xref: /llvm-project/llvm/lib/Transforms/Scalar/JumpThreading.cpp (revision 6a19ed0b8657de1c1d7d7df32c6466bc9d09f950)
1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the Jump Threading pass.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #define DEBUG_TYPE "jump-threading"
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/IntrinsicInst.h"
17 #include "llvm/LLVMContext.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Analysis/InstructionSimplify.h"
20 #include "llvm/Analysis/LazyValueInfo.h"
21 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
22 #include "llvm/Transforms/Utils/Local.h"
23 #include "llvm/Transforms/Utils/SSAUpdater.h"
24 #include "llvm/Target/TargetData.h"
25 #include "llvm/ADT/DenseMap.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/ADT/STLExtras.h"
28 #include "llvm/ADT/SmallPtrSet.h"
29 #include "llvm/ADT/SmallSet.h"
30 #include "llvm/Support/CommandLine.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/ValueHandle.h"
33 #include "llvm/Support/raw_ostream.h"
34 using namespace llvm;
35 
36 STATISTIC(NumThreads, "Number of jumps threaded");
37 STATISTIC(NumFolds,   "Number of terminators folded");
38 STATISTIC(NumDupes,   "Number of branch blocks duplicated to eliminate phi");
39 
40 static cl::opt<unsigned>
41 Threshold("jump-threading-threshold",
42           cl::desc("Max block size to duplicate for jump threading"),
43           cl::init(6), cl::Hidden);
44 
45 // Turn on use of LazyValueInfo.
46 static cl::opt<bool>
47 EnableLVI("enable-jump-threading-lvi", cl::ReallyHidden);
48 
49 
50 
51 namespace {
52   /// This pass performs 'jump threading', which looks at blocks that have
53   /// multiple predecessors and multiple successors.  If one or more of the
54   /// predecessors of the block can be proven to always jump to one of the
55   /// successors, we forward the edge from the predecessor to the successor by
56   /// duplicating the contents of this block.
57   ///
58   /// An example of when this can occur is code like this:
59   ///
60   ///   if () { ...
61   ///     X = 4;
62   ///   }
63   ///   if (X < 3) {
64   ///
65   /// In this case, the unconditional branch at the end of the first if can be
66   /// revectored to the false side of the second if.
67   ///
68   class JumpThreading : public FunctionPass {
69     TargetData *TD;
70     LazyValueInfo *LVI;
71 #ifdef NDEBUG
72     SmallPtrSet<BasicBlock*, 16> LoopHeaders;
73 #else
74     SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
75 #endif
76   public:
77     static char ID; // Pass identification
78     JumpThreading() : FunctionPass(&ID) {}
79 
80     bool runOnFunction(Function &F);
81 
82     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
83       if (EnableLVI)
84         AU.addRequired<LazyValueInfo>();
85     }
86 
87     void FindLoopHeaders(Function &F);
88     bool ProcessBlock(BasicBlock *BB);
89     bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
90                     BasicBlock *SuccBB);
91     bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
92                                           BasicBlock *PredBB);
93 
94     typedef SmallVectorImpl<std::pair<ConstantInt*,
95                                       BasicBlock*> > PredValueInfo;
96 
97     bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
98                                          PredValueInfo &Result);
99     bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB);
100 
101 
102     bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
103     bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
104 
105     bool ProcessBranchOnPHI(PHINode *PN);
106 
107     bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
108   };
109 }
110 
111 char JumpThreading::ID = 0;
112 static RegisterPass<JumpThreading>
113 X("jump-threading", "Jump Threading");
114 
115 // Public interface to the Jump Threading pass
116 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
117 
118 /// runOnFunction - Top level algorithm.
119 ///
120 bool JumpThreading::runOnFunction(Function &F) {
121   DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
122   TD = getAnalysisIfAvailable<TargetData>();
123   LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0;
124 
125   FindLoopHeaders(F);
126 
127   bool Changed, EverChanged = false;
128   do {
129     Changed = false;
130     for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
131       BasicBlock *BB = I;
132       // Thread all of the branches we can over this block.
133       while (ProcessBlock(BB))
134         Changed = true;
135 
136       ++I;
137 
138       // If the block is trivially dead, zap it.  This eliminates the successor
139       // edges which simplifies the CFG.
140       if (pred_begin(BB) == pred_end(BB) &&
141           BB != &BB->getParent()->getEntryBlock()) {
142         DEBUG(dbgs() << "  JT: Deleting dead block '" << BB->getName()
143               << "' with terminator: " << *BB->getTerminator() << '\n');
144         LoopHeaders.erase(BB);
145         DeleteDeadBlock(BB);
146         Changed = true;
147       } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
148         // Can't thread an unconditional jump, but if the block is "almost
149         // empty", we can replace uses of it with uses of the successor and make
150         // this dead.
151         if (BI->isUnconditional() &&
152             BB != &BB->getParent()->getEntryBlock()) {
153           BasicBlock::iterator BBI = BB->getFirstNonPHI();
154           // Ignore dbg intrinsics.
155           while (isa<DbgInfoIntrinsic>(BBI))
156             ++BBI;
157           // If the terminator is the only non-phi instruction, try to nuke it.
158           if (BBI->isTerminator()) {
159             // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
160             // block, we have to make sure it isn't in the LoopHeaders set.  We
161             // reinsert afterward if needed.
162             bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
163             BasicBlock *Succ = BI->getSuccessor(0);
164 
165             if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
166               Changed = true;
167               // If we deleted BB and BB was the header of a loop, then the
168               // successor is now the header of the loop.
169               BB = Succ;
170             }
171 
172             if (ErasedFromLoopHeaders)
173               LoopHeaders.insert(BB);
174           }
175         }
176       }
177     }
178     EverChanged |= Changed;
179   } while (Changed);
180 
181   LoopHeaders.clear();
182   return EverChanged;
183 }
184 
185 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
186 /// thread across it.
187 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
188   /// Ignore PHI nodes, these will be flattened when duplication happens.
189   BasicBlock::const_iterator I = BB->getFirstNonPHI();
190 
191   // FIXME: THREADING will delete values that are just used to compute the
192   // branch, so they shouldn't count against the duplication cost.
