Instruction *, SmallVector<Value *, 4>> &PHIOperands) {
1667 // Prune out obviously bad instructions to move. Each instruction must have
1668 // exactly zero or one use, and we check later that use is by a single, common
1669 // PHI instruction in the successor.
1670 bool HasUse = !Insts.front()->user_empty();
1671 for (auto *I : Insts) {
1672 // These instructions may change or break semantics if moved.
1673 if (isa<PHINode>(I) || I->isEHPad() || isa<AllocaInst>(I) ||
1674 I->getType()->isTokenTy())
1675 return false;
1676
1677 // Do not try to sink an instruction in an infinite loop - it can cause
1678 // this algorithm to infinite loop.
1679 if (I->getParent()->getSingleSuccessor() == I->getParent())
1680 return false;
1681
1682 // Conservatively return false if I is an inline-asm instruction. Sinking
1683 // and merging inline-asm instructions can potentially create arguments
1684 // that cannot satisfy the inline-asm constraints.
1685 // If the instruction has nomerge attribute, return false.
1686 if (const auto *C = dyn_cast<CallBase>(I))
1687 if (C->isInlineAsm() || C->cannotMerge())
1688 return false;
1689
1690 // Each instruction must have zero or one use.
1691 if (HasUse && !I->hasOneUse())
1692 return false;
1693 if (!HasUse && !I->user_empty())
1694 return false;
1695 }
1696
1697 const Instruction *I0 = Insts.front();
1698 for (auto *I : Insts)
1699 if (!I->isSameOperationAs(I0))
1700 return false;
1701
1702 // All instructions in Insts are known to be the same opcode. If they have a
1703 // use, check that the only user is a PHI or in the same block as the
1704 // instruction, because if a user is in the same block as an instruction we're
1705 // contemplating sinking, it must already be determined to be sinkable.
1706 if (HasUse) {
1707 auto *PNUse = dyn_cast<PHINode>(*I0->user_begin());
1708 auto *Succ = I0->getParent()->getTerminator()->getSuccessor(0);
1709 if (!all_of(Insts, [&PNUse,&Succ](const Instruction *I) -> bool {
1710 auto *U = cast<Instruction>(*I->user_begin());
1711 return (PNUse &&
1712 PNUse->getParent() == Succ &&
1713 PNUse->getIncomingValueForBlock(I->getParent()) == I) ||
1714 U->getParent() == I->getParent();
1715 }))
1716 return false;
1717 }
1718
1719 // Because SROA can't handle speculating stores of selects, try not to sink
1720 // loads, stores or lifetime markers of allocas when we'd have to create a
1721 // PHI for the address operand. Also, because it is likely that loads or
1722 // stores of allocas will disappear when Mem2Reg/SROA is run, don't sink
1723 // them.
1724 // This can cause code churn which can have unintended consequences down
1725 // the line - see https://llvm.org/bugs/show_bug.cgi?id=30244.
1726 // FIXME: This is a workaround for a deficiency in SROA - see
1727 // https://llvm.org/bugs/show_bug.cgi?id=30188
1728 if (isa<StoreInst>(I0) && any_of(Insts, [](const Instruction *I) {
1729 return isa<AllocaInst>(I->getOperand(1)->stripPointerCasts());
1730 }))
1731 return false;
1732 if (isa<LoadInst>(I0) && any_of(Insts, [](const Instruction *I) {
1733 return isa<AllocaInst>(I->getOperand(0)->stripPointerCasts());
1734 }))
1735 return false;
1736 if (isLifeTimeMarker(I0) && any_of(Insts, [](const Instruction *I) {
1737 return isa<AllocaInst>(I->getOperand(1)->stripPointerCasts());
1738 }))
1739 return false;
1740
1741 // For calls to be sinkable, they must all be indirect, or have same callee.
1742 // I.e. if we have two direct calls to different callees, we don't want to
1743 // turn that into an indirect call. Likewise, if we have an indirect call,
1744 // and a direct call, we don't actually want to have a single indirect call.
1745 if (isa<CallBase>(I0)) {
1746 auto IsIndirectCall = [](const Instruction *I) {
1747 return cast<CallBase>(I)->isIndirectCall();
1748 };
1749 bool HaveIndirectCalls = any_of(Insts, IsIndirectCall);
1750 bool AllCallsAreIndirect = all_of(Insts, IsIndirectCall);
1751 if (HaveIndirectCalls) {
1752 if (!AllCallsAreIndirect)
1753 return false;
1754 } else {
1755 // All callees must be identical.
1756 Value *Callee = nullptr;
1757 for (const Instruction *I : Insts) {
1758 Value *CurrCallee = cast<CallBase>(I)->getCalledOperand();
1759 if (!Callee)
1760 Callee = CurrCallee;
1761 else if (Callee != CurrCallee)
1762 return false;
1763 }
1764 }
1765 }
1766
1767 for (unsigned OI = 0, OE = I0->getNumOperands(); OI != OE; ++OI) {
1768 Value *Op = I0->getOperand(OI);
1769 if (Op->getType()->isTokenTy())
1770 // Don't touch any operand of token type.
1771 return false;
1772
1773 auto SameAsI0 = [&I0, OI](const Instruction *I) {
1774 assert(I->getNumOperands() == I0->getNumOperands());
1775 return I->getOperand(OI) == I0->getOperand(OI);
1776 };
1777 if (!all_of(Insts, SameAsI0)) {
1778 if ((isa<Constant>(Op) && !replacingOperandWithVariableIsCheap(I0, OI)) ||
1779 !canReplaceOperandWithVariable(I0, OI))
1780 // We can't create a PHI from this GEP.
1781 return false;
1782 for (auto *I : Insts)
1783 PHIOperands[I].push_back(I->getOperand(OI));
1784 }
1785 }
1786 return true;
1787 }
1788
1789 // Assuming canSinkInstructions(Blocks) has returned true, sink the last
1790 // instruction of every block in Blocks to their common successor, commoning
1791 // into one instruction.
sinkLastInstruction(ArrayRef<BasicBlock * > Blocks)1792 static bool sinkLastInstruction(ArrayRef<BasicBlock*> Blocks) {
1793 auto *BBEnd = Blocks[0]->getTerminator()->getSuccessor(0);
1794
1795 // canSinkInstructions returning true guarantees that every block has at
1796 // least one non-terminator instruction.
1797 SmallVector<Instruction*,4> Insts;
1798 for (auto *BB : Blocks) {
1799 Instruction *I = BB->getTerminator();
1800 do {
1801 I = I->getPrevNode();
1802 } while (isa<DbgInfoIntrinsic>(I) && I != &BB->front());
1803 if (!isa<DbgInfoIntrinsic>(I))
1804 Insts.push_back(I);
1805 }
1806
1807 // The only checking we need to do now is that all users of all instructions
1808 // are the same PHI node. canSinkInstructions should have checked this but
1809 // it is slightly over-aggressive - it gets confused by commutative
1810 // instructions so double-check it here.
1811 Instruction *I0 = Insts.front();
1812 if (!I0->user_empty()) {
1813 auto *PNUse = dyn_cast<PHINode>(*I0->user_begin());
1814 if (!all_of(Insts, [&PNUse](const Instruction *I) -> bool {
1815 auto *U = cast<Instruction>(*I->user_begin());
1816 return U == PNUse;
1817 }))
1818 return false;
1819 }
1820
1821 // We don't need to do any more checking here; canSinkInstructions should
1822 // have done it all for us.
1823 SmallVector<Value*, 4> NewOperands;
1824 for (unsigned O = 0, E = I0->getNumOperands(); O != E; ++O) {
1825 // This check is different to that in canSinkInstructions. There, we
1826 // cared about the global view once simplifycfg (and instcombine) have
1827 // completed - it takes into account PHIs that become trivially
1828 // simplifiable. However here we need a more local view; if an operand
1829 // differs we create a PHI and rely on instcombine to clean up the very
1830 // small mess we may make.
1831 bool NeedPHI = any_of(Insts, [&I0, O](const Instruction *I) {
1832 return I->getOperand(O) != I0->getOperand(O);
1833 });
1834 if (!NeedPHI) {
1835 NewOperands.push_back(I0->getOperand(O));
1836 continue;
1837 }
1838
1839 // Create a new PHI in the successor block and populate it.
1840 auto *Op = I0->getOperand(O);
1841 assert(!Op->getType()->isTokenTy() && "Can't PHI tokens!");
1842 auto *PN = PHINode::Create(Op->getType(), Insts.size(),
1843 Op->getName() + ".sink", &BBEnd->front());
1844 for (auto *I : Insts)
1845 PN->addIncoming(I->getOperand(O), I->getParent());
1846 NewOperands.push_back(PN);
1847 }
1848
1849 // Arbitrarily use I0 as the new "common" instruction; remap its operands
1850 // and move it to the start of the successor block.
1851 for (unsigned O = 0, E = I0->getNumOperands(); O != E; ++O)
1852 I0->getOperandUse(O).set(NewOperands[O]);
1853 I0->moveBefore(&*BBEnd->getFirstInsertionPt());
1854
1855 // Update metadata and IR flags, and merge debug locations.
1856 for (auto *I : Insts)
1857 if (I != I0) {
1858 // The debug location for the "common" instruction is the merged locations
1859 // of all the commoned instructions. We start with the original location
1860 // of the "common" instruction and iteratively merge each location in the
1861 // loop below.
1862 // This is an N-way merge, which will be inefficient if I0 is a CallInst.
1863 // However, as N-way merge for CallInst is rare, so we use simplified API
1864 // instead of using complex API for N-way merge.
1865 I0->applyMergedLocation(I0->getDebugLoc(), I->getDebugLoc());
1866 combineMetadataForCSE(I0, I, true);
1867 I0->andIRFlags(I);
1868 }
1869
1870 if (!I0->user_empty()) {
1871 // canSinkLastInstruction checked that all instructions were used by
1872 // one and only one PHI node. Find that now, RAUW it to our common
1873 // instruction and nuke it.
1874 auto *PN = cast<PHINode>(*I0->user_begin());
1875 PN->replaceAllUsesWith(I0);
1876 PN->eraseFromParent();
1877 }
1878
1879 // Finally nuke all instructions apart from the common instruction.
1880 for (auto *I : Insts)
1881 if (I != I0)
1882 I->eraseFromParent();
1883
1884 return true;
1885 }
1886
1887 namespace {
1888
1889 // LockstepReverseIterator - Iterates through instructions
1890 // in a set of blocks in reverse order from the first non-terminator.
1891 // For example (assume all blocks have size n):
1892 // LockstepReverseIterator I([B1, B2, B3]);
1893 // *I-- = [B1[n], B2[n], B3[n]];
1894 // *I-- = [B1[n-1], B2[n-1], B3[n-1]];
1895 // *I-- = [B1[n-2], B2[n-2], B3[n-2]];
1896 // ...
1897 class LockstepReverseIterator {
1898 ArrayRef<BasicBlock*> Blocks;
1899 SmallVector<Instruction*,4> Insts;
1900 bool Fail;
1901
1902 public:
LockstepReverseIterator(ArrayRef<BasicBlock * > Blocks)1903 LockstepReverseIterator(ArrayRef<BasicBlock*> Blocks) : Blocks(Blocks) {
1904 reset();
1905 }
1906
reset()1907 void reset() {
1908 Fail = false;
1909 Insts.clear();
1910 for (auto *BB : Blocks) {
1911 Instruction *Inst = BB->getTerminator();
1912 for (Inst = Inst->getPrevNode(); Inst && isa<DbgInfoIntrinsic>(Inst);)
1913 Inst = Inst->getPrevNode();
1914 if (!Inst) {
1915 // Block wasn't big enough.
1916 Fail = true;
1917 return;
1918 }
1919 Insts.push_back(Inst);
1920 }
1921 }
1922
isValid() const1923 bool isValid() const {
1924 return !Fail;
1925 }
1926
operator --()1927 void operator--() {
1928 if (Fail)
1929 return;
1930 for (auto *&Inst : Insts) {
1931 for (Inst = Inst->getPrevNode(); Inst && isa<DbgInfoIntrinsic>(Inst);)
1932 Inst = Inst->getPrevNode();
1933 // Already at beginning of block.
1934 if (!Inst) {
1935 Fail = true;
1936 return;
1937 }
1938 }
1939 }
1940
operator ++()1941 void operator++() {
1942 if (Fail)
1943 return;
1944 for (auto *&Inst : Insts) {
1945 for (Inst = Inst->getNextNode(); Inst && isa<DbgInfoIntrinsic>(Inst);)
1946 Inst = Inst->getNextNode();
1947 // Already at end of block.
1948 if (!Inst) {
1949 Fail = true;
1950 return;
1951 }
1952 }
1953 }
1954
operator *() const1955 ArrayRef<Instruction*> operator * () const {
1956 return Insts;
1957 }
1958 };
1959
1960 } // end anonymous namespace
1961
1962 /// Check whether BB's predecessors end with unconditional branches. If it is
1963 /// true, sink any common code from the predecessors to BB.
SinkCommonCodeFromPredecessors(BasicBlock * BB,DomTreeUpdater * DTU)1964 static bool SinkCommonCodeFromPredecessors(BasicBlock *BB,
1965 DomTreeUpdater *DTU) {
1966 // We support two situations:
1967 // (1) all incoming arcs are unconditional
1968 // (2) there are non-unconditional incoming arcs
1969 //
1970 // (2) is very common in switch defaults and
1971 // else-if patterns;
1972 //
1973 // if (a) f(1);
1974 // else if (b) f(2);
1975 //
1976 // produces:
1977 //
1978 // [if]
1979 // / \
1980 // [f(1)] [if]
1981 // | | \
1982 // | | |
1983 // | [f(2)]|
1984 // \ | /
1985 // [ end ]
1986 //
1987 // [end] has two unconditional predecessor arcs and one conditional. The
1988 // conditional refers to the implicit empty 'else' arc. This conditional
1989 // arc can also be caused by an empty default block in a switch.
1990 //
1991 // In this case, we attempt to sink code from all *unconditional* arcs.
1992 // If we can sink instructions from these arcs (determined during the scan
1993 // phase below) we insert a common successor for all unconditional arcs and
1994 // connect that to [end], to enable sinking:
1995 //
1996 // [if]
1997 // / \
1998 // [x(1)] [if]
1999 // | | \
2000 // | | \
2001 // | [x(2)] |
2002 // \ / |
2003 // [sink.split] |
2004 // \ /
2005 // [ end ]
2006 //
2007 SmallVector<BasicBlock*,4> UnconditionalPreds;
2008 bool HaveNonUnconditionalPredecessors = false;
2009 for (auto *PredBB : predecessors(BB)) {
2010 auto *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
2011 if (PredBr && PredBr->isUnconditional())
2012 UnconditionalPreds.push_back(PredBB);
2013 else
2014 HaveNonUnconditionalPredecessors = true;
2015 }
2016 if (UnconditionalPreds.size() < 2)
2017 return false;
2018
2019 // We take a two-step approach to tail sinking. First we scan from the end of
2020 // each block upwards in lockstep. If the n'th instruction from the end of each
2021 // block can be sunk, those instructions are added to ValuesToSink and we
2022 // carry on. If we can sink an instruction but need to PHI-merge some operands
2023 // (because they're not identical in each instruction) we add these to
2024 // PHIOperands.
2025 int ScanIdx = 0;
2026 SmallPtrSet<Value*,4> InstructionsToSink;
2027 DenseMap<Instruction*, SmallVector<Value*,4>> PHIOperands;
2028 LockstepReverseIterator LRI(UnconditionalPreds);
2029 while (LRI.isValid() &&
2030 canSinkInstructions(*LRI, PHIOperands)) {
2031 LLVM_DEBUG(dbgs() << "SINK: instruction can be sunk: " << *(*LRI)[0]
2032 << "\n");
2033 InstructionsToSink.insert((*LRI).begin(), (*LRI).end());
2034 ++ScanIdx;
2035 --LRI;
2036 }
2037
2038 // If no instructions can be sunk, early-return.
2039 if (ScanIdx == 0)
2040 return false;
2041
2042 // Okay, we *could* sink last ScanIdx instructions. But how many can we
2043 // actually sink before encountering instruction that is unprofitable to sink?
2044 auto ProfitableToSinkInstruction = [&](LockstepReverseIterator &LRI) {
2045 unsigned NumPHIdValues = 0;
2046 for (auto *I : *LRI)
2047 for (auto *V : PHIOperands[I]) {
2048 if (InstructionsToSink.count(V) == 0)
2049 ++NumPHIdValues;
2050 // FIXME: this check is overly optimistic. We may end up not sinking
2051 // said instruction, due to the very same profitability check.
2052 // See @creating_too_many_phis in sink-common-code.ll.
2053 }
2054 LLVM_DEBUG(dbgs() << "SINK: #phid values: " << NumPHIdValues << "\n");
2055 unsigned NumPHIInsts = NumPHIdValues / UnconditionalPreds.size();
2056 if ((NumPHIdValues % UnconditionalPreds.size()) != 0)
2057 NumPHIInsts++;
2058
2059 return NumPHIInsts <= 1;
2060 };
2061
2062 // If no instructions can be sunk, early-return.
2063 if (ScanIdx == 0)
2064 return false;
2065
2066 // We've determined that we are going to sink last ScanIdx instructions,
2067 // and recorded them in InstructionsToSink. Now, some instructions may be
2068 // unprofitable to sink. But that determination depends on the instructions
2069 // that we are going to sink.
2070
2071 // First, forward scan: find the first instruction unprofitable to sink,
2072 // recording all the ones that are profitable to sink.
2073 // FIXME: would it be better, after we detect that not all are profitable.
2074 // to either record the profitable ones, or erase the unprofitable ones?
2075 // Maybe we need to choose (at runtime) the one that will touch least instrs?
2076 LRI.reset();
2077 int Idx = 0;
2078 SmallPtrSet<Value *, 4> InstructionsProfitableToSink;
2079 while (Idx < ScanIdx) {
2080 if (!ProfitableToSinkInstruction(LRI)) {
2081 // Too many PHIs would be created.
2082 LLVM_DEBUG(
2083 dbgs() << "SINK: stopping here, too many PHIs would be created!\n");
2084 break;
2085 }
2086 InstructionsProfitableToSink.insert((*LRI).begin(), (*LRI).end());
2087 --LRI;
2088 ++Idx;
2089 }
2090
2091 // If no instructions can be sunk, early-return.
2092 if (Idx == 0)
2093 return false;
2094
2095 // Did we determine that (only) some instructions are unprofitable to sink?
2096 if (Idx < ScanIdx) {
2097 // Okay, some instructions are unprofitable.
2098 ScanIdx = Idx;
2099 InstructionsToSink = InstructionsProfitableToSink;
2100
2101 // But, that may make other instructions unprofitable, too.
2102 // So, do a backward scan, do any earlier instructions become unprofitable?
2103 assert(!ProfitableToSinkInstruction(LRI) &&
2104 "We already know that the last instruction is unprofitable to sink");
2105 ++LRI;
2106 --Idx;
2107 while (Idx >= 0) {
2108 // If we detect that an instruction becomes unprofitable to sink,
2109 // all earlier instructions won't be sunk either,
2110 // so preemptively keep InstructionsProfitableToSink in sync.
2111 // FIXME: is this the most performant approach?
2112 for (auto *I : *LRI)
2113 InstructionsProfitableToSink.erase(I);
2114 if (!ProfitableToSinkInstruction(LRI)) {
2115 // Everything starting with this instruction won't be sunk.
2116 ScanIdx = Idx;
2117 InstructionsToSink = InstructionsProfitableToSink;
2118 }
2119 ++LRI;
2120 --Idx;
2121 }
2122 }
2123
2124 // If no instructions can be sunk, early-return.
2125 if (ScanIdx == 0)
2126 return false;
2127
2128 bool Changed = false;
2129
2130 if (HaveNonUnconditionalPredecessors) {
2131 // It is always legal to sink common instructions from unconditional
2132 // predecessors. However, if not all predecessors are unconditional,
2133 // this transformation might be pessimizing. So as a rule of thumb,
2134 // don't do it unless we'd sink at least one non-speculatable instruction.
2135 // See https://bugs.llvm.org/show_bug.cgi?id=30244
2136 LRI.reset();
2137 int Idx = 0;
2138 bool Profitable = false;
2139 while (Idx < ScanIdx) {
2140 if (!isSafeToSpeculativelyExecute((*LRI)[0])) {
2141 Profitable = true;
2142 break;
2143 }
2144 --LRI;
2145 ++Idx;
2146 }
2147 if (!Profitable)
2148 return false;
2149
2150 LLVM_DEBUG(dbgs() << "SINK: Splitting edge\n");
2151 // We have a conditional edge and we're going to sink some instructions.
2152 // Insert a new block postdominating all blocks we're going to sink from.
2153 if (!SplitBlockPredecessors(BB, UnconditionalPreds, ".sink.split", DTU))
2154 // Edges couldn't be split.
2155 return false;
2156 Changed = true;
2157 }
2158
2159 // Now that we've analyzed all potential sinking candidates, perform the
2160 // actual sink. We iteratively sink the last non-terminator of the source
2161 // blocks into their common successor unless doing so would require too
2162 // many PHI instructions to be generated (currently only one PHI is allowed
2163 // per sunk instruction).
2164 //
2165 // We can use InstructionsToSink to discount values needing PHI-merging that will
2166 // actually be sunk in a later iteration. This allows us to be more
2167 // aggressive in what we sink. This does allow a false positive where we
2168 // sink presuming a later value will also be sunk, but stop half way through
2169 // and never actually sink it which means we produce more PHIs than intended.
2170 // This is unlikely in practice though.
2171 int SinkIdx = 0;
2172 for (; SinkIdx != ScanIdx; ++SinkIdx) {
2173 LLVM_DEBUG(dbgs() << "SINK: Sink: "
2174 << *UnconditionalPreds[0]->getTerminator()->getPrevNode()
2175 << "\n");
2176
2177 // Because we've sunk every instruction in turn, the current instruction to
2178 // sink is always at index 0.
2179 LRI.reset();
2180
2181 if (!sinkLastInstruction(UnconditionalPreds)) {
2182 LLVM_DEBUG(
2183 dbgs()
2184 << "SINK: stopping here, failed to actually sink instruction!\n");
2185 break;
2186 }
2187
2188 NumSinkCommonInstrs++;
2189 Changed = true;
2190 }
2191 if (SinkIdx != 0)
2192 ++NumSinkCommonCode;
2193 return Changed;
2194 }
2195
2196 /// Determine if we can hoist sink a sole store instruction out of a
2197 /// conditional block.
2198 ///
2199 /// We are looking for code like the following:
2200 /// BrBB:
2201 /// store i32 %add, i32* %arrayidx2
2202 /// ... // No other stores or function calls (we could be calling a memory
2203 /// ... // function).
2204 /// %cmp = icmp ult %x, %y
2205 /// br i1 %cmp, label %EndBB, label %ThenBB
2206 /// ThenBB:
2207 /// store i32 %add5, i32* %arrayidx2
2208 /// br label EndBB
2209 /// EndBB:
2210 /// ...
2211 /// We are going to transform this into:
2212 /// BrBB:
2213 /// store i32 %add, i32* %arrayidx2
2214 /// ... //
2215 /// %cmp = icmp ult %x, %y
2216 /// %add.add5 = select i1 %cmp, i32 %add, %add5
2217 /// store i32 %add.add5, i32* %arrayidx2
2218 /// ...
2219 ///
2220 /// \return The pointer to the value of the previous store if the store can be
2221 /// hoisted into the predecessor block. 0 otherwise.
isSafeToSpeculateStore(Instruction * I,BasicBlock * BrBB,BasicBlock * StoreBB,BasicBlock * EndBB)2222 static Value *isSafeToSpeculateStore(Instruction *I, BasicBlock *BrBB,
2223 BasicBlock *StoreBB, BasicBlock *EndBB) {
2224 StoreInst *StoreToHoist = dyn_cast<StoreInst>(I);
2225 if (!StoreToHoist)
2226 return nullptr;
2227
2228 // Volatile or atomic.
2229 if (!StoreToHoist->isSimple())
2230 return nullptr;
2231
2232 Value *StorePtr = StoreToHoist->getPointerOperand();
2233
2234 // Look for a store to the same pointer in BrBB.
2235 unsigned MaxNumInstToLookAt = 9;
2236 // Skip pseudo probe intrinsic calls which are not really killing any memory
2237 // accesses.
2238 for (Instruction &CurI : reverse(BrBB->instructionsWithoutDebug(true))) {
2239 if (!MaxNumInstToLookAt)
2240 break;
2241 --MaxNumInstToLookAt;
2242
2243 // Could be calling an instruction that affects memory like free().
2244 if (CurI.mayHaveSideEffects() && !isa<StoreInst>(CurI))
2245 return nullptr;
2246
2247 if (auto *SI = dyn_cast<StoreInst>(&CurI)) {
2248 // Found the previous store make sure it stores to the same location.
2249 if (SI->getPointerOperand() == StorePtr)
2250 // Found the previous store, return its value operand.
2251 return SI->getValueOperand();
2252 return nullptr; // Unknown store.
2253 }
2254 }
2255
2256 return nullptr;
2257 }
2258
2259 /// Estimate the cost of the insertion(s) and check that the PHI nodes can be
2260 /// converted to selects.
validateAndCostRequiredSelects(BasicBlock * BB,BasicBlock * ThenBB,BasicBlock * EndBB,unsigned & SpeculatedInstructions,InstructionCost & Cost,const TargetTransformInfo & TTI)2261 static bool validateAndCostRequiredSelects(BasicBlock *BB, BasicBlock *ThenBB,
2262 BasicBlock *EndBB,
2263 unsigned &SpeculatedInstructions,
2264 InstructionCost &Cost,
2265 const TargetTransformInfo &TTI) {
2266 TargetTransformInfo::TargetCostKind CostKind =
2267 BB->getParent()->hasMinSize()
2268 ? TargetTransformInfo::TCK_CodeSize
2269 : TargetTransformInfo::TCK_SizeAndLatency;
2270
2271 bool HaveRewritablePHIs = false;
2272 for (PHINode &PN : EndBB->phis()) {
2273 Value *OrigV = PN.getIncomingValueForBlock(BB);
2274 Value *ThenV = PN.getIncomingValueForBlock(ThenBB);
2275
2276 // FIXME: Try to remove some of the duplication with HoistThenElseCodeToIf.
2277 // Skip PHIs which are trivial.
2278 if (ThenV == OrigV)
2279 continue;
2280
2281 Cost += TTI.getCmpSelInstrCost(Instruction::Select, PN.getType(), nullptr,
2282 CmpInst::BAD_ICMP_PREDICATE, CostKind);
2283
2284 // Don't convert to selects if we could remove undefined behavior instead.
2285 if (passingValueIsAlwaysUndefined(OrigV, &PN) ||
2286 passingValueIsAlwaysUndefined(ThenV, &PN))
2287 return false;
2288
2289 HaveRewritablePHIs = true;
2290 ConstantExpr *OrigCE = dyn_cast<ConstantExpr>(OrigV);
2291 ConstantExpr *ThenCE = dyn_cast<ConstantExpr>(ThenV);
2292 if (!OrigCE && !ThenCE)
2293 continue; // Known safe and cheap.
2294
2295 if ((ThenCE && !isSafeToSpeculativelyExecute(ThenCE)) ||
2296 (OrigCE && !isSafeToSpeculativelyExecute(OrigCE)))
2297 return false;
2298 InstructionCost OrigCost = OrigCE ? computeSpeculationCost(OrigCE, TTI) : 0;
2299 InstructionCost ThenCost = ThenCE ? computeSpeculationCost(ThenCE, TTI) : 0;
2300 InstructionCost MaxCost =
2301 2 * PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
2302 if (OrigCost + ThenCost > MaxCost)
2303 return false;
2304
2305 // Account for the cost of an unfolded ConstantExpr which could end up
2306 // getting expanded into Instructions.
2307 // FIXME: This doesn't account for how many operations are combined in the
2308 // constant expression.
2309 ++SpeculatedInstructions;
2310 if (SpeculatedInstructions > 1)
2311 return false;
2312 }
2313
2314 return HaveRewritablePHIs;
2315 }
2316
2317 /// Speculate a conditional basic block flattening the CFG.
2318 ///
2319 /// Note that this is a very risky transform currently. Speculating
2320 /// instructions like this is most often not desirable. Instead, there is an MI
2321 /// pass which can do it with full awareness of the resource constraints.
2322 /// However, some cases are "obvious" and we should do directly. An example of
2323 /// this is speculating a single, reasonably cheap instruction.
2324 ///
2325 /// There is only one distinct advantage to flattening the CFG at the IR level:
2326 /// it makes very common but simplistic optimizations such as are common in
2327 /// instcombine and the DAG combiner more powerful by removing CFG edges and
2328 /// modeling their effects with easier to reason about SSA value graphs.
2329 ///
2330 ///
2331 /// An illustration of this transform is turning this IR:
2332 /// \code
2333 /// BB:
2334 /// %cmp = icmp ult %x, %y
2335 /// br i1 %cmp, label %EndBB, label %ThenBB
2336 /// ThenBB:
2337 /// %sub = sub %x, %y
2338 /// br label BB2
2339 /// EndBB:
2340 /// %phi = phi [ %sub, %ThenBB ], [ 0, %EndBB ]
2341 /// ...
2342 /// \endcode
2343 ///
2344 /// Into this IR:
2345 /// \code
2346 /// BB:
2347 /// %cmp = icmp ult %x, %y
2348 /// %sub = sub %x, %y
2349 /// %cond = select i1 %cmp, 0, %sub
2350 /// ...
