xref: /llvm-project/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp (revision 921b04e9a4b9cc854bbefeb644f95fdef9d9dd0c)
1 //===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
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 pass implements a simple loop unroller.  It works best when loops have
11 // been canonicalized by the -indvars pass, allowing it to determine the trip
12 // counts of loops easily.
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/ADT/SetVector.h"
17 #include "llvm/Analysis/GlobalsModRef.h"
18 #include "llvm/Analysis/AssumptionCache.h"
19 #include "llvm/Analysis/CodeMetrics.h"
20 #include "llvm/Analysis/InstructionSimplify.h"
21 #include "llvm/Analysis/LoopPass.h"
22 #include "llvm/Analysis/ScalarEvolution.h"
23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
24 #include "llvm/Analysis/TargetTransformInfo.h"
25 #include "llvm/IR/DataLayout.h"
26 #include "llvm/IR/DiagnosticInfo.h"
27 #include "llvm/IR/Dominators.h"
28 #include "llvm/IR/InstVisitor.h"
29 #include "llvm/IR/IntrinsicInst.h"
30 #include "llvm/IR/Metadata.h"
31 #include "llvm/Support/CommandLine.h"
32 #include "llvm/Support/Debug.h"
33 #include "llvm/Support/raw_ostream.h"
34 #include "llvm/Transforms/Utils/UnrollLoop.h"
35 #include <climits>
36 
37 using namespace llvm;
38 
39 #define DEBUG_TYPE "loop-unroll"
40 
41 static cl::opt<unsigned>
42     UnrollThreshold("unroll-threshold", cl::Hidden,
43                     cl::desc("The baseline cost threshold for loop unrolling"));
44 
45 static cl::opt<unsigned> UnrollPercentDynamicCostSavedThreshold(
46     "unroll-percent-dynamic-cost-saved-threshold", cl::Hidden,
47     cl::desc("The percentage of estimated dynamic cost which must be saved by "
48              "unrolling to allow unrolling up to the max threshold."));
49 
50 static cl::opt<unsigned> UnrollDynamicCostSavingsDiscount(
51     "unroll-dynamic-cost-savings-discount", cl::Hidden,
52     cl::desc("This is the amount discounted from the total unroll cost when "
53              "the unrolled form has a high dynamic cost savings (triggered by "
54              "the '-unroll-perecent-dynamic-cost-saved-threshold' flag)."));
55 
56 static cl::opt<unsigned> UnrollMaxIterationsCountToAnalyze(
57     "unroll-max-iteration-count-to-analyze", cl::init(0), cl::Hidden,
58     cl::desc("Don't allow loop unrolling to simulate more than this number of"
59              "iterations when checking full unroll profitability"));
60 
61 static cl::opt<unsigned>
62 UnrollCount("unroll-count", cl::Hidden,
63   cl::desc("Use this unroll count for all loops including those with "
64            "unroll_count pragma values, for testing purposes"));
65 
66 static cl::opt<bool>
67 UnrollAllowPartial("unroll-allow-partial", cl::Hidden,
68   cl::desc("Allows loops to be partially unrolled until "
69            "-unroll-threshold loop size is reached."));
70 
71 static cl::opt<bool>
72 UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::Hidden,
73   cl::desc("Unroll loops with run-time trip counts"));
74 
75 static cl::opt<unsigned>
76 PragmaUnrollThreshold("pragma-unroll-threshold", cl::init(16 * 1024), cl::Hidden,
77   cl::desc("Unrolled size limit for loops with an unroll(full) or "
78            "unroll_count pragma."));
79 
80 
81 /// A magic value for use with the Threshold parameter to indicate
82 /// that the loop unroll should be performed regardless of how much
83 /// code expansion would result.
84 static const unsigned NoThreshold = UINT_MAX;
85 
86 /// Default unroll count for loops with run-time trip count if
87 /// -unroll-count is not set
88 static const unsigned DefaultUnrollRuntimeCount = 8;
89 
90 /// Gather the various unrolling parameters based on the defaults, compiler
91 /// flags, TTI overrides, pragmas, and user specified parameters.