193 
194 
195   // Sum up the cost of each instruction until we get to the terminator.  Don't
196   // include the terminator because the copy won't include it.
197   unsigned Size = 0;
198   for (; !isa<TerminatorInst>(I); ++I) {
199     // Debugger intrinsics don't incur code size.
200     if (isa<DbgInfoIntrinsic>(I)) continue;
201 
202     // If this is a pointer->pointer bitcast, it is free.
203     if (isa<BitCastInst>(I) && isa<PointerType>(I->getType()))
204       continue;
205 
206     // All other instructions count for at least one unit.
207     ++Size;
208 
209     // Calls are more expensive.  If they are non-intrinsic calls, we model them
210     // as having cost of 4.  If they are a non-vector intrinsic, we model them
211     // as having cost of 2 total, and if they are a vector intrinsic, we model
212     // them as having cost 1.
213     if (const CallInst *CI = dyn_cast<CallInst>(I)) {
214       if (!isa<IntrinsicInst>(CI))
215         Size += 3;
216       else if (!isa<VectorType>(CI->getType()))
217         Size += 1;
218     }
219   }
220 
221   // Threading through a switch statement is particularly profitable.  If this
222   // block ends in a switch, decrease its cost to make it more likely to happen.
223   if (isa<SwitchInst>(I))
224     Size = Size > 6 ? Size-6 : 0;
225 
226   return Size;
227 }
228 
229 /// FindLoopHeaders - We do not want jump threading to turn proper loop
230 /// structures into irreducible loops.  Doing this breaks up the loop nesting
231 /// hierarchy and pessimizes later transformations.  To prevent this from
232 /// happening, we first have to find the loop headers.  Here we approximate this
233 /// by finding targets of backedges in the CFG.
234 ///
235 /// Note that there definitely are cases when we want to allow threading of
236 /// edges across a loop header.  For example, threading a jump from outside the
237 /// loop (the preheader) to an exit block of the loop is definitely profitable.
238 /// It is also almost always profitable to thread backedges from within the loop
239 /// to exit blocks, and is often profitable to thread backedges to other blocks
240 /// within the loop (forming a nested loop).  This simple analysis is not rich
241 /// enough to track all of these properties and keep it up-to-date as the CFG
242 /// mutates, so we don't allow any of these transformations.
243 ///
244 void JumpThreading::FindLoopHeaders(Function &F) {
245   SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
246   FindFunctionBackedges(F, Edges);
247 
248   for (unsigned i = 0, e = Edges.size(); i != e; ++i)
249     LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
250 }
251 
252 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
253 /// if we can infer that the value is a known ConstantInt in any of our
254 /// predecessors.  If so, return the known list of value and pred BB in the
255 /// result vector.  If a value is known to be undef, it is returned as null.
256 ///
257 /// This returns true if there were any known values.
258 ///
259 bool JumpThreading::
260 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
261   // If V is a constantint, then it is known in all predecessors.
262   if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
263     ConstantInt *CI = dyn_cast<ConstantInt>(V);
264 
265     for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
266       Result.push_back(std::make_pair(CI, *PI));
267     return true;
268   }
269 
270   // If V is a non-instruction value, or an instruction in a different block,
271   // then it can't be derived from a PHI.
272   Instruction *I = dyn_cast<Instruction>(V);
273   if (I == 0 || I->getParent() != BB) {
274 
275     // Okay, if this is a live-in value, see if it has a known value at the end
276     // of any of our predecessors.
277     //
278     // FIXME: This should be an edge property, not a block end property.
279     /// TODO: Per PR2563, we could infer value range information about a
280     /// predecessor based on its terminator.
281     //
282     if (LVI) {
283       // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
284       // "I" is a non-local compare-with-a-constant instruction.  This would be
285       // able to handle value inequalities better, for example if the compare is
286       // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
287       // Perhaps getConstantOnEdge should be smart enough to do this?
288 
289       for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
290         // If the value is known by LazyValueInfo to be a constant in a
291         // predecessor, use that information to try to thread this block.
292         Constant *PredCst = LVI->getConstantOnEdge(V, *PI, BB);
293         if (PredCst == 0 ||
294             (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst)))
295           continue;
296 
297         Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), *PI));
298       }
299 
300       return !Result.empty();
301     }
302 
303     return false;
304   }
305 
306   /// If I is a PHI node, then we know the incoming values for any constants.
307   if (PHINode *PN = dyn_cast<PHINode>(I)) {
308     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
309       Value *InVal = PN->getIncomingValue(i);
310       if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
311         ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
312         Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
313       }
314     }
315     return !Result.empty();
316   }
317 
318   SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
319 
320   // Handle some boolean conditions.
321   if (I->getType()->getPrimitiveSizeInBits() == 1) {
322     // X | true -> true
323     // X & false -> false
324     if (I->getOpcode() == Instruction::Or ||
325         I->getOpcode() == Instruction::And) {
326       ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
327       ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
328 
329       if (LHSVals.empty() && RHSVals.empty())
330         return false;
331 
332       ConstantInt *InterestingVal;
333       if (I->getOpcode() == Instruction::Or)
334         InterestingVal = ConstantInt::getTrue(I->getContext());
335       else
336         InterestingVal = ConstantInt::getFalse(I->getContext());
337 
338       // Scan for the sentinel.
339       for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
340         if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0)
341           Result.push_back(LHSVals[i]);
342       for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
343         if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0)
344           Result.push_back(RHSVals[i]);
345       return !Result.empty();
346     }
347 
348     // Handle the NOT form of XOR.
349     if (I->getOpcode() == Instruction::Xor &&
350         isa<ConstantInt>(I->getOperand(1)) &&
351         cast<ConstantInt>(I->getOperand(1))->isOne()) {
352       ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result);
353       if (Result.empty())
354         return false;
355 
356       // Invert the known values.
357       for (unsigned i = 0, e = Result.size(); i != e; ++i)
358         if (Result[i].first)
359           Result[i].first =
360             cast<ConstantInt>(ConstantExpr::getNot(Result[i].first));
361       return true;
362     }
363   }
364 
365   // Handle compare with phi operand, where the PHI is defined in this block.
