Mercurial > hg > Members > tobaru > cbc > CbC_llvm
view lib/Transforms/Scalar/LoopUnrollPass.cpp @ 83:60c9769439b8
LLVM 3.7
author | Tatsuki IHA <e125716@ie.u-ryukyu.ac.jp> |
---|---|
date | Wed, 18 Feb 2015 14:55:36 +0900 |
parents | 54457678186b |
children | afa8332a0e37 |
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//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This pass implements a simple loop unroller. It works best when loops have // been canonicalized by the -indvars pass, allowing it to determine the trip // counts of loops easily. //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/ADT/SetVector.h" #include "llvm/Analysis/AssumptionCache.h" #include "llvm/Analysis/CodeMetrics.h" #include "llvm/Analysis/LoopPass.h" #include "llvm/Analysis/ScalarEvolution.h" #include "llvm/Analysis/ScalarEvolutionExpressions.h" #include "llvm/Analysis/TargetTransformInfo.h" #include "llvm/IR/DataLayout.h" #include "llvm/IR/DiagnosticInfo.h" #include "llvm/IR/Dominators.h" #include "llvm/IR/IntrinsicInst.h" #include "llvm/IR/Metadata.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" #include "llvm/Transforms/Utils/UnrollLoop.h" #include "llvm/IR/InstVisitor.h" #include "llvm/Analysis/InstructionSimplify.h" #include <climits> using namespace llvm; #define DEBUG_TYPE "loop-unroll" static cl::opt<unsigned> UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden, cl::desc("The cut-off point for automatic loop unrolling")); static cl::opt<unsigned> UnrollMaxIterationsCountToAnalyze( "unroll-max-iteration-count-to-analyze", cl::init(0), cl::Hidden, cl::desc("Don't allow loop unrolling to simulate more than this number of" "iterations when checking full unroll profitability")); static cl::opt<unsigned> UnrollMinPercentOfOptimized( "unroll-percent-of-optimized-for-complete-unroll", cl::init(20), cl::Hidden, cl::desc("If complete unrolling could trigger further optimizations, and, " "by that, remove the given percent of instructions, perform the " "complete unroll even if it's beyond the threshold")); static cl::opt<unsigned> UnrollAbsoluteThreshold( "unroll-absolute-threshold", cl::init(2000), cl::Hidden, cl::desc("Don't unroll if the unrolled size is bigger than this threshold," " even if we can remove big portion of instructions later.")); static cl::opt<unsigned> UnrollCount("unroll-count", cl::init(0), cl::Hidden, cl::desc("Use this unroll count for all loops including those with " "unroll_count pragma values, for testing purposes")); static cl::opt<bool> UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden, cl::desc("Allows loops to be partially unrolled until " "-unroll-threshold loop size is reached.")); static cl::opt<bool> UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden, cl::desc("Unroll loops with run-time trip counts")); static cl::opt<unsigned> PragmaUnrollThreshold("pragma-unroll-threshold", cl::init(16 * 1024), cl::Hidden, cl::desc("Unrolled size limit for loops with an unroll(full) or " "unroll_count pragma.")); namespace { class LoopUnroll : public LoopPass { public: static char ID; // Pass ID, replacement for typeid LoopUnroll(int T = -1, int C = -1, int P = -1, int R = -1) : LoopPass(ID) { CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T); CurrentAbsoluteThreshold = UnrollAbsoluteThreshold; CurrentMinPercentOfOptimized = UnrollMinPercentOfOptimized; CurrentCount = (C == -1) ? UnrollCount : unsigned(C); CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P; CurrentRuntime = (R == -1) ? UnrollRuntime : (bool)R; UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0); UserAbsoluteThreshold = (UnrollAbsoluteThreshold.getNumOccurrences() > 0); UserPercentOfOptimized = (UnrollMinPercentOfOptimized.getNumOccurrences() > 0); UserAllowPartial = (P != -1) || (UnrollAllowPartial.getNumOccurrences() > 0); UserRuntime = (R != -1) || (UnrollRuntime.getNumOccurrences() > 0); UserCount = (C != -1) || (UnrollCount.getNumOccurrences() > 0); initializeLoopUnrollPass(*PassRegistry::getPassRegistry()); } /// A magic value for use with the Threshold parameter to indicate /// that the loop unroll should be performed regardless of how much /// code expansion would result. static const unsigned NoThreshold = UINT_MAX; // Threshold to use when optsize is specified (and there is no // explicit -unroll-threshold). static const unsigned OptSizeUnrollThreshold = 50; // Default unroll count for loops with run-time trip count if // -unroll-count is not set static const unsigned UnrollRuntimeCount = 8; unsigned CurrentCount; unsigned CurrentThreshold; unsigned CurrentAbsoluteThreshold; unsigned CurrentMinPercentOfOptimized; bool CurrentAllowPartial; bool CurrentRuntime; bool UserCount; // CurrentCount is user-specified. bool UserThreshold; // CurrentThreshold is user-specified. bool UserAbsoluteThreshold; // CurrentAbsoluteThreshold is // user-specified. bool UserPercentOfOptimized; // CurrentMinPercentOfOptimized is // user-specified. bool UserAllowPartial; // CurrentAllowPartial is user-specified. bool UserRuntime; // CurrentRuntime is user-specified. bool runOnLoop(Loop *L, LPPassManager &LPM) override; /// This transformation requires natural loop information & requires that /// loop preheaders be inserted into the CFG... /// void getAnalysisUsage(AnalysisUsage &AU) const override { AU.addRequired<AssumptionCacheTracker>(); AU.addRequired<LoopInfoWrapperPass>(); AU.addPreserved<LoopInfoWrapperPass>(); AU.addRequiredID(LoopSimplifyID); AU.addPreservedID(LoopSimplifyID); AU.addRequiredID(LCSSAID); AU.addPreservedID(LCSSAID); AU.addRequired<ScalarEvolution>(); AU.addPreserved<ScalarEvolution>(); AU.addRequired<TargetTransformInfoWrapperPass>(); // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info. // If loop unroll does not preserve dom info then LCSSA pass on next // loop will receive invalid dom info. // For now, recreate dom info, if loop is unrolled. AU.addPreserved<DominatorTreeWrapperPass>(); } // Fill in the UnrollingPreferences parameter with values from the // TargetTransformationInfo. void getUnrollingPreferences(Loop *L, const TargetTransformInfo &TTI, TargetTransformInfo::UnrollingPreferences &UP) { UP.Threshold = CurrentThreshold; UP.AbsoluteThreshold = CurrentAbsoluteThreshold; UP.MinPercentOfOptimized = CurrentMinPercentOfOptimized; UP.OptSizeThreshold = OptSizeUnrollThreshold; UP.PartialThreshold = CurrentThreshold; UP.PartialOptSizeThreshold = OptSizeUnrollThreshold; UP.Count = CurrentCount; UP.MaxCount = UINT_MAX; UP.Partial = CurrentAllowPartial; UP.Runtime = CurrentRuntime; TTI.getUnrollingPreferences(L, UP); } // Select and return an unroll count based on parameters from // user, unroll preferences, unroll pragmas, or a heuristic. // SetExplicitly is set to true if the unroll count is is set by // the user or a pragma rather than selected heuristically. unsigned selectUnrollCount(const Loop *L, unsigned TripCount, bool PragmaFullUnroll, unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP, bool &SetExplicitly); // Select threshold values used to limit unrolling based on a // total unrolled size. Parameters Threshold and PartialThreshold // are set to the maximum unrolled size for fully and partially // unrolled loops respectively. void selectThresholds(const Loop *L, bool HasPragma, const TargetTransformInfo::UnrollingPreferences &UP, unsigned &Threshold, unsigned &PartialThreshold, unsigned NumberOfOptimizedInstructions) { // Determine the current unrolling threshold. While this is // normally set from UnrollThreshold, it is overridden to a // smaller value if the current function is marked as // optimize-for-size, and the unroll threshold was not user // specified. Threshold = UserThreshold ? CurrentThreshold : UP.Threshold; // If we are allowed to completely unroll if we can remove M% of // instructions, and we know that with complete unrolling we'll be able // to kill N instructions, then we can afford to completely unroll loops // with unrolled size up to N*100/M. // Adjust the threshold according to that: unsigned PercentOfOptimizedForCompleteUnroll = UserPercentOfOptimized ? CurrentMinPercentOfOptimized : UP.MinPercentOfOptimized; unsigned AbsoluteThreshold = UserAbsoluteThreshold ? CurrentAbsoluteThreshold : UP.AbsoluteThreshold; if (PercentOfOptimizedForCompleteUnroll) Threshold = std::max<unsigned>(Threshold, NumberOfOptimizedInstructions * 100 / PercentOfOptimizedForCompleteUnroll); // But don't allow unrolling loops bigger than absolute threshold. Threshold = std::min<unsigned>(Threshold, AbsoluteThreshold); PartialThreshold = UserThreshold ? CurrentThreshold : UP.PartialThreshold; if (!UserThreshold && L->getHeader()->getParent()->hasFnAttribute( Attribute::OptimizeForSize)) { Threshold = UP.OptSizeThreshold; PartialThreshold = UP.PartialOptSizeThreshold; } if (HasPragma) { // If the loop has an unrolling pragma, we want to be more // aggressive with unrolling limits. Set thresholds to at // least the PragmaTheshold value which is larger than the // default limits. if (Threshold != NoThreshold) Threshold = std::max<unsigned>(Threshold, PragmaUnrollThreshold); if (PartialThreshold != NoThreshold) PartialThreshold = std::max<unsigned>(PartialThreshold, PragmaUnrollThreshold); } } }; } char LoopUnroll::ID = 0; INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false) INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) INITIALIZE_PASS_DEPENDENCY(LoopSimplify) INITIALIZE_PASS_DEPENDENCY(LCSSA) INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false) Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial, int Runtime) { return new LoopUnroll(Threshold, Count, AllowPartial, Runtime); } Pass *llvm::createSimpleLoopUnrollPass() { return llvm::createLoopUnrollPass(-1, -1, 0, 0); } static bool isLoadFromConstantInitializer(Value *V) { if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) if (GV->isConstant() && GV->hasDefinitiveInitializer()) return GV->getInitializer(); return false; } struct FindConstantPointers { bool LoadCanBeConstantFolded; bool IndexIsConstant; APInt Step; APInt StartValue; Value *BaseAddress; const Loop *L; ScalarEvolution &SE; FindConstantPointers(const Loop *loop, ScalarEvolution &SE) : LoadCanBeConstantFolded(true), IndexIsConstant(true), L(loop), SE(SE) {} bool follow(const SCEV *S) { if (const SCEVUnknown *SC = dyn_cast<SCEVUnknown>(S)) { // We've reached the leaf node of SCEV, it's most probably just a // variable. Now it's time to see if it corresponds to a global constant // global (in which case we can eliminate the load), or not. BaseAddress = SC->getValue(); LoadCanBeConstantFolded = IndexIsConstant && isLoadFromConstantInitializer(BaseAddress); return false; } if (isa<SCEVConstant>(S)) return true; if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { // If the current SCEV expression is AddRec, and its loop isn't the loop // we are about to unroll, then we won't get a constant address after // unrolling, and thus, won't be able to eliminate the load. if (AR->getLoop() != L) return IndexIsConstant = false; // If the step isn't constant, we won't get constant addresses in unrolled // version. Bail out. if (const SCEVConstant *StepSE = dyn_cast<SCEVConstant>(AR->getStepRecurrence(SE))) Step = StepSE->getValue()->getValue(); else return IndexIsConstant = false; return IndexIsConstant; } // If Result is true, continue traversal. // Otherwise, we have found something that prevents us from (possible) load // elimination. return IndexIsConstant; } bool isDone() const { return !IndexIsConstant; } }; // This class is used to get an estimate of the optimization effects that we // could get from complete loop unrolling. It comes from the fact that some // loads might be replaced with concrete constant values and that could trigger // a chain of instruction simplifications. // // E.g. we might have: // int a[] = {0, 1, 0}; // v = 0; // for (i = 0; i < 3; i ++) // v += b[i]*a[i]; // If we completely unroll the loop, we would get: // v = b[0]*a[0] + b[1]*a[1] + b[2]*a[2] // Which then will be simplified to: // v = b[0]* 0 + b[1]* 1 + b[2]* 0 // And finally: // v = b[1] class UnrollAnalyzer : public InstVisitor<UnrollAnalyzer, bool> { typedef InstVisitor<UnrollAnalyzer, bool> Base; friend class InstVisitor<UnrollAnalyzer, bool>; const Loop *L; unsigned TripCount; ScalarEvolution &SE; const TargetTransformInfo &TTI; DenseMap<Value *, Constant *> SimplifiedValues; DenseMap<LoadInst *, Value *> LoadBaseAddresses; SmallPtrSet<Instruction *, 32> CountedInstructions; /// \brief Count the number of optimized instructions. unsigned NumberOfOptimizedInstructions; // Provide base case for our instruction visit. bool visitInstruction(Instruction &I) { return false; }; // TODO: We should also visit ICmp, FCmp, GetElementPtr, Trunc, ZExt, SExt, // FPTrunc, FPExt, FPToUI, FPToSI, UIToFP, SIToFP, BitCast, Select, // ExtractElement, InsertElement, ShuffleVector, ExtractValue, InsertValue. // // Probaly