92 static TargetTransformInfo::UnrollingPreferences gatherUnrollingPreferences(
93     Loop *L, const TargetTransformInfo &TTI, Optional<unsigned> UserThreshold,
94     Optional<unsigned> UserCount, Optional<bool> UserAllowPartial,
95     Optional<bool> UserRuntime, unsigned PragmaCount, bool PragmaFullUnroll,
96     bool PragmaEnableUnroll, unsigned TripCount) {
97   TargetTransformInfo::UnrollingPreferences UP;
98 
99   // Set up the defaults
100   UP.Threshold = 150;
101   UP.PercentDynamicCostSavedThreshold = 20;
102   UP.DynamicCostSavingsDiscount = 2000;
103   UP.OptSizeThreshold = 50;
104   UP.PartialThreshold = UP.Threshold;
105   UP.PartialOptSizeThreshold = UP.OptSizeThreshold;
106   UP.Count = 0;
107   UP.MaxCount = UINT_MAX;
108   UP.Partial = false;
109   UP.Runtime = false;
110   UP.AllowExpensiveTripCount = false;
111 
112   // Override with any target specific settings
113   TTI.getUnrollingPreferences(L, UP);
114 
115   // Apply size attributes
116   if (L->getHeader()->getParent()->optForSize()) {
117     UP.Threshold = UP.OptSizeThreshold;
118     UP.PartialThreshold = UP.PartialOptSizeThreshold;
119   }
120 
121   // Apply unroll count pragmas
122   if (PragmaCount)
123     UP.Count = PragmaCount;
124   else if (PragmaFullUnroll)
125     UP.Count = TripCount;
126 
127   // Apply any user values specified by cl::opt
128   if (UnrollThreshold.getNumOccurrences() > 0) {
129     UP.Threshold = UnrollThreshold;
130     UP.PartialThreshold = UnrollThreshold;
131   }
132   if (UnrollPercentDynamicCostSavedThreshold.getNumOccurrences() > 0)
133     UP.PercentDynamicCostSavedThreshold =
134         UnrollPercentDynamicCostSavedThreshold;
135   if (UnrollDynamicCostSavingsDiscount.getNumOccurrences() > 0)
136     UP.DynamicCostSavingsDiscount = UnrollDynamicCostSavingsDiscount;
137   if (UnrollCount.getNumOccurrences() > 0)
138     UP.Count = UnrollCount;
139   if (UnrollAllowPartial.getNumOccurrences() > 0)
140     UP.Partial = UnrollAllowPartial;
141   if (UnrollRuntime.getNumOccurrences() > 0)
142     UP.Runtime = UnrollRuntime;
143 
144   // Apply user values provided by argument
145   if (UserThreshold.hasValue()) {
146     UP.Threshold = *UserThreshold;
147     UP.PartialThreshold = *UserThreshold;
148   }
149   if (UserCount.hasValue())
150     UP.Count = *UserCount;
151   if (UserAllowPartial.hasValue())
152     UP.Partial = *UserAllowPartial;
153   if (UserRuntime.hasValue())
154     UP.Runtime = *UserRuntime;
155 
156   if (PragmaCount > 0 ||
157       ((PragmaFullUnroll || PragmaEnableUnroll) && TripCount != 0)) {
158     // If the loop has an unrolling pragma, we want to be more aggressive with
159     // unrolling limits. Set thresholds to at least the PragmaTheshold value
160     // which is larger than the default limits.
161     if (UP.Threshold != NoThreshold)
162       UP.Threshold = std::max<unsigned>(UP.Threshold, PragmaUnrollThreshold);
163     if (UP.PartialThreshold != NoThreshold)
164       UP.PartialThreshold =
165           std::max<unsigned>(UP.PartialThreshold, PragmaUnrollThreshold);
166   }
167 
168   return UP;
169 }
170 
171 namespace {
172   class LoopUnroll : public LoopPass {
173   public:
174     static char ID; // Pass ID, replacement for typeid
175     LoopUnroll(Optional<unsigned> Threshold = None,
176                Optional<unsigned> Count = None,
177                Optional<bool> AllowPartial = None,
178                Optional<bool> Runtime = None)
179         : LoopPass(ID), ProvidedCount(Count), ProvidedThreshold(Threshold),
180           ProvidedAllowPartial(AllowPartial), ProvidedRuntime(Runtime) {
181       initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
182     }
183 
184     Optional<unsigned> ProvidedCount;
185     Optional<unsigned> ProvidedThreshold;
186     Optional<bool> ProvidedAllowPartial;
187     Optional<bool> ProvidedRuntime;
188 
189     bool runOnLoop(Loop *L, LPPassManager &) override;
190 
191     /// This transformation requires natural loop information & requires that
192     /// loop preheaders be inserted into the CFG...
193     ///
194     void getAnalysisUsage(AnalysisUsage &AU) const override {
195       AU.addRequired<AssumptionCacheTracker>();
196       AU.addRequired<DominatorTreeWrapperPass>();
197       AU.addRequired<LoopInfoWrapperPass>();
198       AU.addPreserved<LoopInfoWrapperPass>();
199       AU.addRequiredID(LoopSimplifyID);
200       AU.addPreservedID(LoopSimplifyID);
201       AU.addRequiredID(LCSSAID);
202       AU.addPreservedID(LCSSAID);
203       AU.addRequired<ScalarEvolutionWrapperPass>();
204       AU.addPreserved<ScalarEvolutionWrapperPass>();
205       AU.addRequired<TargetTransformInfoWrapperPass>();
206       // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
207       // If loop unroll does not preserve dom info then LCSSA pass on next
208       // loop will receive invalid dom info.
209       // For now, recreate dom info, if loop is unrolled.
210       AU.addPreserved<DominatorTreeWrapperPass>();
211       AU.addPreserved<GlobalsAAWrapperPass>();
212     }
213   };
214 }
215 
216 char LoopUnroll::ID = 0;
217 INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
218 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
219 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
220 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
221 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
222 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
223 INITIALIZE_PASS_DEPENDENCY(LCSSA)
224 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
225 INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
226 
227 Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial,
228                                  int Runtime) {
229   // TODO: It would make more sense for this function to take the optionals
230   // directly, but that's dangerous since it would silently break out of tree
231   // callers.
232   return new LoopUnroll(Threshold == -1 ? None : Optional<unsigned>(Threshold),
233                         Count == -1 ? None : Optional<unsigned>(Count),
234                         AllowPartial == -1 ? None
235                                            : Optional<bool>(AllowPartial),
236                         Runtime == -1 ? None : Optional<bool>(Runtime));
237 }
238 
239 Pass *llvm::createSimpleLoopUnrollPass() {
240   return llvm::createLoopUnrollPass(-1, -1, 0, 0);
241 }
242 
243 namespace {
244 // This class is used to get an estimate of the optimization effects that we
245 // could get from complete loop unrolling. It comes from the fact that some
246 // loads might be replaced with concrete constant values and that could trigger
247 // a chain of instruction simplifications.