366   if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
367     PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
368     if (PN && PN->getParent() == BB) {
369       // We can do this simplification if any comparisons fold to true or false.
370       // See if any do.
371       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
372         BasicBlock *PredBB = PN->getIncomingBlock(i);
373         Value *LHS = PN->getIncomingValue(i);
374         Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
375 
376         Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
377         if (Res == 0) {
378           if (!LVI || !isa<Constant>(RHS))
379             continue;
380 
381           LazyValueInfo::Tristate
382             ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
383                                            cast<Constant>(RHS), PredBB, BB);
384           if (ResT == LazyValueInfo::Unknown)
385             continue;
386           Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
387         }
388 
389         if (isa<UndefValue>(Res))
390           Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
391         else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
392           Result.push_back(std::make_pair(CI, PredBB));
393       }
394 
395       return !Result.empty();
396     }
397 
398 
399     // If comparing a live-in value against a constant, see if we know the
400     // live-in value on any predecessors.
401     if (LVI && isa<Constant>(Cmp->getOperand(1)) &&
402         Cmp->getType()->isInteger() && // Not vector compare.
403         (!isa<Instruction>(Cmp->getOperand(0)) ||
404          cast<Instruction>(Cmp->getOperand(0))->getParent() != BB)) {
405       Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
406 
407       for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
408         // If the value is known by LazyValueInfo to be a constant in a
409         // predecessor, use that information to try to thread this block.
410         LazyValueInfo::Tristate
411           Res = LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
412                                         RHSCst, *PI, BB);
413         if (Res == LazyValueInfo::Unknown)
414           continue;
415 
416         Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
417         Result.push_back(std::make_pair(cast<ConstantInt>(ResC), *PI));
418       }
419 
420       return !Result.empty();
421     }
422   }
423   return false;
424 }
425 
426 
427 
428 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
429 /// in an undefined jump, decide which block is best to revector to.
430 ///
431 /// Since we can pick an arbitrary destination, we pick the successor with the
432 /// fewest predecessors.  This should reduce the in-degree of the others.
433 ///
434 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
435   TerminatorInst *BBTerm = BB->getTerminator();
436   unsigned MinSucc = 0;
437   BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
438   // Compute the successor with the minimum number of predecessors.
439   unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
440   for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
441     TestBB = BBTerm->getSuccessor(i);
442     unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
443     if (NumPreds < MinNumPreds)
444       MinSucc = i;
445   }
446 
447   return MinSucc;
448 }
449 
450 /// ProcessBlock - If there are any predecessors whose control can be threaded
451 /// through to a successor, transform them now.
452 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
453   // If this block has a single predecessor, and if that pred has a single
454   // successor, merge the blocks.  This encourages recursive jump threading
455   // because now the condition in this block can be threaded through
456   // predecessors of our predecessor block.
457   if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
458     if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
459         SinglePred != BB) {
460       // If SinglePred was a loop header, BB becomes one.
461       if (LoopHeaders.erase(SinglePred))
462         LoopHeaders.insert(BB);
463 
464       // Remember if SinglePred was the entry block of the function.  If so, we
465       // will need to move BB back to the entry position.
466       bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
467       MergeBasicBlockIntoOnlyPred(BB);
468 
469       if (isEntry && BB != &BB->getParent()->getEntryBlock())
470         BB->moveBefore(&BB->getParent()->getEntryBlock());
471       return true;
472     }
473   }
474 
475   // Look to see if the terminator is a branch of switch, if not we can't thread
476   // it.
477   Value *Condition;
478   if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
479     // Can't thread an unconditional jump.
480     if (BI->isUnconditional()) return false;
481     Condition = BI->getCondition();
482   } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
483     Condition = SI->getCondition();
484   else
485     return false; // Must be an invoke.
486 
487   // If the terminator of this block is branching on a constant, simplify the
488   // terminator to an unconditional branch.  This can occur due to threading in
489   // other blocks.
490   if (isa<ConstantInt>(Condition)) {
491     DEBUG(dbgs() << "  In block '" << BB->getName()
492           << "' folding terminator: " << *BB->getTerminator() << '\n');
493     ++NumFolds;
494     ConstantFoldTerminator(BB);
495     return true;
496   }
497 
498   // If the terminator is branching on an undef, we can pick any of the
499   // successors to branch to.  Let GetBestDestForJumpOnUndef decide.
500   if (isa<UndefValue>(Condition)) {
501     unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
502 
503     // Fold the branch/switch.
504     TerminatorInst *BBTerm = BB->getTerminator();
505     for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
506       if (i == BestSucc) continue;
507       RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
508     }
509 
510     DEBUG(dbgs() << "  In block '" << BB->getName()
511           << "' folding undef terminator: " << *BBTerm << '\n');
512     BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
513     BBTerm->eraseFromParent();
514     return true;
515   }
516 
517   Instruction *CondInst = dyn_cast<Instruction>(Condition);
518 
519   // If the condition is an instruction defined in another block, see if a
520   // predecessor has the same condition:
521   //     br COND, BBX, BBY
522   //  BBX:
523   //     br COND, BBZ, BBW
524   if (!LVI &&
525       !Condition->hasOneUse() && // Multiple uses.
526       (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
527     pred_iterator PI = pred_begin(BB), E = pred_end(BB);
528     if (isa<BranchInst>(BB->getTerminator())) {
529       for (; PI != E; ++PI)
530         if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
531           if (PBI->isConditional() && PBI->getCondition() == Condition &&
532               ProcessBranchOnDuplicateCond(*PI, BB))
533             return true;
534     } else {
535       assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
536       for (; PI != E; ++PI)
537         if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator()))
538           if (PSI->getCondition() == Condition &&
539               ProcessSwitchOnDuplicateCond(*PI, BB))
540             return true;
541     }
542   }
543 
544   // All the rest of our checks depend on the condition being an instruction.
545   if (CondInst == 0) {
546     // FIXME: Unify this with code below.