2351 /// \endcode
2352 ///
2353 /// \returns true if the conditional block is removed.
SpeculativelyExecuteBB(BranchInst * BI,BasicBlock * ThenBB,const TargetTransformInfo & TTI)2354 bool SimplifyCFGOpt::SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
2355 const TargetTransformInfo &TTI) {
2356 // Be conservative for now. FP select instruction can often be expensive.
2357 Value *BrCond = BI->getCondition();
2358 if (isa<FCmpInst>(BrCond))
2359 return false;
2360
2361 BasicBlock *BB = BI->getParent();
2362 BasicBlock *EndBB = ThenBB->getTerminator()->getSuccessor(0);
2363 InstructionCost Budget =
2364 PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
2365
2366 // If ThenBB is actually on the false edge of the conditional branch, remember
2367 // to swap the select operands later.
2368 bool Invert = false;
2369 if (ThenBB != BI->getSuccessor(0)) {
2370 assert(ThenBB == BI->getSuccessor(1) && "No edge from 'if' block?");
2371 Invert = true;
2372 }
2373 assert(EndBB == BI->getSuccessor(!Invert) && "No edge from to end block");
2374
2375 // Keep a count of how many times instructions are used within ThenBB when
2376 // they are candidates for sinking into ThenBB. Specifically:
2377 // - They are defined in BB, and
2378 // - They have no side effects, and
2379 // - All of their uses are in ThenBB.
2380 SmallDenseMap<Instruction *, unsigned, 4> SinkCandidateUseCounts;
2381
2382 SmallVector<Instruction *, 4> SpeculatedDbgIntrinsics;
2383
2384 unsigned SpeculatedInstructions = 0;
2385 Value *SpeculatedStoreValue = nullptr;
2386 StoreInst *SpeculatedStore = nullptr;
2387 for (BasicBlock::iterator BBI = ThenBB->begin(),
2388 BBE = std::prev(ThenBB->end());
2389 BBI != BBE; ++BBI) {
2390 Instruction *I = &*BBI;
2391 // Skip debug info.
2392 if (isa<DbgInfoIntrinsic>(I)) {
2393 SpeculatedDbgIntrinsics.push_back(I);
2394 continue;
2395 }
2396
2397 // Skip pseudo probes. The consequence is we lose track of the branch
2398 // probability for ThenBB, which is fine since the optimization here takes
2399 // place regardless of the branch probability.
2400 if (isa<PseudoProbeInst>(I)) {
2401 continue;
2402 }
2403
2404 // Only speculatively execute a single instruction (not counting the
2405 // terminator) for now.
2406 ++SpeculatedInstructions;
2407 if (SpeculatedInstructions > 1)
2408 return false;
2409
2410 // Don't hoist the instruction if it's unsafe or expensive.
2411 if (!isSafeToSpeculativelyExecute(I) &&
2412 !(HoistCondStores && (SpeculatedStoreValue = isSafeToSpeculateStore(
2413 I, BB, ThenBB, EndBB))))
2414 return false;
2415 if (!SpeculatedStoreValue &&
2416 computeSpeculationCost(I, TTI) >
2417 PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic)
2418 return false;
2419
2420 // Store the store speculation candidate.
2421 if (SpeculatedStoreValue)
2422 SpeculatedStore = cast<StoreInst>(I);
2423
2424 // Do not hoist the instruction if any of its operands are defined but not
2425 // used in BB. The transformation will prevent the operand from
2426 // being sunk into the use block.
2427 for (Use &Op : I->operands()) {
2428 Instruction *OpI = dyn_cast<Instruction>(Op);
2429 if (!OpI || OpI->getParent() != BB || OpI->mayHaveSideEffects())
2430 continue; // Not a candidate for sinking.
2431
2432 ++SinkCandidateUseCounts[OpI];
2433 }
2434 }
2435
2436 // Consider any sink candidates which are only used in ThenBB as costs for
2437 // speculation. Note, while we iterate over a DenseMap here, we are summing
2438 // and so iteration order isn't significant.
2439 for (SmallDenseMap<Instruction *, unsigned, 4>::iterator
2440 I = SinkCandidateUseCounts.begin(),
2441 E = SinkCandidateUseCounts.end();
2442 I != E; ++I)
2443 if (I->first->hasNUses(I->second)) {
2444 ++SpeculatedInstructions;
2445 if (SpeculatedInstructions > 1)
2446 return false;
2447 }
2448
2449 // Check that we can insert the selects and that it's not too expensive to do
2450 // so.
2451 bool Convert = SpeculatedStore != nullptr;
2452 InstructionCost Cost = 0;
2453 Convert |= validateAndCostRequiredSelects(BB, ThenBB, EndBB,
2454 SpeculatedInstructions,
2455 Cost, TTI);
2456 if (!Convert || Cost > Budget)
2457 return false;
2458
2459 // If we get here, we can hoist the instruction and if-convert.
2460 LLVM_DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *ThenBB << "\n";);
2461
2462 // Insert a select of the value of the speculated store.
2463 if (SpeculatedStoreValue) {
2464 IRBuilder<NoFolder> Builder(BI);
2465 Value *TrueV = SpeculatedStore->getValueOperand();
2466 Value *FalseV = SpeculatedStoreValue;
2467 if (Invert)
2468 std::swap(TrueV, FalseV);
2469 Value *S = Builder.CreateSelect(
2470 BrCond, TrueV, FalseV, "spec.store.select", BI);
2471 SpeculatedStore->setOperand(0, S);
2472 SpeculatedStore->applyMergedLocation(BI->getDebugLoc(),
2473 SpeculatedStore->getDebugLoc());
2474 }
2475
2476 // A hoisted conditional probe should be treated as dangling so that it will
2477 // not be over-counted when the samples collected on the non-conditional path
2478 // are counted towards the conditional path. We leave it for the counts
2479 // inference algorithm to figure out a proper count for a danglng probe.
2480 moveAndDanglePseudoProbes(ThenBB, BI);
2481
2482 // Metadata can be dependent on the condition we are hoisting above.
2483 // Conservatively strip all metadata on the instruction. Drop the debug loc
2484 // to avoid making it appear as if the condition is a constant, which would
2485 // be misleading while debugging.
2486 for (auto &I : *ThenBB) {
2487 assert(!isa<PseudoProbeInst>(I) &&
2488 "Should not drop debug info from any pseudo probes.");
2489 if (!SpeculatedStoreValue || &I != SpeculatedStore)
2490 I.setDebugLoc(DebugLoc());
2491 I.dropUnknownNonDebugMetadata();
2492 }
2493
2494 // Hoist the instructions.
2495 BB->getInstList().splice(BI->getIterator(), ThenBB->getInstList(),
2496 ThenBB->begin(), std::prev(ThenBB->end()));
2497
2498 // Insert selects and rewrite the PHI operands.
2499 IRBuilder<NoFolder> Builder(BI);
2500 for (PHINode &PN : EndBB->phis()) {
2501 unsigned OrigI = PN.getBasicBlockIndex(BB);
2502 unsigned ThenI = PN.getBasicBlockIndex(ThenBB);
2503 Value *OrigV = PN.getIncomingValue(OrigI);
2504 Value *ThenV = PN.getIncomingValue(ThenI);
2505
2506 // Skip PHIs which are trivial.
2507 if (OrigV == ThenV)
2508 continue;
2509
2510 // Create a select whose true value is the speculatively executed value and
2511 // false value is the pre-existing value. Swap them if the branch
2512 // destinations were inverted.
2513 Value *TrueV = ThenV, *FalseV = OrigV;
2514 if (Invert)
2515 std::swap(TrueV, FalseV);
2516 Value *V = Builder.CreateSelect(BrCond, TrueV, FalseV, "spec.select", BI);
2517 PN.setIncomingValue(OrigI, V);
2518 PN.setIncomingValue(ThenI, V);
2519 }
2520
2521 // Remove speculated dbg intrinsics.
2522 // FIXME: Is it possible to do this in a more elegant way? Moving/merging the
2523 // dbg value for the different flows and inserting it after the select.
2524 for (Instruction *I : SpeculatedDbgIntrinsics)
2525 I->eraseFromParent();
2526
2527 ++NumSpeculations;
2528 return true;
2529 }
2530
2531 /// Return true if we can thread a branch across this block.
BlockIsSimpleEnoughToThreadThrough(BasicBlock * BB)2532 static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
2533 int Size = 0;
2534
2535 SmallPtrSet<const Value *, 32> EphValues;
2536 auto IsEphemeral = [&](const Value *V) {
2537 if (isa<AssumeInst>(V))
2538 return true;
2539 return isSafeToSpeculativelyExecute(V) &&
2540 all_of(V->users(),
2541 [&](const User *U) { return EphValues.count(U); });
2542 };
2543
2544 // Walk the loop in reverse so that we can identify ephemeral values properly
2545 // (values only feeding assumes).
2546 for (Instruction &I : reverse(BB->instructionsWithoutDebug())) {
2547 // Can't fold blocks that contain noduplicate or convergent calls.
2548 if (CallInst *CI = dyn_cast<CallInst>(&I))
2549 if (CI->cannotDuplicate() || CI->isConvergent())
2550 return false;
2551
2552 // Ignore ephemeral values which are deleted during codegen.
2553 if (IsEphemeral(&I))
2554 EphValues.insert(&I);
2555 // We will delete Phis while threading, so Phis should not be accounted in
2556 // block's size.
2557 else if (!isa<PHINode>(I)) {
2558 if (Size++ > MaxSmallBlockSize)
2559 return false; // Don't clone large BB's.
2560 }
2561
2562 // We can only support instructions that do not define values that are
2563 // live outside of the current basic block.
2564 for (User *U : I.users()) {
2565 Instruction *UI = cast<Instruction>(U);
2566 if (UI->getParent() != BB || isa<PHINode>(UI))
2567 return false;
2568 }
2569
2570 // Looks ok, continue checking.
2571 }
2572
2573 return true;
2574 }
2575
2576 /// If we have a conditional branch on a PHI node value that is defined in the
2577 /// same block as the branch and if any PHI entries are constants, thread edges
2578 /// corresponding to that entry to be branches to their ultimate destination.
FoldCondBranchOnPHI(BranchInst * BI,DomTreeUpdater * DTU,const DataLayout & DL,AssumptionCache * AC)2579 static bool FoldCondBranchOnPHI(BranchInst *BI, DomTreeUpdater *DTU,
2580 const DataLayout &DL, AssumptionCache *AC) {
2581 BasicBlock *BB = BI->getParent();
2582 PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
2583 // NOTE: we currently cannot transform this case if the PHI node is used
2584 // outside of the block.
2585 if (!PN || PN->getParent() != BB || !PN->hasOneUse())
2586 return false;
2587
2588 // Degenerate case of a single entry PHI.
2589 if (PN->getNumIncomingValues() == 1) {
2590 FoldSingleEntryPHINodes(PN->getParent());
2591 return true;
2592 }
2593
2594 // Now we know that this block has multiple preds and two succs.
2595 if (!BlockIsSimpleEnoughToThreadThrough(BB))
2596 return false;
2597
2598 // Okay, this is a simple enough basic block. See if any phi values are
2599 // constants.
2600 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
2601 ConstantInt *CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i));
2602 if (!CB || !CB->getType()->isIntegerTy(1))
2603 continue;
2604
2605 // Okay, we now know that all edges from PredBB should be revectored to
2606 // branch to RealDest.
2607 BasicBlock *PredBB = PN->getIncomingBlock(i);
2608 BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue());
2609
2610 if (RealDest == BB)
2611 continue; // Skip self loops.
2612 // Skip if the predecessor's terminator is an indirect branch.
2613 if (isa<IndirectBrInst>(PredBB->getTerminator()))
2614 continue;
2615
2616 SmallVector<DominatorTree::UpdateType, 3> Updates;
2617
2618 // The dest block might have PHI nodes, other predecessors and other
2619 // difficult cases. Instead of being smart about this, just insert a new
2620 // block that jumps to the destination block, effectively splitting
2621 // the edge we are about to create.
2622 BasicBlock *EdgeBB =
2623 BasicBlock::Create(BB->getContext(), RealDest->getName() + ".critedge",
2624 RealDest->getParent(), RealDest);
2625 BranchInst *CritEdgeBranch = BranchInst::Create(RealDest, EdgeBB);
2626 if (DTU)
2627 Updates.push_back({DominatorTree::Insert, EdgeBB, RealDest});
2628 CritEdgeBranch->setDebugLoc(BI->getDebugLoc());
2629
2630 // Update PHI nodes.
2631 AddPredecessorToBlock(RealDest, EdgeBB, BB);
2632
2633 // BB may have instructions that are being threaded over. Clone these
2634 // instructions into EdgeBB. We know that there will be no uses of the
2635 // cloned instructions outside of EdgeBB.
2636 BasicBlock::iterator InsertPt = EdgeBB->begin();
2637 DenseMap<Value *, Value *> TranslateMap; // Track translated values.
2638 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
2639 if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
2640 TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB);
2641 continue;
2642 }
2643 // Clone the instruction.
2644 Instruction *N = BBI->clone();
2645 if (BBI->hasName())
2646 N->setName(BBI->getName() + ".c");
2647
2648 // Update operands due to translation.
2649 for (Use &Op : N->operands()) {
2650 DenseMap<Value *, Value *>::iterator PI = TranslateMap.find(Op);
2651 if (PI != TranslateMap.end())
2652 Op = PI->second;
2653 }
2654
2655 // Check for trivial simplification.
2656 if (Value *V = SimplifyInstruction(N, {DL, nullptr, nullptr, AC})) {
2657 if (!BBI->use_empty())
2658 TranslateMap[&*BBI] = V;
2659 if (!N->mayHaveSideEffects()) {
2660 N->deleteValue(); // Instruction folded away, don't need actual inst
2661 N = nullptr;
2662 }
2663 } else {
2664 if (!BBI->use_empty())
2665 TranslateMap[&*BBI] = N;
2666 }
2667 if (N) {
2668 // Insert the new instruction into its new home.
2669 EdgeBB->getInstList().insert(InsertPt, N);
2670
2671 // Register the new instruction with the assumption cache if necessary.
2672 if (auto *Assume = dyn_cast<AssumeInst>(N))
2673 if (AC)
2674 AC->registerAssumption(Assume);
2675 }
2676 }
2677
2678 // Loop over all of the edges from PredBB to BB, changing them to branch
2679 // to EdgeBB instead.
2680 Instruction *PredBBTI = PredBB->getTerminator();
2681 for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i)
2682 if (PredBBTI->getSuccessor(i) == BB) {
2683 BB->removePredecessor(PredBB);
2684 PredBBTI->setSuccessor(i, EdgeBB);
2685 }
2686
2687 if (DTU) {
2688 Updates.push_back({DominatorTree::Insert, PredBB, EdgeBB});
2689 Updates.push_back({DominatorTree::Delete, PredBB, BB});
2690
2691 DTU->applyUpdates(Updates);
2692 }
2693
2694 // Recurse, simplifying any other constants.
2695 return FoldCondBranchOnPHI(BI, DTU, DL, AC) || true;
2696 }
2697
2698 return false;
2699 }
2700
2701 /// Given a BB that starts with the specified two-entry PHI node,
2702 /// see if we can eliminate it.
FoldTwoEntryPHINode(PHINode * PN,const TargetTransformInfo & TTI,DomTreeUpdater * DTU,const DataLayout & DL)2703 static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
2704 DomTreeUpdater *DTU, const DataLayout &DL) {
2705 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
2706 // statement", which has a very simple dominance structure. Basically, we
2707 // are trying to find the condition that is being branched on, which
2708 // subsequently causes this merge to happen. We really want control
2709 // dependence information for this check, but simplifycfg can't keep it up
2710 // to date, and this catches most of the cases we care about anyway.
2711 BasicBlock *BB = PN->getParent();
2712
2713 BasicBlock *IfTrue, *IfFalse;
2714 Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
2715 if (!IfCond ||
2716 // Don't bother if the branch will be constant folded trivially.
2717 isa<ConstantInt>(IfCond))
2718 return false;
2719
2720 // Okay, we found that we can merge this two-entry phi node into a select.
2721 // Doing so would require us to fold *all* two entry phi nodes in this block.
2722 // At some point this becomes non-profitable (particularly if the target
2723 // doesn't support cmov's). Only do this transformation if there are two or
2724 // fewer PHI nodes in this block.
2725 unsigned NumPhis = 0;
2726 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I)
2727 if (NumPhis > 2)
2728 return false;
2729
2730 // Loop over the PHI's seeing if we can promote them all to select
2731 // instructions. While we are at it, keep track of the instructions
2732 // that need to be moved to the dominating block.
2733 SmallPtrSet<Instruction *, 4> AggressiveInsts;
2734 InstructionCost Cost = 0;
2735 InstructionCost Budget =
2736 TwoEntryPHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
2737
2738 bool Changed = false;
2739 for (BasicBlock::iterator II = BB->begin(); isa<PHINode>(II);) {
2740 PHINode *PN = cast<PHINode>(II++);
2741 if (Value *V = SimplifyInstruction(PN, {DL, PN})) {
2742 PN->replaceAllUsesWith(V);
2743 PN->eraseFromParent();
2744 Changed = true;
2745 continue;
2746 }
2747
2748 if (!dominatesMergePoint(PN->getIncomingValue(0), BB, AggressiveInsts,
2749 Cost, Budget, TTI) ||
2750 !dominatesMergePoint(PN->getIncomingValue(1), BB, AggressiveInsts,
2751 Cost, Budget, TTI))
2752 return Changed;
2753 }
2754
2755 // If we folded the first phi, PN dangles at this point. Refresh it. If
2756 // we ran out of PHIs then we simplified them all.
2757 PN = dyn_cast<PHINode>(BB->begin());
2758 if (!PN)
2759 return true;
2760
2761 // Return true if at least one of these is a 'not', and another is either
2762 // a 'not' too, or a constant.
2763 auto CanHoistNotFromBothValues = [](Value *V0, Value *V1) {
2764 if (!match(V0, m_Not(m_Value())))
2765 std::swap(V0, V1);
2766 auto Invertible = m_CombineOr(m_Not(m_Value()), m_AnyIntegralConstant());
2767 return match(V0, m_Not(m_Value())) && match(V1, Invertible);
2768 };
2769
2770 // Don't fold i1 branches on PHIs which contain binary operators or
2771 // select form of or/ands, unless one of the incoming values is an 'not' and
2772 // another one is freely invertible.
2773 // These can often be turned into switches and other things.
2774 auto IsBinOpOrAnd = [](Value *V) {
2775 return match(
2776 V, m_CombineOr(m_BinOp(), m_CombineOr(m_LogicalAnd(), m_LogicalOr())));
2777 };
2778 if (PN->getType()->isIntegerTy(1) &&
2779 (IsBinOpOrAnd(PN->getIncomingValue(0)) ||
2780 IsBinOpOrAnd(PN->getIncomingValue(1)) || IsBinOpOrAnd(IfCond)) &&
2781 !CanHoistNotFromBothValues(PN->getIncomingValue(0),
2782 PN->getIncomingValue(1)))
2783 return Changed;
2784
2785 // If all PHI nodes are promotable, check to make sure that all instructions
2786 // in the predecessor blocks can be promoted as well. If not, we won't be able
2787 // to get rid of the control flow, so it's not worth promoting to select
2788 // instructions.
2789 BasicBlock *DomBlock = nullptr;
2790 BasicBlock *IfBlock1 = PN->getIncomingBlock(0);
2791 BasicBlock *IfBlock2 = PN->getIncomingBlock(1);
2792 if (cast<BranchInst>(IfBlock1->getTerminator())->isConditional()) {
2793 IfBlock1 = nullptr;
2794 } else {
2795 DomBlock = *pred_begin(IfBlock1);
2796 for (BasicBlock::iterator I = IfBlock1->begin(); !I->isTerminator(); ++I)
2797 if (!AggressiveInsts.count(&*I) && !isa<DbgInfoIntrinsic>(I) &&
2798 !isa<PseudoProbeInst>(I)) {
2799 // This is not an aggressive instruction that we can promote.
2800 // Because of this, we won't be able to get rid of the control flow, so
2801 // the xform is not worth it.
2802 return Changed;
2803 }
2804 }
2805
2806 if (cast<BranchInst>(IfBlock2->getTerminator())->isConditional()) {
2807 IfBlock2 = nullptr;
2808 } else {
2809 DomBlock = *pred_begin(IfBlock2);
2810 for (BasicBlock::iterator I = IfBlock2->begin(); !I->isTerminator(); ++I)
2811 if (!AggressiveInsts.count(&*I) && !isa<DbgInfoIntrinsic>(I) &&
2812 !isa<PseudoProbeInst>(I)) {
2813 // This is not an aggressive instruction that we can promote.
2814 // Because of this, we won't be able to get rid of the control flow, so
2815 // the xform is not worth it.
2816 return Changed;
2817 }
2818 }
2819 assert(DomBlock && "Failed to find root DomBlock");
2820
2821 LLVM_DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond
2822 << " T: " << IfTrue->getName()
2823 << " F: " << IfFalse->getName() << "\n");
2824
2825 // If we can still promote the PHI nodes after this gauntlet of tests,
2826 // do all of the PHI's now.
2827 Instruction *InsertPt = DomBlock->getTerminator();
2828 IRBuilder<NoFolder> Builder(InsertPt);
2829
2830 // Move all 'aggressive' instructions, which are defined in the
2831 // conditional parts of the if's up to the dominating block.
2832 if (IfBlock1)
2833 hoistAllInstructionsInto(DomBlock, InsertPt, IfBlock1);
2834 if (IfBlock2)
2835 hoistAllInstructionsInto(DomBlock, InsertPt, IfBlock2);
2836
2837 // Propagate fast-math-flags from phi nodes to replacement selects.
2838 IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
2839 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
2840 if (isa<FPMathOperator>(PN))
2841 Builder.setFastMathFlags(PN->getFastMathFlags());
2842
2843 // Change the PHI node into a select instruction.
2844 Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
2845 Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
2846
2847 Value *Sel = Builder.CreateSelect(IfCond, TrueVal, FalseVal, "", InsertPt);
2848 PN->replaceAllUsesWith(Sel);
2849 Sel->takeName(PN);
2850 PN->eraseFromParent();
2851 }
2852
2853 // At this point, IfBlock1 and IfBlock2 are both empty, so our if statement
2854 // has been flattened. Change DomBlock to jump directly to our new block to
2855 // avoid other simplifycfg's kicking in on the diamond.
2856 Instruction *OldTI = DomBlock->getTerminator();
2857 Builder.SetInsertPoint(OldTI);
2858 Builder.CreateBr(BB);
2859
2860 SmallVector<DominatorTree::UpdateType, 3> Updates;
2861 if (DTU) {
2862 Updates.push_back({DominatorTree::Insert, DomBlock, BB});
2863 for (auto *Successor : successors(DomBlock))
2864 Updates.push_back({DominatorTree::Delete, DomBlock, Successor});
2865 }
2866
2867 OldTI->eraseFromParent();
2868 if (DTU)
2869 DTU->applyUpdates(Updates);
2870
2871 return true;
2872 }
2873
2874 /// If we found a conditional branch that goes to two returning blocks,
2875 /// try to merge them together into one return,
2876 /// introducing a select if the return values disagree.
SimplifyCondBranchToTwoReturns(BranchInst * BI,IRBuilder<> & Builder)2877 bool SimplifyCFGOpt::SimplifyCondBranchToTwoReturns(BranchInst *BI,
2878 IRBuilder<> &Builder) {
2879 auto *BB = BI->getParent();
2880 assert(BI->isConditional() && "Must be a conditional branch");
2881 BasicBlock *TrueSucc = BI->getSuccessor(0);
2882 BasicBlock *FalseSucc = BI->getSuccessor(1);
2883 // NOTE: destinations may match, this could be degenerate uncond branch.
2884 ReturnInst *TrueRet = cast<ReturnInst>(TrueSucc->getTerminator());
2885 ReturnInst *FalseRet = cast<ReturnInst>(FalseSucc->getTerminator());
2886
2887 // Check to ensure both blocks are empty (just a return) or optionally empty
2888 // with PHI nodes. If there are other instructions, merging would cause extra
2889 // computation on one path or the other.
2890 if (!TrueSucc->getFirstNonPHIOrDbg()->isTerminator())
2891 return false;
2892 if (!FalseSucc->getFirstNonPHIOrDbg()->isTerminator())
2893 return false;
2894
2895 Builder.SetInsertPoint(BI);
2896 // Okay, we found a branch that is going to two return nodes. If
2897 // there is no return value for this function, just change the
2898 // branch into a return.
2899 if (FalseRet->getNumOperands() == 0) {
2900 TrueSucc->removePredecessor(BB);
2901 FalseSucc->removePredecessor(BB);
2902 Builder.CreateRetVoid();
2903 EraseTerminatorAndDCECond(BI);
2904 if (DTU) {
2905 SmallVector<DominatorTree::UpdateType, 2> Updates;
2906 Updates.push_back({DominatorTree::Delete, BB, TrueSucc});
2907 if (TrueSucc != FalseSucc)
2908 Updates.push_back({DominatorTree::Delete, BB, FalseSucc});
2909 DTU->applyUpdates(Updates);
2910 }
2911 return true;
2912 }
2913
2914 // Otherwise, figure out what the true and false return values are
2915 // so we can insert a new select instruction.
2916 Value *TrueValue = TrueRet->getReturnValue();
2917 Value *FalseValue = FalseRet->getReturnValue();
2918
2919 // Unwrap any PHI nodes in the return blocks.
2920 if (PHINode *TVPN = dyn_cast_or_null<PHINode>(TrueValue))
2921 if (TVPN->getParent() == TrueSucc)
2922 TrueValue = TVPN->getIncomingValueForBlock(BB);
2923 if (PHINode *FVPN = dyn_cast_or_null<PHINode>(FalseValue))
2924 if (FVPN->getParent() == FalseSucc)
2925 FalseValue = FVPN->getIncomingValueForBlock(BB);
2926
2927 // In order for this transformation to be safe, we must be able to
2928 // unconditionally execute both operands to the return. This is
2929 // normally the case, but we could have a potentially-trapping
2930 // constant expression that prevents this transformation from being
2931 // safe.
2932 if (ConstantExpr *TCV = dyn_cast_or_null<ConstantExpr>(TrueValue))
2933 if (TCV->canTrap())
2934 return false;
2935 if (ConstantExpr *FCV = dyn_cast_or_null<ConstantExpr>(FalseValue))
2936 if (FCV->canTrap())
2937 return false;
2938
2939 // Okay, we collected all the mapped values and checked them for sanity, and
2940 // defined to really do this transformation. First, update the CFG.
2941 TrueSucc->removePredecessor(BB);
2942 FalseSucc->removePredecessor(BB);
2943
2944 // Insert select instructions where needed.
2945 Value *BrCond = BI->getCondition();
2946 if (TrueValue) {
2947 // Insert a select if the results differ.
2948 if (TrueValue == FalseValue || isa<UndefValue>(FalseValue)) {
2949 } else if (isa<UndefValue>(TrueValue)) {
2950 TrueValue = FalseValue;
2951 } else {
2952 TrueValue =
2953 Builder.CreateSelect(BrCond, TrueValue, FalseValue, "retval", BI);
2954 }
2955 }
2956
2957 Value *RI =
2958 !TrueValue ? Builder.CreateRetVoid() : Builder.CreateRet(TrueValue);
2959
2960 (void)RI;
2961
2962 LLVM_DEBUG(dbgs() << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:"
2963 << "\n " << *BI << "\nNewRet = " << *RI << "\nTRUEBLOCK: "
2964 << *TrueSucc << "\nFALSEBLOCK: " << *FalseSucc);
2965
2966 EraseTerminatorAndDCECond(BI);
2967 if (DTU) {
2968 SmallVector<DominatorTree::UpdateType, 2> Updates;
2969 Updates.push_back({DominatorTree::Delete, BB, TrueSucc});
2970 if (TrueSucc != FalseSucc)
2971 Updates.push_back({DominatorTree::Delete, BB, FalseSucc});
2972 DTU->applyUpdates(Updates);
2973 }
2974
2975 return true;
2976 }
2977
createLogicalOp(IRBuilderBase & Builder,Instruction::BinaryOps Opc,Value * LHS,Value * RHS,const Twine & Name="")2978 static Value *createLogicalOp(IRBuilderBase &Builder,
2979 Instruction::BinaryOps Opc, Value *LHS,
2980 Value *RHS, const Twine &Name = "") {
2981 // Try to relax logical op to binary op.
2982 if (impliesPoison(RHS, LHS))
2983 return Builder.CreateBinOp(Opc, LHS, RHS, Name);
2984 if (Opc == Instruction::And)
2985 return Builder.CreateLogicalAnd(LHS, RHS, Name);
2986 if (Opc == Instruction::Or)
2987 return Builder.CreateLogicalOr(LHS, RHS, Name);
2988 llvm_unreachable("Invalid logical opcode");
2989 }
2990
2991 /// Return true if either PBI or BI has branch weight available, and store
2992 /// the weights in {Pred|Succ}{True|False}Weight. If one of PBI and BI does
2993 /// not have branch weight, use 1:1 as its weight.
extractPredSuccWeights(BranchInst * PBI,BranchInst * BI,uint64_t & PredTrueWeight,uint64_t & PredFalseWeight,uint64_t & SuccTrueWeight,uint64_t & SuccFalseWeight)2994 static bool extractPredSuccWeights(BranchInst *PBI, BranchInst *BI,
2995 uint64_t &PredTrueWeight,
2996 uint64_t &PredFalseWeight,
2997 uint64_t &SuccTrueWeight,
2998 uint64_t &SuccFalseWeight) {
2999 bool PredHasWeights =
3000 PBI->extractProfMetadata(PredTrueWeight, PredFalseWeight);
3001 bool SuccHasWeights =
3002 BI->extractProfMetadata(SuccTrueWeight, SuccFalseWeight);
3003 if (PredHasWeights || SuccHasWeights) {
3004 if (!PredHasWeights)
3005 PredTrueWeight = PredFalseWeight = 1;
3006 if (!SuccHasWeights)
3007 SuccTrueWeight = SuccFalseWeight = 1;
3008 return true;
3009 } else {
3010 return false;
3011 }
3012 }
3013
3014 /// Determine if the two branches share a common destination and deduce a glue
3015 /// that joins the branches' conditions to arrive at the common destination if
3016 /// that would be profitable.