248 //
249 // E.g. we might have:
250 //   int a[] = {0, 1, 0};
251 //   v = 0;
252 //   for (i = 0; i < 3; i ++)
253 //     v += b[i]*a[i];
254 // If we completely unroll the loop, we would get:
255 //   v = b[0]*a[0] + b[1]*a[1] + b[2]*a[2]
256 // Which then will be simplified to:
257 //   v = b[0]* 0 + b[1]* 1 + b[2]* 0
258 // And finally:
259 //   v = b[1]
260 class UnrolledInstAnalyzer : private InstVisitor<UnrolledInstAnalyzer, bool> {
261   typedef InstVisitor<UnrolledInstAnalyzer, bool> Base;
262   friend class InstVisitor<UnrolledInstAnalyzer, bool>;
263   struct SimplifiedAddress {
264     Value *Base = nullptr;
265     ConstantInt *Offset = nullptr;
266   };
267 
268 public:
269   UnrolledInstAnalyzer(unsigned Iteration,
270                        DenseMap<Value *, Constant *> &SimplifiedValues,
271                        ScalarEvolution &SE)
272       : SimplifiedValues(SimplifiedValues), SE(SE) {
273       IterationNumber = SE.getConstant(APInt(64, Iteration));
274   }
275 
276   // Allow access to the initial visit method.
277   using Base::visit;
278 
279 private:
280   /// \brief A cache of pointer bases and constant-folded offsets corresponding
281   /// to GEP (or derived from GEP) instructions.
282   ///
283   /// In order to find the base pointer one needs to perform non-trivial
284   /// traversal of the corresponding SCEV expression, so it's good to have the
285   /// results saved.
286   DenseMap<Value *, SimplifiedAddress> SimplifiedAddresses;
287 
288   /// \brief SCEV expression corresponding to number of currently simulated
289   /// iteration.
290   const SCEV *IterationNumber;
291 
292   /// \brief A Value->Constant map for keeping values that we managed to
293   /// constant-fold on the given iteration.
294   ///
295   /// While we walk the loop instructions, we build up and maintain a mapping
296   /// of simplified values specific to this iteration.  The idea is to propagate
297   /// any special information we have about loads that can be replaced with
298   /// constants after complete unrolling, and account for likely simplifications
299   /// post-unrolling.
300   DenseMap<Value *, Constant *> &SimplifiedValues;
301 
302   ScalarEvolution &SE;
303 
304   /// \brief Try to simplify instruction \param I using its SCEV expression.
305   ///
306   /// The idea is that some AddRec expressions become constants, which then
307   /// could trigger folding of other instructions. However, that only happens
308   /// for expressions whose start value is also constant, which isn't always the
309   /// case. In another common and important case the start value is just some
310   /// address (i.e. SCEVUnknown) - in this case we compute the offset and save
311   /// it along with the base address instead.
312   bool simplifyInstWithSCEV(Instruction *I) {
313     if (!SE.isSCEVable(I->getType()))
314       return false;
315 
316     const SCEV *S = SE.getSCEV(I);
317     if (auto *SC = dyn_cast<SCEVConstant>(S)) {
318       SimplifiedValues[I] = SC->getValue();
319       return true;
320     }
321 
322     auto *AR = dyn_cast<SCEVAddRecExpr>(S);
323     if (!AR)
324       return false;
325 
326     const SCEV *ValueAtIteration = AR->evaluateAtIteration(IterationNumber, SE);
327     // Check if the AddRec expression becomes a constant.
328     if (auto *SC = dyn_cast<SCEVConstant>(ValueAtIteration)) {
329       SimplifiedValues[I] = SC->getValue();
330       return true;
331     }
332 
333     // Check if the offset from the base address becomes a constant.
334     auto *Base = dyn_cast<SCEVUnknown>(SE.getPointerBase(S));
335     if (!Base)
336       return false;
337     auto *Offset =
338         dyn_cast<SCEVConstant>(SE.getMinusSCEV(ValueAtIteration, Base));
339     if (!Offset)
340       return false;
341     SimplifiedAddress Address;
342     Address.Base = Base->getValue();
343     Address.Offset = Offset->getValue();
344     SimplifiedAddresses[I] = Address;
345     return true;
346   }
347 
348   /// Base case for the instruction visitor.
349   bool visitInstruction(Instruction &I) {
350     return simplifyInstWithSCEV(&I);
351   }
352 
353   /// Try to simplify binary operator I.
354   ///
355   /// TODO: Probably it's worth to hoist the code for estimating the
356   /// simplifications effects to a separate class, since we have a very similar
357   /// code in InlineCost already.
358   bool visitBinaryOperator(BinaryOperator &I) {
359     Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
360     if (!isa<Constant>(LHS))
361       if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
362         LHS = SimpleLHS;
363     if (!isa<Constant>(RHS))
364       if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
365         RHS = SimpleRHS;
366 
367     Value *SimpleV = nullptr;
368     const DataLayout &DL = I.getModule()->getDataLayout();
369     if (auto FI = dyn_cast<FPMathOperator>(&I))
370       SimpleV =
371           SimplifyFPBinOp(I.getOpcode(), LHS, RHS, FI->getFastMathFlags(), DL);
372     else
373       SimpleV = SimplifyBinOp(I.getOpcode(), LHS, RHS, DL);
374 
375     if (Constant *C = dyn_cast_or_null<Constant>(SimpleV))
376       SimplifiedValues[&I] = C;
377 
378     if (SimpleV)
379       return true;
380     return Base::visitBinaryOperator(I);
381   }
382 
383   /// Try to fold load I.