547     if (LVI && ProcessThreadableEdges(Condition, BB))
548       return true;
549     return false;
550   }
551 
552 
553   if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
554     if (!LVI &&
555         (!isa<PHINode>(CondCmp->getOperand(0)) ||
556          cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) {
557       // If we have a comparison, loop over the predecessors to see if there is
558       // a condition with a lexically identical value.
559       pred_iterator PI = pred_begin(BB), E = pred_end(BB);
560       for (; PI != E; ++PI)
561         if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
562           if (PBI->isConditional() && *PI != BB) {
563             if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
564               if (CI->getOperand(0) == CondCmp->getOperand(0) &&
565                   CI->getOperand(1) == CondCmp->getOperand(1) &&
566                   CI->getPredicate() == CondCmp->getPredicate()) {
567                 // TODO: Could handle things like (x != 4) --> (x == 17)
568                 if (ProcessBranchOnDuplicateCond(*PI, BB))
569                   return true;
570               }
571             }
572           }
573     }
574   }
575 
576   // Check for some cases that are worth simplifying.  Right now we want to look
577   // for loads that are used by a switch or by the condition for the branch.  If
578   // we see one, check to see if it's partially redundant.  If so, insert a PHI
579   // which can then be used to thread the values.
580   //
581   Value *SimplifyValue = CondInst;
582   if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
583     if (isa<Constant>(CondCmp->getOperand(1)))
584       SimplifyValue = CondCmp->getOperand(0);
585 
586   // TODO: There are other places where load PRE would be profitable, such as
587   // more complex comparisons.
588   if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
589     if (SimplifyPartiallyRedundantLoad(LI))
590       return true;
591 
592 
593   // Handle a variety of cases where we are branching on something derived from
594   // a PHI node in the current block.  If we can prove that any predecessors
595   // compute a predictable value based on a PHI node, thread those predecessors.
596   //
597   if (ProcessThreadableEdges(CondInst, BB))
598     return true;
599 
600   // If this is an otherwise-unfoldable branch on a phi node in the current
601   // block, see if we can simplify.
602   if (PHINode *PN = dyn_cast<PHINode>(CondInst))
603     if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
604       return ProcessBranchOnPHI(PN);
605 
606   // TODO: If we have: "br (X > 0)"  and we have a predecessor where we know
607   // "(X == 4)", thread through this block.
608 
609   return false;
610 }
611 
612 /// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
613 /// block that jump on exactly the same condition.  This means that we almost
614 /// always know the direction of the edge in the DESTBB:
615 ///  PREDBB:
616 ///     br COND, DESTBB, BBY
617 ///  DESTBB:
618 ///     br COND, BBZ, BBW
619 ///
620 /// If DESTBB has multiple predecessors, we can't just constant fold the branch
621 /// in DESTBB, we have to thread over it.
622 bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
623                                                  BasicBlock *BB) {
624   BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
625 
626   // If both successors of PredBB go to DESTBB, we don't know anything.  We can
627   // fold the branch to an unconditional one, which allows other recursive
628   // simplifications.
629   bool BranchDir;
630   if (PredBI->getSuccessor(1) != BB)
631     BranchDir = true;
632   else if (PredBI->getSuccessor(0) != BB)
633     BranchDir = false;
634   else {
635     DEBUG(dbgs() << "  In block '" << PredBB->getName()
636           << "' folding terminator: " << *PredBB->getTerminator() << '\n');
637     ++NumFolds;
638     ConstantFoldTerminator(PredBB);
639     return true;
640   }
641 
642   BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
643 
644   // If the dest block has one predecessor, just fix the branch condition to a
645   // constant and fold it.
646   if (BB->getSinglePredecessor()) {
647     DEBUG(dbgs() << "  In block '" << BB->getName()
648           << "' folding condition to '" << BranchDir << "': "
649           << *BB->getTerminator() << '\n');
650     ++NumFolds;
651     Value *OldCond = DestBI->getCondition();
652     DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
653                                           BranchDir));
654     ConstantFoldTerminator(BB);
655     RecursivelyDeleteTriviallyDeadInstructions(OldCond);
656     return true;
657   }
658 
659 
660   // Next, figure out which successor we are threading to.
661   BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
662 
663   SmallVector<BasicBlock*, 2> Preds;
664   Preds.push_back(PredBB);
665 
666   // Ok, try to thread it!
667   return ThreadEdge(BB, Preds, SuccBB);
668 }
669 
670 /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
671 /// block that switch on exactly the same condition.  This means that we almost
672 /// always know the direction of the edge in the DESTBB:
673 ///  PREDBB:
674 ///     switch COND [... DESTBB, BBY ... ]
675 ///  DESTBB:
676 ///     switch COND [... BBZ, BBW ]
677 ///
678 /// Optimizing switches like this is very important, because simplifycfg builds
679 /// switches out of repeated 'if' conditions.
680 bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
681                                                  BasicBlock *DestBB) {
682   // Can't thread edge to self.
683   if (PredBB == DestBB)
684     return false;
685 
686   SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
687   SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
688 
689   // There are a variety of optimizations that we can potentially do on these
690   // blocks: we order them from most to least preferable.
691 
692   // If DESTBB *just* contains the switch, then we can forward edges from PREDBB
693   // directly to their destination.  This does not introduce *any* code size
694   // growth.  Skip debug info first.
695   BasicBlock::iterator BBI = DestBB->begin();
696   while (isa<DbgInfoIntrinsic>(BBI))
697     BBI++;
698 
699   // FIXME: Thread if it just contains a PHI.
700   if (isa<SwitchInst>(BBI)) {
701     bool MadeChange = false;
702     // Ignore the default edge for now.
703     for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
704       ConstantInt *DestVal = DestSI->getCaseValue(i);
705       BasicBlock *DestSucc = DestSI->getSuccessor(i);
706 
707       // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'.  See if
708       // PredSI has an explicit case for it.  If so, forward.  If it is covered
709       // by the default case, we can't update PredSI.
710       unsigned PredCase = PredSI->findCaseValue(DestVal);
711       if (PredCase == 0) continue;
712 
713       // If PredSI doesn't go to DestBB on this value, then it won't reach the
714       // case on this condition.