3017 static Optional<std::pair<Instruction::BinaryOps, bool>>
shouldFoldCondBranchesToCommonDestination(BranchInst * BI,BranchInst * PBI,const TargetTransformInfo * TTI)3018 shouldFoldCondBranchesToCommonDestination(BranchInst *BI, BranchInst *PBI,
3019 const TargetTransformInfo *TTI) {
3020 assert(BI && PBI && BI->isConditional() && PBI->isConditional() &&
3021 "Both blocks must end with a conditional branches.");
3022 assert(is_contained(predecessors(BI->getParent()), PBI->getParent()) &&
3023 "PredBB must be a predecessor of BB.");
3024
3025 // We have the potential to fold the conditions together, but if the
3026 // predecessor branch is predictable, we may not want to merge them.
3027 uint64_t PTWeight, PFWeight;
3028 BranchProbability PBITrueProb, Likely;
3029 if (TTI && PBI->extractProfMetadata(PTWeight, PFWeight) &&
3030 (PTWeight + PFWeight) != 0) {
3031 PBITrueProb =
3032 BranchProbability::getBranchProbability(PTWeight, PTWeight + PFWeight);
3033 Likely = TTI->getPredictableBranchThreshold();
3034 }
3035
3036 if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
3037 // Speculate the 2nd condition unless the 1st is probably true.
3038 if (PBITrueProb.isUnknown() || PBITrueProb < Likely)
3039 return {{Instruction::Or, false}};
3040 } else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
3041 // Speculate the 2nd condition unless the 1st is probably false.
3042 if (PBITrueProb.isUnknown() || PBITrueProb.getCompl() < Likely)
3043 return {{Instruction::And, false}};
3044 } else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
3045 // Speculate the 2nd condition unless the 1st is probably true.
3046 if (PBITrueProb.isUnknown() || PBITrueProb < Likely)
3047 return {{Instruction::And, true}};
3048 } else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
3049 // Speculate the 2nd condition unless the 1st is probably false.
3050 if (PBITrueProb.isUnknown() || PBITrueProb.getCompl() < Likely)
3051 return {{Instruction::Or, true}};
3052 }
3053 return None;
3054 }
3055
performBranchToCommonDestFolding(BranchInst * BI,BranchInst * PBI,DomTreeUpdater * DTU,MemorySSAUpdater * MSSAU,const TargetTransformInfo * TTI)3056 static bool performBranchToCommonDestFolding(BranchInst *BI, BranchInst *PBI,
3057 DomTreeUpdater *DTU,
3058 MemorySSAUpdater *MSSAU,
3059 const TargetTransformInfo *TTI) {
3060 BasicBlock *BB = BI->getParent();
3061 BasicBlock *PredBlock = PBI->getParent();
3062
3063 // Determine if the two branches share a common destination.
3064 Instruction::BinaryOps Opc;
3065 bool InvertPredCond;
3066 std::tie(Opc, InvertPredCond) =
3067 *shouldFoldCondBranchesToCommonDestination(BI, PBI, TTI);
3068
3069 LLVM_DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB);
3070
3071 IRBuilder<> Builder(PBI);
3072 // The builder is used to create instructions to eliminate the branch in BB.
3073 // If BB's terminator has !annotation metadata, add it to the new
3074 // instructions.
3075 Builder.CollectMetadataToCopy(BB->getTerminator(),
3076 {LLVMContext::MD_annotation});
3077
3078 // If we need to invert the condition in the pred block to match, do so now.
3079 if (InvertPredCond) {
3080 Value *NewCond = PBI->getCondition();
3081 if (NewCond->hasOneUse() && isa<CmpInst>(NewCond)) {
3082 CmpInst *CI = cast<CmpInst>(NewCond);
3083 CI->setPredicate(CI->getInversePredicate());
3084 } else {
3085 NewCond =
3086 Builder.CreateNot(NewCond, PBI->getCondition()->getName() + ".not");
3087 }
3088
3089 PBI->setCondition(NewCond);
3090 PBI->swapSuccessors();
3091 }
3092
3093 BasicBlock *UniqueSucc =
3094 PBI->getSuccessor(0) == BB ? BI->getSuccessor(0) : BI->getSuccessor(1);
3095
3096 // Before cloning instructions, notify the successor basic block that it
3097 // is about to have a new predecessor. This will update PHI nodes,
3098 // which will allow us to update live-out uses of bonus instructions.
3099 AddPredecessorToBlock(UniqueSucc, PredBlock, BB, MSSAU);
3100
3101 // Try to update branch weights.
3102 uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
3103 if (extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
3104 SuccTrueWeight, SuccFalseWeight)) {
3105 SmallVector<uint64_t, 8> NewWeights;
3106
3107 if (PBI->getSuccessor(0) == BB) {
3108 // PBI: br i1 %x, BB, FalseDest
3109 // BI: br i1 %y, UniqueSucc, FalseDest
3110 // TrueWeight is TrueWeight for PBI * TrueWeight for BI.
3111 NewWeights.push_back(PredTrueWeight * SuccTrueWeight);
3112 // FalseWeight is FalseWeight for PBI * TotalWeight for BI +
3113 // TrueWeight for PBI * FalseWeight for BI.
3114 // We assume that total weights of a BranchInst can fit into 32 bits.
3115 // Therefore, we will not have overflow using 64-bit arithmetic.
3116 NewWeights.push_back(PredFalseWeight *
3117 (SuccFalseWeight + SuccTrueWeight) +
3118 PredTrueWeight * SuccFalseWeight);
3119 } else {
3120 // PBI: br i1 %x, TrueDest, BB
3121 // BI: br i1 %y, TrueDest, UniqueSucc
3122 // TrueWeight is TrueWeight for PBI * TotalWeight for BI +
3123 // FalseWeight for PBI * TrueWeight for BI.
3124 NewWeights.push_back(PredTrueWeight * (SuccFalseWeight + SuccTrueWeight) +
3125 PredFalseWeight * SuccTrueWeight);
3126 // FalseWeight is FalseWeight for PBI * FalseWeight for BI.
3127 NewWeights.push_back(PredFalseWeight * SuccFalseWeight);
3128 }
3129
3130 // Halve the weights if any of them cannot fit in an uint32_t
3131 FitWeights(NewWeights);
3132
3133 SmallVector<uint32_t, 8> MDWeights(NewWeights.begin(), NewWeights.end());
3134 setBranchWeights(PBI, MDWeights[0], MDWeights[1]);
3135
3136 // TODO: If BB is reachable from all paths through PredBlock, then we
3137 // could replace PBI's branch probabilities with BI's.
3138 } else
3139 PBI->setMetadata(LLVMContext::MD_prof, nullptr);
3140
3141 // Now, update the CFG.
3142 PBI->setSuccessor(PBI->getSuccessor(0) != BB, UniqueSucc);
3143
3144 if (DTU)
3145 DTU->applyUpdates({{DominatorTree::Insert, PredBlock, UniqueSucc},
3146 {DominatorTree::Delete, PredBlock, BB}});
3147
3148 // If BI was a loop latch, it may have had associated loop metadata.
3149 // We need to copy it to the new latch, that is, PBI.
3150 if (MDNode *LoopMD = BI->getMetadata(LLVMContext::MD_loop))
3151 PBI->setMetadata(LLVMContext::MD_loop, LoopMD);
3152
3153 ValueToValueMapTy VMap; // maps original values to cloned values
3154 CloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(BB, PredBlock, VMap);
3155
3156 // Now that the Cond was cloned into the predecessor basic block,
3157 // or/and the two conditions together.
3158 Value *BICond = VMap[BI->getCondition()];
3159 PBI->setCondition(
3160 createLogicalOp(Builder, Opc, PBI->getCondition(), BICond, "or.cond"));
3161
3162 // Copy any debug value intrinsics into the end of PredBlock.
3163 for (Instruction &I : *BB) {
3164 if (isa<DbgInfoIntrinsic>(I)) {
3165 Instruction *NewI = I.clone();
3166 RemapInstruction(NewI, VMap,
3167 RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
3168 NewI->insertBefore(PBI);
3169 }
3170 }
3171
3172 ++NumFoldBranchToCommonDest;
3173 return true;
3174 }
3175
3176 /// If this basic block is simple enough, and if a predecessor branches to us
3177 /// and one of our successors, fold the block into the predecessor and use
3178 /// logical operations to pick the right destination.
FoldBranchToCommonDest(BranchInst * BI,DomTreeUpdater * DTU,MemorySSAUpdater * MSSAU,const TargetTransformInfo * TTI,unsigned BonusInstThreshold)3179 bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
3180 MemorySSAUpdater *MSSAU,
3181 const TargetTransformInfo *TTI,
3182 unsigned BonusInstThreshold) {
3183 // If this block ends with an unconditional branch,
3184 // let SpeculativelyExecuteBB() deal with it.
3185 if (!BI->isConditional())
3186 return false;
3187
3188 BasicBlock *BB = BI->getParent();
3189
3190 bool Changed = false;
3191
3192 TargetTransformInfo::TargetCostKind CostKind =
3193 BB->getParent()->hasMinSize() ? TargetTransformInfo::TCK_CodeSize
3194 : TargetTransformInfo::TCK_SizeAndLatency;
3195
3196 Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
3197
3198 if (!Cond || (!isa<CmpInst>(Cond) && !isa<BinaryOperator>(Cond)) ||
3199 Cond->getParent() != BB || !Cond->hasOneUse())
3200 return Changed;
3201
3202 // Cond is known to be a compare or binary operator. Check to make sure that
3203 // neither operand is a potentially-trapping constant expression.
3204 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(0)))
3205 if (CE->canTrap())
3206 return Changed;
3207 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(1)))
3208 if (CE->canTrap())
3209 return Changed;
3210
3211 // Finally, don't infinitely unroll conditional loops.
3212 if (is_contained(successors(BB), BB))
3213 return Changed;
3214
3215 // With which predecessors will we want to deal with?
3216 SmallVector<BasicBlock *, 8> Preds;
3217 for (BasicBlock *PredBlock : predecessors(BB)) {
3218 BranchInst *PBI = dyn_cast<BranchInst>(PredBlock->getTerminator());
3219
3220 // Check that we have two conditional branches. If there is a PHI node in
3221 // the common successor, verify that the same value flows in from both
3222 // blocks.
3223 if (!PBI || PBI->isUnconditional() || !SafeToMergeTerminators(BI, PBI))
3224 continue;
3225
3226 // Determine if the two branches share a common destination.
3227 Instruction::BinaryOps Opc;
3228 bool InvertPredCond;
3229 if (auto Recipe = shouldFoldCondBranchesToCommonDestination(BI, PBI, TTI))
3230 std::tie(Opc, InvertPredCond) = *Recipe;
3231 else
3232 continue;
3233
3234 // Check the cost of inserting the necessary logic before performing the
3235 // transformation.
3236 if (TTI) {
3237 Type *Ty = BI->getCondition()->getType();
3238 InstructionCost Cost = TTI->getArithmeticInstrCost(Opc, Ty, CostKind);
3239 if (InvertPredCond && (!PBI->getCondition()->hasOneUse() ||
3240 !isa<CmpInst>(PBI->getCondition())))
3241 Cost += TTI->getArithmeticInstrCost(Instruction::Xor, Ty, CostKind);
3242
3243 if (Cost > BranchFoldThreshold)
3244 continue;
3245 }
3246
3247 // Ok, we do want to deal with this predecessor. Record it.
3248 Preds.emplace_back(PredBlock);
3249 }
3250
3251 // If there aren't any predecessors into which we can fold,
3252 // don't bother checking the cost.
3253 if (Preds.empty())
3254 return Changed;
3255
3256 // Only allow this transformation if computing the condition doesn't involve
3257 // too many instructions and these involved instructions can be executed
3258 // unconditionally. We denote all involved instructions except the condition
3259 // as "bonus instructions", and only allow this transformation when the
3260 // number of the bonus instructions we'll need to create when cloning into
3261 // each predecessor does not exceed a certain threshold.
3262 unsigned NumBonusInsts = 0;
3263 const unsigned PredCount = Preds.size();
3264 for (Instruction &I : *BB) {
3265 // Don't check the branch condition comparison itself.
3266 if (&I == Cond)
3267 continue;
3268 // Ignore dbg intrinsics, and the terminator.
3269 if (isa<DbgInfoIntrinsic>(I) || isa<BranchInst>(I))
3270 continue;
3271 // I must be safe to execute unconditionally.
3272 if (!isSafeToSpeculativelyExecute(&I))
3273 return Changed;
3274
3275 // Account for the cost of duplicating this instruction into each
3276 // predecessor.
3277 NumBonusInsts += PredCount;
3278 // Early exits once we reach the limit.
3279 if (NumBonusInsts > BonusInstThreshold)
3280 return Changed;
3281 }
3282
3283 // Ok, we have the budget. Perform the transformation.
3284 for (BasicBlock *PredBlock : Preds) {
3285 auto *PBI = cast<BranchInst>(PredBlock->getTerminator());
3286 return performBranchToCommonDestFolding(BI, PBI, DTU, MSSAU, TTI);
3287 }
3288 return Changed;
3289 }
3290
3291 // If there is only one store in BB1 and BB2, return it, otherwise return
3292 // nullptr.
findUniqueStoreInBlocks(BasicBlock * BB1,BasicBlock * BB2)3293 static StoreInst *findUniqueStoreInBlocks(BasicBlock *BB1, BasicBlock *BB2) {
3294 StoreInst *S = nullptr;
3295 for (auto *BB : {BB1, BB2}) {
3296 if (!BB)
3297 continue;
3298 for (auto &I : *BB)
3299 if (auto *SI = dyn_cast<StoreInst>(&I)) {
3300 if (S)
3301 // Multiple stores seen.
3302 return nullptr;
3303 else
3304 S = SI;
3305 }
3306 }
3307 return S;
3308 }
3309
ensureValueAvailableInSuccessor(Value * V,BasicBlock * BB,Value * AlternativeV=nullptr)3310 static Value *ensureValueAvailableInSuccessor(Value *V, BasicBlock *BB,
3311 Value *AlternativeV = nullptr) {
3312 // PHI is going to be a PHI node that allows the value V that is defined in
3313 // BB to be referenced in BB's only successor.
3314 //
3315 // If AlternativeV is nullptr, the only value we care about in PHI is V. It
3316 // doesn't matter to us what the other operand is (it'll never get used). We
3317 // could just create a new PHI with an undef incoming value, but that could
3318 // increase register pressure if EarlyCSE/InstCombine can't fold it with some
3319 // other PHI. So here we directly look for some PHI in BB's successor with V
3320 // as an incoming operand. If we find one, we use it, else we create a new
3321 // one.
3322 //
3323 // If AlternativeV is not nullptr, we care about both incoming values in PHI.
3324 // PHI must be exactly: phi <ty> [ %BB, %V ], [ %OtherBB, %AlternativeV]
3325 // where OtherBB is the single other predecessor of BB's only successor.
3326 PHINode *PHI = nullptr;
3327 BasicBlock *Succ = BB->getSingleSuccessor();
3328
3329 for (auto I = Succ->begin(); isa<PHINode>(I); ++I)
3330 if (cast<PHINode>(I)->getIncomingValueForBlock(BB) == V) {
3331 PHI = cast<PHINode>(I);
3332 if (!AlternativeV)
3333 break;
3334
3335 assert(Succ->hasNPredecessors(2));
3336 auto PredI = pred_begin(Succ);
3337 BasicBlock *OtherPredBB = *PredI == BB ? *++PredI : *PredI;
3338 if (PHI->getIncomingValueForBlock(OtherPredBB) == AlternativeV)
3339 break;
3340 PHI = nullptr;
3341 }
3342 if (PHI)
3343 return PHI;
3344
3345 // If V is not an instruction defined in BB, just return it.
3346 if (!AlternativeV &&
3347 (!isa<Instruction>(V) || cast<Instruction>(V)->getParent() != BB))
3348 return V;
3349
3350 PHI = PHINode::Create(V->getType(), 2, "simplifycfg.merge", &Succ->front());
3351 PHI->addIncoming(V, BB);
3352 for (BasicBlock *PredBB : predecessors(Succ))
3353 if (PredBB != BB)
3354 PHI->addIncoming(
3355 AlternativeV ? AlternativeV : UndefValue::get(V->getType()), PredBB);
3356 return PHI;
3357 }
3358
mergeConditionalStoreToAddress(BasicBlock * PTB,BasicBlock * PFB,BasicBlock * QTB,BasicBlock * QFB,BasicBlock * PostBB,Value * Address,bool InvertPCond,bool InvertQCond,DomTreeUpdater * DTU,const DataLayout & DL,const TargetTransformInfo & TTI)3359 static bool mergeConditionalStoreToAddress(
3360 BasicBlock *PTB, BasicBlock *PFB, BasicBlock *QTB, BasicBlock *QFB,
3361 BasicBlock *PostBB, Value *Address, bool InvertPCond, bool InvertQCond,
3362 DomTreeUpdater *DTU, const DataLayout &DL, const TargetTransformInfo &TTI) {
3363 // For every pointer, there must be exactly two stores, one coming from
3364 // PTB or PFB, and the other from QTB or QFB. We don't support more than one
3365 // store (to any address) in PTB,PFB or QTB,QFB.
3366 // FIXME: We could relax this restriction with a bit more work and performance
3367 // testing.
3368 StoreInst *PStore = findUniqueStoreInBlocks(PTB, PFB);
3369 StoreInst *QStore = findUniqueStoreInBlocks(QTB, QFB);
3370 if (!PStore || !QStore)
3371 return false;
3372
3373 // Now check the stores are compatible.
3374 if (!QStore->isUnordered() || !PStore->isUnordered())
3375 return false;
3376
3377 // Check that sinking the store won't cause program behavior changes. Sinking
3378 // the store out of the Q blocks won't change any behavior as we're sinking
3379 // from a block to its unconditional successor. But we're moving a store from
3380 // the P blocks down through the middle block (QBI) and past both QFB and QTB.
3381 // So we need to check that there are no aliasing loads or stores in
3382 // QBI, QTB and QFB. We also need to check there are no conflicting memory
3383 // operations between PStore and the end of its parent block.
3384 //
3385 // The ideal way to do this is to query AliasAnalysis, but we don't
3386 // preserve AA currently so that is dangerous. Be super safe and just
3387 // check there are no other memory operations at all.
3388 for (auto &I : *QFB->getSinglePredecessor())
3389 if (I.mayReadOrWriteMemory())
3390 return false;
3391 for (auto &I : *QFB)
3392 if (&I != QStore && I.mayReadOrWriteMemory())
3393 return false;
3394 if (QTB)
3395 for (auto &I : *QTB)
3396 if (&I != QStore && I.mayReadOrWriteMemory())
3397 return false;
3398 for (auto I = BasicBlock::iterator(PStore), E = PStore->getParent()->end();
3399 I != E; ++I)
3400 if (&*I != PStore && I->mayReadOrWriteMemory())
3401 return false;
3402
3403 // If we're not in aggressive mode, we only optimize if we have some
3404 // confidence that by optimizing we'll allow P and/or Q to be if-converted.
3405 auto IsWorthwhile = [&](BasicBlock *BB, ArrayRef<StoreInst *> FreeStores) {
3406 if (!BB)
3407 return true;
3408 // Heuristic: if the block can be if-converted/phi-folded and the
3409 // instructions inside are all cheap (arithmetic/GEPs), it's worthwhile to
3410 // thread this store.
3411 InstructionCost Cost = 0;
3412 InstructionCost Budget =
3413 PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
3414 for (auto &I : BB->instructionsWithoutDebug()) {
3415 // Consider terminator instruction to be free.
3416 if (I.isTerminator())
3417 continue;
3418 // If this is one the stores that we want to speculate out of this BB,
3419 // then don't count it's cost, consider it to be free.
3420 if (auto *S = dyn_cast<StoreInst>(&I))
3421 if (llvm::find(FreeStores, S))
3422 continue;
3423 // Else, we have a white-list of instructions that we are ak speculating.
3424 if (!isa<BinaryOperator>(I) && !isa<GetElementPtrInst>(I))
3425 return false; // Not in white-list - not worthwhile folding.
3426 // And finally, if this is a non-free instruction that we are okay
3427 // speculating, ensure that we consider the speculation budget.
3428 Cost += TTI.getUserCost(&I, TargetTransformInfo::TCK_SizeAndLatency);
3429 if (Cost > Budget)
3430 return false; // Eagerly refuse to fold as soon as we're out of budget.
3431 }
3432 assert(Cost <= Budget &&
3433 "When we run out of budget we will eagerly return from within the "
3434 "per-instruction loop.");
3435 return true;
3436 };
3437
3438 const std::array<StoreInst *, 2> FreeStores = {PStore, QStore};
3439 if (!MergeCondStoresAggressively &&
3440 (!IsWorthwhile(PTB, FreeStores) || !IsWorthwhile(PFB, FreeStores) ||
3441 !IsWorthwhile(QTB, FreeStores) || !IsWorthwhile(QFB, FreeStores)))
3442 return false;
3443
3444 // If PostBB has more than two predecessors, we need to split it so we can
3445 // sink the store.
3446 if (std::next(pred_begin(PostBB), 2) != pred_end(PostBB)) {
3447 // We know that QFB's only successor is PostBB. And QFB has a single
3448 // predecessor. If QTB exists, then its only successor is also PostBB.
3449 // If QTB does not exist, then QFB's only predecessor has a conditional
3450 // branch to QFB and PostBB.
3451 BasicBlock *TruePred = QTB ? QTB : QFB->getSinglePredecessor();
3452 BasicBlock *NewBB =
3453 SplitBlockPredecessors(PostBB, {QFB, TruePred}, "condstore.split", DTU);
3454 if (!NewBB)
3455 return false;
3456 PostBB = NewBB;
3457 }
3458
3459 // OK, we're going to sink the stores to PostBB. The store has to be
3460 // conditional though, so first create the predicate.
3461 Value *PCond = cast<BranchInst>(PFB->getSinglePredecessor()->getTerminator())
3462 ->getCondition();
3463 Value *QCond = cast<BranchInst>(QFB->getSinglePredecessor()->getTerminator())
3464 ->getCondition();
3465
3466 Value *PPHI = ensureValueAvailableInSuccessor(PStore->getValueOperand(),
3467 PStore->getParent());
3468 Value *QPHI = ensureValueAvailableInSuccessor(QStore->getValueOperand(),
3469 QStore->getParent(), PPHI);
3470
3471 IRBuilder<> QB(&*PostBB->getFirstInsertionPt());
3472
3473 Value *PPred = PStore->getParent() == PTB ? PCond : QB.CreateNot(PCond);
3474 Value *QPred = QStore->getParent() == QTB ? QCond : QB.CreateNot(QCond);
3475
3476 if (InvertPCond)
3477 PPred = QB.CreateNot(PPred);
3478 if (InvertQCond)
3479 QPred = QB.CreateNot(QPred);
3480 Value *CombinedPred = QB.CreateOr(PPred, QPred);
3481
3482 auto *T = SplitBlockAndInsertIfThen(CombinedPred, &*QB.GetInsertPoint(),
3483 /*Unreachable=*/false,
3484 /*BranchWeights=*/nullptr, DTU);
3485 QB.SetInsertPoint(T);
3486 StoreInst *SI = cast<StoreInst>(QB.CreateStore(QPHI, Address));
3487 AAMDNodes AAMD;
3488 PStore->getAAMetadata(AAMD, /*Merge=*/false);
3489 PStore->getAAMetadata(AAMD, /*Merge=*/true);
3490 SI->setAAMetadata(AAMD);
3491 // Choose the minimum alignment. If we could prove both stores execute, we
3492 // could use biggest one. In this case, though, we only know that one of the
3493 // stores executes. And we don't know it's safe to take the alignment from a
3494 // store that doesn't execute.
3495 SI->setAlignment(std::min(PStore->getAlign(), QStore->getAlign()));
3496
3497 QStore->eraseFromParent();
3498 PStore->eraseFromParent();
3499
3500 return true;
3501 }
3502
mergeConditionalStores(BranchInst * PBI,BranchInst * QBI,DomTreeUpdater * DTU,const DataLayout & DL,const TargetTransformInfo & TTI)3503 static bool mergeConditionalStores(BranchInst *PBI, BranchInst *QBI,
3504 DomTreeUpdater *DTU, const DataLayout &DL,
3505 const TargetTransformInfo &TTI) {
3506 // The intention here is to find diamonds or triangles (see below) where each
3507 // conditional block contains a store to the same address. Both of these
3508 // stores are conditional, so they can't be unconditionally sunk. But it may
3509 // be profitable to speculatively sink the stores into one merged store at the
3510 // end, and predicate the merged store on the union of the two conditions of
3511 // PBI and QBI.
3512 //
3513 // This can reduce the number of stores executed if both of the conditions are
3514 // true, and can allow the blocks to become small enough to be if-converted.
3515 // This optimization will also chain, so that ladders of test-and-set
3516 // sequences can be if-converted away.
3517 //
3518 // We only deal with simple diamonds or triangles:
3519 //
3520 // PBI or PBI or a combination of the two
3521 // / \ | \
3522 // PTB PFB | PFB
3523 // \ / | /
3524 // QBI QBI
3525 // / \ | \
3526 // QTB QFB | QFB
3527 // \ / | /
3528 // PostBB PostBB
3529 //
3530 // We model triangles as a type of diamond with a nullptr "true" block.
3531 // Triangles are canonicalized so that the fallthrough edge is represented by
3532 // a true condition, as in the diagram above.
3533 BasicBlock *PTB = PBI->getSuccessor(0);
3534 BasicBlock *PFB = PBI->getSuccessor(1);
3535 BasicBlock *QTB = QBI->getSuccessor(0);
3536 BasicBlock *QFB = QBI->getSuccessor(1);
3537 BasicBlock *PostBB = QFB->getSingleSuccessor();
3538
3539 // Make sure we have a good guess for PostBB. If QTB's only successor is
3540 // QFB, then QFB is a better PostBB.
3541 if (QTB->getSingleSuccessor() == QFB)
3542 PostBB = QFB;
3543
3544 // If we couldn't find a good PostBB, stop.
3545 if (!PostBB)
3546 return false;
3547
3548 bool InvertPCond = false, InvertQCond = false;
3549 // Canonicalize fallthroughs to the true branches.
3550 if (PFB == QBI->getParent()) {
3551 std::swap(PFB, PTB);
3552 InvertPCond = true;
3553 }
3554 if (QFB == PostBB) {
3555 std::swap(QFB, QTB);
3556 InvertQCond = true;
3557 }
3558
3559 // From this point on we can assume PTB or QTB may be fallthroughs but PFB
3560 // and QFB may not. Model fallthroughs as a nullptr block.
3561 if (PTB == QBI->getParent())
3562 PTB = nullptr;
3563 if (QTB == PostBB)
3564 QTB = nullptr;
3565
3566 // Legality bailouts. We must have at least the non-fallthrough blocks and
3567 // the post-dominating block, and the non-fallthroughs must only have one
3568 // predecessor.
3569 auto HasOnePredAndOneSucc = [](BasicBlock *BB, BasicBlock *P, BasicBlock *S) {
3570 return BB->getSinglePredecessor() == P && BB->getSingleSuccessor() == S;
3571 };
3572 if (!HasOnePredAndOneSucc(PFB, PBI->getParent(), QBI->getParent()) ||
3573 !HasOnePredAndOneSucc(QFB, QBI->getParent(), PostBB))
3574 return false;
3575 if ((PTB && !HasOnePredAndOneSucc(PTB, PBI->getParent(), QBI->getParent())) ||
3576 (QTB && !HasOnePredAndOneSucc(QTB, QBI->getParent(), PostBB)))
3577 return false;
3578 if (!QBI->getParent()->hasNUses(2))
3579 return false;
3580
3581 // OK, this is a sequence of two diamonds or triangles.
3582 // Check if there are stores in PTB or PFB that are repeated in QTB or QFB.
3583 SmallPtrSet<Value *, 4> PStoreAddresses, QStoreAddresses;
3584 for (auto *BB : {PTB, PFB}) {
3585 if (!BB)
3586 continue;
3587 for (auto &I : *BB)
3588 if (StoreInst *SI = dyn_cast<StoreInst>(&I))
3589 PStoreAddresses.insert(SI->getPointerOperand());
3590 }
3591 for (auto *BB : {QTB, QFB}) {
3592 if (!BB)
3593 continue;
3594 for (auto &I : *BB)
3595 if (StoreInst *SI = dyn_cast<StoreInst>(&I))
3596 QStoreAddresses.insert(SI->getPointerOperand());
3597 }
3598
3599 set_intersect(PStoreAddresses, QStoreAddresses);
3600 // set_intersect mutates PStoreAddresses in place. Rename it here to make it
3601 // clear what it contains.