384   bool visitLoad(LoadInst &I) {
385     Value *AddrOp = I.getPointerOperand();
386 
387     auto AddressIt = SimplifiedAddresses.find(AddrOp);
388     if (AddressIt == SimplifiedAddresses.end())
389       return false;
390     ConstantInt *SimplifiedAddrOp = AddressIt->second.Offset;
391 
392     auto *GV = dyn_cast<GlobalVariable>(AddressIt->second.Base);
393     // We're only interested in loads that can be completely folded to a
394     // constant.
395     if (!GV || !GV->hasDefinitiveInitializer() || !GV->isConstant())
396       return false;
397 
398     ConstantDataSequential *CDS =
399         dyn_cast<ConstantDataSequential>(GV->getInitializer());
400     if (!CDS)
401       return false;
402 
403     // We might have a vector load from an array. FIXME: for now we just bail
404     // out in this case, but we should be able to resolve and simplify such
405     // loads.
406     if(!CDS->isElementTypeCompatible(I.getType()))
407       return false;
408 
409     int ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
410     assert(SimplifiedAddrOp->getValue().getActiveBits() < 64 &&
411            "Unexpectedly large index value.");
412     int64_t Index = SimplifiedAddrOp->getSExtValue() / ElemSize;
413     if (Index >= CDS->getNumElements()) {
414       // FIXME: For now we conservatively ignore out of bound accesses, but
415       // we're allowed to perform the optimization in this case.
416       return false;
417     }
418 
419     Constant *CV = CDS->getElementAsConstant(Index);
420     assert(CV && "Constant expected.");
421     SimplifiedValues[&I] = CV;
422 
423     return true;
424   }
425 
426   bool visitCastInst(CastInst &I) {
427     // Propagate constants through casts.
428     Constant *COp = dyn_cast<Constant>(I.getOperand(0));
429     if (!COp)
430       COp = SimplifiedValues.lookup(I.getOperand(0));
431     if (COp)
432       if (Constant *C =
433               ConstantExpr::getCast(I.getOpcode(), COp, I.getType())) {
434         SimplifiedValues[&I] = C;
435         return true;
436       }
437 
438     return Base::visitCastInst(I);
439   }
440 
441   bool visitCmpInst(CmpInst &I) {
442     Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
443 
444     // First try to handle simplified comparisons.
445     if (!isa<Constant>(LHS))
446       if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
447         LHS = SimpleLHS;
448     if (!isa<Constant>(RHS))
449       if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
450         RHS = SimpleRHS;
451 
452     if (!isa<Constant>(LHS) && !isa<Constant>(RHS)) {
453       auto SimplifiedLHS = SimplifiedAddresses.find(LHS);
454       if (SimplifiedLHS != SimplifiedAddresses.end()) {
455         auto SimplifiedRHS = SimplifiedAddresses.find(RHS);
456         if (SimplifiedRHS != SimplifiedAddresses.end()) {
457           SimplifiedAddress &LHSAddr = SimplifiedLHS->second;
458           SimplifiedAddress &RHSAddr = SimplifiedRHS->second;
459           if (LHSAddr.Base == RHSAddr.Base) {
460             LHS = LHSAddr.Offset;
461             RHS = RHSAddr.Offset;
462           }
463         }
464       }
465     }
466 
467     if (Constant *CLHS = dyn_cast<Constant>(LHS)) {
468       if (Constant *CRHS = dyn_cast<Constant>(RHS)) {
469         if (Constant *C = ConstantExpr::getCompare(I.getPredicate(), CLHS, CRHS)) {
470           SimplifiedValues[&I] = C;
471           return true;
472         }
473       }
474     }
475 
476     return Base::visitCmpInst(I);
477   }
478 };
479 } // namespace
480 
481 
482 namespace {
483 struct EstimatedUnrollCost {
484   /// \brief The estimated cost after unrolling.
485   int UnrolledCost;
486 
487   /// \brief The estimated dynamic cost of executing the instructions in the
488   /// rolled form.
489   int RolledDynamicCost;
490 };
491 }
492 
493 /// \brief Figure out if the loop is worth full unrolling.
494 ///
495 /// Complete loop unrolling can make some loads constant, and we need to know
496 /// if that would expose any further optimization opportunities.  This routine
497 /// estimates this optimization.  It computes cost of unrolled loop
498 /// (UnrolledCost) and dynamic cost of the original loop (RolledDynamicCost). By
499 /// dynamic cost we mean that we won't count costs of blocks that are known not
500 /// to be executed (i.e. if we have a branch in the loop and we know that at the
501 /// given iteration its condition would be resolved to true, we won't add up the
502 /// cost of the 'false'-block).
503 /// \returns Optional value, holding the RolledDynamicCost and UnrolledCost. If
504 /// the analysis failed (no benefits expected from the unrolling, or the loop is
505 /// too big to analyze), the returned value is None.
506 static Optional<EstimatedUnrollCost>
507 analyzeLoopUnrollCost(const Loop *L, unsigned TripCount, DominatorTree &DT,
508                       ScalarEvolution &SE, const TargetTransformInfo &TTI,
509                       int MaxUnrolledLoopSize) {
510   // We want to be able to scale offsets by the trip count and add more offsets
511   // to them without checking for overflows, and we already don't want to
512   // analyze *massive* trip counts, so we force the max to be reasonably small.
513   assert(UnrollMaxIterationsCountToAnalyze < (INT_MAX / 2) &&
514          "The unroll iterations max is too large!");
515 
516   // Don't simulate loops with a big or unknown tripcount
517   if (!UnrollMaxIterationsCountToAnalyze || !TripCount ||
518       TripCount > UnrollMaxIterationsCountToAnalyze)
519     return None;
520 
521   SmallSetVector<BasicBlock *, 16> BBWorklist;
522   DenseMap<Value *, Constant *> SimplifiedValues;
523   SmallVector<std::pair<Value *, Constant *>, 4> SimplifiedInputValues;
524 
525   // The estimated cost of the unrolled form of the loop. We try to estimate
526   // this by simplifying as much as we can while computing the estimate.