715       if (PredSI->getSuccessor(PredCase) != DestBB &&
716           DestSI->getSuccessor(i) != DestBB)
717         continue;
718 
719       // Do not forward this if it already goes to this destination, this would
720       // be an infinite loop.
721       if (PredSI->getSuccessor(PredCase) == DestSucc)
722         continue;
723 
724       // Otherwise, we're safe to make the change.  Make sure that the edge from
725       // DestSI to DestSucc is not critical and has no PHI nodes.
726       DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << "   FROM: " << *PredSI);
727       DEBUG(dbgs() << "THROUGH: " << *DestSI);
728 
729       // If the destination has PHI nodes, just split the edge for updating
730       // simplicity.
731       if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
732         SplitCriticalEdge(DestSI, i, this);
733         DestSucc = DestSI->getSuccessor(i);
734       }
735       FoldSingleEntryPHINodes(DestSucc);
736       PredSI->setSuccessor(PredCase, DestSucc);
737       MadeChange = true;
738     }
739 
740     if (MadeChange)
741       return true;
742   }
743 
744   return false;
745 }
746 
747 
748 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
749 /// load instruction, eliminate it by replacing it with a PHI node.  This is an
750 /// important optimization that encourages jump threading, and needs to be run
751 /// interlaced with other jump threading tasks.
752 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
753   // Don't hack volatile loads.
754   if (LI->isVolatile()) return false;
755 
756   // If the load is defined in a block with exactly one predecessor, it can't be
757   // partially redundant.
758   BasicBlock *LoadBB = LI->getParent();
759   if (LoadBB->getSinglePredecessor())
760     return false;
761 
762   Value *LoadedPtr = LI->getOperand(0);
763 
764   // If the loaded operand is defined in the LoadBB, it can't be available.
765   // TODO: Could do simple PHI translation, that would be fun :)
766   if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
767     if (PtrOp->getParent() == LoadBB)
768       return false;
769 
770   // Scan a few instructions up from the load, to see if it is obviously live at
771   // the entry to its block.
772   BasicBlock::iterator BBIt = LI;
773 
774   if (Value *AvailableVal =
775         FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
776     // If the value if the load is locally available within the block, just use
777     // it.  This frequently occurs for reg2mem'd allocas.
778     //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
779 
780     // If the returned value is the load itself, replace with an undef. This can
781     // only happen in dead loops.
782     if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
783     LI->replaceAllUsesWith(AvailableVal);
784     LI->eraseFromParent();
785     return true;
786   }
787 
788   // Otherwise, if we scanned the whole block and got to the top of the block,
789   // we know the block is locally transparent to the load.  If not, something
790   // might clobber its value.
791   if (BBIt != LoadBB->begin())
792     return false;
793 
794 
795   SmallPtrSet<BasicBlock*, 8> PredsScanned;
796   typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
797   AvailablePredsTy AvailablePreds;
798   BasicBlock *OneUnavailablePred = 0;
799 
800   // If we got here, the loaded value is transparent through to the start of the
801   // block.  Check to see if it is available in any of the predecessor blocks.
802   for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
803        PI != PE; ++PI) {
804     BasicBlock *PredBB = *PI;
805 
806     // If we already scanned this predecessor, skip it.
807     if (!PredsScanned.insert(PredBB))
808       continue;
809 
810     // Scan the predecessor to see if the value is available in the pred.
811     BBIt = PredBB->end();
812     Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
813     if (!PredAvailable) {
814       OneUnavailablePred = PredBB;
815       continue;
816     }
817 
818     // If so, this load is partially redundant.  Remember this info so that we
819     // can create a PHI node.
820     AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
821   }
822 
823   // If the loaded value isn't available in any predecessor, it isn't partially
824   // redundant.
825   if (AvailablePreds.empty()) return false;
826 
827   // Okay, the loaded value is available in at least one (and maybe all!)
828   // predecessors.  If the value is unavailable in more than one unique
829   // predecessor, we want to insert a merge block for those common predecessors.
830   // This ensures that we only have to insert one reload, thus not increasing
831   // code size.
832   BasicBlock *UnavailablePred = 0;
833 
834   // If there is exactly one predecessor where the value is unavailable, the
835   // already computed 'OneUnavailablePred' block is it.  If it ends in an
836   // unconditional branch, we know that it isn't a critical edge.
837   if (PredsScanned.size() == AvailablePreds.size()+1 &&
838       OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
839     UnavailablePred = OneUnavailablePred;
840   } else if (PredsScanned.size() != AvailablePreds.size()) {
841     // Otherwise, we had multiple unavailable predecessors or we had a critical
842     // edge from the one.
843     SmallVector<BasicBlock*, 8> PredsToSplit;
844     SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
845 
846     for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
847       AvailablePredSet.insert(AvailablePreds[i].first);
848 
849     // Add all the unavailable predecessors to the PredsToSplit list.
850     for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
851          PI != PE; ++PI)
852       if (!AvailablePredSet.count(*PI))
853         PredsToSplit.push_back(*PI);
854 
855     // Split them out to their own block.
856     UnavailablePred =
857       SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
858                              "thread-pre-split", this);
859   }
860 
861   // If the value isn't available in all predecessors, then there will be
862   // exactly one where it isn't available.  Insert a load on that edge and add
863   // it to the AvailablePreds list.
864   if (UnavailablePred) {
865     assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
866            "Can't handle critical edge here!");
867     Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
868                                  LI->getAlignment(),
869                                  UnavailablePred->getTerminator());
870     AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
871   }
872 
873   // Now we know that each predecessor of this block has a value in
874   // AvailablePreds, sort them for efficient access as we're walking the preds.
875   array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
876 
877   // Create a PHI node at the start of the block for the PRE'd load value.
878   PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
879   PN->takeName(LI);
880 
881   // Insert new entries into the PHI for each predecessor.  A single block may
882   // have multiple entries here.