3602 auto &CommonAddresses = PStoreAddresses;
3603
3604 bool Changed = false;
3605 for (auto *Address : CommonAddresses)
3606 Changed |=
3607 mergeConditionalStoreToAddress(PTB, PFB, QTB, QFB, PostBB, Address,
3608 InvertPCond, InvertQCond, DTU, DL, TTI);
3609 return Changed;
3610 }
3611
3612 /// If the previous block ended with a widenable branch, determine if reusing
3613 /// the target block is profitable and legal. This will have the effect of
3614 /// "widening" PBI, but doesn't require us to reason about hosting safety.
tryWidenCondBranchToCondBranch(BranchInst * PBI,BranchInst * BI,DomTreeUpdater * DTU)3615 static bool tryWidenCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
3616 DomTreeUpdater *DTU) {
3617 // TODO: This can be generalized in two important ways:
3618 // 1) We can allow phi nodes in IfFalseBB and simply reuse all the input
3619 // values from the PBI edge.
3620 // 2) We can sink side effecting instructions into BI's fallthrough
3621 // successor provided they doesn't contribute to computation of
3622 // BI's condition.
3623 Value *CondWB, *WC;
3624 BasicBlock *IfTrueBB, *IfFalseBB;
3625 if (!parseWidenableBranch(PBI, CondWB, WC, IfTrueBB, IfFalseBB) ||
3626 IfTrueBB != BI->getParent() || !BI->getParent()->getSinglePredecessor())
3627 return false;
3628 if (!IfFalseBB->phis().empty())
3629 return false; // TODO
3630 // Use lambda to lazily compute expensive condition after cheap ones.
3631 auto NoSideEffects = [](BasicBlock &BB) {
3632 return !llvm::any_of(BB, [](const Instruction &I) {
3633 return I.mayWriteToMemory() || I.mayHaveSideEffects();
3634 });
3635 };
3636 if (BI->getSuccessor(1) != IfFalseBB && // no inf looping
3637 BI->getSuccessor(1)->getTerminatingDeoptimizeCall() && // profitability
3638 NoSideEffects(*BI->getParent())) {
3639 auto *OldSuccessor = BI->getSuccessor(1);
3640 OldSuccessor->removePredecessor(BI->getParent());
3641 BI->setSuccessor(1, IfFalseBB);
3642 if (DTU)
3643 DTU->applyUpdates(
3644 {{DominatorTree::Insert, BI->getParent(), IfFalseBB},
3645 {DominatorTree::Delete, BI->getParent(), OldSuccessor}});
3646 return true;
3647 }
3648 if (BI->getSuccessor(0) != IfFalseBB && // no inf looping
3649 BI->getSuccessor(0)->getTerminatingDeoptimizeCall() && // profitability
3650 NoSideEffects(*BI->getParent())) {
3651 auto *OldSuccessor = BI->getSuccessor(0);
3652 OldSuccessor->removePredecessor(BI->getParent());
3653 BI->setSuccessor(0, IfFalseBB);
3654 if (DTU)
3655 DTU->applyUpdates(
3656 {{DominatorTree::Insert, BI->getParent(), IfFalseBB},
3657 {DominatorTree::Delete, BI->getParent(), OldSuccessor}});
3658 return true;
3659 }
3660 return false;
3661 }
3662
3663 /// If we have a conditional branch as a predecessor of another block,
3664 /// this function tries to simplify it. We know
3665 /// that PBI and BI are both conditional branches, and BI is in one of the
3666 /// successor blocks of PBI - PBI branches to BI.
SimplifyCondBranchToCondBranch(BranchInst * PBI,BranchInst * BI,DomTreeUpdater * DTU,const DataLayout & DL,const TargetTransformInfo & TTI)3667 static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
3668 DomTreeUpdater *DTU,
3669 const DataLayout &DL,
3670 const TargetTransformInfo &TTI) {
3671 assert(PBI->isConditional() && BI->isConditional());
3672 BasicBlock *BB = BI->getParent();
3673
3674 // If this block ends with a branch instruction, and if there is a
3675 // predecessor that ends on a branch of the same condition, make
3676 // this conditional branch redundant.
3677 if (PBI->getCondition() == BI->getCondition() &&
3678 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
3679 // Okay, the outcome of this conditional branch is statically
3680 // knowable. If this block had a single pred, handle specially.
3681 if (BB->getSinglePredecessor()) {
3682 // Turn this into a branch on constant.
3683 bool CondIsTrue = PBI->getSuccessor(0) == BB;
3684 BI->setCondition(
3685 ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue));
3686 return true; // Nuke the branch on constant.
3687 }
3688
3689 // Otherwise, if there are multiple predecessors, insert a PHI that merges
3690 // in the constant and simplify the block result. Subsequent passes of
3691 // simplifycfg will thread the block.
3692 if (BlockIsSimpleEnoughToThreadThrough(BB)) {
3693 pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
3694 PHINode *NewPN = PHINode::Create(
3695 Type::getInt1Ty(BB->getContext()), std::distance(PB, PE),
3696 BI->getCondition()->getName() + ".pr", &BB->front());
3697 // Okay, we're going to insert the PHI node. Since PBI is not the only
3698 // predecessor, compute the PHI'd conditional value for all of the preds.
3699 // Any predecessor where the condition is not computable we keep symbolic.
3700 for (pred_iterator PI = PB; PI != PE; ++PI) {
3701 BasicBlock *P = *PI;
3702 if ((PBI = dyn_cast<BranchInst>(P->getTerminator())) && PBI != BI &&
3703 PBI->isConditional() && PBI->getCondition() == BI->getCondition() &&
3704 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
3705 bool CondIsTrue = PBI->getSuccessor(0) == BB;
3706 NewPN->addIncoming(
3707 ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue),
3708 P);
3709 } else {
3710 NewPN->addIncoming(BI->getCondition(), P);
3711 }
3712 }
3713
3714 BI->setCondition(NewPN);
3715 return true;
3716 }
3717 }
3718
3719 // If the previous block ended with a widenable branch, determine if reusing
3720 // the target block is profitable and legal. This will have the effect of
3721 // "widening" PBI, but doesn't require us to reason about hosting safety.
3722 if (tryWidenCondBranchToCondBranch(PBI, BI, DTU))
3723 return true;
3724
3725 if (auto *CE = dyn_cast<ConstantExpr>(BI->getCondition()))
3726 if (CE->canTrap())
3727 return false;
3728
3729 // If both branches are conditional and both contain stores to the same
3730 // address, remove the stores from the conditionals and create a conditional
3731 // merged store at the end.
3732 if (MergeCondStores && mergeConditionalStores(PBI, BI, DTU, DL, TTI))
3733 return true;
3734
3735 // If this is a conditional branch in an empty block, and if any
3736 // predecessors are a conditional branch to one of our destinations,
3737 // fold the conditions into logical ops and one cond br.
3738
3739 // Ignore dbg intrinsics.
3740 if (&*BB->instructionsWithoutDebug().begin() != BI)
3741 return false;
3742
3743 int PBIOp, BIOp;
3744 if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
3745 PBIOp = 0;
3746 BIOp = 0;
3747 } else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
3748 PBIOp = 0;
3749 BIOp = 1;
3750 } else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
3751 PBIOp = 1;
3752 BIOp = 0;
3753 } else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
3754 PBIOp = 1;
3755 BIOp = 1;
3756 } else {
3757 return false;
3758 }
3759
3760 // Check to make sure that the other destination of this branch
3761 // isn't BB itself. If so, this is an infinite loop that will
3762 // keep getting unwound.
3763 if (PBI->getSuccessor(PBIOp) == BB)
3764 return false;
3765
3766 // Do not perform this transformation if it would require
3767 // insertion of a large number of select instructions. For targets
3768 // without predication/cmovs, this is a big pessimization.
3769
3770 // Also do not perform this transformation if any phi node in the common
3771 // destination block can trap when reached by BB or PBB (PR17073). In that
3772 // case, it would be unsafe to hoist the operation into a select instruction.
3773
3774 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
3775 BasicBlock *RemovedDest = PBI->getSuccessor(PBIOp ^ 1);
3776 unsigned NumPhis = 0;
3777 for (BasicBlock::iterator II = CommonDest->begin(); isa<PHINode>(II);
3778 ++II, ++NumPhis) {
3779 if (NumPhis > 2) // Disable this xform.
3780 return false;
3781
3782 PHINode *PN = cast<PHINode>(II);
3783 Value *BIV = PN->getIncomingValueForBlock(BB);
3784 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BIV))
3785 if (CE->canTrap())
3786 return false;
3787
3788 unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
3789 Value *PBIV = PN->getIncomingValue(PBBIdx);
3790 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(PBIV))
3791 if (CE->canTrap())
3792 return false;
3793 }
3794
3795 // Finally, if everything is ok, fold the branches to logical ops.
3796 BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
3797
3798 LLVM_DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
3799 << "AND: " << *BI->getParent());
3800
3801 SmallVector<DominatorTree::UpdateType, 5> Updates;
3802
3803 // If OtherDest *is* BB, then BB is a basic block with a single conditional
3804 // branch in it, where one edge (OtherDest) goes back to itself but the other
3805 // exits. We don't *know* that the program avoids the infinite loop
3806 // (even though that seems likely). If we do this xform naively, we'll end up
3807 // recursively unpeeling the loop. Since we know that (after the xform is
3808 // done) that the block *is* infinite if reached, we just make it an obviously
3809 // infinite loop with no cond branch.
3810 if (OtherDest == BB) {
3811 // Insert it at the end of the function, because it's either code,
3812 // or it won't matter if it's hot. :)
3813 BasicBlock *InfLoopBlock =
3814 BasicBlock::Create(BB->getContext(), "infloop", BB->getParent());
3815 BranchInst::Create(InfLoopBlock, InfLoopBlock);
3816 if (DTU)
3817 Updates.push_back({DominatorTree::Insert, InfLoopBlock, InfLoopBlock});
3818 OtherDest = InfLoopBlock;
3819 }
3820
3821 LLVM_DEBUG(dbgs() << *PBI->getParent()->getParent());
3822
3823 // BI may have other predecessors. Because of this, we leave
3824 // it alone, but modify PBI.
3825
3826 // Make sure we get to CommonDest on True&True directions.
3827 Value *PBICond = PBI->getCondition();
3828 IRBuilder<NoFolder> Builder(PBI);
3829 if (PBIOp)
3830 PBICond = Builder.CreateNot(PBICond, PBICond->getName() + ".not");
3831
3832 Value *BICond = BI->getCondition();
3833 if (BIOp)
3834 BICond = Builder.CreateNot(BICond, BICond->getName() + ".not");
3835
3836 // Merge the conditions.
3837 Value *Cond =
3838 createLogicalOp(Builder, Instruction::Or, PBICond, BICond, "brmerge");
3839
3840 // Modify PBI to branch on the new condition to the new dests.
3841 PBI->setCondition(Cond);
3842 PBI->setSuccessor(0, CommonDest);
3843 PBI->setSuccessor(1, OtherDest);
3844
3845 if (DTU) {
3846 Updates.push_back({DominatorTree::Insert, PBI->getParent(), OtherDest});
3847 Updates.push_back({DominatorTree::Delete, PBI->getParent(), RemovedDest});
3848
3849 DTU->applyUpdates(Updates);
3850 }
3851
3852 // Update branch weight for PBI.
3853 uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
3854 uint64_t PredCommon, PredOther, SuccCommon, SuccOther;
3855 bool HasWeights =
3856 extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
3857 SuccTrueWeight, SuccFalseWeight);
3858 if (HasWeights) {
3859 PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight;
3860 PredOther = PBIOp ? PredTrueWeight : PredFalseWeight;
3861 SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight;
3862 SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight;
3863 // The weight to CommonDest should be PredCommon * SuccTotal +
3864 // PredOther * SuccCommon.
3865 // The weight to OtherDest should be PredOther * SuccOther.
3866 uint64_t NewWeights[2] = {PredCommon * (SuccCommon + SuccOther) +
3867 PredOther * SuccCommon,
3868 PredOther * SuccOther};
3869 // Halve the weights if any of them cannot fit in an uint32_t
3870 FitWeights(NewWeights);
3871
3872 setBranchWeights(PBI, NewWeights[0], NewWeights[1]);
3873 }
3874
3875 // OtherDest may have phi nodes. If so, add an entry from PBI's
3876 // block that are identical to the entries for BI's block.
3877 AddPredecessorToBlock(OtherDest, PBI->getParent(), BB);
3878
3879 // We know that the CommonDest already had an edge from PBI to
3880 // it. If it has PHIs though, the PHIs may have different
3881 // entries for BB and PBI's BB. If so, insert a select to make
3882 // them agree.
3883 for (PHINode &PN : CommonDest->phis()) {
3884 Value *BIV = PN.getIncomingValueForBlock(BB);
3885 unsigned PBBIdx = PN.getBasicBlockIndex(PBI->getParent());
3886 Value *PBIV = PN.getIncomingValue(PBBIdx);
3887 if (BIV != PBIV) {
3888 // Insert a select in PBI to pick the right value.
3889 SelectInst *NV = cast<SelectInst>(
3890 Builder.CreateSelect(PBICond, PBIV, BIV, PBIV->getName() + ".mux"));
3891 PN.setIncomingValue(PBBIdx, NV);
3892 // Although the select has the same condition as PBI, the original branch
3893 // weights for PBI do not apply to the new select because the select's
3894 // 'logical' edges are incoming edges of the phi that is eliminated, not
3895 // the outgoing edges of PBI.
3896 if (HasWeights) {
3897 uint64_t PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight;
3898 uint64_t PredOther = PBIOp ? PredTrueWeight : PredFalseWeight;
3899 uint64_t SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight;
3900 uint64_t SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight;
3901 // The weight to PredCommonDest should be PredCommon * SuccTotal.
3902 // The weight to PredOtherDest should be PredOther * SuccCommon.
3903 uint64_t NewWeights[2] = {PredCommon * (SuccCommon + SuccOther),
3904 PredOther * SuccCommon};
3905
3906 FitWeights(NewWeights);
3907
3908 setBranchWeights(NV, NewWeights[0], NewWeights[1]);
3909 }
3910 }
3911 }
3912
3913 LLVM_DEBUG(dbgs() << "INTO: " << *PBI->getParent());
3914 LLVM_DEBUG(dbgs() << *PBI->getParent()->getParent());
3915
3916 // This basic block is probably dead. We know it has at least
3917 // one fewer predecessor.
3918 return true;
3919 }
3920
3921 // Simplifies a terminator by replacing it with a branch to TrueBB if Cond is
3922 // true or to FalseBB if Cond is false.
3923 // Takes care of updating the successors and removing the old terminator.
3924 // Also makes sure not to introduce new successors by assuming that edges to
3925 // non-successor TrueBBs and FalseBBs aren't reachable.
SimplifyTerminatorOnSelect(Instruction * OldTerm,Value * Cond,BasicBlock * TrueBB,BasicBlock * FalseBB,uint32_t TrueWeight,uint32_t FalseWeight)3926 bool SimplifyCFGOpt::SimplifyTerminatorOnSelect(Instruction *OldTerm,
3927 Value *Cond, BasicBlock *TrueBB,
3928 BasicBlock *FalseBB,
3929 uint32_t TrueWeight,
3930 uint32_t FalseWeight) {
3931 auto *BB = OldTerm->getParent();
3932 // Remove any superfluous successor edges from the CFG.
3933 // First, figure out which successors to preserve.
3934 // If TrueBB and FalseBB are equal, only try to preserve one copy of that
3935 // successor.
3936 BasicBlock *KeepEdge1 = TrueBB;
3937 BasicBlock *KeepEdge2 = TrueBB != FalseBB ? FalseBB : nullptr;
3938
3939 SmallPtrSet<BasicBlock *, 2> RemovedSuccessors;
3940
3941 // Then remove the rest.
3942 for (BasicBlock *Succ : successors(OldTerm)) {
3943 // Make sure only to keep exactly one copy of each edge.
3944 if (Succ == KeepEdge1)
3945 KeepEdge1 = nullptr;
3946 else if (Succ == KeepEdge2)
3947 KeepEdge2 = nullptr;
3948 else {
3949 Succ->removePredecessor(BB,
3950 /*KeepOneInputPHIs=*/true);
3951
3952 if (Succ != TrueBB && Succ != FalseBB)
3953 RemovedSuccessors.insert(Succ);
3954 }
3955 }
3956
3957 IRBuilder<> Builder(OldTerm);
3958 Builder.SetCurrentDebugLocation(OldTerm->getDebugLoc());
3959
3960 // Insert an appropriate new terminator.
3961 if (!KeepEdge1 && !KeepEdge2) {
3962 if (TrueBB == FalseBB) {
3963 // We were only looking for one successor, and it was present.
3964 // Create an unconditional branch to it.
3965 Builder.CreateBr(TrueBB);
3966 } else {
3967 // We found both of the successors we were looking for.
3968 // Create a conditional branch sharing the condition of the select.
3969 BranchInst *NewBI = Builder.CreateCondBr(Cond, TrueBB, FalseBB);
3970 if (TrueWeight != FalseWeight)
3971 setBranchWeights(NewBI, TrueWeight, FalseWeight);
3972 }
3973 } else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) {
3974 // Neither of the selected blocks were successors, so this
3975 // terminator must be unreachable.
3976 new UnreachableInst(OldTerm->getContext(), OldTerm);
3977 } else {
3978 // One of the selected values was a successor, but the other wasn't.
3979 // Insert an unconditional branch to the one that was found;
3980 // the edge to the one that wasn't must be unreachable.
3981 if (!KeepEdge1) {
3982 // Only TrueBB was found.
3983 Builder.CreateBr(TrueBB);
3984 } else {
3985 // Only FalseBB was found.
3986 Builder.CreateBr(FalseBB);
3987 }
3988 }
3989
3990 EraseTerminatorAndDCECond(OldTerm);
3991
3992 if (DTU) {
3993 SmallVector<DominatorTree::UpdateType, 2> Updates;
3994 Updates.reserve(RemovedSuccessors.size());
3995 for (auto *RemovedSuccessor : RemovedSuccessors)
3996 Updates.push_back({DominatorTree::Delete, BB, RemovedSuccessor});
3997 DTU->applyUpdates(Updates);
3998 }
3999
4000 return true;
4001 }
4002
4003 // Replaces
4004 // (switch (select cond, X, Y)) on constant X, Y
4005 // with a branch - conditional if X and Y lead to distinct BBs,
4006 // unconditional otherwise.
SimplifySwitchOnSelect(SwitchInst * SI,SelectInst * Select)4007 bool SimplifyCFGOpt::SimplifySwitchOnSelect(SwitchInst *SI,
4008 SelectInst *Select) {
4009 // Check for constant integer values in the select.
4010 ConstantInt *TrueVal = dyn_cast<ConstantInt>(Select->getTrueValue());
4011 ConstantInt *FalseVal = dyn_cast<ConstantInt>(Select->getFalseValue());
4012 if (!TrueVal || !FalseVal)
4013 return false;
4014
4015 // Find the relevant condition and destinations.
4016 Value *Condition = Select->getCondition();
4017 BasicBlock *TrueBB = SI->findCaseValue(TrueVal)->getCaseSuccessor();
4018 BasicBlock *FalseBB = SI->findCaseValue(FalseVal)->getCaseSuccessor();
4019
4020 // Get weight for TrueBB and FalseBB.
4021 uint32_t TrueWeight = 0, FalseWeight = 0;
4022 SmallVector<uint64_t, 8> Weights;
4023 bool HasWeights = HasBranchWeights(SI);
4024 if (HasWeights) {
4025 GetBranchWeights(SI, Weights);
4026 if (Weights.size() == 1 + SI->getNumCases()) {
4027 TrueWeight =
4028 (uint32_t)Weights[SI->findCaseValue(TrueVal)->getSuccessorIndex()];
4029 FalseWeight =
4030 (uint32_t)Weights[SI->findCaseValue(FalseVal)->getSuccessorIndex()];
4031 }
4032 }
4033
4034 // Perform the actual simplification.
4035 return SimplifyTerminatorOnSelect(SI, Condition, TrueBB, FalseBB, TrueWeight,
4036 FalseWeight);
4037 }
4038
4039 // Replaces
4040 // (indirectbr (select cond, blockaddress(@fn, BlockA),
4041 // blockaddress(@fn, BlockB)))
4042 // with
4043 // (br cond, BlockA, BlockB).
SimplifyIndirectBrOnSelect(IndirectBrInst * IBI,SelectInst * SI)4044 bool SimplifyCFGOpt::SimplifyIndirectBrOnSelect(IndirectBrInst *IBI,
4045 SelectInst *SI) {
4046 // Check that both operands of the select are block addresses.
4047 BlockAddress *TBA = dyn_cast<BlockAddress>(SI->getTrueValue());
4048 BlockAddress *FBA = dyn_cast<BlockAddress>(SI->getFalseValue());
4049 if (!TBA || !FBA)
4050 return false;
4051
4052 // Extract the actual blocks.
4053 BasicBlock *TrueBB = TBA->getBasicBlock();
4054 BasicBlock *FalseBB = FBA->getBasicBlock();
4055
4056 // Perform the actual simplification.
4057 return SimplifyTerminatorOnSelect(IBI, SI->getCondition(), TrueBB, FalseBB, 0,
4058 0);
4059 }
4060
4061 /// This is called when we find an icmp instruction
4062 /// (a seteq/setne with a constant) as the only instruction in a
4063 /// block that ends with an uncond branch. We are looking for a very specific
4064 /// pattern that occurs when "A == 1 || A == 2 || A == 3" gets simplified. In
4065 /// this case, we merge the first two "or's of icmp" into a switch, but then the
4066 /// default value goes to an uncond block with a seteq in it, we get something
4067 /// like:
4068 ///
4069 /// switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ]
4070 /// DEFAULT:
4071 /// %tmp = icmp eq i8 %A, 92
4072 /// br label %end
4073 /// end:
4074 /// ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ]
4075 ///
4076 /// We prefer to split the edge to 'end' so that there is a true/false entry to
4077 /// the PHI, merging the third icmp into the switch.
tryToSimplifyUncondBranchWithICmpInIt(ICmpInst * ICI,IRBuilder<> & Builder)4078 bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
4079 ICmpInst *ICI, IRBuilder<> &Builder) {
4080 BasicBlock *BB = ICI->getParent();
4081
4082 // If the block has any PHIs in it or the icmp has multiple uses, it is too
4083 // complex.
4084 if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse())
4085 return false;
4086
4087 Value *V = ICI->getOperand(0);
4088 ConstantInt *Cst = cast<ConstantInt>(ICI->getOperand(1));
4089
4090 // The pattern we're looking for is where our only predecessor is a switch on
4091 // 'V' and this block is the default case for the switch. In this case we can
4092 // fold the compared value into the switch to simplify things.
4093 BasicBlock *Pred = BB->getSinglePredecessor();
4094 if (!Pred || !isa<SwitchInst>(Pred->getTerminator()))
4095 return false;
4096
4097 SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
4098 if (SI->getCondition() != V)
4099 return false;
4100
4101 // If BB is reachable on a non-default case, then we simply know the value of
4102 // V in this block. Substitute it and constant fold the icmp instruction
4103 // away.
4104 if (SI->getDefaultDest() != BB) {
4105 ConstantInt *VVal = SI->findCaseDest(BB);
4106 assert(VVal && "Should have a unique destination value");
4107 ICI->setOperand(0, VVal);
4108
4109 if (Value *V = SimplifyInstruction(ICI, {DL, ICI})) {
4110 ICI->replaceAllUsesWith(V);
4111 ICI->eraseFromParent();
4112 }
4113 // BB is now empty, so it is likely to simplify away.
4114 return requestResimplify();
4115 }
4116
4117 // Ok, the block is reachable from the default dest. If the constant we're
4118 // comparing exists in one of the other edges, then we can constant fold ICI
4119 // and zap it.
4120 if (SI->findCaseValue(Cst) != SI->case_default()) {
4121 Value *V;
4122 if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
4123 V = ConstantInt::getFalse(BB->getContext());
4124 else
4125 V = ConstantInt::getTrue(BB->getContext());
4126
4127 ICI->replaceAllUsesWith(V);
4128 ICI->eraseFromParent();
4129 // BB is now empty, so it is likely to simplify away.
4130 return requestResimplify();
4131 }
4132
4133 // The use of the icmp has to be in the 'end' block, by the only PHI node in
4134 // the block.
4135 BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
4136 PHINode *PHIUse = dyn_cast<PHINode>(ICI->user_back());
4137 if (PHIUse == nullptr || PHIUse != &SuccBlock->front() ||
4138 isa<PHINode>(++BasicBlock::iterator(PHIUse)))
4139 return false;
4140
4141 // If the icmp is a SETEQ, then the default dest gets false, the new edge gets
4142 // true in the PHI.
4143 Constant *DefaultCst = ConstantInt::getTrue(BB->getContext());
4144 Constant *NewCst = ConstantInt::getFalse(BB->getContext());
4145
4146 if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
4147 std::swap(DefaultCst, NewCst);
4148
4149 // Replace ICI (which is used by the PHI for the default value) with true or
4150 // false depending on if it is EQ or NE.
4151 ICI->replaceAllUsesWith(DefaultCst);
4152 ICI->eraseFromParent();
4153
4154 SmallVector<DominatorTree::UpdateType, 2> Updates;
4155
4156 // Okay, the switch goes to this block on a default value. Add an edge from
4157 // the switch to the merge point on the compared value.
4158 BasicBlock *NewBB =
4159 BasicBlock::Create(BB->getContext(), "switch.edge", BB->getParent(), BB);
4160 {
4161 SwitchInstProfUpdateWrapper SIW(*SI);
4162 auto W0 = SIW.getSuccessorWeight(0);
4163 SwitchInstProfUpdateWrapper::CaseWeightOpt NewW;
4164 if (W0) {
4165 NewW = ((uint64_t(*W0) + 1) >> 1);
4166 SIW.setSuccessorWeight(0, *NewW);
4167 }
4168 SIW.addCase(Cst, NewBB, NewW);
4169 if (DTU)
4170 Updates.push_back({DominatorTree::Insert, Pred, NewBB});
4171 }
4172
4173 // NewBB branches to the phi block, add the uncond branch and the phi entry.
4174 Builder.SetInsertPoint(NewBB);
4175 Builder.SetCurrentDebugLocation(SI->getDebugLoc());
4176 Builder.CreateBr(SuccBlock);
4177 PHIUse->addIncoming(NewCst, NewBB);
4178 if (DTU) {
4179 Updates.push_back({DominatorTree::Insert, NewBB, SuccBlock});
4180 DTU->applyUpdates(Updates);
4181 }
4182 return true;
4183 }
4184
4185 /// The specified branch is a conditional branch.
4186 /// Check to see if it is branching on an or/and chain of icmp instructions, and
4187 /// fold it into a switch instruction if so.
SimplifyBranchOnICmpChain(BranchInst * BI,IRBuilder<> & Builder,const DataLayout & DL)4188 bool SimplifyCFGOpt::SimplifyBranchOnICmpChain(BranchInst *BI,
4189 IRBuilder<> &Builder,
4190 const DataLayout &DL) {
4191 Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
4192 if (!Cond)
4193 return false;
4194
4195 // Change br (X == 0 | X == 1), T, F into a switch instruction.
4196 // If this is a bunch of seteq's or'd together, or if it's a bunch of
4197 // 'setne's and'ed together, collect them.
4198
4199 // Try to gather values from a chain of and/or to be turned into a switch
4200 ConstantComparesGatherer ConstantCompare(Cond, DL);
4201 // Unpack the result
4202 SmallVectorImpl<ConstantInt *> &Values = ConstantCompare.Vals;
4203 Value *CompVal = ConstantCompare.CompValue;
4204 unsigned UsedICmps = ConstantCompare.UsedICmps;
4205 Value *ExtraCase = ConstantCompare.Extra;
4206
4207 // If we didn't have a multiply compared value, fail.
4208 if (!CompVal)
4209 return false;
4210
4211 // Avoid turning single icmps into a switch.
4212 if (UsedICmps <= 1)
4213 return false;
4214
4215 bool TrueWhenEqual = match(Cond, m_LogicalOr(m_Value(), m_Value()));
4216
4217 // There might be duplicate constants in the list, which the switch
4218 // instruction can't handle, remove them now.
4219 array_pod_sort(Values.begin(), Values.end(), ConstantIntSortPredicate);
4220 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
4221
4222 // If Extra was used, we require at least two switch values to do the
4223 // transformation. A switch with one value is just a conditional branch.
4224 if (ExtraCase && Values.size() < 2)
4225 return false;
4226
4227 // TODO: Preserve branch weight metadata, similarly to how
4228 // FoldValueComparisonIntoPredecessors preserves it.
4229
4230 // Figure out which block is which destination.
4231 BasicBlock *DefaultBB = BI->getSuccessor(1);
4232 BasicBlock *EdgeBB = BI->getSuccessor(0);
4233 if (!TrueWhenEqual)
4234 std::swap(DefaultBB, EdgeBB);
4235
4236 BasicBlock *BB = BI->getParent();
4237
4238 // MSAN does not like undefs as branch condition which can be introduced
4239 // with "explicit branch".
4240 if (ExtraCase && BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory))
4241 return false;
4242
4243 LLVM_DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size()
4244 << " cases into SWITCH. BB is:\n"
4245 << *BB);
4246
4247 SmallVector<DominatorTree::UpdateType, 2> Updates;
4248
4249 // If there are any extra values that couldn't be folded into the switch
4250 // then we evaluate them with an explicit branch first. Split the block
4251 // right before the condbr to handle it.