527   int UnrolledCost = 0;
528   // We also track the estimated dynamic (that is, actually executed) cost in
529   // the rolled form. This helps identify cases when the savings from unrolling
530   // aren't just exposing dead control flows, but actual reduced dynamic
531   // instructions due to the simplifications which we expect to occur after
532   // unrolling.
533   int RolledDynamicCost = 0;
534 
535   // Ensure that we don't violate the loop structure invariants relied on by
536   // this analysis.
537   assert(L->isLoopSimplifyForm() && "Must put loop into normal form first.");
538   assert(L->isLCSSAForm(DT) &&
539          "Must have loops in LCSSA form to track live-out values.");
540 
541   DEBUG(dbgs() << "Starting LoopUnroll profitability analysis...\n");
542 
543   // Simulate execution of each iteration of the loop counting instructions,
544   // which would be simplified.
545   // Since the same load will take different values on different iterations,
546   // we literally have to go through all loop's iterations.
547   for (unsigned Iteration = 0; Iteration < TripCount; ++Iteration) {
548     DEBUG(dbgs() << " Analyzing iteration " << Iteration << "\n");
549 
550     // Prepare for the iteration by collecting any simplified entry or backedge
551     // inputs.
552     for (Instruction &I : *L->getHeader()) {
553       auto *PHI = dyn_cast<PHINode>(&I);
554       if (!PHI)
555         break;
556 
557       // The loop header PHI nodes must have exactly two input: one from the
558       // loop preheader and one from the loop latch.
559       assert(
560           PHI->getNumIncomingValues() == 2 &&
561           "Must have an incoming value only for the preheader and the latch.");
562 
563       Value *V = PHI->getIncomingValueForBlock(
564           Iteration == 0 ? L->getLoopPreheader() : L->getLoopLatch());
565       Constant *C = dyn_cast<Constant>(V);
566       if (Iteration != 0 && !C)
567         C = SimplifiedValues.lookup(V);
568       if (C)
569         SimplifiedInputValues.push_back({PHI, C});
570     }
571 
572     // Now clear and re-populate the map for the next iteration.
573     SimplifiedValues.clear();
574     while (!SimplifiedInputValues.empty())
575       SimplifiedValues.insert(SimplifiedInputValues.pop_back_val());
576 
577     UnrolledInstAnalyzer Analyzer(Iteration, SimplifiedValues, SE);
578 
579     BBWorklist.clear();
580     BBWorklist.insert(L->getHeader());
581     // Note that we *must not* cache the size, this loop grows the worklist.
582     for (unsigned Idx = 0; Idx != BBWorklist.size(); ++Idx) {
583       BasicBlock *BB = BBWorklist[Idx];
584 
585       // Visit all instructions in the given basic block and try to simplify
586       // it.  We don't change the actual IR, just count optimization
587       // opportunities.
588       for (Instruction &I : *BB) {
589         int InstCost = TTI.getUserCost(&I);
590 
591         // Visit the instruction to analyze its loop cost after unrolling,
592         // and if the visitor returns false, include this instruction in the
593         // unrolled cost.
594         if (!Analyzer.visit(I))
595           UnrolledCost += InstCost;
596         else {
597           DEBUG(dbgs() << "  " << I
598                        << " would be simplified if loop is unrolled.\n");
599           (void)0;
600         }
601 
602         // Also track this instructions expected cost when executing the rolled
603         // loop form.
604         RolledDynamicCost += InstCost;
605 
606         // If unrolled body turns out to be too big, bail out.
607         if (UnrolledCost > MaxUnrolledLoopSize) {
608           DEBUG(dbgs() << "  Exceeded threshold.. exiting.\n"
609                        << "  UnrolledCost: " << UnrolledCost
610                        << ", MaxUnrolledLoopSize: " << MaxUnrolledLoopSize
611                        << "\n");
612           return None;
613         }
614       }
615 
616       TerminatorInst *TI = BB->getTerminator();
617 
618       // Add in the live successors by first checking whether we have terminator
619       // that may be simplified based on the values simplified by this call.
620       if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
621         if (BI->isConditional()) {
622           if (Constant *SimpleCond =
623                   SimplifiedValues.lookup(BI->getCondition())) {
624             BasicBlock *Succ = nullptr;
625             // Just take the first successor if condition is undef
626             if (isa<UndefValue>(SimpleCond))
627               Succ = BI->getSuccessor(0);
628             else
629               Succ = BI->getSuccessor(
630                   cast<ConstantInt>(SimpleCond)->isZero() ? 1 : 0);
631             if (L->contains(Succ))
632               BBWorklist.insert(Succ);
633             continue;
634           }
635         }
636       } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
637         if (Constant *SimpleCond =
638                 SimplifiedValues.lookup(SI->getCondition())) {
639           BasicBlock *Succ = nullptr;
640           // Just take the first successor if condition is undef
641           if (isa<UndefValue>(SimpleCond))
642             Succ = SI->getSuccessor(0);
643           else
644             Succ = SI->findCaseValue(cast<ConstantInt>(SimpleCond))
645                        .getCaseSuccessor();
646           if (L->contains(Succ))
647             BBWorklist.insert(Succ);
648           continue;
649         }
650       }
651 
652       // Add BB's successors to the worklist.