883   for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
884        ++PI) {
885     AvailablePredsTy::iterator I =
886       std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
887                        std::make_pair(*PI, (Value*)0));
888 
889     assert(I != AvailablePreds.end() && I->first == *PI &&
890            "Didn't find entry for predecessor!");
891 
892     PN->addIncoming(I->second, I->first);
893   }
894 
895   //cerr << "PRE: " << *LI << *PN << "\n";
896 
897   LI->replaceAllUsesWith(PN);
898   LI->eraseFromParent();
899 
900   return true;
901 }
902 
903 /// FindMostPopularDest - The specified list contains multiple possible
904 /// threadable destinations.  Pick the one that occurs the most frequently in
905 /// the list.
906 static BasicBlock *
907 FindMostPopularDest(BasicBlock *BB,
908                     const SmallVectorImpl<std::pair<BasicBlock*,
909                                   BasicBlock*> > &PredToDestList) {
910   assert(!PredToDestList.empty());
911 
912   // Determine popularity.  If there are multiple possible destinations, we
913   // explicitly choose to ignore 'undef' destinations.  We prefer to thread
914   // blocks with known and real destinations to threading undef.  We'll handle
915   // them later if interesting.
916   DenseMap<BasicBlock*, unsigned> DestPopularity;
917   for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
918     if (PredToDestList[i].second)
919       DestPopularity[PredToDestList[i].second]++;
920 
921   // Find the most popular dest.
922   DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
923   BasicBlock *MostPopularDest = DPI->first;
924   unsigned Popularity = DPI->second;
925   SmallVector<BasicBlock*, 4> SamePopularity;
926 
927   for (++DPI; DPI != DestPopularity.end(); ++DPI) {
928     // If the popularity of this entry isn't higher than the popularity we've
929     // seen so far, ignore it.
930     if (DPI->second < Popularity)
931       ; // ignore.
932     else if (DPI->second == Popularity) {
933       // If it is the same as what we've seen so far, keep track of it.
934       SamePopularity.push_back(DPI->first);
935     } else {
936       // If it is more popular, remember it.
937       SamePopularity.clear();
938       MostPopularDest = DPI->first;
939       Popularity = DPI->second;
940     }
941   }
942 
943   // Okay, now we know the most popular destination.  If there is more than
944   // destination, we need to determine one.  This is arbitrary, but we need
945   // to make a deterministic decision.  Pick the first one that appears in the
946   // successor list.
947   if (!SamePopularity.empty()) {
948     SamePopularity.push_back(MostPopularDest);
949     TerminatorInst *TI = BB->getTerminator();
950     for (unsigned i = 0; ; ++i) {
951       assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
952 
953       if (std::find(SamePopularity.begin(), SamePopularity.end(),
954                     TI->getSuccessor(i)) == SamePopularity.end())
955         continue;
956 
957       MostPopularDest = TI->getSuccessor(i);
958       break;
959     }
960   }
961 
962   // Okay, we have finally picked the most popular destination.
963   return MostPopularDest;
964 }
965 
966 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) {
967   // If threading this would thread across a loop header, don't even try to
968   // thread the edge.
969   if (LoopHeaders.count(BB))
970     return false;
971 
972   SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
973   if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues))
974     return false;
975   assert(!PredValues.empty() &&
976          "ComputeValueKnownInPredecessors returned true with no values");
977 
978   DEBUG(dbgs() << "IN BB: " << *BB;
979         for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
980           dbgs() << "  BB '" << BB->getName() << "': FOUND condition = ";
981           if (PredValues[i].first)
982             dbgs() << *PredValues[i].first;
983           else
984             dbgs() << "UNDEF";
985           dbgs() << " for pred '" << PredValues[i].second->getName()
986           << "'.\n";
987         });
988 
989   // Decide what we want to thread through.  Convert our list of known values to
990   // a list of known destinations for each pred.  This also discards duplicate
991   // predecessors and keeps track of the undefined inputs (which are represented
992   // as a null dest in the PredToDestList).
993   SmallPtrSet<BasicBlock*, 16> SeenPreds;
994   SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
995 
996   BasicBlock *OnlyDest = 0;
997   BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
998 
999   for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1000     BasicBlock *Pred = PredValues[i].second;
1001     if (!SeenPreds.insert(Pred))
1002       continue;  // Duplicate predecessor entry.
1003 
1004     // If the predecessor ends with an indirect goto, we can't change its
1005     // destination.
1006     if (isa<IndirectBrInst>(Pred->getTerminator()))
1007       continue;
1008 
1009     ConstantInt *Val = PredValues[i].first;
1010 
1011     BasicBlock *DestBB;
1012     if (Val == 0)      // Undef.
1013       DestBB = 0;
1014     else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1015       DestBB = BI->getSuccessor(Val->isZero());
1016     else {
1017       SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
1018       DestBB = SI->getSuccessor(SI->findCaseValue(Val));
1019     }
1020 
1021     // If we have exactly one destination, remember it for efficiency below.
1022     if (i == 0)
1023       OnlyDest = DestBB;
1024     else if (OnlyDest != DestBB)
1025       OnlyDest = MultipleDestSentinel;
1026 
1027     PredToDestList.push_back(std::make_pair(Pred, DestBB));
1028   }
1029 
1030   // If all edges were unthreadable, we fail.
1031   if (PredToDestList.empty())
1032     return false;
1033 
1034   // Determine which is the most common successor.  If we have many inputs and
1035   // this block is a switch, we want to start by threading the batch that goes
1036   // to the most popular destination first.  If we only know about one
1037   // threadable destination (the common case) we can avoid this.
1038   BasicBlock *MostPopularDest = OnlyDest;
1039 
1040   if (MostPopularDest == MultipleDestSentinel)
1041     MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1042 
1043   // Now that we know what the most popular destination is, factor all
1044   // predecessors that will jump to it into a single predecessor.
1045   SmallVector<BasicBlock*, 16> PredsToFactor;
1046   for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1047     if (PredToDestList[i].second == MostPopularDest) {
1048       BasicBlock *Pred = PredToDestList[i].first;
1049 
1050       // This predecessor may be a switch or something else that has multiple
1051       // edges to the block.  Factor each of these edges by listing them
1052       // according to # occurrences in PredsToFactor.
1053       TerminatorInst *PredTI = Pred->getTerminator();
1054       for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1055         if (PredTI->getSuccessor(i) == BB)
1056           PredsToFactor.push_back(Pred);
1057     }
1058 
1059   // If the threadable edges are branching on an undefined value, we get to pick
1060   // the destination that these predecessors should get to.