4252 if (ExtraCase) {
4253 BasicBlock *NewBB = SplitBlock(BB, BI, DTU, /*LI=*/nullptr,
4254 /*MSSAU=*/nullptr, "switch.early.test");
4255
4256 // Remove the uncond branch added to the old block.
4257 Instruction *OldTI = BB->getTerminator();
4258 Builder.SetInsertPoint(OldTI);
4259
4260 if (TrueWhenEqual)
4261 Builder.CreateCondBr(ExtraCase, EdgeBB, NewBB);
4262 else
4263 Builder.CreateCondBr(ExtraCase, NewBB, EdgeBB);
4264
4265 OldTI->eraseFromParent();
4266
4267 if (DTU)
4268 Updates.push_back({DominatorTree::Insert, BB, EdgeBB});
4269
4270 // If there are PHI nodes in EdgeBB, then we need to add a new entry to them
4271 // for the edge we just added.
4272 AddPredecessorToBlock(EdgeBB, BB, NewBB);
4273
4274 LLVM_DEBUG(dbgs() << " ** 'icmp' chain unhandled condition: " << *ExtraCase
4275 << "\nEXTRABB = " << *BB);
4276 BB = NewBB;
4277 }
4278
4279 Builder.SetInsertPoint(BI);
4280 // Convert pointer to int before we switch.
4281 if (CompVal->getType()->isPointerTy()) {
4282 CompVal = Builder.CreatePtrToInt(
4283 CompVal, DL.getIntPtrType(CompVal->getType()), "magicptr");
4284 }
4285
4286 // Create the new switch instruction now.
4287 SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size());
4288
4289 // Add all of the 'cases' to the switch instruction.
4290 for (unsigned i = 0, e = Values.size(); i != e; ++i)
4291 New->addCase(Values[i], EdgeBB);
4292
4293 // We added edges from PI to the EdgeBB. As such, if there were any
4294 // PHI nodes in EdgeBB, they need entries to be added corresponding to
4295 // the number of edges added.
4296 for (BasicBlock::iterator BBI = EdgeBB->begin(); isa<PHINode>(BBI); ++BBI) {
4297 PHINode *PN = cast<PHINode>(BBI);
4298 Value *InVal = PN->getIncomingValueForBlock(BB);
4299 for (unsigned i = 0, e = Values.size() - 1; i != e; ++i)
4300 PN->addIncoming(InVal, BB);
4301 }
4302
4303 // Erase the old branch instruction.
4304 EraseTerminatorAndDCECond(BI);
4305 if (DTU)
4306 DTU->applyUpdates(Updates);
4307
4308 LLVM_DEBUG(dbgs() << " ** 'icmp' chain result is:\n" << *BB << '\n');
4309 return true;
4310 }
4311
simplifyResume(ResumeInst * RI,IRBuilder<> & Builder)4312 bool SimplifyCFGOpt::simplifyResume(ResumeInst *RI, IRBuilder<> &Builder) {
4313 if (isa<PHINode>(RI->getValue()))
4314 return simplifyCommonResume(RI);
4315 else if (isa<LandingPadInst>(RI->getParent()->getFirstNonPHI()) &&
4316 RI->getValue() == RI->getParent()->getFirstNonPHI())
4317 // The resume must unwind the exception that caused control to branch here.
4318 return simplifySingleResume(RI);
4319
4320 return false;
4321 }
4322
4323 // Check if cleanup block is empty
isCleanupBlockEmpty(iterator_range<BasicBlock::iterator> R)4324 static bool isCleanupBlockEmpty(iterator_range<BasicBlock::iterator> R) {
4325 for (Instruction &I : R) {
4326 auto *II = dyn_cast<IntrinsicInst>(&I);
4327 if (!II)
4328 return false;
4329
4330 Intrinsic::ID IntrinsicID = II->getIntrinsicID();
4331 switch (IntrinsicID) {
4332 case Intrinsic::dbg_declare:
4333 case Intrinsic::dbg_value:
4334 case Intrinsic::dbg_label:
4335 case Intrinsic::lifetime_end:
4336 break;
4337 default:
4338 return false;
4339 }
4340 }
4341 return true;
4342 }
4343
4344 // Simplify resume that is shared by several landing pads (phi of landing pad).
simplifyCommonResume(ResumeInst * RI)4345 bool SimplifyCFGOpt::simplifyCommonResume(ResumeInst *RI) {
4346 BasicBlock *BB = RI->getParent();
4347
4348 // Check that there are no other instructions except for debug and lifetime
4349 // intrinsics between the phi's and resume instruction.
4350 if (!isCleanupBlockEmpty(
4351 make_range(RI->getParent()->getFirstNonPHI(), BB->getTerminator())))
4352 return false;
4353
4354 SmallSetVector<BasicBlock *, 4> TrivialUnwindBlocks;
4355 auto *PhiLPInst = cast<PHINode>(RI->getValue());
4356
4357 // Check incoming blocks to see if any of them are trivial.
4358 for (unsigned Idx = 0, End = PhiLPInst->getNumIncomingValues(); Idx != End;
4359 Idx++) {
4360 auto *IncomingBB = PhiLPInst->getIncomingBlock(Idx);
4361 auto *IncomingValue = PhiLPInst->getIncomingValue(Idx);
4362
4363 // If the block has other successors, we can not delete it because
4364 // it has other dependents.
4365 if (IncomingBB->getUniqueSuccessor() != BB)
4366 continue;
4367
4368 auto *LandingPad = dyn_cast<LandingPadInst>(IncomingBB->getFirstNonPHI());
4369 // Not the landing pad that caused the control to branch here.
4370 if (IncomingValue != LandingPad)
4371 continue;
4372
4373 if (isCleanupBlockEmpty(
4374 make_range(LandingPad->getNextNode(), IncomingBB->getTerminator())))
4375 TrivialUnwindBlocks.insert(IncomingBB);
4376 }
4377
4378 // If no trivial unwind blocks, don't do any simplifications.
4379 if (TrivialUnwindBlocks.empty())
4380 return false;
4381
4382 // Turn all invokes that unwind here into calls.
4383 for (auto *TrivialBB : TrivialUnwindBlocks) {
4384 // Blocks that will be simplified should be removed from the phi node.
4385 // Note there could be multiple edges to the resume block, and we need
4386 // to remove them all.
4387 while (PhiLPInst->getBasicBlockIndex(TrivialBB) != -1)
4388 BB->removePredecessor(TrivialBB, true);
4389
4390 for (BasicBlock *Pred :
4391 llvm::make_early_inc_range(predecessors(TrivialBB))) {
4392 removeUnwindEdge(Pred, DTU);
4393 ++NumInvokes;
4394 }
4395
4396 // In each SimplifyCFG run, only the current processed block can be erased.
4397 // Otherwise, it will break the iteration of SimplifyCFG pass. So instead
4398 // of erasing TrivialBB, we only remove the branch to the common resume
4399 // block so that we can later erase the resume block since it has no
4400 // predecessors.
4401 TrivialBB->getTerminator()->eraseFromParent();
4402 new UnreachableInst(RI->getContext(), TrivialBB);
4403 if (DTU)
4404 DTU->applyUpdates({{DominatorTree::Delete, TrivialBB, BB}});
4405 }
4406
4407 // Delete the resume block if all its predecessors have been removed.
4408 if (pred_empty(BB))
4409 DeleteDeadBlock(BB, DTU);
4410
4411 return !TrivialUnwindBlocks.empty();
4412 }
4413
4414 // Simplify resume that is only used by a single (non-phi) landing pad.
simplifySingleResume(ResumeInst * RI)4415 bool SimplifyCFGOpt::simplifySingleResume(ResumeInst *RI) {
4416 BasicBlock *BB = RI->getParent();
4417 auto *LPInst = cast<LandingPadInst>(BB->getFirstNonPHI());
4418 assert(RI->getValue() == LPInst &&
4419 "Resume must unwind the exception that caused control to here");
4420
4421 // Check that there are no other instructions except for debug intrinsics.
4422 if (!isCleanupBlockEmpty(
4423 make_range<Instruction *>(LPInst->getNextNode(), RI)))
4424 return false;
4425
4426 // Turn all invokes that unwind here into calls and delete the basic block.
4427 for (BasicBlock *Pred : llvm::make_early_inc_range(predecessors(BB))) {
4428 removeUnwindEdge(Pred, DTU);
4429 ++NumInvokes;
4430 }
4431
4432 // The landingpad is now unreachable. Zap it.
4433 DeleteDeadBlock(BB, DTU);
4434 return true;
4435 }
4436
removeEmptyCleanup(CleanupReturnInst * RI,DomTreeUpdater * DTU)4437 static bool removeEmptyCleanup(CleanupReturnInst *RI, DomTreeUpdater *DTU) {
4438 // If this is a trivial cleanup pad that executes no instructions, it can be
4439 // eliminated. If the cleanup pad continues to the caller, any predecessor
4440 // that is an EH pad will be updated to continue to the caller and any
4441 // predecessor that terminates with an invoke instruction will have its invoke
4442 // instruction converted to a call instruction. If the cleanup pad being
4443 // simplified does not continue to the caller, each predecessor will be
4444 // updated to continue to the unwind destination of the cleanup pad being
4445 // simplified.
4446 BasicBlock *BB = RI->getParent();
4447 CleanupPadInst *CPInst = RI->getCleanupPad();
4448 if (CPInst->getParent() != BB)
4449 // This isn't an empty cleanup.
4450 return false;
4451
4452 // We cannot kill the pad if it has multiple uses. This typically arises
4453 // from unreachable basic blocks.
4454 if (!CPInst->hasOneUse())
4455 return false;
4456
4457 // Check that there are no other instructions except for benign intrinsics.
4458 if (!isCleanupBlockEmpty(
4459 make_range<Instruction *>(CPInst->getNextNode(), RI)))
4460 return false;
4461
4462 // If the cleanup return we are simplifying unwinds to the caller, this will
4463 // set UnwindDest to nullptr.
4464 BasicBlock *UnwindDest = RI->getUnwindDest();
4465 Instruction *DestEHPad = UnwindDest ? UnwindDest->getFirstNonPHI() : nullptr;
4466
4467 // We're about to remove BB from the control flow. Before we do, sink any
4468 // PHINodes into the unwind destination. Doing this before changing the
4469 // control flow avoids some potentially slow checks, since we can currently
4470 // be certain that UnwindDest and BB have no common predecessors (since they
4471 // are both EH pads).
4472 if (UnwindDest) {
4473 // First, go through the PHI nodes in UnwindDest and update any nodes that
4474 // reference the block we are removing
4475 for (PHINode &DestPN : UnwindDest->phis()) {
4476 int Idx = DestPN.getBasicBlockIndex(BB);
4477 // Since BB unwinds to UnwindDest, it has to be in the PHI node.
4478 assert(Idx != -1);
4479 // This PHI node has an incoming value that corresponds to a control
4480 // path through the cleanup pad we are removing. If the incoming
4481 // value is in the cleanup pad, it must be a PHINode (because we
4482 // verified above that the block is otherwise empty). Otherwise, the
4483 // value is either a constant or a value that dominates the cleanup
4484 // pad being removed.
4485 //
4486 // Because BB and UnwindDest are both EH pads, all of their
4487 // predecessors must unwind to these blocks, and since no instruction
4488 // can have multiple unwind destinations, there will be no overlap in
4489 // incoming blocks between SrcPN and DestPN.
4490 Value *SrcVal = DestPN.getIncomingValue(Idx);
4491 PHINode *SrcPN = dyn_cast<PHINode>(SrcVal);
4492
4493 bool NeedPHITranslation = SrcPN && SrcPN->getParent() == BB;
4494 for (auto *Pred : predecessors(BB)) {
4495 Value *Incoming =
4496 NeedPHITranslation ? SrcPN->getIncomingValueForBlock(Pred) : SrcVal;
4497 DestPN.addIncoming(Incoming, Pred);
4498 }
4499 }
4500
4501 // Sink any remaining PHI nodes directly into UnwindDest.
4502 Instruction *InsertPt = DestEHPad;
4503 for (PHINode &PN : BB->phis()) {
4504 // The iterator must be incremented here because the instructions are
4505 // being moved to another block.
4506 if (PN.use_empty() || !PN.isUsedOutsideOfBlock(BB))
4507 // If the PHI node has no uses or all of its uses are in this basic
4508 // block (meaning they are debug or lifetime intrinsics), just leave
4509 // it. It will be erased when we erase BB below.
4510 continue;
4511
4512 // Otherwise, sink this PHI node into UnwindDest.
4513 // Any predecessors to UnwindDest which are not already represented
4514 // must be back edges which inherit the value from the path through
4515 // BB. In this case, the PHI value must reference itself.
4516 for (auto *pred : predecessors(UnwindDest))
4517 if (pred != BB)
4518 PN.addIncoming(&PN, pred);
4519 PN.moveBefore(InsertPt);
4520 // Also, add a dummy incoming value for the original BB itself,
4521 // so that the PHI is well-formed until we drop said predecessor.
4522 PN.addIncoming(UndefValue::get(PN.getType()), BB);
4523 }
4524 }
4525
4526 std::vector<DominatorTree::UpdateType> Updates;
4527
4528 // We use make_early_inc_range here because we will remove all predecessors.
4529 for (BasicBlock *PredBB : llvm::make_early_inc_range(predecessors(BB))) {
4530 if (UnwindDest == nullptr) {
4531 if (DTU) {
4532 DTU->applyUpdates(Updates);
4533 Updates.clear();
4534 }
4535 removeUnwindEdge(PredBB, DTU);
4536 ++NumInvokes;
4537 } else {
4538 BB->removePredecessor(PredBB);
4539 Instruction *TI = PredBB->getTerminator();
4540 TI->replaceUsesOfWith(BB, UnwindDest);
4541 if (DTU) {
4542 Updates.push_back({DominatorTree::Insert, PredBB, UnwindDest});
4543 Updates.push_back({DominatorTree::Delete, PredBB, BB});
4544 }
4545 }
4546 }
4547
4548 if (DTU)
4549 DTU->applyUpdates(Updates);
4550
4551 DeleteDeadBlock(BB, DTU);
4552
4553 return true;
4554 }
4555
4556 // Try to merge two cleanuppads together.
mergeCleanupPad(CleanupReturnInst * RI)4557 static bool mergeCleanupPad(CleanupReturnInst *RI) {
4558 // Skip any cleanuprets which unwind to caller, there is nothing to merge
4559 // with.
4560 BasicBlock *UnwindDest = RI->getUnwindDest();
4561 if (!UnwindDest)
4562 return false;
4563
4564 // This cleanupret isn't the only predecessor of this cleanuppad, it wouldn't
4565 // be safe to merge without code duplication.
4566 if (UnwindDest->getSinglePredecessor() != RI->getParent())
4567 return false;
4568
4569 // Verify that our cleanuppad's unwind destination is another cleanuppad.
4570 auto *SuccessorCleanupPad = dyn_cast<CleanupPadInst>(&UnwindDest->front());
4571 if (!SuccessorCleanupPad)
4572 return false;
4573
4574 CleanupPadInst *PredecessorCleanupPad = RI->getCleanupPad();
4575 // Replace any uses of the successor cleanupad with the predecessor pad
4576 // The only cleanuppad uses should be this cleanupret, it's cleanupret and
4577 // funclet bundle operands.
4578 SuccessorCleanupPad->replaceAllUsesWith(PredecessorCleanupPad);
4579 // Remove the old cleanuppad.
4580 SuccessorCleanupPad->eraseFromParent();
4581 // Now, we simply replace the cleanupret with a branch to the unwind
4582 // destination.
4583 BranchInst::Create(UnwindDest, RI->getParent());
4584 RI->eraseFromParent();
4585
4586 return true;
4587 }
4588
simplifyCleanupReturn(CleanupReturnInst * RI)4589 bool SimplifyCFGOpt::simplifyCleanupReturn(CleanupReturnInst *RI) {
4590 // It is possible to transiantly have an undef cleanuppad operand because we
4591 // have deleted some, but not all, dead blocks.
4592 // Eventually, this block will be deleted.
4593 if (isa<UndefValue>(RI->getOperand(0)))
4594 return false;
4595
4596 if (mergeCleanupPad(RI))
4597 return true;
4598
4599 if (removeEmptyCleanup(RI, DTU))
4600 return true;
4601
4602 return false;
4603 }
4604
simplifyReturn(ReturnInst * RI,IRBuilder<> & Builder)4605 bool SimplifyCFGOpt::simplifyReturn(ReturnInst *RI, IRBuilder<> &Builder) {
4606 BasicBlock *BB = RI->getParent();
4607 if (!BB->getFirstNonPHIOrDbg()->isTerminator())
4608 return false;
4609
4610 // Find predecessors that end with branches.
4611 SmallVector<BasicBlock *, 8> UncondBranchPreds;
4612 SmallVector<BranchInst *, 8> CondBranchPreds;
4613 for (BasicBlock *P : predecessors(BB)) {
4614 Instruction *PTI = P->getTerminator();
4615 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) {
4616 if (BI->isUnconditional())
4617 UncondBranchPreds.push_back(P);
4618 else
4619 CondBranchPreds.push_back(BI);
4620 }
4621 }
4622
4623 // If we found some, do the transformation!
4624 if (!UncondBranchPreds.empty() && DupRet) {
4625 while (!UncondBranchPreds.empty()) {
4626 BasicBlock *Pred = UncondBranchPreds.pop_back_val();
4627 LLVM_DEBUG(dbgs() << "FOLDING: " << *BB
4628 << "INTO UNCOND BRANCH PRED: " << *Pred);
4629 (void)FoldReturnIntoUncondBranch(RI, BB, Pred, DTU);
4630 }
4631
4632 // If we eliminated all predecessors of the block, delete the block now.
4633 if (pred_empty(BB))
4634 DeleteDeadBlock(BB, DTU);
4635
4636 return true;
4637 }
4638
4639 // Check out all of the conditional branches going to this return
4640 // instruction. If any of them just select between returns, change the
4641 // branch itself into a select/return pair.
4642 while (!CondBranchPreds.empty()) {
4643 BranchInst *BI = CondBranchPreds.pop_back_val();
4644
4645 // Check to see if the non-BB successor is also a return block.
4646 if (isa<ReturnInst>(BI->getSuccessor(0)->getTerminator()) &&
4647 isa<ReturnInst>(BI->getSuccessor(1)->getTerminator()) &&
4648 SimplifyCondBranchToTwoReturns(BI, Builder))
4649 return true;
4650 }
4651 return false;
4652 }
4653
simplifyUnreachable(UnreachableInst * UI)4654 bool SimplifyCFGOpt::simplifyUnreachable(UnreachableInst *UI) {
4655 BasicBlock *BB = UI->getParent();
4656
4657 bool Changed = false;
4658
4659 // If there are any instructions immediately before the unreachable that can
4660 // be removed, do so.
4661 while (UI->getIterator() != BB->begin()) {
4662 BasicBlock::iterator BBI = UI->getIterator();
4663 --BBI;
4664 // Do not delete instructions that can have side effects which might cause
4665 // the unreachable to not be reachable; specifically, calls and volatile
4666 // operations may have this effect.
4667 if (isa<CallInst>(BBI) && !isa<DbgInfoIntrinsic>(BBI))
4668 break;
4669
4670 if (BBI->mayHaveSideEffects()) {
4671 if (auto *SI = dyn_cast<StoreInst>(BBI)) {
4672 if (SI->isVolatile())
4673 break;
4674 } else if (auto *LI = dyn_cast<LoadInst>(BBI)) {
4675 if (LI->isVolatile())
4676 break;
4677 } else if (auto *RMWI = dyn_cast<AtomicRMWInst>(BBI)) {
4678 if (RMWI->isVolatile())
4679 break;
4680 } else if (auto *CXI = dyn_cast<AtomicCmpXchgInst>(BBI)) {
4681 if (CXI->isVolatile())
4682 break;
4683 } else if (isa<CatchPadInst>(BBI)) {
4684 // A catchpad may invoke exception object constructors and such, which
4685 // in some languages can be arbitrary code, so be conservative by
4686 // default.
4687 // For CoreCLR, it just involves a type test, so can be removed.
4688 if (classifyEHPersonality(BB->getParent()->getPersonalityFn()) !=
4689 EHPersonality::CoreCLR)
4690 break;
4691 } else if (!isa<FenceInst>(BBI) && !isa<VAArgInst>(BBI) &&
4692 !isa<LandingPadInst>(BBI)) {
4693 break;
4694 }
4695 // Note that deleting LandingPad's here is in fact okay, although it
4696 // involves a bit of subtle reasoning. If this inst is a LandingPad,
4697 // all the predecessors of this block will be the unwind edges of Invokes,
4698 // and we can therefore guarantee this block will be erased.
4699 }
4700
4701 // Delete this instruction (any uses are guaranteed to be dead)
4702 if (!BBI->use_empty())
4703 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
4704 BBI->eraseFromParent();
4705 Changed = true;
4706 }
4707
4708 // If the unreachable instruction is the first in the block, take a gander
4709 // at all of the predecessors of this instruction, and simplify them.
4710 if (&BB->front() != UI)
4711 return Changed;
4712
4713 std::vector<DominatorTree::UpdateType> Updates;
4714
4715 SmallSetVector<BasicBlock *, 8> Preds(pred_begin(BB), pred_end(BB));
4716 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
4717 auto *Predecessor = Preds[i];
4718 Instruction *TI = Predecessor->getTerminator();
4719 IRBuilder<> Builder(TI);
4720 if (auto *BI = dyn_cast<BranchInst>(TI)) {
4721 // We could either have a proper unconditional branch,
4722 // or a degenerate conditional branch with matching destinations.
4723 if (all_of(BI->successors(),
4724 [BB](auto *Successor) { return Successor == BB; })) {
4725 new UnreachableInst(TI->getContext(), TI);
4726 TI->eraseFromParent();
4727 Changed = true;
4728 } else {
4729 assert(BI->isConditional() && "Can't get here with an uncond branch.");
4730 Value* Cond = BI->getCondition();
4731 assert(BI->getSuccessor(0) != BI->getSuccessor(1) &&
4732 "The destinations are guaranteed to be different here.");
4733 if (BI->getSuccessor(0) == BB) {
4734 Builder.CreateAssumption(Builder.CreateNot(Cond));
4735 Builder.CreateBr(BI->getSuccessor(1));
4736 } else {
4737 assert(BI->getSuccessor(1) == BB && "Incorrect CFG");
4738 Builder.CreateAssumption(Cond);
4739 Builder.CreateBr(BI->getSuccessor(0));
4740 }
4741 EraseTerminatorAndDCECond(BI);
4742 Changed = true;
4743 }
4744 if (DTU)
4745 Updates.push_back({DominatorTree::Delete, Predecessor, BB});
4746 } else if (auto *SI = dyn_cast<SwitchInst>(TI)) {
4747 SwitchInstProfUpdateWrapper SU(*SI);
4748 for (auto i = SU->case_begin(), e = SU->case_end(); i != e;) {
4749 if (i->getCaseSuccessor() != BB) {
4750 ++i;
4751 continue;
4752 }
4753 BB->removePredecessor(SU->getParent());
4754 i = SU.removeCase(i);
4755 e = SU->case_end();
4756 Changed = true;
4757 }
4758 // Note that the default destination can't be removed!
4759 if (DTU && SI->getDefaultDest() != BB)
4760 Updates.push_back({DominatorTree::Delete, Predecessor, BB});
4761 } else if (auto *II = dyn_cast<InvokeInst>(TI)) {
4762 if (II->getUnwindDest() == BB) {
4763 if (DTU) {
4764 DTU->applyUpdates(Updates);
4765 Updates.clear();
4766 }
4767 removeUnwindEdge(TI->getParent(), DTU);
4768 Changed = true;
4769 }
4770 } else if (auto *CSI = dyn_cast<CatchSwitchInst>(TI)) {
4771 if (CSI->getUnwindDest() == BB) {
4772 if (DTU) {
4773 DTU->applyUpdates(Updates);
4774 Updates.clear();
4775 }
4776 removeUnwindEdge(TI->getParent(), DTU);
4777 Changed = true;
4778 continue;
4779 }
4780
4781 for (CatchSwitchInst::handler_iterator I = CSI->handler_begin(),
4782 E = CSI->handler_end();
4783 I != E; ++I) {
4784 if (*I == BB) {
4785 CSI->removeHandler(I);
4786 --I;
4787 --E;
4788 Changed = true;
4789 }
4790 }
4791 if (DTU)
4792 Updates.push_back({DominatorTree::Delete, Predecessor, BB});
4793 if (CSI->getNumHandlers() == 0) {
4794 if (CSI->hasUnwindDest()) {
4795 // Redirect all predecessors of the block containing CatchSwitchInst
4796 // to instead branch to the CatchSwitchInst's unwind destination.
4797 if (DTU) {
4798 for (auto *PredecessorOfPredecessor : predecessors(Predecessor)) {
4799 Updates.push_back({DominatorTree::Insert,
4800 PredecessorOfPredecessor,
4801 CSI->getUnwindDest()});
4802 Updates.push_back({DominatorTree::Delete,
4803 PredecessorOfPredecessor, Predecessor});
4804 }
4805 }
4806 Predecessor->replaceAllUsesWith(CSI->getUnwindDest());
4807 } else {
4808 // Rewrite all preds to unwind to caller (or from invoke to call).
4809 if (DTU) {
4810 DTU->applyUpdates(Updates);
4811 Updates.clear();
4812 }
4813 SmallVector<BasicBlock *, 8> EHPreds(predecessors(Predecessor));
4814 for (BasicBlock *EHPred : EHPreds)
4815 removeUnwindEdge(EHPred, DTU);
4816 }
4817 // The catchswitch is no longer reachable.
4818 new UnreachableInst(CSI->getContext(), CSI);
4819 CSI->eraseFromParent();
4820 Changed = true;
4821 }
4822 } else if (auto *CRI = dyn_cast<CleanupReturnInst>(TI)) {
4823 (void)CRI;
4824 assert(CRI->hasUnwindDest() && CRI->getUnwindDest() == BB &&
4825 "Expected to always have an unwind to BB.");
4826 if (DTU)
4827 Updates.push_back({DominatorTree::Delete, Predecessor, BB});
4828 new UnreachableInst(TI->getContext(), TI);
4829 TI->eraseFromParent();
4830 Changed = true;
4831 }
4832 }
4833
4834 if (DTU)
4835 DTU->applyUpdates(Updates);
4836
4837 // If this block is now dead, remove it.
4838 if (pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()) {
4839 DeleteDeadBlock(BB, DTU);
4840 return true;
4841 }
4842
4843 return Changed;
4844 }
4845
CasesAreContiguous(SmallVectorImpl<ConstantInt * > & Cases)4846 static bool CasesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
4847 assert(Cases.size() >= 1);
4848
4849 array_pod_sort(Cases.begin(), Cases.end(), ConstantIntSortPredicate);
4850 for (size_t I = 1, E = Cases.size(); I != E; ++I) {
4851 if (Cases[I - 1]->getValue() != Cases[I]->getValue() + 1)
4852 return false;
4853 }
4854 return true;
4855 }
4856
createUnreachableSwitchDefault(SwitchInst * Switch,DomTreeUpdater * DTU)4857 static void createUnreachableSwitchDefault(SwitchInst *Switch,
4858 DomTreeUpdater *DTU) {
4859 LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
4860 auto *BB = Switch->getParent();
4861 BasicBlock *NewDefaultBlock = SplitBlockPredecessors(
4862 Switch->getDefaultDest(), Switch->getParent(), "", DTU);
4863 auto *OrigDefaultBlock = Switch->getDefaultDest();
4864 Switch->setDefaultDest(&*NewDefaultBlock);
4865 if (DTU)
4866 DTU->applyUpdates({{DominatorTree::Insert, BB, &*NewDefaultBlock},
4867 {DominatorTree::Delete, BB, OrigDefaultBlock}});
4868 SplitBlock(&*NewDefaultBlock, &NewDefaultBlock->front(), DTU);
4869 SmallVector<DominatorTree::UpdateType, 2> Updates;
4870 if (DTU)
4871 for (auto *Successor : successors(NewDefaultBlock))
4872 Updates.push_back({DominatorTree::Delete, NewDefaultBlock, Successor});
4873 auto *NewTerminator = NewDefaultBlock->getTerminator();
4874 new UnreachableInst(Switch->getContext(), NewTerminator);
4875 EraseTerminatorAndDCECond(NewTerminator);
4876 if (DTU)
4877 DTU->applyUpdates(Updates);
4878 }
4879
4880 /// Turn a switch with two reachable destinations into an integer range
4881 /// comparison and branch.
TurnSwitchRangeIntoICmp(SwitchInst * SI,IRBuilder<> & Builder)4882 bool SimplifyCFGOpt::TurnSwitchRangeIntoICmp(SwitchInst *SI,
4883 IRBuilder<> &Builder) {
4884 assert(SI->getNumCases() > 1 && "Degenerate switch?");
4885
4886 bool HasDefault =
4887 !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
4888
4889 auto *BB = SI->getParent();
4890
4891 // Partition the cases into two sets with different destinations.