it's worth to hoist the code for estimating the simplifications // effects to a separate class, since we have a very similar code in // InlineCost already. bool visitBinaryOperator(BinaryOperator &I) { Value *LHS = I.getOperand(0), *RHS = I.getOperand(1); if (!isa<Constant>(LHS)) if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS)) LHS = SimpleLHS; if (!isa<Constant>(RHS)) if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS)) RHS = SimpleRHS; Value *SimpleV = nullptr; if (auto FI = dyn_cast<FPMathOperator>(&I)) SimpleV = SimplifyFPBinOp(I.getOpcode(), LHS, RHS, FI->getFastMathFlags()); else SimpleV = SimplifyBinOp(I.getOpcode(), LHS, RHS); if (SimpleV && CountedInstructions.insert(&I).second) NumberOfOptimizedInstructions += TTI.getUserCost(&I); if (Constant *C = dyn_cast_or_null<Constant>(SimpleV)) { SimplifiedValues[&I] = C; return true; } return false; } Constant *computeLoadValue(LoadInst *LI, unsigned Iteration) { if (!LI) return nullptr; Value *BaseAddr = LoadBaseAddresses[LI]; if (!BaseAddr) return nullptr; auto GV = dyn_cast<GlobalVariable>(BaseAddr); if (!GV) return nullptr; ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(GV->getInitializer()); if (!CDS) return nullptr; const SCEV *BaseAddrSE = SE.getSCEV(BaseAddr); const SCEV *S = SE.getSCEV(LI->getPointerOperand()); const SCEV *OffSE = SE.getMinusSCEV(S, BaseAddrSE); APInt StepC, StartC; const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(OffSE); if (!AR) return nullptr; if (const SCEVConstant *StepSE = dyn_cast<SCEVConstant>(AR->getStepRecurrence(SE))) StepC = StepSE->getValue()->getValue(); else return nullptr; if (const SCEVConstant *StartSE = dyn_cast<SCEVConstant>(AR->getStart())) StartC = StartSE->getValue()->getValue(); else return nullptr; unsigned ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U; unsigned Start = StartC.getLimitedValue(); unsigned Step = StepC.getLimitedValue(); unsigned Index = (Start + Step * Iteration) / ElemSize; if (Index >= CDS->getNumElements()) return nullptr; Constant *CV = CDS->getElementAsConstant(Index); return CV; } public: UnrollAnalyzer(const Loop *L, unsigned TripCount, ScalarEvolution &SE, const TargetTransformInfo &TTI) : L(L), TripCount(TripCount), SE(SE), TTI(TTI), NumberOfOptimizedInstructions(0) {} // Visit all loads the loop L, and for those that, after complete loop // unrolling, would have a constant address and it will point to a known // constant initializer, record its base address for future use. It is used // when we estimate number of potentially simplified instructions. void findConstFoldableLoads() { for (auto BB : L->getBlocks()) { for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { if (LoadInst *LI = dyn_cast<LoadInst>(I)) { if (!LI->isSimple()) continue; Value *AddrOp = LI->getPointerOperand(); const SCEV *S = SE.getSCEV(AddrOp); FindConstantPointers Visitor(L, SE); SCEVTraversal<FindConstantPointers> T(Visitor); T.visitAll(S); if (Visitor.IndexIsConstant && Visitor.LoadCanBeConstantFolded) { LoadBaseAddresses[LI] = Visitor.BaseAddress; } } } } } // Given a list of loads that could be constant-folded (LoadBaseAddresses), // estimate number of optimized instructions after substituting the concrete // values for the given Iteration. Also track how many instructions become // dead through this process. unsigned estimateNumberOfOptimizedInstructions(unsigned Iteration) { // We keep a set vector for the worklist so that we don't wast space in the // worklist queuing up the same instruction repeatedly. This can happen due // to multiple operands being the same instruction or due to the same // instruction being an operand of lots of things that end up dead or // simplified. SmallSetVector<Instruction *, 8> Worklist; // Clear the simplified values and counts for this iteration. SimplifiedValues.clear(); CountedInstructions.clear(); NumberOfOptimizedInstructions = 0; // We start by adding all loads to the worklist. for (auto &LoadDescr : LoadBaseAddresses) { LoadInst *LI = LoadDescr.first; SimplifiedValues[LI] = computeLoadValue(LI, Iteration); if (CountedInstructions.insert(LI).second) NumberOfOptimizedInstructions += TTI.getUserCost(LI); for (User *U : LI->users()) Worklist.insert(cast<Instruction>(U)); } // And