653       for (BasicBlock *Succ : successors(BB))
654         if (L->contains(Succ))
655           BBWorklist.insert(Succ);
656     }
657 
658     // If we found no optimization opportunities on the first iteration, we
659     // won't find them on later ones too.
660     if (UnrolledCost == RolledDynamicCost) {
661       DEBUG(dbgs() << "  No opportunities found.. exiting.\n"
662                    << "  UnrolledCost: " << UnrolledCost << "\n");
663       return None;
664     }
665   }
666   DEBUG(dbgs() << "Analysis finished:\n"
667                << "UnrolledCost: " << UnrolledCost << ", "
668                << "RolledDynamicCost: " << RolledDynamicCost << "\n");
669   return {{UnrolledCost, RolledDynamicCost}};
670 }
671 
672 /// ApproximateLoopSize - Approximate the size of the loop.
673 static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
674                                     bool &NotDuplicatable,
675                                     const TargetTransformInfo &TTI,
676                                     AssumptionCache *AC) {
677   SmallPtrSet<const Value *, 32> EphValues;
678   CodeMetrics::collectEphemeralValues(L, AC, EphValues);
679 
680   CodeMetrics Metrics;
681   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
682        I != E; ++I)
683     Metrics.analyzeBasicBlock(*I, TTI, EphValues);
684   NumCalls = Metrics.NumInlineCandidates;
685   NotDuplicatable = Metrics.notDuplicatable;
686 
687   unsigned LoopSize = Metrics.NumInsts;
688 
689   // Don't allow an estimate of size zero.  This would allows unrolling of loops
690   // with huge iteration counts, which is a compile time problem even if it's
691   // not a problem for code quality. Also, the code using this size may assume
692   // that each loop has at least three instructions (likely a conditional
693   // branch, a comparison feeding that branch, and some kind of loop increment
694   // feeding that comparison instruction).
695   LoopSize = std::max(LoopSize, 3u);
696 
697   return LoopSize;
698 }
699 
700 // Returns the loop hint metadata node with the given name (for example,
701 // "llvm.loop.unroll.count").  If no such metadata node exists, then nullptr is
702 // returned.
703 static MDNode *GetUnrollMetadataForLoop(const Loop *L, StringRef Name) {
704   if (MDNode *LoopID = L->getLoopID())
705     return GetUnrollMetadata(LoopID, Name);
706   return nullptr;
707 }
708 
709 // Returns true if the loop has an unroll(full) pragma.
710 static bool HasUnrollFullPragma(const Loop *L) {
711   return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.full");
712 }
713 
714 // Returns true if the loop has an unroll(enable) pragma. This metadata is used
715 // for both "#pragma unroll" and "#pragma clang loop unroll(enable)" directives.
716 static bool HasUnrollEnablePragma(const Loop *L) {
717   return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.enable");
718 }
719 
720 // Returns true if the loop has an unroll(disable) pragma.
721 static bool HasUnrollDisablePragma(const Loop *L) {
722   return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.disable");
723 }
724 
725 // Returns true if the loop has an runtime unroll(disable) pragma.
726 static bool HasRuntimeUnrollDisablePragma(const Loop *L) {
727   return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.runtime.disable");
728 }
729 
730 // If loop has an unroll_count pragma return the (necessarily
731 // positive) value from the pragma.  Otherwise return 0.
732 static unsigned UnrollCountPragmaValue(const Loop *L) {
733   MDNode *MD = GetUnrollMetadataForLoop(L, "llvm.loop.unroll.count");
734   if (MD) {
735     assert(MD->getNumOperands() == 2 &&
736            "Unroll count hint metadata should have two operands.");
737     unsigned Count =
738         mdconst::extract<ConstantInt>(MD->getOperand(1))->getZExtValue();
739     assert(Count >= 1 && "Unroll count must be positive.");
740     return Count;
741   }
742   return 0;
743 }
744 
745 // Remove existing unroll metadata and add unroll disable metadata to
746 // indicate the loop has already been unrolled.  This prevents a loop
747 // from being unrolled more than is directed by a pragma if the loop
748 // unrolling pass is run more than once (which it generally is).
749 static void SetLoopAlreadyUnrolled(Loop *L) {
750   MDNode *LoopID = L->getLoopID();
751   if (!LoopID) return;
752 
753   // First remove any existing loop unrolling metadata.
754   SmallVector<Metadata *, 4> MDs;
755   // Reserve first location for self reference to the LoopID metadata node.
756   MDs.push_back(nullptr);
757   for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
758     bool IsUnrollMetadata = false;
759     MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
760     if (MD) {
761       const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
762       IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll.");
763     }
764     if (!IsUnrollMetadata)
765       MDs.push_back(LoopID->getOperand(i));
766   }
767 
768   // Add unroll(disable) metadata to disable future unrolling.
769   LLVMContext &Context = L->getHeader()->getContext();
770   SmallVector<Metadata *, 1> DisableOperands;
771   DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.disable"));
772   MDNode *DisableNode = MDNode::get(Context, DisableOperands);
773   MDs.push_back(DisableNode);
774 
775   MDNode *NewLoopID = MDNode::get(Context, MDs);
776   // Set operand 0 to refer to the loop id itself.