4892 BasicBlock *DestA = HasDefault ? SI->getDefaultDest() : nullptr;
4893 BasicBlock *DestB = nullptr;
4894 SmallVector<ConstantInt *, 16> CasesA;
4895 SmallVector<ConstantInt *, 16> CasesB;
4896
4897 for (auto Case : SI->cases()) {
4898 BasicBlock *Dest = Case.getCaseSuccessor();
4899 if (!DestA)
4900 DestA = Dest;
4901 if (Dest == DestA) {
4902 CasesA.push_back(Case.getCaseValue());
4903 continue;
4904 }
4905 if (!DestB)
4906 DestB = Dest;
4907 if (Dest == DestB) {
4908 CasesB.push_back(Case.getCaseValue());
4909 continue;
4910 }
4911 return false; // More than two destinations.
4912 }
4913
4914 assert(DestA && DestB &&
4915 "Single-destination switch should have been folded.");
4916 assert(DestA != DestB);
4917 assert(DestB != SI->getDefaultDest());
4918 assert(!CasesB.empty() && "There must be non-default cases.");
4919 assert(!CasesA.empty() || HasDefault);
4920
4921 // Figure out if one of the sets of cases form a contiguous range.
4922 SmallVectorImpl<ConstantInt *> *ContiguousCases = nullptr;
4923 BasicBlock *ContiguousDest = nullptr;
4924 BasicBlock *OtherDest = nullptr;
4925 if (!CasesA.empty() && CasesAreContiguous(CasesA)) {
4926 ContiguousCases = &CasesA;
4927 ContiguousDest = DestA;
4928 OtherDest = DestB;
4929 } else if (CasesAreContiguous(CasesB)) {
4930 ContiguousCases = &CasesB;
4931 ContiguousDest = DestB;
4932 OtherDest = DestA;
4933 } else
4934 return false;
4935
4936 // Start building the compare and branch.
4937
4938 Constant *Offset = ConstantExpr::getNeg(ContiguousCases->back());
4939 Constant *NumCases =
4940 ConstantInt::get(Offset->getType(), ContiguousCases->size());
4941
4942 Value *Sub = SI->getCondition();
4943 if (!Offset->isNullValue())
4944 Sub = Builder.CreateAdd(Sub, Offset, Sub->getName() + ".off");
4945
4946 Value *Cmp;
4947 // If NumCases overflowed, then all possible values jump to the successor.
4948 if (NumCases->isNullValue() && !ContiguousCases->empty())
4949 Cmp = ConstantInt::getTrue(SI->getContext());
4950 else
4951 Cmp = Builder.CreateICmpULT(Sub, NumCases, "switch");
4952 BranchInst *NewBI = Builder.CreateCondBr(Cmp, ContiguousDest, OtherDest);
4953
4954 // Update weight for the newly-created conditional branch.
4955 if (HasBranchWeights(SI)) {
4956 SmallVector<uint64_t, 8> Weights;
4957 GetBranchWeights(SI, Weights);
4958 if (Weights.size() == 1 + SI->getNumCases()) {
4959 uint64_t TrueWeight = 0;
4960 uint64_t FalseWeight = 0;
4961 for (size_t I = 0, E = Weights.size(); I != E; ++I) {
4962 if (SI->getSuccessor(I) == ContiguousDest)
4963 TrueWeight += Weights[I];
4964 else
4965 FalseWeight += Weights[I];
4966 }
4967 while (TrueWeight > UINT32_MAX || FalseWeight > UINT32_MAX) {
4968 TrueWeight /= 2;
4969 FalseWeight /= 2;
4970 }
4971 setBranchWeights(NewBI, TrueWeight, FalseWeight);
4972 }
4973 }
4974
4975 // Prune obsolete incoming values off the successors' PHI nodes.
4976 for (auto BBI = ContiguousDest->begin(); isa<PHINode>(BBI); ++BBI) {
4977 unsigned PreviousEdges = ContiguousCases->size();
4978 if (ContiguousDest == SI->getDefaultDest())
4979 ++PreviousEdges;
4980 for (unsigned I = 0, E = PreviousEdges - 1; I != E; ++I)
4981 cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
4982 }
4983 for (auto BBI = OtherDest->begin(); isa<PHINode>(BBI); ++BBI) {
4984 unsigned PreviousEdges = SI->getNumCases() - ContiguousCases->size();
4985 if (OtherDest == SI->getDefaultDest())
4986 ++PreviousEdges;
4987 for (unsigned I = 0, E = PreviousEdges - 1; I != E; ++I)
4988 cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
4989 }
4990
4991 // Clean up the default block - it may have phis or other instructions before
4992 // the unreachable terminator.
4993 if (!HasDefault)
4994 createUnreachableSwitchDefault(SI, DTU);
4995
4996 auto *UnreachableDefault = SI->getDefaultDest();
4997
4998 // Drop the switch.
4999 SI->eraseFromParent();
5000
5001 if (!HasDefault && DTU)
5002 DTU->applyUpdates({{DominatorTree::Delete, BB, UnreachableDefault}});
5003
5004 return true;
5005 }
5006
5007 /// Compute masked bits for the condition of a switch
5008 /// and use it to remove dead cases.
eliminateDeadSwitchCases(SwitchInst * SI,DomTreeUpdater * DTU,AssumptionCache * AC,const DataLayout & DL)5009 static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
5010 AssumptionCache *AC,
5011 const DataLayout &DL) {
5012 Value *Cond = SI->getCondition();
5013 unsigned Bits = Cond->getType()->getIntegerBitWidth();
5014 KnownBits Known = computeKnownBits(Cond, DL, 0, AC, SI);
5015
5016 // We can also eliminate cases by determining that their values are outside of
5017 // the limited range of the condition based on how many significant (non-sign)
5018 // bits are in the condition value.
5019 unsigned ExtraSignBits = ComputeNumSignBits(Cond, DL, 0, AC, SI) - 1;
5020 unsigned MaxSignificantBitsInCond = Bits - ExtraSignBits;
5021
5022 // Gather dead cases.
5023 SmallVector<ConstantInt *, 8> DeadCases;
5024 SmallDenseMap<BasicBlock *, int, 8> NumPerSuccessorCases;
5025 for (auto &Case : SI->cases()) {
5026 auto *Successor = Case.getCaseSuccessor();
5027 if (DTU)
5028 ++NumPerSuccessorCases[Successor];
5029 const APInt &CaseVal = Case.getCaseValue()->getValue();
5030 if (Known.Zero.intersects(CaseVal) || !Known.One.isSubsetOf(CaseVal) ||
5031 (CaseVal.getMinSignedBits() > MaxSignificantBitsInCond)) {
5032 DeadCases.push_back(Case.getCaseValue());
5033 if (DTU)
5034 --NumPerSuccessorCases[Successor];
5035 LLVM_DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal
5036 << " is dead.\n");
5037 }
5038 }
5039
5040 // If we can prove that the cases must cover all possible values, the
5041 // default destination becomes dead and we can remove it. If we know some
5042 // of the bits in the value, we can use that to more precisely compute the
5043 // number of possible unique case values.
5044 bool HasDefault =
5045 !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
5046 const unsigned NumUnknownBits =
5047 Bits - (Known.Zero | Known.One).countPopulation();
5048 assert(NumUnknownBits <= Bits);
5049 if (HasDefault && DeadCases.empty() &&
5050 NumUnknownBits < 64 /* avoid overflow */ &&
5051 SI->getNumCases() == (1ULL << NumUnknownBits)) {
5052 createUnreachableSwitchDefault(SI, DTU);
5053 return true;
5054 }
5055
5056 if (DeadCases.empty())
5057 return false;
5058
5059 SwitchInstProfUpdateWrapper SIW(*SI);
5060 for (ConstantInt *DeadCase : DeadCases) {
5061 SwitchInst::CaseIt CaseI = SI->findCaseValue(DeadCase);
5062 assert(CaseI != SI->case_default() &&
5063 "Case was not found. Probably mistake in DeadCases forming.");
5064 // Prune unused values from PHI nodes.
5065 CaseI->getCaseSuccessor()->removePredecessor(SI->getParent());
5066 SIW.removeCase(CaseI);
5067 }
5068
5069 if (DTU) {
5070 std::vector<DominatorTree::UpdateType> Updates;
5071 for (const std::pair<BasicBlock *, int> &I : NumPerSuccessorCases)
5072 if (I.second == 0)
5073 Updates.push_back({DominatorTree::Delete, SI->getParent(), I.first});
5074 DTU->applyUpdates(Updates);
5075 }
5076
5077 return true;
5078 }
5079
5080 /// If BB would be eligible for simplification by
5081 /// TryToSimplifyUncondBranchFromEmptyBlock (i.e. it is empty and terminated
5082 /// by an unconditional branch), look at the phi node for BB in the successor
5083 /// block and see if the incoming value is equal to CaseValue. If so, return
5084 /// the phi node, and set PhiIndex to BB's index in the phi node.
FindPHIForConditionForwarding(ConstantInt * CaseValue,BasicBlock * BB,int * PhiIndex)5085 static PHINode *FindPHIForConditionForwarding(ConstantInt *CaseValue,
5086 BasicBlock *BB, int *PhiIndex) {
5087 if (BB->getFirstNonPHIOrDbg() != BB->getTerminator())
5088 return nullptr; // BB must be empty to be a candidate for simplification.
5089 if (!BB->getSinglePredecessor())
5090 return nullptr; // BB must be dominated by the switch.
5091
5092 BranchInst *Branch = dyn_cast<BranchInst>(BB->getTerminator());
5093 if (!Branch || !Branch->isUnconditional())
5094 return nullptr; // Terminator must be unconditional branch.
5095
5096 BasicBlock *Succ = Branch->getSuccessor(0);
5097
5098 for (PHINode &PHI : Succ->phis()) {
5099 int Idx = PHI.getBasicBlockIndex(BB);
5100 assert(Idx >= 0 && "PHI has no entry for predecessor?");
5101
5102 Value *InValue = PHI.getIncomingValue(Idx);
5103 if (InValue != CaseValue)
5104 continue;
5105
5106 *PhiIndex = Idx;
5107 return &PHI;
5108 }
5109
5110 return nullptr;
5111 }
5112
5113 /// Try to forward the condition of a switch instruction to a phi node
5114 /// dominated by the switch, if that would mean that some of the destination
5115 /// blocks of the switch can be folded away. Return true if a change is made.
ForwardSwitchConditionToPHI(SwitchInst * SI)5116 static bool ForwardSwitchConditionToPHI(SwitchInst *SI) {
5117 using ForwardingNodesMap = DenseMap<PHINode *, SmallVector<int, 4>>;
5118
5119 ForwardingNodesMap ForwardingNodes;
5120 BasicBlock *SwitchBlock = SI->getParent();
5121 bool Changed = false;
5122 for (auto &Case : SI->cases()) {
5123 ConstantInt *CaseValue = Case.getCaseValue();
5124 BasicBlock *CaseDest = Case.getCaseSuccessor();
5125
5126 // Replace phi operands in successor blocks that are using the constant case
5127 // value rather than the switch condition variable:
5128 // switchbb:
5129 // switch i32 %x, label %default [
5130 // i32 17, label %succ
5131 // ...
5132 // succ:
5133 // %r = phi i32 ... [ 17, %switchbb ] ...
5134 // -->
5135 // %r = phi i32 ... [ %x, %switchbb ] ...
5136
5137 for (PHINode &Phi : CaseDest->phis()) {
5138 // This only works if there is exactly 1 incoming edge from the switch to
5139 // a phi. If there is >1, that means multiple cases of the switch map to 1
5140 // value in the phi, and that phi value is not the switch condition. Thus,
5141 // this transform would not make sense (the phi would be invalid because
5142 // a phi can't have different incoming values from the same block).
5143 int SwitchBBIdx = Phi.getBasicBlockIndex(SwitchBlock);
5144 if (Phi.getIncomingValue(SwitchBBIdx) == CaseValue &&
5145 count(Phi.blocks(), SwitchBlock) == 1) {
5146 Phi.setIncomingValue(SwitchBBIdx, SI->getCondition());
5147 Changed = true;
5148 }
5149 }
5150
5151 // Collect phi nodes that are indirectly using this switch's case constants.
5152 int PhiIdx;
5153 if (auto *Phi = FindPHIForConditionForwarding(CaseValue, CaseDest, &PhiIdx))
5154 ForwardingNodes[Phi].push_back(PhiIdx);
5155 }
5156
5157 for (auto &ForwardingNode : ForwardingNodes) {
5158 PHINode *Phi = ForwardingNode.first;
5159 SmallVectorImpl<int> &Indexes = ForwardingNode.second;
5160 if (Indexes.size() < 2)
5161 continue;
5162
5163 for (int Index : Indexes)
5164 Phi->setIncomingValue(Index, SI->getCondition());
5165 Changed = true;
5166 }
5167
5168 return Changed;
5169 }
5170
5171 /// Return true if the backend will be able to handle
5172 /// initializing an array of constants like C.
ValidLookupTableConstant(Constant * C,const TargetTransformInfo & TTI)5173 static bool ValidLookupTableConstant(Constant *C, const TargetTransformInfo &TTI) {
5174 if (C->isThreadDependent())
5175 return false;
5176 if (C->isDLLImportDependent())
5177 return false;
5178
5179 if (!isa<ConstantFP>(C) && !isa<ConstantInt>(C) &&
5180 !isa<ConstantPointerNull>(C) && !isa<GlobalValue>(C) &&
5181 !isa<UndefValue>(C) && !isa<ConstantExpr>(C))
5182 return false;
5183
5184 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
5185 if (!CE->isGEPWithNoNotionalOverIndexing())
5186 return false;
5187 if (!ValidLookupTableConstant(CE->getOperand(0), TTI))
5188 return false;
5189 }
5190
5191 if (!TTI.shouldBuildLookupTablesForConstant(C))
5192 return false;
5193
5194 return true;
5195 }
5196
5197 /// If V is a Constant, return it. Otherwise, try to look up
5198 /// its constant value in ConstantPool, returning 0 if it's not there.
5199 static Constant *
LookupConstant(Value * V,const SmallDenseMap<Value *,Constant * > & ConstantPool)5200 LookupConstant(Value *V,
5201 const SmallDenseMap<Value *, Constant *> &ConstantPool) {
5202 if (Constant *C = dyn_cast<Constant>(V))
5203 return C;
5204 return ConstantPool.lookup(V);
5205 }
5206
5207 /// Try to fold instruction I into a constant. This works for
5208 /// simple instructions such as binary operations where both operands are
5209 /// constant or can be replaced by constants from the ConstantPool. Returns the
5210 /// resulting constant on success, 0 otherwise.
5211 static Constant *
ConstantFold(Instruction * I,const DataLayout & DL,const SmallDenseMap<Value *,Constant * > & ConstantPool)5212 ConstantFold(Instruction *I, const DataLayout &DL,
5213 const SmallDenseMap<Value *, Constant *> &ConstantPool) {
5214 if (SelectInst *Select = dyn_cast<SelectInst>(I)) {
5215 Constant *A = LookupConstant(Select->getCondition(), ConstantPool);
5216 if (!A)
5217 return nullptr;
5218 if (A->isAllOnesValue())
5219 return LookupConstant(Select->getTrueValue(), ConstantPool);
5220 if (A->isNullValue())
5221 return LookupConstant(Select->getFalseValue(), ConstantPool);
5222 return nullptr;
5223 }
5224
5225 SmallVector<Constant *, 4> COps;
5226 for (unsigned N = 0, E = I->getNumOperands(); N != E; ++N) {
5227 if (Constant *A = LookupConstant(I->getOperand(N), ConstantPool))
5228 COps.push_back(A);
5229 else
5230 return nullptr;
5231 }
5232
5233 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
5234 return ConstantFoldCompareInstOperands(Cmp->getPredicate(), COps[0],
5235 COps[1], DL);
5236 }
5237
5238 return ConstantFoldInstOperands(I, COps, DL);
5239 }
5240
5241 /// Try to determine the resulting constant values in phi nodes
5242 /// at the common destination basic block, *CommonDest, for one of the case
5243 /// destionations CaseDest corresponding to value CaseVal (0 for the default
5244 /// case), of a switch instruction SI.
5245 static bool
GetCaseResults(SwitchInst * SI,ConstantInt * CaseVal,BasicBlock * CaseDest,BasicBlock ** CommonDest,SmallVectorImpl<std::pair<PHINode *,Constant * >> & Res,const DataLayout & DL,const TargetTransformInfo & TTI)5246 GetCaseResults(SwitchInst *SI, ConstantInt *CaseVal, BasicBlock *CaseDest,
5247 BasicBlock **CommonDest,
5248 SmallVectorImpl<std::pair<PHINode *, Constant *>> &Res,
5249 const DataLayout &DL, const TargetTransformInfo &TTI) {
5250 // The block from which we enter the common destination.
5251 BasicBlock *Pred = SI->getParent();
5252
5253 // If CaseDest is empty except for some side-effect free instructions through
5254 // which we can constant-propagate the CaseVal, continue to its successor.
5255 SmallDenseMap<Value *, Constant *> ConstantPool;
5256 ConstantPool.insert(std::make_pair(SI->getCondition(), CaseVal));
5257 for (Instruction &I :CaseDest->instructionsWithoutDebug()) {
5258 if (I.isTerminator()) {
5259 // If the terminator is a simple branch, continue to the next block.
5260 if (I.getNumSuccessors() != 1 || I.isExceptionalTerminator())
5261 return false;
5262 Pred = CaseDest;
5263 CaseDest = I.getSuccessor(0);
5264 } else if (Constant *C = ConstantFold(&I, DL, ConstantPool)) {
5265 // Instruction is side-effect free and constant.
5266
5267 // If the instruction has uses outside this block or a phi node slot for
5268 // the block, it is not safe to bypass the instruction since it would then
5269 // no longer dominate all its uses.
5270 for (auto &Use : I.uses()) {
5271 User *User = Use.getUser();
5272 if (Instruction *I = dyn_cast<Instruction>(User))
5273 if (I->getParent() == CaseDest)
5274 continue;
5275 if (PHINode *Phi = dyn_cast<PHINode>(User))
5276 if (Phi->getIncomingBlock(Use) == CaseDest)
5277 continue;
5278 return false;
5279 }
5280
5281 ConstantPool.insert(std::make_pair(&I, C));
5282 } else {
5283 break;
5284 }
5285 }
5286
5287 // If we did not have a CommonDest before, use the current one.
5288 if (!*CommonDest)
5289 *CommonDest = CaseDest;
5290 // If the destination isn't the common one, abort.
5291 if (CaseDest != *CommonDest)
5292 return false;
5293
5294 // Get the values for this case from phi nodes in the destination block.
5295 for (PHINode &PHI : (*CommonDest)->phis()) {
5296 int Idx = PHI.getBasicBlockIndex(Pred);
5297 if (Idx == -1)
5298 continue;
5299
5300 Constant *ConstVal =
5301 LookupConstant(PHI.getIncomingValue(Idx), ConstantPool);
5302 if (!ConstVal)
5303 return false;
5304
5305 // Be conservative about which kinds of constants we support.
5306 if (!ValidLookupTableConstant(ConstVal, TTI))
5307 return false;
5308
5309 Res.push_back(std::make_pair(&PHI, ConstVal));
5310 }
5311
5312 return Res.size() > 0;
5313 }
5314
5315 // Helper function used to add CaseVal to the list of cases that generate
5316 // Result. Returns the updated number of cases that generate this result.
MapCaseToResult(ConstantInt * CaseVal,SwitchCaseResultVectorTy & UniqueResults,Constant * Result)5317 static uintptr_t MapCaseToResult(ConstantInt *CaseVal,
5318 SwitchCaseResultVectorTy &UniqueResults,
5319 Constant *Result) {
5320 for (auto &I : UniqueResults) {
5321 if (I.first == Result) {
5322 I.second.push_back(CaseVal);
5323 return I.second.size();
5324 }
5325 }
5326 UniqueResults.push_back(
5327 std::make_pair(Result, SmallVector<ConstantInt *, 4>(1, CaseVal)));
5328 return 1;
5329 }
5330
5331 // Helper function that initializes a map containing
5332 // results for the PHI node of the common destination block for a switch
5333 // instruction. Returns false if multiple PHI nodes have been found or if
5334 // there is not a common destination block for the switch.
5335 static bool
InitializeUniqueCases(SwitchInst * SI,PHINode * & PHI,BasicBlock * & CommonDest,SwitchCaseResultVectorTy & UniqueResults,Constant * & DefaultResult,const DataLayout & DL,const TargetTransformInfo & TTI,uintptr_t MaxUniqueResults,uintptr_t MaxCasesPerResult)5336 InitializeUniqueCases(SwitchInst *SI, PHINode *&PHI, BasicBlock *&CommonDest,
5337 SwitchCaseResultVectorTy &UniqueResults,
5338 Constant *&DefaultResult, const DataLayout &DL,
5339 const TargetTransformInfo &TTI,
5340 uintptr_t MaxUniqueResults, uintptr_t MaxCasesPerResult) {
5341 for (auto &I : SI->cases()) {
5342 ConstantInt *CaseVal = I.getCaseValue();
5343
5344 // Resulting value at phi nodes for this case value.
5345 SwitchCaseResultsTy Results;
5346 if (!GetCaseResults(SI, CaseVal, I.getCaseSuccessor(), &CommonDest, Results,
5347 DL, TTI))
5348 return false;
5349
5350 // Only one value per case is permitted.
5351 if (Results.size() > 1)
5352 return false;
5353
5354 // Add the case->result mapping to UniqueResults.
5355 const uintptr_t NumCasesForResult =
5356 MapCaseToResult(CaseVal, UniqueResults, Results.begin()->second);
5357
5358 // Early out if there are too many cases for this result.
5359 if (NumCasesForResult > MaxCasesPerResult)
5360 return false;
5361
5362 // Early out if there are too many unique results.
5363 if (UniqueResults.size() > MaxUniqueResults)
5364 return false;
5365
5366 // Check the PHI consistency.
5367 if (!PHI)
5368 PHI = Results[0].first;
5369 else if (PHI != Results[0].first)
5370 return false;
5371 }
5372 // Find the default result value.
5373 SmallVector<std::pair<PHINode *, Constant *>, 1> DefaultResults;
5374 BasicBlock *DefaultDest = SI->getDefaultDest();
5375 GetCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest, DefaultResults,
5376 DL, TTI);
5377 // If the default value is not found abort unless the default destination
5378 // is unreachable.
5379 DefaultResult =
5380 DefaultResults.size() == 1 ? DefaultResults.begin()->second : nullptr;
5381 if ((!DefaultResult &&
5382 !isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg())))
5383 return false;
5384
5385 return true;
5386 }
5387
5388 // Helper function that checks if it is possible to transform a switch with only
5389 // two cases (or two cases + default) that produces a result into a select.
5390 // Example:
5391 // switch (a) {
5392 // case 10: %0 = icmp eq i32 %a, 10
5393 // return 10; %1 = select i1 %0, i32 10, i32 4
5394 // case 20: ----> %2 = icmp eq i32 %a, 20
5395 // return 2; %3 = select i1 %2, i32 2, i32 %1
5396 // default:
5397 // return 4;
5398 // }
ConvertTwoCaseSwitch(const SwitchCaseResultVectorTy & ResultVector,Constant * DefaultResult,Value * Condition,IRBuilder<> & Builder)5399 static Value *ConvertTwoCaseSwitch(const SwitchCaseResultVectorTy &ResultVector,
5400 Constant *DefaultResult, Value *Condition,
5401 IRBuilder<> &Builder) {
5402 // If we are selecting between only two cases transform into a simple
5403 // select or a two-way select if default is possible.
5404 if (ResultVector.size() == 2 && ResultVector[0].second.size() == 1 &&
5405 ResultVector[1].second.size() == 1) {
5406 ConstantInt *const FirstCase = ResultVector[0].second[0];
5407 ConstantInt *const SecondCase = ResultVector[1].second[0];
5408
5409 bool DefaultCanTrigger = DefaultResult;
5410 Value *SelectValue = ResultVector[1].first;
5411 if (DefaultCanTrigger) {
5412 Value *const ValueCompare =
5413 Builder.CreateICmpEQ(Condition, SecondCase, "switch.selectcmp");
5414 SelectValue = Builder.CreateSelect(ValueCompare, ResultVector[1].first,
5415 DefaultResult, "switch.select");
5416 }
5417 Value *const ValueCompare =
5418 Builder.CreateICmpEQ(Condition, FirstCase, "switch.selectcmp");
5419 return Builder.CreateSelect(ValueCompare, ResultVector[0].first,
5420 SelectValue, "switch.select");
5421 }
5422
5423 // Handle the degenerate case where two cases have the same value.
5424 if (ResultVector.size() == 1 && ResultVector[0].second.size() == 2 &&
5425 DefaultResult) {
5426 Value *Cmp1 = Builder.CreateICmpEQ(
5427 Condition, ResultVector[0].second[0], "switch.selectcmp.case1");
5428 Value *Cmp2 = Builder.CreateICmpEQ(
5429 Condition, ResultVector[0].second[1], "switch.selectcmp.case2");
5430 Value *Cmp = Builder.CreateOr(Cmp1, Cmp2, "switch.selectcmp");
5431 return Builder.CreateSelect(Cmp, ResultVector[0].first, DefaultResult);
5432 }
5433
5434 return nullptr;
5435 }
5436
5437 // Helper function to cleanup a switch instruction that has been converted into
5438 // a select, fixing up PHI nodes and basic blocks.
RemoveSwitchAfterSelectConversion(SwitchInst * SI,PHINode * PHI,Value * SelectValue,IRBuilder<> & Builder,DomTreeUpdater * DTU)5439 static void RemoveSwitchAfterSelectConversion(SwitchInst *SI, PHINode *PHI,
5440 Value *SelectValue,
5441 IRBuilder<> &Builder,
5442 DomTreeUpdater *DTU) {
5443 std::vector<DominatorTree::UpdateType> Updates;
5444
5445 BasicBlock *SelectBB = SI->getParent();
5446 BasicBlock *DestBB = PHI->getParent();
5447
5448 if (DTU && !is_contained(predecessors(DestBB), SelectBB))
5449 Updates.push_back({DominatorTree::Insert, SelectBB, DestBB});
5450 Builder.CreateBr(DestBB);
5451
5452 // Remove the switch.
5453
5454 while (PHI->getBasicBlockIndex(SelectBB) >= 0)
5455 PHI->removeIncomingValue(SelectBB);
5456 PHI->addIncoming(SelectValue, SelectBB);
5457
5458 SmallPtrSet<BasicBlock *, 4> RemovedSuccessors;
5459 for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
5460 BasicBlock *Succ = SI->getSuccessor(i);
5461
5462 if (Succ == DestBB)
5463 continue;
5464 Succ->removePredecessor(SelectBB);
5465 if (DTU && RemovedSuccessors.insert(Succ).second)
5466 Updates.push_back({DominatorTree::Delete, SelectBB, Succ});
5467 }
5468 SI->eraseFromParent();
5469 if (DTU)
5470 DTU->applyUpdates(Updates);
5471 }
5472
5473 /// If the switch is only used to initialize one or more
5474 /// phi nodes in a common successor block with only two different
5475 /// constant values, replace the switch with select.
switchToSelect(SwitchInst * SI,IRBuilder<> & Builder,DomTreeUpdater * DTU,const DataLayout & DL,const TargetTransformInfo & TTI)5476 static bool switchToSelect(SwitchInst *SI, IRBuilder<> &Builder,
5477 DomTreeUpdater *DTU, const DataLayout &DL,
5478 const TargetTransformInfo &TTI) {
5479 Value *const Cond = SI->getCondition();
5480 PHINode *PHI = nullptr;
5481 BasicBlock *CommonDest = nullptr;
5482 Constant *DefaultResult;
5483 SwitchCaseResultVectorTy UniqueResults;
5484 // Collect all the cases that will deliver the same value from the switch.
5485 if (!InitializeUniqueCases(SI, PHI, CommonDest, UniqueResults, DefaultResult,
5486 DL, TTI, /*MaxUniqueResults*/2,
5487 /*MaxCasesPerResult*/2))
5488 return false;
5489 assert(PHI != nullptr && "PHI for value select not found");
5490
5491 Builder.SetInsertPoint(SI);
5492 Value *SelectValue =
5493 ConvertTwoCaseSwitch(UniqueResults, DefaultResult, Cond, Builder);
5494 if (SelectValue) {
5495 RemoveSwitchAfterSelectConversion(SI, PHI, SelectValue, Builder, DTU);
5496 return true;
5497 }
5498 // The switch couldn't be converted into a select.
5499 return false;
5500 }
5501
5502 namespace {
5503
5504 /// This class represents a lookup table that can be used to replace a switch.
5505 class SwitchLookupTable {
5506 public:
5507 /// Create a lookup table to use as a switch replacement with the contents
5508 /// of Values, using DefaultValue to fill any holes in the table.
5509 SwitchLookupTable(
5510 Module &M, uint64_t TableSize, ConstantInt *Offset,
5511 const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values,
5512 Constant *DefaultValue, const DataLayout &DL, const StringRef &FuncName);
5513
5514 /// Build instructions with Builder to retrieve the value at
5515 /// the position given by Index in the lookup table.
5516 Value *BuildLookup(Value *Index, IRBuilder<> &Builder);
5517
5518 /// Return true if a table with TableSize elements of
5519 /// type ElementType would fit in a target-legal register.
5520 static bool WouldFitInRegister(const DataLayout &DL, uint64_t TableSize,
5521 Type *ElementType);
5522
5523 private:
5524 // Depending on the contents of the table, it can be represented in
5525 // different ways.
5526 enum {
5527 // For tables where each element contains the same value, we just have to
5528 // store that single value and return it for each lookup.
5529 SingleValueKind,
5530
5531 // For tables where there is a linear relationship between table index
5532 // and values. We calculate the result with a simple multiplication
5533 // and addition instead of a table lookup.