1061   if (MostPopularDest == 0)
1062     MostPopularDest = BB->getTerminator()->
1063                             getSuccessor(GetBestDestForJumpOnUndef(BB));
1064 
1065   // Ok, try to thread it!
1066   return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1067 }
1068 
1069 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1070 /// a PHI node in the current block.  See if there are any simplifications we
1071 /// can do based on inputs to the phi node.
1072 ///
1073 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1074   BasicBlock *BB = PN->getParent();
1075 
1076   // If any of the predecessor blocks end in an unconditional branch, we can
1077   // *duplicate* the conditional branch into that block in order to further
1078   // encourage jump threading and to eliminate cases where we have branch on a
1079   // phi of an icmp (branch on icmp is much better).
1080   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1081     BasicBlock *PredBB = PN->getIncomingBlock(i);
1082     if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1083       if (PredBr->isUnconditional() &&
1084           // Try to duplicate BB into PredBB.
1085           DuplicateCondBranchOnPHIIntoPred(BB, PredBB))
1086         return true;
1087   }
1088 
1089   return false;
1090 }
1091 
1092 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1093 /// predecessor to the PHIBB block.  If it has PHI nodes, add entries for
1094 /// NewPred using the entries from OldPred (suitably mapped).
1095 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1096                                             BasicBlock *OldPred,
1097                                             BasicBlock *NewPred,
1098                                      DenseMap<Instruction*, Value*> &ValueMap) {
1099   for (BasicBlock::iterator PNI = PHIBB->begin();
1100        PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1101     // Ok, we have a PHI node.  Figure out what the incoming value was for the
1102     // DestBlock.
1103     Value *IV = PN->getIncomingValueForBlock(OldPred);
1104 
1105     // Remap the value if necessary.
1106     if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1107       DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1108       if (I != ValueMap.end())
1109         IV = I->second;
1110     }
1111 
1112     PN->addIncoming(IV, NewPred);
1113   }
1114 }
1115 
1116 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1117 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1118 /// across BB.  Transform the IR to reflect this change.
1119 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1120                                const SmallVectorImpl<BasicBlock*> &PredBBs,
1121                                BasicBlock *SuccBB) {
1122   // If threading to the same block as we come from, we would infinite loop.
1123   if (SuccBB == BB) {
1124     DEBUG(dbgs() << "  Not threading across BB '" << BB->getName()
1125           << "' - would thread to self!\n");
1126     return false;
1127   }
1128 
1129   // If threading this would thread across a loop header, don't thread the edge.
1130   // See the comments above FindLoopHeaders for justifications and caveats.
1131   if (LoopHeaders.count(BB)) {
1132     DEBUG(dbgs() << "  Not threading across loop header BB '" << BB->getName()
1133           << "' to dest BB '" << SuccBB->getName()
1134           << "' - it might create an irreducible loop!\n");
1135     return false;
1136   }
1137 
1138   unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1139   if (JumpThreadCost > Threshold) {
1140     DEBUG(dbgs() << "  Not threading BB '" << BB->getName()
1141           << "' - Cost is too high: " << JumpThreadCost << "\n");
1142     return false;
1143   }
1144 
1145   // And finally, do it!  Start by factoring the predecessors is needed.
1146   BasicBlock *PredBB;
1147   if (PredBBs.size() == 1)
1148     PredBB = PredBBs[0];
1149   else {
1150     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
1151           << " common predecessors.\n");
1152     PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1153                                     ".thr_comm", this);
1154   }
1155 
1156   // And finally, do it!
1157   DEBUG(dbgs() << "  Threading edge from '" << PredBB->getName() << "' to '"
1158         << SuccBB->getName() << "' with cost: " << JumpThreadCost
1159         << ", across block:\n    "
1160         << *BB << "\n");
1161 
1162   // We are going to have to map operands from the original BB block to the new
1163   // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
1164   // account for entry from PredBB.
1165   DenseMap<Instruction*, Value*> ValueMapping;
1166 
1167   BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1168                                          BB->getName()+".thread",
1169                                          BB->getParent(), BB);
1170   NewBB->moveAfter(PredBB);
1171 
1172   BasicBlock::iterator BI = BB->begin();
1173   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1174     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1175 
1176   // Clone the non-phi instructions of BB into NewBB, keeping track of the
1177   // mapping and using it to remap operands in the cloned instructions.
1178   for (; !isa<TerminatorInst>(BI); ++BI) {
1179     Instruction *New = BI->clone();
1180     New->setName(BI->getName());
1181     NewBB->getInstList().push_back(New);
1182     ValueMapping[BI] = New;
1183 
1184     // Remap operands to patch up intra-block references.
1185     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1186       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1187         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1188         if (I != ValueMapping.end())
1189           New->setOperand(i, I->second);
1190       }
1191   }
1192 
1193   // We didn't copy the terminator from BB over to NewBB, because there is now
1194   // an unconditional jump to SuccBB.  Insert the unconditional jump.
1195   BranchInst::Create(SuccBB, NewBB);
1196 
1197   // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1198   // PHI nodes for NewBB now.
1199   AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1200 
1201   // If there were values defined in BB that are used outside the block, then we
1202   // now have to update all uses of the value to use either the original value,
1203   // the cloned value, or some PHI derived value.  This can require arbitrary
1204   // PHI insertion, of which we are prepared to do, clean these up now.
1205   SSAUpdater SSAUpdate;
1206   SmallVector<Use*, 16> UsesToRename;
1207   for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1208     // Scan all uses of this instruction to see if it is used outside of its
1209     // block, and if so, record them in UsesToRename.
1210     for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1211          ++UI) {
1212       Instruction *User = cast<Instruction>(*UI);
1213       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1214         if (UserPN->getIncomingBlock(UI) == BB)
1215           continue;
1216       } else if (User->getParent() == BB)
1217         continue;
1218 
1219       UsesToRename.push_back(&UI.getUse());
1220     }
1221 
1222     // If there are no uses outside the block, we're done with this instruction.