777   NewLoopID->replaceOperandWith(0, NewLoopID);
778   L->setLoopID(NewLoopID);
779 }
780 
781 static bool canUnrollCompletely(Loop *L, unsigned Threshold,
782                                 unsigned PercentDynamicCostSavedThreshold,
783                                 unsigned DynamicCostSavingsDiscount,
784                                 uint64_t UnrolledCost,
785                                 uint64_t RolledDynamicCost) {
786   if (Threshold == NoThreshold) {
787     DEBUG(dbgs() << "  Can fully unroll, because no threshold is set.\n");
788     return true;
789   }
790 
791   if (UnrolledCost <= Threshold) {
792     DEBUG(dbgs() << "  Can fully unroll, because unrolled cost: "
793                  << UnrolledCost << "<" << Threshold << "\n");
794     return true;
795   }
796 
797   assert(UnrolledCost && "UnrolledCost can't be 0 at this point.");
798   assert(RolledDynamicCost >= UnrolledCost &&
799          "Cannot have a higher unrolled cost than a rolled cost!");
800 
801   // Compute the percentage of the dynamic cost in the rolled form that is
802   // saved when unrolled. If unrolling dramatically reduces the estimated
803   // dynamic cost of the loop, we use a higher threshold to allow more
804   // unrolling.
805   unsigned PercentDynamicCostSaved =
806       (uint64_t)(RolledDynamicCost - UnrolledCost) * 100ull / RolledDynamicCost;
807 
808   if (PercentDynamicCostSaved >= PercentDynamicCostSavedThreshold &&
809       (int64_t)UnrolledCost - (int64_t)DynamicCostSavingsDiscount <=
810           (int64_t)Threshold) {
811     DEBUG(dbgs() << "  Can fully unroll, because unrolling will reduce the "
812                     "expected dynamic cost by " << PercentDynamicCostSaved
813                  << "% (threshold: " << PercentDynamicCostSavedThreshold
814                  << "%)\n"
815                  << "  and the unrolled cost (" << UnrolledCost
816                  << ") is less than the max threshold ("
817                  << DynamicCostSavingsDiscount << ").\n");
818     return true;
819   }
820 
821   DEBUG(dbgs() << "  Too large to fully unroll:\n");
822   DEBUG(dbgs() << "    Threshold: " << Threshold << "\n");
823   DEBUG(dbgs() << "    Max threshold: " << DynamicCostSavingsDiscount << "\n");
824   DEBUG(dbgs() << "    Percent cost saved threshold: "
825                << PercentDynamicCostSavedThreshold << "%\n");
826   DEBUG(dbgs() << "    Unrolled cost: " << UnrolledCost << "\n");
827   DEBUG(dbgs() << "    Rolled dynamic cost: " << RolledDynamicCost << "\n");
828   DEBUG(dbgs() << "    Percent cost saved: " << PercentDynamicCostSaved
829                << "\n");
830   return false;
831 }
832 
833 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &) {
834   if (skipOptnoneFunction(L))
835     return false;
836 
837   Function &F = *L->getHeader()->getParent();
838 
839   auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
840   LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
841   ScalarEvolution *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
842   const TargetTransformInfo &TTI =
843       getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
844   auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
845   bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
846 
847   BasicBlock *Header = L->getHeader();
848   DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
849         << "] Loop %" << Header->getName() << "\n");
850 
851   if (HasUnrollDisablePragma(L)) {
852     return false;
853   }
854   bool PragmaFullUnroll = HasUnrollFullPragma(L);
855   bool PragmaEnableUnroll = HasUnrollEnablePragma(L);
856   unsigned PragmaCount = UnrollCountPragmaValue(L);
857   bool HasPragma = PragmaFullUnroll || PragmaEnableUnroll || PragmaCount > 0;
858 
859   // Find trip count and trip multiple if count is not available
860   unsigned TripCount = 0;
861   unsigned TripMultiple = 1;
862   // If there are multiple exiting blocks but one of them is the latch, use the
863   // latch for the trip count estimation. Otherwise insist on a single exiting
864   // block for the trip count estimation.
865   BasicBlock *ExitingBlock = L->getLoopLatch();
866   if (!ExitingBlock || !L->isLoopExiting(ExitingBlock))
867     ExitingBlock = L->getExitingBlock();
868   if (ExitingBlock) {
869     TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);
870     TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock);
871   }
872 
873   TargetTransformInfo::UnrollingPreferences UP = gatherUnrollingPreferences(
874       L, TTI, ProvidedThreshold, ProvidedCount, ProvidedAllowPartial,
875       ProvidedRuntime, PragmaCount, PragmaFullUnroll, PragmaEnableUnroll,
876       TripCount);
877 
878   unsigned Count = UP.Count;
879   bool CountSetExplicitly = Count != 0;
880   // Use a heuristic count if we didn't set anything explicitly.
881   if (!CountSetExplicitly)
882     Count = TripCount == 0 ? DefaultUnrollRuntimeCount : TripCount;
883   if (TripCount && Count > TripCount)
884     Count = TripCount;
885 
886   unsigned NumInlineCandidates;
887   bool notDuplicatable;
888   unsigned LoopSize =
889       ApproximateLoopSize(L, NumInlineCandidates, notDuplicatable, TTI, &AC);
890   DEBUG(dbgs() << "  Loop Size = " << LoopSize << "\n");
891 
892   // When computing the unrolled size, note that the conditional branch on the
893   // backedge and the comparison feeding it are not replicated like the rest of
894   // the loop body (which is why 2 is subtracted).
895   uint64_t UnrolledSize = (uint64_t)(LoopSize-2) * Count + 2;
896   if (notDuplicatable) {
897     DEBUG(dbgs() << "  Not unrolling loop which contains non-duplicatable"
898                  << " instructions.\n");
899     return false;
900   }
901   if (NumInlineCandidates != 0) {
902     DEBUG(dbgs() << "  Not unrolling loop with inlinable calls.\n");
903     return false;
904   }
905 
906   // Given Count, TripCount and thresholds determine the type of
907   // unrolling which is to be performed.