5534 LinearMapKind,
5535
5536 // For small tables with integer elements, we can pack them into a bitmap
5537 // that fits into a target-legal register. Values are retrieved by
5538 // shift and mask operations.
5539 BitMapKind,
5540
5541 // The table is stored as an array of values. Values are retrieved by load
5542 // instructions from the table.
5543 ArrayKind
5544 } Kind;
5545
5546 // For SingleValueKind, this is the single value.
5547 Constant *SingleValue = nullptr;
5548
5549 // For BitMapKind, this is the bitmap.
5550 ConstantInt *BitMap = nullptr;
5551 IntegerType *BitMapElementTy = nullptr;
5552
5553 // For LinearMapKind, these are the constants used to derive the value.
5554 ConstantInt *LinearOffset = nullptr;
5555 ConstantInt *LinearMultiplier = nullptr;
5556
5557 // For ArrayKind, this is the array.
5558 GlobalVariable *Array = nullptr;
5559 };
5560
5561 } // end anonymous namespace
5562
SwitchLookupTable(Module & M,uint64_t TableSize,ConstantInt * Offset,const SmallVectorImpl<std::pair<ConstantInt *,Constant * >> & Values,Constant * DefaultValue,const DataLayout & DL,const StringRef & FuncName)5563 SwitchLookupTable::SwitchLookupTable(
5564 Module &M, uint64_t TableSize, ConstantInt *Offset,
5565 const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values,
5566 Constant *DefaultValue, const DataLayout &DL, const StringRef &FuncName) {
5567 assert(Values.size() && "Can't build lookup table without values!");
5568 assert(TableSize >= Values.size() && "Can't fit values in table!");
5569
5570 // If all values in the table are equal, this is that value.
5571 SingleValue = Values.begin()->second;
5572
5573 Type *ValueType = Values.begin()->second->getType();
5574
5575 // Build up the table contents.
5576 SmallVector<Constant *, 64> TableContents(TableSize);
5577 for (size_t I = 0, E = Values.size(); I != E; ++I) {
5578 ConstantInt *CaseVal = Values[I].first;
5579 Constant *CaseRes = Values[I].second;
5580 assert(CaseRes->getType() == ValueType);
5581
5582 uint64_t Idx = (CaseVal->getValue() - Offset->getValue()).getLimitedValue();
5583 TableContents[Idx] = CaseRes;
5584
5585 if (CaseRes != SingleValue)
5586 SingleValue = nullptr;
5587 }
5588
5589 // Fill in any holes in the table with the default result.
5590 if (Values.size() < TableSize) {
5591 assert(DefaultValue &&
5592 "Need a default value to fill the lookup table holes.");
5593 assert(DefaultValue->getType() == ValueType);
5594 for (uint64_t I = 0; I < TableSize; ++I) {
5595 if (!TableContents[I])
5596 TableContents[I] = DefaultValue;
5597 }
5598
5599 if (DefaultValue != SingleValue)
5600 SingleValue = nullptr;
5601 }
5602
5603 // If each element in the table contains the same value, we only need to store
5604 // that single value.
5605 if (SingleValue) {
5606 Kind = SingleValueKind;
5607 return;
5608 }
5609
5610 // Check if we can derive the value with a linear transformation from the
5611 // table index.
5612 if (isa<IntegerType>(ValueType)) {
5613 bool LinearMappingPossible = true;
5614 APInt PrevVal;
5615 APInt DistToPrev;
5616 assert(TableSize >= 2 && "Should be a SingleValue table.");
5617 // Check if there is the same distance between two consecutive values.
5618 for (uint64_t I = 0; I < TableSize; ++I) {
5619 ConstantInt *ConstVal = dyn_cast<ConstantInt>(TableContents[I]);
5620 if (!ConstVal) {
5621 // This is an undef. We could deal with it, but undefs in lookup tables
5622 // are very seldom. It's probably not worth the additional complexity.
5623 LinearMappingPossible = false;
5624 break;
5625 }
5626 const APInt &Val = ConstVal->getValue();
5627 if (I != 0) {
5628 APInt Dist = Val - PrevVal;
5629 if (I == 1) {
5630 DistToPrev = Dist;
5631 } else if (Dist != DistToPrev) {
5632 LinearMappingPossible = false;
5633 break;
5634 }
5635 }
5636 PrevVal = Val;
5637 }
5638 if (LinearMappingPossible) {
5639 LinearOffset = cast<ConstantInt>(TableContents[0]);
5640 LinearMultiplier = ConstantInt::get(M.getContext(), DistToPrev);
5641 Kind = LinearMapKind;
5642 ++NumLinearMaps;
5643 return;
5644 }
5645 }
5646
5647 // If the type is integer and the table fits in a register, build a bitmap.
5648 if (WouldFitInRegister(DL, TableSize, ValueType)) {
5649 IntegerType *IT = cast<IntegerType>(ValueType);
5650 APInt TableInt(TableSize * IT->getBitWidth(), 0);
5651 for (uint64_t I = TableSize; I > 0; --I) {
5652 TableInt <<= IT->getBitWidth();
5653 // Insert values into the bitmap. Undef values are set to zero.
5654 if (!isa<UndefValue>(TableContents[I - 1])) {
5655 ConstantInt *Val = cast<ConstantInt>(TableContents[I - 1]);
5656 TableInt |= Val->getValue().zext(TableInt.getBitWidth());
5657 }
5658 }
5659 BitMap = ConstantInt::get(M.getContext(), TableInt);
5660 BitMapElementTy = IT;
5661 Kind = BitMapKind;
5662 ++NumBitMaps;
5663 return;
5664 }
5665
5666 // Store the table in an array.
5667 ArrayType *ArrayTy = ArrayType::get(ValueType, TableSize);
5668 Constant *Initializer = ConstantArray::get(ArrayTy, TableContents);
5669
5670 Array = new GlobalVariable(M, ArrayTy, /*isConstant=*/true,
5671 GlobalVariable::PrivateLinkage, Initializer,
5672 "switch.table." + FuncName);
5673 Array->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
5674 // Set the alignment to that of an array items. We will be only loading one
5675 // value out of it.
5676 Array->setAlignment(Align(DL.getPrefTypeAlignment(ValueType)));
5677 Kind = ArrayKind;
5678 }
5679
BuildLookup(Value * Index,IRBuilder<> & Builder)5680 Value *SwitchLookupTable::BuildLookup(Value *Index, IRBuilder<> &Builder) {
5681 switch (Kind) {
5682 case SingleValueKind:
5683 return SingleValue;
5684 case LinearMapKind: {
5685 // Derive the result value from the input value.
5686 Value *Result = Builder.CreateIntCast(Index, LinearMultiplier->getType(),
5687 false, "switch.idx.cast");
5688 if (!LinearMultiplier->isOne())
5689 Result = Builder.CreateMul(Result, LinearMultiplier, "switch.idx.mult");
5690 if (!LinearOffset->isZero())
5691 Result = Builder.CreateAdd(Result, LinearOffset, "switch.offset");
5692 return Result;
5693 }
5694 case BitMapKind: {
5695 // Type of the bitmap (e.g. i59).
5696 IntegerType *MapTy = BitMap->getType();
5697
5698 // Cast Index to the same type as the bitmap.
5699 // Note: The Index is <= the number of elements in the table, so
5700 // truncating it to the width of the bitmask is safe.
5701 Value *ShiftAmt = Builder.CreateZExtOrTrunc(Index, MapTy, "switch.cast");
5702
5703 // Multiply the shift amount by the element width.
5704 ShiftAmt = Builder.CreateMul(
5705 ShiftAmt, ConstantInt::get(MapTy, BitMapElementTy->getBitWidth()),
5706 "switch.shiftamt");
5707
5708 // Shift down.
5709 Value *DownShifted =
5710 Builder.CreateLShr(BitMap, ShiftAmt, "switch.downshift");
5711 // Mask off.
5712 return Builder.CreateTrunc(DownShifted, BitMapElementTy, "switch.masked");
5713 }
5714 case ArrayKind: {
5715 // Make sure the table index will not overflow when treated as signed.
5716 IntegerType *IT = cast<IntegerType>(Index->getType());
5717 uint64_t TableSize =
5718 Array->getInitializer()->getType()->getArrayNumElements();
5719 if (TableSize > (1ULL << (IT->getBitWidth() - 1)))
5720 Index = Builder.CreateZExt(
5721 Index, IntegerType::get(IT->getContext(), IT->getBitWidth() + 1),
5722 "switch.tableidx.zext");
5723
5724 Value *GEPIndices[] = {Builder.getInt32(0), Index};
5725 Value *GEP = Builder.CreateInBoundsGEP(Array->getValueType(), Array,
5726 GEPIndices, "switch.gep");
5727 return Builder.CreateLoad(
5728 cast<ArrayType>(Array->getValueType())->getElementType(), GEP,
5729 "switch.load");
5730 }
5731 }
5732 llvm_unreachable("Unknown lookup table kind!");
5733 }
5734
WouldFitInRegister(const DataLayout & DL,uint64_t TableSize,Type * ElementType)5735 bool SwitchLookupTable::WouldFitInRegister(const DataLayout &DL,
5736 uint64_t TableSize,
5737 Type *ElementType) {
5738 auto *IT = dyn_cast<IntegerType>(ElementType);
5739 if (!IT)
5740 return false;
5741 // FIXME: If the type is wider than it needs to be, e.g. i8 but all values
5742 // are <= 15, we could try to narrow the type.
5743
5744 // Avoid overflow, fitsInLegalInteger uses unsigned int for the width.
5745 if (TableSize >= UINT_MAX / IT->getBitWidth())
5746 return false;
5747 return DL.fitsInLegalInteger(TableSize * IT->getBitWidth());
5748 }
5749
5750 /// Determine whether a lookup table should be built for this switch, based on
5751 /// the number of cases, size of the table, and the types of the results.
5752 static bool
ShouldBuildLookupTable(SwitchInst * SI,uint64_t TableSize,const TargetTransformInfo & TTI,const DataLayout & DL,const SmallDenseMap<PHINode *,Type * > & ResultTypes)5753 ShouldBuildLookupTable(SwitchInst *SI, uint64_t TableSize,
5754 const TargetTransformInfo &TTI, const DataLayout &DL,
5755 const SmallDenseMap<PHINode *, Type *> &ResultTypes) {
5756 if (SI->getNumCases() > TableSize || TableSize >= UINT64_MAX / 10)
5757 return false; // TableSize overflowed, or mul below might overflow.
5758
5759 bool AllTablesFitInRegister = true;
5760 bool HasIllegalType = false;
5761 for (const auto &I : ResultTypes) {
5762 Type *Ty = I.second;
5763
5764 // Saturate this flag to true.
5765 HasIllegalType = HasIllegalType || !TTI.isTypeLegal(Ty);
5766
5767 // Saturate this flag to false.
5768 AllTablesFitInRegister =
5769 AllTablesFitInRegister &&
5770 SwitchLookupTable::WouldFitInRegister(DL, TableSize, Ty);
5771
5772 // If both flags saturate, we're done. NOTE: This *only* works with
5773 // saturating flags, and all flags have to saturate first due to the
5774 // non-deterministic behavior of iterating over a dense map.
5775 if (HasIllegalType && !AllTablesFitInRegister)
5776 break;
5777 }
5778
5779 // If each table would fit in a register, we should build it anyway.
5780 if (AllTablesFitInRegister)
5781 return true;
5782
5783 // Don't build a table that doesn't fit in-register if it has illegal types.
5784 if (HasIllegalType)
5785 return false;
5786
5787 // The table density should be at least 40%. This is the same criterion as for
5788 // jump tables, see SelectionDAGBuilder::handleJTSwitchCase.
5789 // FIXME: Find the best cut-off.
5790 return SI->getNumCases() * 10 >= TableSize * 4;
5791 }
5792
5793 /// Try to reuse the switch table index compare. Following pattern:
5794 /// \code
5795 /// if (idx < tablesize)
5796 /// r = table[idx]; // table does not contain default_value
5797 /// else
5798 /// r = default_value;
5799 /// if (r != default_value)
5800 /// ...
5801 /// \endcode
5802 /// Is optimized to:
5803 /// \code
5804 /// cond = idx < tablesize;
5805 /// if (cond)
5806 /// r = table[idx];
5807 /// else
5808 /// r = default_value;
5809 /// if (cond)
5810 /// ...
5811 /// \endcode
5812 /// Jump threading will then eliminate the second if(cond).
reuseTableCompare(User * PhiUser,BasicBlock * PhiBlock,BranchInst * RangeCheckBranch,Constant * DefaultValue,const SmallVectorImpl<std::pair<ConstantInt *,Constant * >> & Values)5813 static void reuseTableCompare(
5814 User *PhiUser, BasicBlock *PhiBlock, BranchInst *RangeCheckBranch,
5815 Constant *DefaultValue,
5816 const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values) {
5817 ICmpInst *CmpInst = dyn_cast<ICmpInst>(PhiUser);
5818 if (!CmpInst)
5819 return;
5820
5821 // We require that the compare is in the same block as the phi so that jump
5822 // threading can do its work afterwards.
5823 if (CmpInst->getParent() != PhiBlock)
5824 return;
5825
5826 Constant *CmpOp1 = dyn_cast<Constant>(CmpInst->getOperand(1));
5827 if (!CmpOp1)
5828 return;
5829
5830 Value *RangeCmp = RangeCheckBranch->getCondition();
5831 Constant *TrueConst = ConstantInt::getTrue(RangeCmp->getType());
5832 Constant *FalseConst = ConstantInt::getFalse(RangeCmp->getType());
5833
5834 // Check if the compare with the default value is constant true or false.
5835 Constant *DefaultConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
5836 DefaultValue, CmpOp1, true);
5837 if (DefaultConst != TrueConst && DefaultConst != FalseConst)
5838 return;
5839
5840 // Check if the compare with the case values is distinct from the default
5841 // compare result.
5842 for (auto ValuePair : Values) {
5843 Constant *CaseConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
5844 ValuePair.second, CmpOp1, true);
5845 if (!CaseConst || CaseConst == DefaultConst || isa<UndefValue>(CaseConst))
5846 return;
5847 assert((CaseConst == TrueConst || CaseConst == FalseConst) &&
5848 "Expect true or false as compare result.");
5849 }
5850
5851 // Check if the branch instruction dominates the phi node. It's a simple
5852 // dominance check, but sufficient for our needs.
5853 // Although this check is invariant in the calling loops, it's better to do it
5854 // at this late stage. Practically we do it at most once for a switch.
5855 BasicBlock *BranchBlock = RangeCheckBranch->getParent();
5856 for (BasicBlock *Pred : predecessors(PhiBlock)) {
5857 if (Pred != BranchBlock && Pred->getUniquePredecessor() != BranchBlock)
5858 return;
5859 }
5860
5861 if (DefaultConst == FalseConst) {
5862 // The compare yields the same result. We can replace it.
5863 CmpInst->replaceAllUsesWith(RangeCmp);
5864 ++NumTableCmpReuses;
5865 } else {
5866 // The compare yields the same result, just inverted. We can replace it.
5867 Value *InvertedTableCmp = BinaryOperator::CreateXor(
5868 RangeCmp, ConstantInt::get(RangeCmp->getType(), 1), "inverted.cmp",
5869 RangeCheckBranch);
5870 CmpInst->replaceAllUsesWith(InvertedTableCmp);
5871 ++NumTableCmpReuses;
5872 }
5873 }
5874
5875 /// If the switch is only used to initialize one or more phi nodes in a common
5876 /// successor block with different constant values, replace the switch with
5877 /// lookup tables.
SwitchToLookupTable(SwitchInst * SI,IRBuilder<> & Builder,DomTreeUpdater * DTU,const DataLayout & DL,const TargetTransformInfo & TTI)5878 static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
5879 DomTreeUpdater *DTU, const DataLayout &DL,
5880 const TargetTransformInfo &TTI) {
5881 assert(SI->getNumCases() > 1 && "Degenerate switch?");
5882
5883 BasicBlock *BB = SI->getParent();
5884 Function *Fn = BB->getParent();
5885 // Only build lookup table when we have a target that supports it or the
5886 // attribute is not set.
5887 if (!TTI.shouldBuildLookupTables() ||
5888 (Fn->getFnAttribute("no-jump-tables").getValueAsBool()))
5889 return false;
5890
5891 // FIXME: If the switch is too sparse for a lookup table, perhaps we could
5892 // split off a dense part and build a lookup table for that.
5893
5894 // FIXME: This creates arrays of GEPs to constant strings, which means each
5895 // GEP needs a runtime relocation in PIC code. We should just build one big
5896 // string and lookup indices into that.
5897
5898 // Ignore switches with less than three cases. Lookup tables will not make
5899 // them faster, so we don't analyze them.
5900 if (SI->getNumCases() < 3)
5901 return false;
5902
5903 // Figure out the corresponding result for each case value and phi node in the
5904 // common destination, as well as the min and max case values.
5905 assert(!SI->cases().empty());
5906 SwitchInst::CaseIt CI = SI->case_begin();
5907 ConstantInt *MinCaseVal = CI->getCaseValue();
5908 ConstantInt *MaxCaseVal = CI->getCaseValue();
5909
5910 BasicBlock *CommonDest = nullptr;
5911
5912 using ResultListTy = SmallVector<std::pair<ConstantInt *, Constant *>, 4>;
5913 SmallDenseMap<PHINode *, ResultListTy> ResultLists;
5914
5915 SmallDenseMap<PHINode *, Constant *> DefaultResults;
5916 SmallDenseMap<PHINode *, Type *> ResultTypes;
5917 SmallVector<PHINode *, 4> PHIs;
5918
5919 for (SwitchInst::CaseIt E = SI->case_end(); CI != E; ++CI) {
5920 ConstantInt *CaseVal = CI->getCaseValue();
5921 if (CaseVal->getValue().slt(MinCaseVal->getValue()))
5922 MinCaseVal = CaseVal;
5923 if (CaseVal->getValue().sgt(MaxCaseVal->getValue()))
5924 MaxCaseVal = CaseVal;
5925
5926 // Resulting value at phi nodes for this case value.
5927 using ResultsTy = SmallVector<std::pair<PHINode *, Constant *>, 4>;
5928 ResultsTy Results;
5929 if (!GetCaseResults(SI, CaseVal, CI->getCaseSuccessor(), &CommonDest,
5930 Results, DL, TTI))
5931 return false;
5932
5933 // Append the result from this case to the list for each phi.
5934 for (const auto &I : Results) {
5935 PHINode *PHI = I.first;
5936 Constant *Value = I.second;
5937 if (!ResultLists.count(PHI))
5938 PHIs.push_back(PHI);
5939 ResultLists[PHI].push_back(std::make_pair(CaseVal, Value));
5940 }
5941 }
5942
5943 // Keep track of the result types.
5944 for (PHINode *PHI : PHIs) {
5945 ResultTypes[PHI] = ResultLists[PHI][0].second->getType();
5946 }
5947
5948 uint64_t NumResults = ResultLists[PHIs[0]].size();
5949 APInt RangeSpread = MaxCaseVal->getValue() - MinCaseVal->getValue();
5950 uint64_t TableSize = RangeSpread.getLimitedValue() + 1;
5951 bool TableHasHoles = (NumResults < TableSize);
5952
5953 // If the table has holes, we need a constant result for the default case
5954 // or a bitmask that fits in a register.
5955 SmallVector<std::pair<PHINode *, Constant *>, 4> DefaultResultsList;
5956 bool HasDefaultResults =
5957 GetCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest,
5958 DefaultResultsList, DL, TTI);
5959
5960 bool NeedMask = (TableHasHoles && !HasDefaultResults);
5961 if (NeedMask) {
5962 // As an extra penalty for the validity test we require more cases.
5963 if (SI->getNumCases() < 4) // FIXME: Find best threshold value (benchmark).
5964 return false;
5965 if (!DL.fitsInLegalInteger(TableSize))
5966 return false;
5967 }
5968
5969 for (const auto &I : DefaultResultsList) {
5970 PHINode *PHI = I.first;
5971 Constant *Result = I.second;
5972 DefaultResults[PHI] = Result;
5973 }
5974
5975 if (!ShouldBuildLookupTable(SI, TableSize, TTI, DL, ResultTypes))
5976 return false;
5977
5978 std::vector<DominatorTree::UpdateType> Updates;
5979
5980 // Create the BB that does the lookups.
5981 Module &Mod = *CommonDest->getParent()->getParent();
5982 BasicBlock *LookupBB = BasicBlock::Create(
5983 Mod.getContext(), "switch.lookup", CommonDest->getParent(), CommonDest);
5984
5985 // Compute the table index value.
5986 Builder.SetInsertPoint(SI);
5987 Value *TableIndex;
5988 if (MinCaseVal->isNullValue())
5989 TableIndex = SI->getCondition();
5990 else
5991 TableIndex = Builder.CreateSub(SI->getCondition(), MinCaseVal,
5992 "switch.tableidx");
5993
5994 // Compute the maximum table size representable by the integer type we are
5995 // switching upon.
5996 unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits();
5997 uint64_t MaxTableSize = CaseSize > 63 ? UINT64_MAX : 1ULL << CaseSize;
5998 assert(MaxTableSize >= TableSize &&
5999 "It is impossible for a switch to have more entries than the max "
6000 "representable value of its input integer type's size.");
6001
6002 // If the default destination is unreachable, or if the lookup table covers
6003 // all values of the conditional variable, branch directly to the lookup table
6004 // BB. Otherwise, check that the condition is within the case range.
6005 const bool DefaultIsReachable =
6006 !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
6007 const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
6008 BranchInst *RangeCheckBranch = nullptr;
6009
6010 if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
6011 Builder.CreateBr(LookupBB);
6012 if (DTU)
6013 Updates.push_back({DominatorTree::Insert, BB, LookupBB});
6014 // Note: We call removeProdecessor later since we need to be able to get the
6015 // PHI value for the default case in case we're using a bit mask.
6016 } else {
6017 Value *Cmp = Builder.CreateICmpULT(
6018 TableIndex, ConstantInt::get(MinCaseVal->getType(), TableSize));
6019 RangeCheckBranch =
6020 Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
6021 if (DTU)
6022 Updates.push_back({DominatorTree::Insert, BB, LookupBB});
6023 }
6024
6025 // Populate the BB that does the lookups.
6026 Builder.SetInsertPoint(LookupBB);
6027
6028 if (NeedMask) {
6029 // Before doing the lookup, we do the hole check. The LookupBB is therefore
6030 // re-purposed to do the hole check, and we create a new LookupBB.
6031 BasicBlock *MaskBB = LookupBB;
6032 MaskBB->setName("switch.hole_check");
6033 LookupBB = BasicBlock::Create(Mod.getContext(), "switch.lookup",
6034 CommonDest->getParent(), CommonDest);
6035
6036 // Make the mask's bitwidth at least 8-bit and a power-of-2 to avoid
6037 // unnecessary illegal types.
6038 uint64_t TableSizePowOf2 = NextPowerOf2(std::max(7ULL, TableSize - 1ULL));
6039 APInt MaskInt(TableSizePowOf2, 0);
6040 APInt One(TableSizePowOf2, 1);
6041 // Build bitmask; fill in a 1 bit for every case.
6042 const ResultListTy &ResultList = ResultLists[PHIs[0]];
6043 for (size_t I = 0, E = ResultList.size(); I != E; ++I) {
6044 uint64_t Idx = (ResultList[I].first->getValue() - MinCaseVal->getValue())
6045 .getLimitedValue();
6046 MaskInt |= One << Idx;
6047 }
6048 ConstantInt *TableMask = ConstantInt::get(Mod.getContext(), MaskInt);
6049
6050 // Get the TableIndex'th bit of the bitmask.
6051 // If this bit is 0 (meaning hole) jump to the default destination,
6052 // else continue with table lookup.
6053 IntegerType *MapTy = TableMask->getType();
6054 Value *MaskIndex =
6055 Builder.CreateZExtOrTrunc(TableIndex, MapTy, "switch.maskindex");
6056 Value *Shifted = Builder.CreateLShr(TableMask, MaskIndex, "switch.shifted");
6057 Value *LoBit = Builder.CreateTrunc(
6058 Shifted, Type::getInt1Ty(Mod.getContext()), "switch.lobit");
6059 Builder.CreateCondBr(LoBit, LookupBB, SI->getDefaultDest());
6060 if (DTU) {
6061 Updates.push_back({DominatorTree::Insert, MaskBB, LookupBB});
6062 Updates.push_back({DominatorTree::Insert, MaskBB, SI->getDefaultDest()});
6063 }
6064 Builder.SetInsertPoint(LookupBB);
6065 AddPredecessorToBlock(SI->getDefaultDest(), MaskBB, BB);
6066 }
6067
6068 if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
6069 // We cached PHINodes in PHIs. To avoid accessing deleted PHINodes later,
6070 // do not delete PHINodes here.
6071 SI->getDefaultDest()->removePredecessor(BB,
6072 /*KeepOneInputPHIs=*/true);
6073 if (DTU)
6074 Updates.push_back({DominatorTree::Delete, BB, SI->getDefaultDest()});
6075 }
6076
6077 bool ReturnedEarly = false;
6078 for (PHINode *PHI : PHIs) {
6079 const ResultListTy &ResultList = ResultLists[PHI];
6080
6081 // If using a bitmask, use any value to fill the lookup table holes.
6082 Constant *DV = NeedMask ? ResultLists[PHI][0].second : DefaultResults[PHI];
6083 StringRef FuncName = Fn->getName();
6084 SwitchLookupTable Table(Mod, TableSize, MinCaseVal, ResultList, DV, DL,
6085 FuncName);
6086
6087 Value *Result = Table.BuildLookup(TableIndex, Builder);
6088
6089 // If the result is used to return immediately from the function, we want to
6090 // do that right here.
6091 if (PHI->hasOneUse() && isa<ReturnInst>(*PHI->user_begin()) &&
6092 PHI->user_back() == CommonDest->getFirstNonPHIOrDbg()) {
6093 Builder.CreateRet(Result);
6094 ReturnedEarly = true;
6095 break;
6096 }
6097
6098 // Do a small peephole optimization: re-use the switch table compare if
6099 // possible.
6100 if (!TableHasHoles && HasDefaultResults && RangeCheckBranch) {
6101 BasicBlock *PhiBlock = PHI->getParent();
6102 // Search for compare instructions which use the phi.
6103 for (auto *User : PHI->users()) {
6104 reuseTableCompare(User, PhiBlock, RangeCheckBranch, DV, ResultList);
6105 }
6106 }
6107
6108 PHI->addIncoming(Result, LookupBB);
6109 }
6110
6111 if (!ReturnedEarly) {
6112 Builder.CreateBr(CommonDest);
6113 if (DTU)
6114 Updates.push_back({DominatorTree::Insert, LookupBB, CommonDest});
6115 }
6116
6117 // Remove the switch.
6118 SmallPtrSet<BasicBlock *, 8> RemovedSuccessors;
6119 for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
6120 BasicBlock *Succ = SI->getSuccessor(i);
6121
6122 if (Succ == SI->getDefaultDest())
6123 continue;
6124 Succ->removePredecessor(BB);
6125 RemovedSuccessors.insert(Succ);
6126 }
6127 SI->eraseFromParent();
6128
6129 if (DTU) {
6130 for (BasicBlock *RemovedSuccessor : RemovedSuccessors)
6131 Updates.push_back({DominatorTree::Delete, BB, RemovedSuccessor});
6132 DTU->applyUpdates(Updates);
6133 }
6134
6135 ++NumLookupTables;
6136 if (NeedMask)
6137 ++NumLookupTablesHoles;
6138 return true;
6139 }
6140
isSwitchDense(ArrayRef<int64_t> Values)6141 static bool isSwitchDense(ArrayRef<int64_t> Values) {
6142 // See also SelectionDAGBuilder::isDense(), which this function was based on.
6143 uint64_t Diff = (uint64_t)Values.back() - (uint64_t)Values.front();
6144 uint64_t Range = Diff + 1;
6145 uint64_t NumCases = Values.size();
6146 // 40% is the default density for building a jump table in optsize/minsize mode.
6147 uint64_t MinDensity = 40;
6148
6149 return NumCases * 100 >= Range * MinDensity;
6150 }
6151
6152 /// Try to transform a switch that has "holes" in it to a contiguous sequence
6153 /// of cases.
6154 ///
6155 /// A switch such as: switch(i) {case 5: case 9: case 13: case 17:} can be
6156 /// range-reduced to: switch ((i-5) / 4) {case 0: case 1: case 2: case 3:}.
6157 ///
6158 /// This converts a sparse switch into a dense switch which allows better
6159 /// lowering and could also allow transforming into a lookup table.
ReduceSwitchRange(SwitchInst * SI,IRBuilder<> & Builder,const DataLayout & DL,const TargetTransformInfo & TTI)6160 static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
6161 const DataLayout &DL,
6162 const TargetTransformInfo &TTI) {
6163 auto *CondTy = cast<IntegerType>(SI->getCondition()->getType());
6164 if (CondTy->getIntegerBitWidth() > 64 ||
6165 !DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
6166 return false;
6167 // Only bother with this optimization if there are more than 3 switch cases;
6168 // SDAG will only bother creating jump tables for 4 or more cases.
6169 if (SI->getNumCases() < 4)
6170 return false;
6171
6172 // This transform is agnostic to the signedness of the input or case values. We
6173 // can treat the case values as signed or unsigned. We can optimize more common
6174 // cases such as a sequence crossing zero {-4,0,4,8} if we interpret case values
6175 // as signed.