1223     if (UsesToRename.empty())
1224       continue;
1225 
1226     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1227 
1228     // We found a use of I outside of BB.  Rename all uses of I that are outside
1229     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1230     // with the two values we know.
1231     SSAUpdate.Initialize(I);
1232     SSAUpdate.AddAvailableValue(BB, I);
1233     SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1234 
1235     while (!UsesToRename.empty())
1236       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1237     DEBUG(dbgs() << "\n");
1238   }
1239 
1240 
1241   // Ok, NewBB is good to go.  Update the terminator of PredBB to jump to
1242   // NewBB instead of BB.  This eliminates predecessors from BB, which requires
1243   // us to simplify any PHI nodes in BB.
1244   TerminatorInst *PredTerm = PredBB->getTerminator();
1245   for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1246     if (PredTerm->getSuccessor(i) == BB) {
1247       RemovePredecessorAndSimplify(BB, PredBB, TD);
1248       PredTerm->setSuccessor(i, NewBB);
1249     }
1250 
1251   // At this point, the IR is fully up to date and consistent.  Do a quick scan
1252   // over the new instructions and zap any that are constants or dead.  This
1253   // frequently happens because of phi translation.
1254   BI = NewBB->begin();
1255   for (BasicBlock::iterator E = NewBB->end(); BI != E; ) {
1256     Instruction *Inst = BI++;
1257 
1258     if (Value *V = SimplifyInstruction(Inst, TD)) {
1259       WeakVH BIHandle(BI);
1260       ReplaceAndSimplifyAllUses(Inst, V, TD);
1261       if (BIHandle == 0)
1262         BI = NewBB->begin();
1263       continue;
1264     }
1265 
1266     RecursivelyDeleteTriviallyDeadInstructions(Inst);
1267   }
1268 
1269   // Threaded an edge!
1270   ++NumThreads;
1271   return true;
1272 }
1273 
1274 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1275 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1276 /// If we can duplicate the contents of BB up into PredBB do so now, this
1277 /// improves the odds that the branch will be on an analyzable instruction like
1278 /// a compare.
1279 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1280                                                      BasicBlock *PredBB) {
1281   // If BB is a loop header, then duplicating this block outside the loop would
1282   // cause us to transform this into an irreducible loop, don't do this.
1283   // See the comments above FindLoopHeaders for justifications and caveats.
1284   if (LoopHeaders.count(BB)) {
1285     DEBUG(dbgs() << "  Not duplicating loop header '" << BB->getName()
1286           << "' into predecessor block '" << PredBB->getName()
1287           << "' - it might create an irreducible loop!\n");
1288     return false;
1289   }
1290 
1291   unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1292   if (DuplicationCost > Threshold) {
1293     DEBUG(dbgs() << "  Not duplicating BB '" << BB->getName()
1294           << "' - Cost is too high: " << DuplicationCost << "\n");
1295     return false;
1296   }
1297 
1298   // Okay, we decided to do this!  Clone all the instructions in BB onto the end
1299   // of PredBB.
1300   DEBUG(dbgs() << "  Duplicating block '" << BB->getName() << "' into end of '"
1301         << PredBB->getName() << "' to eliminate branch on phi.  Cost: "
1302         << DuplicationCost << " block is:" << *BB << "\n");
1303 
1304   // We are going to have to map operands from the original BB block into the
1305   // PredBB block.  Evaluate PHI nodes in BB.
1306   DenseMap<Instruction*, Value*> ValueMapping;
1307 
1308   BasicBlock::iterator BI = BB->begin();
1309   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1310     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1311 
1312   BranchInst *OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1313 
1314   // Clone the non-phi instructions of BB into PredBB, keeping track of the
1315   // mapping and using it to remap operands in the cloned instructions.
1316   for (; BI != BB->end(); ++BI) {
1317     Instruction *New = BI->clone();
1318     New->setName(BI->getName());
1319     PredBB->getInstList().insert(OldPredBranch, New);
1320     ValueMapping[BI] = New;
1321 
1322     // Remap operands to patch up intra-block references.
1323     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1324       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1325         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1326         if (I != ValueMapping.end())
1327           New->setOperand(i, I->second);
1328       }
1329   }
1330 
1331   // Check to see if the targets of the branch had PHI nodes. If so, we need to
1332   // add entries to the PHI nodes for branch from PredBB now.
1333   BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1334   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1335                                   ValueMapping);
1336   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1337                                   ValueMapping);
1338 
1339   // If there were values defined in BB that are used outside the block, then we
1340   // now have to update all uses of the value to use either the original value,
1341   // the cloned value, or some PHI derived value.  This can require arbitrary
1342   // PHI insertion, of which we are prepared to do, clean these up now.
1343   SSAUpdater SSAUpdate;
1344   SmallVector<Use*, 16> UsesToRename;
1345   for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1346     // Scan all uses of this instruction to see if it is used outside of its
1347     // block, and if so, record them in UsesToRename.
1348     for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1349          ++UI) {
1350       Instruction *User = cast<Instruction>(*UI);
1351       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1352         if (UserPN->getIncomingBlock(UI) == BB)
1353           continue;
1354       } else if (User->getParent() == BB)
1355         continue;
1356 
1357       UsesToRename.push_back(&UI.getUse());
1358     }
1359 
1360     // If there are no uses outside the block, we're done with this instruction.
1361     if (UsesToRename.empty())
1362       continue;
1363 
1364     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1365 
1366     // We found a use of I outside of BB.  Rename all uses of I that are outside
1367     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1368     // with the two values we know.
1369     SSAUpdate.Initialize(I);
1370     SSAUpdate.AddAvailableValue(BB, I);
1371     SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1372 
1373     while (!UsesToRename.empty())
1374       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1375     DEBUG(dbgs() << "\n");
1376   }
1377 
1378   // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1379   // that we nuked.
1380   RemovePredecessorAndSimplify(BB, PredBB, TD);
1381 
1382   // Remove the unconditional branch at the end of the PredBB block.
1383   OldPredBranch->eraseFromParent();
1384 
1385   ++NumDupes;
1386   return true;
1387 }
1388 
1389 
1390