908   enum { Full = 0, Partial = 1, Runtime = 2 };
909   int Unrolling;
910   if (TripCount && Count == TripCount) {
911     Unrolling = Partial;
912     // If the loop is really small, we don't need to run an expensive analysis.
913     if (canUnrollCompletely(L, UP.Threshold, 100, UP.DynamicCostSavingsDiscount,
914                             UnrolledSize, UnrolledSize)) {
915       Unrolling = Full;
916     } else {
917       // The loop isn't that small, but we still can fully unroll it if that
918       // helps to remove a significant number of instructions.
919       // To check that, run additional analysis on the loop.
920       if (Optional<EstimatedUnrollCost> Cost = analyzeLoopUnrollCost(
921               L, TripCount, DT, *SE, TTI,
922               UP.Threshold + UP.DynamicCostSavingsDiscount))
923         if (canUnrollCompletely(L, UP.Threshold,
924                                 UP.PercentDynamicCostSavedThreshold,
925                                 UP.DynamicCostSavingsDiscount,
926                                 Cost->UnrolledCost, Cost->RolledDynamicCost)) {
927           Unrolling = Full;
928         }
929     }
930   } else if (TripCount && Count < TripCount) {
931     Unrolling = Partial;
932   } else {
933     Unrolling = Runtime;
934   }
935 
936   // Reduce count based on the type of unrolling and the threshold values.
937   unsigned OriginalCount = Count;
938   bool AllowRuntime = PragmaEnableUnroll || (PragmaCount > 0) || UP.Runtime;
939   // Don't unroll a runtime trip count loop with unroll full pragma.
940   if (HasRuntimeUnrollDisablePragma(L) || PragmaFullUnroll) {
941     AllowRuntime = false;
942   }
943   if (Unrolling == Partial) {
944     bool AllowPartial = PragmaEnableUnroll || UP.Partial;
945     if (!AllowPartial && !CountSetExplicitly) {
946       DEBUG(dbgs() << "  will not try to unroll partially because "
947                    << "-unroll-allow-partial not given\n");
948       return false;
949     }
950     if (UP.PartialThreshold != NoThreshold &&
951         UnrolledSize > UP.PartialThreshold) {
952       // Reduce unroll count to be modulo of TripCount for partial unrolling.
953       Count = (std::max(UP.PartialThreshold, 3u) - 2) / (LoopSize - 2);
954       while (Count != 0 && TripCount % Count != 0)
955         Count--;
956     }
957   } else if (Unrolling == Runtime) {
958     if (!AllowRuntime && !CountSetExplicitly) {
959       DEBUG(dbgs() << "  will not try to unroll loop with runtime trip count "
960                    << "-unroll-runtime not given\n");
961       return false;
962     }
963     // Reduce unroll count to be the largest power-of-two factor of
964     // the original count which satisfies the threshold limit.
965     while (Count != 0 && UnrolledSize > UP.PartialThreshold) {
966       Count >>= 1;
967       UnrolledSize = (LoopSize-2) * Count + 2;
968     }
969     if (Count > UP.MaxCount)
970       Count = UP.MaxCount;
971     DEBUG(dbgs() << "  partially unrolling with count: " << Count << "\n");
972   }
973 
974   if (HasPragma) {
975     if (PragmaCount != 0)
976       // If loop has an unroll count pragma mark loop as unrolled to prevent
977       // unrolling beyond that requested by the pragma.
978       SetLoopAlreadyUnrolled(L);
979 
980     // Emit optimization remarks if we are unable to unroll the loop
981     // as directed by a pragma.
982     DebugLoc LoopLoc = L->getStartLoc();
983     Function *F = Header->getParent();
984     LLVMContext &Ctx = F->getContext();
985     if ((PragmaCount > 0) && Count != OriginalCount) {
986       emitOptimizationRemarkMissed(
987           Ctx, DEBUG_TYPE, *F, LoopLoc,
988           "Unable to unroll loop the number of times directed by "
989           "unroll_count pragma because unrolled size is too large.");
990     } else if (PragmaFullUnroll && !TripCount) {
991       emitOptimizationRemarkMissed(
992           Ctx, DEBUG_TYPE, *F, LoopLoc,
993           "Unable to fully unroll loop as directed by unroll(full) pragma "
994           "because loop has a runtime trip count.");
995     } else if (PragmaEnableUnroll && Count != TripCount && Count < 2) {
996       emitOptimizationRemarkMissed(
997           Ctx, DEBUG_TYPE, *F, LoopLoc,
998           "Unable to unroll loop as directed by unroll(enable) pragma because "
999           "unrolled size is too large.");
1000     } else if ((PragmaFullUnroll || PragmaEnableUnroll) && TripCount &&
1001                Count != TripCount) {
1002       emitOptimizationRemarkMissed(
1003           Ctx, DEBUG_TYPE, *F, LoopLoc,
1004           "Unable to fully unroll loop as directed by unroll pragma because "
1005           "unrolled size is too large.");
1006     }
1007   }
1008 
1009   if (Unrolling != Full && Count < 2) {
1010     // Partial unrolling by 1 is a nop.  For full unrolling, a factor
1011     // of 1 makes sense because loop control can be eliminated.
1012     return false;
1013   }
1014 
1015   // Unroll the loop.
1016   if (!UnrollLoop(L, Count, TripCount, AllowRuntime, UP.AllowExpensiveTripCount,
1017                   TripMultiple, LI, SE, &DT, &AC, PreserveLCSSA))
1018     return false;
1019 
1020   return true;
1021 }
1022