6176 SmallVector<int64_t,4> Values;
6177 for (auto &C : SI->cases())
6178 Values.push_back(C.getCaseValue()->getValue().getSExtValue());
6179 llvm::sort(Values);
6180
6181 // If the switch is already dense, there's nothing useful to do here.
6182 if (isSwitchDense(Values))
6183 return false;
6184
6185 // First, transform the values such that they start at zero and ascend.
6186 int64_t Base = Values[0];
6187 for (auto &V : Values)
6188 V -= (uint64_t)(Base);
6189
6190 // Now we have signed numbers that have been shifted so that, given enough
6191 // precision, there are no negative values. Since the rest of the transform
6192 // is bitwise only, we switch now to an unsigned representation.
6193
6194 // This transform can be done speculatively because it is so cheap - it
6195 // results in a single rotate operation being inserted.
6196 // FIXME: It's possible that optimizing a switch on powers of two might also
6197 // be beneficial - flag values are often powers of two and we could use a CLZ
6198 // as the key function.
6199
6200 // countTrailingZeros(0) returns 64. As Values is guaranteed to have more than
6201 // one element and LLVM disallows duplicate cases, Shift is guaranteed to be
6202 // less than 64.
6203 unsigned Shift = 64;
6204 for (auto &V : Values)
6205 Shift = std::min(Shift, countTrailingZeros((uint64_t)V));
6206 assert(Shift < 64);
6207 if (Shift > 0)
6208 for (auto &V : Values)
6209 V = (int64_t)((uint64_t)V >> Shift);
6210
6211 if (!isSwitchDense(Values))
6212 // Transform didn't create a dense switch.
6213 return false;
6214
6215 // The obvious transform is to shift the switch condition right and emit a
6216 // check that the condition actually cleanly divided by GCD, i.e.
6217 // C & (1 << Shift - 1) == 0
6218 // inserting a new CFG edge to handle the case where it didn't divide cleanly.
6219 //
6220 // A cheaper way of doing this is a simple ROTR(C, Shift). This performs the
6221 // shift and puts the shifted-off bits in the uppermost bits. If any of these
6222 // are nonzero then the switch condition will be very large and will hit the
6223 // default case.
6224
6225 auto *Ty = cast<IntegerType>(SI->getCondition()->getType());
6226 Builder.SetInsertPoint(SI);
6227 auto *ShiftC = ConstantInt::get(Ty, Shift);
6228 auto *Sub = Builder.CreateSub(SI->getCondition(), ConstantInt::get(Ty, Base));
6229 auto *LShr = Builder.CreateLShr(Sub, ShiftC);
6230 auto *Shl = Builder.CreateShl(Sub, Ty->getBitWidth() - Shift);
6231 auto *Rot = Builder.CreateOr(LShr, Shl);
6232 SI->replaceUsesOfWith(SI->getCondition(), Rot);
6233
6234 for (auto Case : SI->cases()) {
6235 auto *Orig = Case.getCaseValue();
6236 auto Sub = Orig->getValue() - APInt(Ty->getBitWidth(), Base);
6237 Case.setValue(
6238 cast<ConstantInt>(ConstantInt::get(Ty, Sub.lshr(ShiftC->getValue()))));
6239 }
6240 return true;
6241 }
6242
simplifySwitch(SwitchInst * SI,IRBuilder<> & Builder)6243 bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
6244 BasicBlock *BB = SI->getParent();
6245
6246 if (isValueEqualityComparison(SI)) {
6247 // If we only have one predecessor, and if it is a branch on this value,
6248 // see if that predecessor totally determines the outcome of this switch.
6249 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
6250 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred, Builder))
6251 return requestResimplify();
6252
6253 Value *Cond = SI->getCondition();
6254 if (SelectInst *Select = dyn_cast<SelectInst>(Cond))
6255 if (SimplifySwitchOnSelect(SI, Select))
6256 return requestResimplify();
6257
6258 // If the block only contains the switch, see if we can fold the block
6259 // away into any preds.
6260 if (SI == &*BB->instructionsWithoutDebug().begin())
6261 if (FoldValueComparisonIntoPredecessors(SI, Builder))
6262 return requestResimplify();
6263 }
6264
6265 // Try to transform the switch into an icmp and a branch.
6266 if (TurnSwitchRangeIntoICmp(SI, Builder))
6267 return requestResimplify();
6268
6269 // Remove unreachable cases.
6270 if (eliminateDeadSwitchCases(SI, DTU, Options.AC, DL))
6271 return requestResimplify();
6272
6273 if (switchToSelect(SI, Builder, DTU, DL, TTI))
6274 return requestResimplify();
6275
6276 if (Options.ForwardSwitchCondToPhi && ForwardSwitchConditionToPHI(SI))
6277 return requestResimplify();
6278
6279 // The conversion from switch to lookup tables results in difficult-to-analyze
6280 // code and makes pruning branches much harder. This is a problem if the
6281 // switch expression itself can still be restricted as a result of inlining or
6282 // CVP. Therefore, only apply this transformation during late stages of the
6283 // optimisation pipeline.
6284 if (Options.ConvertSwitchToLookupTable &&
6285 SwitchToLookupTable(SI, Builder, DTU, DL, TTI))
6286 return requestResimplify();
6287
6288 if (ReduceSwitchRange(SI, Builder, DL, TTI))
6289 return requestResimplify();
6290
6291 return false;
6292 }
6293
simplifyIndirectBr(IndirectBrInst * IBI)6294 bool SimplifyCFGOpt::simplifyIndirectBr(IndirectBrInst *IBI) {
6295 BasicBlock *BB = IBI->getParent();
6296 bool Changed = false;
6297
6298 // Eliminate redundant destinations.
6299 SmallPtrSet<Value *, 8> Succs;
6300 SmallPtrSet<BasicBlock *, 8> RemovedSuccs;
6301 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
6302 BasicBlock *Dest = IBI->getDestination(i);
6303 if (!Dest->hasAddressTaken() || !Succs.insert(Dest).second) {
6304 if (!Dest->hasAddressTaken())
6305 RemovedSuccs.insert(Dest);
6306 Dest->removePredecessor(BB);
6307 IBI->removeDestination(i);
6308 --i;
6309 --e;
6310 Changed = true;
6311 }
6312 }
6313
6314 if (DTU) {
6315 std::vector<DominatorTree::UpdateType> Updates;
6316 Updates.reserve(RemovedSuccs.size());
6317 for (auto *RemovedSucc : RemovedSuccs)
6318 Updates.push_back({DominatorTree::Delete, BB, RemovedSucc});
6319 DTU->applyUpdates(Updates);
6320 }
6321
6322 if (IBI->getNumDestinations() == 0) {
6323 // If the indirectbr has no successors, change it to unreachable.
6324 new UnreachableInst(IBI->getContext(), IBI);
6325 EraseTerminatorAndDCECond(IBI);
6326 return true;
6327 }
6328
6329 if (IBI->getNumDestinations() == 1) {
6330 // If the indirectbr has one successor, change it to a direct branch.
6331 BranchInst::Create(IBI->getDestination(0), IBI);
6332 EraseTerminatorAndDCECond(IBI);
6333 return true;
6334 }
6335
6336 if (SelectInst *SI = dyn_cast<SelectInst>(IBI->getAddress())) {
6337 if (SimplifyIndirectBrOnSelect(IBI, SI))
6338 return requestResimplify();
6339 }
6340 return Changed;
6341 }
6342
6343 /// Given an block with only a single landing pad and a unconditional branch
6344 /// try to find another basic block which this one can be merged with. This
6345 /// handles cases where we have multiple invokes with unique landing pads, but
6346 /// a shared handler.
6347 ///
6348 /// We specifically choose to not worry about merging non-empty blocks
6349 /// here. That is a PRE/scheduling problem and is best solved elsewhere. In
6350 /// practice, the optimizer produces empty landing pad blocks quite frequently
6351 /// when dealing with exception dense code. (see: instcombine, gvn, if-else
6352 /// sinking in this file)
6353 ///
6354 /// This is primarily a code size optimization. We need to avoid performing
6355 /// any transform which might inhibit optimization (such as our ability to
6356 /// specialize a particular handler via tail commoning). We do this by not
6357 /// merging any blocks which require us to introduce a phi. Since the same
6358 /// values are flowing through both blocks, we don't lose any ability to
6359 /// specialize. If anything, we make such specialization more likely.
6360 ///
6361 /// TODO - This transformation could remove entries from a phi in the target
6362 /// block when the inputs in the phi are the same for the two blocks being
6363 /// merged. In some cases, this could result in removal of the PHI entirely.
TryToMergeLandingPad(LandingPadInst * LPad,BranchInst * BI,BasicBlock * BB,DomTreeUpdater * DTU)6364 static bool TryToMergeLandingPad(LandingPadInst *LPad, BranchInst *BI,
6365 BasicBlock *BB, DomTreeUpdater *DTU) {
6366 auto Succ = BB->getUniqueSuccessor();
6367 assert(Succ);
6368 // If there's a phi in the successor block, we'd likely have to introduce
6369 // a phi into the merged landing pad block.
6370 if (isa<PHINode>(*Succ->begin()))
6371 return false;
6372
6373 for (BasicBlock *OtherPred : predecessors(Succ)) {
6374 if (BB == OtherPred)
6375 continue;
6376 BasicBlock::iterator I = OtherPred->begin();
6377 LandingPadInst *LPad2 = dyn_cast<LandingPadInst>(I);
6378 if (!LPad2 || !LPad2->isIdenticalTo(LPad))
6379 continue;
6380 for (++I; isa<DbgInfoIntrinsic>(I); ++I)
6381 ;
6382 BranchInst *BI2 = dyn_cast<BranchInst>(I);
6383 if (!BI2 || !BI2->isIdenticalTo(BI))
6384 continue;
6385
6386 std::vector<DominatorTree::UpdateType> Updates;
6387
6388 // We've found an identical block. Update our predecessors to take that
6389 // path instead and make ourselves dead.
6390 SmallPtrSet<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
6391 for (BasicBlock *Pred : Preds) {
6392 InvokeInst *II = cast<InvokeInst>(Pred->getTerminator());
6393 assert(II->getNormalDest() != BB && II->getUnwindDest() == BB &&
6394 "unexpected successor");
6395 II->setUnwindDest(OtherPred);
6396 if (DTU) {
6397 Updates.push_back({DominatorTree::Insert, Pred, OtherPred});
6398 Updates.push_back({DominatorTree::Delete, Pred, BB});
6399 }
6400 }
6401
6402 // The debug info in OtherPred doesn't cover the merged control flow that
6403 // used to go through BB. We need to delete it or update it.
6404 for (auto I = OtherPred->begin(), E = OtherPred->end(); I != E;) {
6405 Instruction &Inst = *I;
6406 I++;
6407 if (isa<DbgInfoIntrinsic>(Inst))
6408 Inst.eraseFromParent();
6409 }
6410
6411 SmallPtrSet<BasicBlock *, 16> Succs(succ_begin(BB), succ_end(BB));
6412 for (BasicBlock *Succ : Succs) {
6413 Succ->removePredecessor(BB);
6414 if (DTU)
6415 Updates.push_back({DominatorTree::Delete, BB, Succ});
6416 }
6417
6418 IRBuilder<> Builder(BI);
6419 Builder.CreateUnreachable();
6420 BI->eraseFromParent();
6421 if (DTU)
6422 DTU->applyUpdates(Updates);
6423 return true;
6424 }
6425 return false;
6426 }
6427
simplifyBranch(BranchInst * Branch,IRBuilder<> & Builder)6428 bool SimplifyCFGOpt::simplifyBranch(BranchInst *Branch, IRBuilder<> &Builder) {
6429 return Branch->isUnconditional() ? simplifyUncondBranch(Branch, Builder)
6430 : simplifyCondBranch(Branch, Builder);
6431 }
6432
simplifyUncondBranch(BranchInst * BI,IRBuilder<> & Builder)6433 bool SimplifyCFGOpt::simplifyUncondBranch(BranchInst *BI,
6434 IRBuilder<> &Builder) {
6435 BasicBlock *BB = BI->getParent();
6436 BasicBlock *Succ = BI->getSuccessor(0);
6437
6438 // If the Terminator is the only non-phi instruction, simplify the block.
6439 // If LoopHeader is provided, check if the block or its successor is a loop
6440 // header. (This is for early invocations before loop simplify and
6441 // vectorization to keep canonical loop forms for nested loops. These blocks
6442 // can be eliminated when the pass is invoked later in the back-end.)
6443 // Note that if BB has only one predecessor then we do not introduce new
6444 // backedge, so we can eliminate BB.
6445 bool NeedCanonicalLoop =
6446 Options.NeedCanonicalLoop &&
6447 (!LoopHeaders.empty() && BB->hasNPredecessorsOrMore(2) &&
6448 (is_contained(LoopHeaders, BB) || is_contained(LoopHeaders, Succ)));
6449 BasicBlock::iterator I = BB->getFirstNonPHIOrDbg(true)->getIterator();
6450 if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&
6451 !NeedCanonicalLoop && TryToSimplifyUncondBranchFromEmptyBlock(BB, DTU))
6452 return true;
6453
6454 // If the only instruction in the block is a seteq/setne comparison against a
6455 // constant, try to simplify the block.
6456 if (ICmpInst *ICI = dyn_cast<ICmpInst>(I))
6457 if (ICI->isEquality() && isa<ConstantInt>(ICI->getOperand(1))) {
6458 for (++I; isa<DbgInfoIntrinsic>(I); ++I)
6459 ;
6460 if (I->isTerminator() &&
6461 tryToSimplifyUncondBranchWithICmpInIt(ICI, Builder))
6462 return true;
6463 }
6464
6465 // See if we can merge an empty landing pad block with another which is
6466 // equivalent.
6467 if (LandingPadInst *LPad = dyn_cast<LandingPadInst>(I)) {
6468 for (++I; isa<DbgInfoIntrinsic>(I); ++I)
6469 ;
6470 if (I->isTerminator() && TryToMergeLandingPad(LPad, BI, BB, DTU))
6471 return true;
6472 }
6473
6474 // If this basic block is ONLY a compare and a branch, and if a predecessor
6475 // branches to us and our successor, fold the comparison into the
6476 // predecessor and use logical operations to update the incoming value
6477 // for PHI nodes in common successor.
6478 if (FoldBranchToCommonDest(BI, DTU, /*MSSAU=*/nullptr, &TTI,
6479 Options.BonusInstThreshold))
6480 return requestResimplify();
6481 return false;
6482 }
6483
allPredecessorsComeFromSameSource(BasicBlock * BB)6484 static BasicBlock *allPredecessorsComeFromSameSource(BasicBlock *BB) {
6485 BasicBlock *PredPred = nullptr;
6486 for (auto *P : predecessors(BB)) {
6487 BasicBlock *PPred = P->getSinglePredecessor();
6488 if (!PPred || (PredPred && PredPred != PPred))
6489 return nullptr;
6490 PredPred = PPred;
6491 }
6492 return PredPred;
6493 }
6494
simplifyCondBranch(BranchInst * BI,IRBuilder<> & Builder)6495 bool SimplifyCFGOpt::simplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
6496 BasicBlock *BB = BI->getParent();
6497 if (!Options.SimplifyCondBranch)
6498 return false;
6499
6500 // Conditional branch
6501 if (isValueEqualityComparison(BI)) {
6502 // If we only have one predecessor, and if it is a branch on this value,
6503 // see if that predecessor totally determines the outcome of this
6504 // switch.
6505 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
6506 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred, Builder))
6507 return requestResimplify();
6508
6509 // This block must be empty, except for the setcond inst, if it exists.
6510 // Ignore dbg and pseudo intrinsics.
6511 auto I = BB->instructionsWithoutDebug(true).begin();
6512 if (&*I == BI) {
6513 if (FoldValueComparisonIntoPredecessors(BI, Builder))
6514 return requestResimplify();
6515 } else if (&*I == cast<Instruction>(BI->getCondition())) {
6516 ++I;
6517 if (&*I == BI && FoldValueComparisonIntoPredecessors(BI, Builder))
6518 return requestResimplify();
6519 }
6520 }
6521
6522 // Try to turn "br (X == 0 | X == 1), T, F" into a switch instruction.
6523 if (SimplifyBranchOnICmpChain(BI, Builder, DL))
6524 return true;
6525
6526 // If this basic block has dominating predecessor blocks and the dominating
6527 // blocks' conditions imply BI's condition, we know the direction of BI.
6528 Optional<bool> Imp = isImpliedByDomCondition(BI->getCondition(), BI, DL);
6529 if (Imp) {
6530 // Turn this into a branch on constant.
6531 auto *OldCond = BI->getCondition();
6532 ConstantInt *TorF = *Imp ? ConstantInt::getTrue(BB->getContext())
6533 : ConstantInt::getFalse(BB->getContext());
6534 BI->setCondition(TorF);
6535 RecursivelyDeleteTriviallyDeadInstructions(OldCond);
6536 return requestResimplify();
6537 }
6538
6539 // If this basic block is ONLY a compare and a branch, and if a predecessor
6540 // branches to us and one of our successors, fold the comparison into the
6541 // predecessor and use logical operations to pick the right destination.
6542 if (FoldBranchToCommonDest(BI, DTU, /*MSSAU=*/nullptr, &TTI,
6543 Options.BonusInstThreshold))
6544 return requestResimplify();
6545
6546 // We have a conditional branch to two blocks that are only reachable
6547 // from BI. We know that the condbr dominates the two blocks, so see if
6548 // there is any identical code in the "then" and "else" blocks. If so, we
6549 // can hoist it up to the branching block.
6550 if (BI->getSuccessor(0)->getSinglePredecessor()) {
6551 if (BI->getSuccessor(1)->getSinglePredecessor()) {
6552 if (HoistCommon &&
6553 HoistThenElseCodeToIf(BI, TTI, !Options.HoistCommonInsts))
6554 return requestResimplify();
6555 } else {
6556 // If Successor #1 has multiple preds, we may be able to conditionally
6557 // execute Successor #0 if it branches to Successor #1.
6558 Instruction *Succ0TI = BI->getSuccessor(0)->getTerminator();
6559 if (Succ0TI->getNumSuccessors() == 1 &&
6560 Succ0TI->getSuccessor(0) == BI->getSuccessor(1))
6561 if (SpeculativelyExecuteBB(BI, BI->getSuccessor(0), TTI))
6562 return requestResimplify();
6563 }
6564 } else if (BI->getSuccessor(1)->getSinglePredecessor()) {
6565 // If Successor #0 has multiple preds, we may be able to conditionally
6566 // execute Successor #1 if it branches to Successor #0.
6567 Instruction *Succ1TI = BI->getSuccessor(1)->getTerminator();
6568 if (Succ1TI->getNumSuccessors() == 1 &&
6569 Succ1TI->getSuccessor(0) == BI->getSuccessor(0))
6570 if (SpeculativelyExecuteBB(BI, BI->getSuccessor(1), TTI))
6571 return requestResimplify();
6572 }
6573
6574 // If this is a branch on a phi node in the current block, thread control
6575 // through this block if any PHI node entries are constants.
6576 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
6577 if (PN->getParent() == BI->getParent())
6578 if (FoldCondBranchOnPHI(BI, DTU, DL, Options.AC))
6579 return requestResimplify();
6580
6581 // Scan predecessor blocks for conditional branches.
6582 for (BasicBlock *Pred : predecessors(BB))
6583 if (BranchInst *PBI = dyn_cast<BranchInst>(Pred->getTerminator()))
6584 if (PBI != BI && PBI->isConditional())
6585 if (SimplifyCondBranchToCondBranch(PBI, BI, DTU, DL, TTI))
6586 return requestResimplify();
6587
6588 // Look for diamond patterns.
6589 if (MergeCondStores)
6590 if (BasicBlock *PrevBB = allPredecessorsComeFromSameSource(BB))
6591 if (BranchInst *PBI = dyn_cast<BranchInst>(PrevBB->getTerminator()))
6592 if (PBI != BI && PBI->isConditional())
6593 if (mergeConditionalStores(PBI, BI, DTU, DL, TTI))
6594 return requestResimplify();
6595
6596 return false;
6597 }
6598
6599 /// Check if passing a value to an instruction will cause undefined behavior.
passingValueIsAlwaysUndefined(Value * V,Instruction * I,bool PtrValueMayBeModified)6600 static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValueMayBeModified) {
6601 Constant *C = dyn_cast<Constant>(V);
6602 if (!C)
6603 return false;
6604
6605 if (I->use_empty())
6606 return false;
6607
6608 if (C->isNullValue() || isa<UndefValue>(C)) {
6609 // Only look at the first use, avoid hurting compile time with long uselists
6610 User *Use = *I->user_begin();
6611
6612 // Now make sure that there are no instructions in between that can alter
6613 // control flow (eg. calls)
6614 for (BasicBlock::iterator
6615 i = ++BasicBlock::iterator(I),
6616 UI = BasicBlock::iterator(dyn_cast<Instruction>(Use));
6617 i != UI; ++i) {
6618 if (i == I->getParent()->end())
6619 return false;
6620 if (!isGuaranteedToTransferExecutionToSuccessor(&*i))
6621 return false;
6622 }
6623
6624 // Look through GEPs. A load from a GEP derived from NULL is still undefined
6625 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))
6626 if (GEP->getPointerOperand() == I) {
6627 if (!GEP->isInBounds() || !GEP->hasAllZeroIndices())
6628 PtrValueMayBeModified = true;
6629 return passingValueIsAlwaysUndefined(V, GEP, PtrValueMayBeModified);
6630 }
6631
6632 // Look through bitcasts.
6633 if (BitCastInst *BC = dyn_cast<BitCastInst>(Use))
6634 return passingValueIsAlwaysUndefined(V, BC, PtrValueMayBeModified);
6635
6636 // Load from null is undefined.
6637 if (LoadInst *LI = dyn_cast<LoadInst>(Use))
6638 if (!LI->isVolatile())
6639 return !NullPointerIsDefined(LI->getFunction(),
6640 LI->getPointerAddressSpace());
6641
6642 // Store to null is undefined.
6643 if (StoreInst *SI = dyn_cast<StoreInst>(Use))
6644 if (!SI->isVolatile())
6645 return (!NullPointerIsDefined(SI->getFunction(),
6646 SI->getPointerAddressSpace())) &&
6647 SI->getPointerOperand() == I;
6648
6649 if (auto *CB = dyn_cast<CallBase>(Use)) {
6650 if (C->isNullValue() && NullPointerIsDefined(CB->getFunction()))
6651 return false;
6652 // A call to null is undefined.
6653 if (CB->getCalledOperand() == I)
6654 return true;
6655
6656 if (C->isNullValue()) {
6657 for (const llvm::Use &Arg : CB->args())
6658 if (Arg == I) {
6659 unsigned ArgIdx = CB->getArgOperandNo(&Arg);
6660 if (CB->isPassingUndefUB(ArgIdx) &&
6661 CB->paramHasAttr(ArgIdx, Attribute::NonNull)) {
6662 // Passing null to a nonnnull+noundef argument is undefined.
6663 return !PtrValueMayBeModified;
6664 }
6665 }
6666 } else if (isa<UndefValue>(C)) {
6667 // Passing undef to a noundef argument is undefined.
6668 for (const llvm::Use &Arg : CB->args())
6669 if (Arg == I) {
6670 unsigned ArgIdx = CB->getArgOperandNo(&Arg);
6671 if (CB->isPassingUndefUB(ArgIdx)) {
6672 // Passing undef to a noundef argument is undefined.
6673 return true;
6674 }
6675 }
6676 }
6677 }
6678 }
6679 return false;
6680 }
6681
6682 /// If BB has an incoming value that will always trigger undefined behavior
6683 /// (eg. null pointer dereference), remove the branch leading here.
removeUndefIntroducingPredecessor(BasicBlock * BB,DomTreeUpdater * DTU)6684 static bool removeUndefIntroducingPredecessor(BasicBlock *BB,
6685 DomTreeUpdater *DTU) {
6686 for (PHINode &PHI : BB->phis())
6687 for (unsigned i = 0, e = PHI.getNumIncomingValues(); i != e; ++i)
6688 if (passingValueIsAlwaysUndefined(PHI.getIncomingValue(i), &PHI)) {
6689 BasicBlock *Predecessor = PHI.getIncomingBlock(i);
6690 Instruction *T = Predecessor->getTerminator();
6691 IRBuilder<> Builder(T);
6692 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
6693 BB->removePredecessor(Predecessor);
6694 // Turn uncoditional branches into unreachables and remove the dead
6695 // destination from conditional branches.
6696 if (BI->isUnconditional())
6697 Builder.CreateUnreachable();
6698 else
6699 Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1)
6700 : BI->getSuccessor(0));
6701 BI->eraseFromParent();
6702 if (DTU)
6703 DTU->applyUpdates({{DominatorTree::Delete, Predecessor, BB}});
6704 return true;
6705 }
6706 // TODO: SwitchInst.
6707 }
6708
6709 return false;
6710 }
6711
simplifyOnceImpl(BasicBlock * BB)6712 bool SimplifyCFGOpt::simplifyOnceImpl(BasicBlock *BB) {
6713 bool Changed = false;
6714
6715 assert(BB && BB->getParent() && "Block not embedded in function!");
6716 assert(BB->getTerminator() && "Degenerate basic block encountered!");
6717
6718 // Remove basic blocks that have no predecessors (except the entry block)...
6719 // or that just have themself as a predecessor. These are unreachable.
6720 if ((pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()) ||
6721 BB->getSinglePredecessor() == BB) {
6722 LLVM_DEBUG(dbgs() << "Removing BB: \n" << *BB);
6723 DeleteDeadBlock(BB, DTU);
6724 return true;
6725 }
6726
6727 // Check to see if we can constant propagate this terminator instruction
6728 // away...
6729 Changed |= ConstantFoldTerminator(BB, /*DeleteDeadConditions=*/true,
6730 /*TLI=*/nullptr, DTU);
6731
6732 // Check for and eliminate duplicate PHI nodes in this block.
6733 Changed |= EliminateDuplicatePHINodes(BB);
6734
6735 // Check for and remove branches that will always cause undefined behavior.
6736 Changed |= removeUndefIntroducingPredecessor(BB, DTU);
6737
6738 // Merge basic blocks into their predecessor if there is only one distinct
6739 // pred, and if there is only one distinct successor of the predecessor, and
6740 // if there are no PHI nodes.
6741 if (MergeBlockIntoPredecessor(BB, DTU))
6742 return true;
6743
6744 if (SinkCommon && Options.SinkCommonInsts)
6745 Changed |= SinkCommonCodeFromPredecessors(BB, DTU);
6746
6747 IRBuilder<> Builder(BB);
6748
6749 if (Options.FoldTwoEntryPHINode) {
6750 // If there is a trivial two-entry PHI node in this basic block, and we can
6751 // eliminate it, do so now.
6752 if (auto *PN = dyn_cast<PHINode>(BB->begin()))
6753 if (PN->getNumIncomingValues() == 2)
6754 Changed |= FoldTwoEntryPHINode(PN, TTI, DTU, DL);
6755 }
6756
6757 Instruction *Terminator = BB->getTerminator();
6758 Builder.SetInsertPoint(Terminator);
6759 switch (Terminator->getOpcode()) {
6760 case Instruction::Br:
6761 Changed |= simplifyBranch(cast<BranchInst>(Terminator), Builder);
6762 break;
6763 case Instruction::Ret:
6764 Changed |= simplifyReturn(cast<ReturnInst>(Terminator), Builder);
6765 break;
6766 case Instruction::Resume:
6767 Changed |= simplifyResume(cast<ResumeInst>(Terminator), Builder);
6768 break;
6769 case Instruction::CleanupRet:
6770 Changed |= simplifyCleanupReturn(cast<CleanupReturnInst>(Terminator));
6771 break;
6772 case Instruction::Switch:
6773 Changed |= simplifySwitch(cast<SwitchInst>(Terminator), Builder);
6774 break;
6775 case Instruction::Unreachable:
6776 Changed |= simplifyUnreachable(cast<UnreachableInst>(Terminator));
6777 break;
6778 case Instruction::IndirectBr:
6779 Changed |= simplifyIndirectBr(cast<IndirectBrInst>(Terminator));
6780 break;
6781 }
6782
6783 return Changed;
6784 }
6785
simplifyOnce(BasicBlock * BB)6786 bool SimplifyCFGOpt::simplifyOnce(BasicBlock *BB) {
6787 bool Changed = simplifyOnceImpl(BB);
6788
6789 return Changed;
6790 }
6791
run(BasicBlock * BB)6792 bool SimplifyCFGOpt::run(BasicBlock *BB) {
6793 bool Changed = false;
6794
6795 // Repeated simplify BB as long as resimplification is requested.
6796 do {
6797 Resimplify = false;
6798
6799 // Perform one round of simplifcation. Resimplify flag will be set if
6800 // another iteration is requested.
6801 Changed |= simplifyOnce(BB);
6802 } while (Resimplify);
6803
6804 return Changed;
6805 }
6806
simplifyCFG(BasicBlock * BB,const TargetTransformInfo & TTI,DomTreeUpdater * DTU,const SimplifyCFGOptions & Options,ArrayRef<WeakVH> LoopHeaders)6807 bool llvm::simplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI,
6808 DomTreeUpdater *DTU, const SimplifyCFGOptions &Options,
6809 ArrayRef<WeakVH> LoopHeaders) {
6810 return SimplifyCFGOpt(TTI, DTU, BB->getModule()->getDataLayout(), LoopHeaders,
6811 Options)
6812 .run(BB);
6813 }
6814