then we try to simplify every user of every instruction from the // worklist. If we do simplify a user, add it to the worklist to process // its users as well. while (!Worklist.empty()) { Instruction *I = Worklist.pop_back_val(); if (!L->contains(I)) continue; if (!visit(I)) continue; for (User *U : I->users()) Worklist.insert(cast<Instruction>(U)); } // Now that we know the potentially simplifed instructions, estimate number // of instructions that would become dead if we do perform the // simplification. // The dead instructions are held in a separate set. This is used to // prevent us from re-examining instructions and make sure we only count // the benifit once. The worklist's internal set handles insertion // deduplication. SmallPtrSet<Instruction *, 16> DeadInstructions; // Lambda to enque operands onto the worklist. auto EnqueueOperands = [&](Instruction &I) { for (auto *Op : I.operand_values()) if (auto *OpI = dyn_cast<Instruction>(Op)) if (!OpI->use_empty()) Worklist.insert(OpI); }; // Start by initializing worklist with simplified instructions. for (auto &FoldedKeyValue : SimplifiedValues) if (auto *FoldedInst = dyn_cast<Instruction>(FoldedKeyValue.first)) { DeadInstructions.insert(FoldedInst); // Add each instruction operand of this dead instruction to the // worklist. EnqueueOperands(*FoldedInst); } // If a definition of an insn is only used by simplified or dead // instructions, it's also dead. Check defs of all instructions from the // worklist. while (!Worklist.empty()) { Instruction *I = Worklist.pop_back_val(); if (!L->contains(I)) continue; if (DeadInstructions.count(I)) continue; if (std::all_of(I->user_begin(), I->user_end(), [&](User *U) { return DeadInstructions.count(cast<Instruction>(U)); })) { NumberOfOptimizedInstructions += TTI.getUserCost(I); DeadInstructions.insert(I); EnqueueOperands(*I); } } return NumberOfOptimizedInstructions; } }; // Complete loop unrolling can make some loads constant, and we need to know if // that would expose any further optimization opportunities. // This routine estimates this optimization effect and returns the number of // instructions, that potentially might be optimized away. static unsigned approximateNumberOfOptimizedInstructions(const Loop *L, ScalarEvolution &SE, unsigned TripCount, const TargetTransformInfo &TTI) { if (!TripCount || !UnrollMaxIterationsCountToAnalyze) return 0; UnrollAnalyzer UA(L, TripCount, SE, TTI); UA.findConstFoldableLoads(); // Estimate number of instructions, that could be simplified if we replace a // load with the corresponding constant. Since the same load will take // different values on different iterations, we have to go through all loop's // iterations here. To limit ourselves here, we check only first N // iterations, and then scale the found number, if necessary. unsigned IterationsNumberForEstimate = std::min<unsigned>(UnrollMaxIterationsCountToAnalyze, TripCount); unsigned NumberOfOptimizedInstructions = 0; for (unsigned i = 0; i < IterationsNumberForEstimate; ++i) NumberOfOptimizedInstructions += UA.estimateNumberOfOptimizedInstructions(i); NumberOfOptimizedInstructions *= TripCount / IterationsNumberForEstimate; return NumberOfOptimizedInstructions; } /// ApproximateLoopSize - Approximate the size of the loop. static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls, bool &NotDuplicatable, const TargetTransformInfo &TTI, AssumptionCache *AC) { SmallPtrSet<const Value *, 32> EphValues; CodeMetrics::collectEphemeralValues(L, AC, EphValues); CodeMetrics Metrics; for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); I != E; ++I) Metrics.analyzeBasicBlock(*I, TTI, EphValues); NumCalls = Metrics.NumInlineCandidates; NotDuplicatable = Metrics.notDuplicatable; unsigned LoopSize = Metrics.NumInsts; // Don't allow an estimate of size zero. This would allows unrolling of loops // with huge iteration counts, which is a compile time problem even if it's // not a problem for code quality. Also, the code using this size may assume // that each loop has at least three instructions (likely a conditional // branch, a comparison feeding that branch, and some kind of loop increment // feeding that comparison instruction). LoopSize = std::max(LoopSize, 3u); return LoopSize; } // Returns the loop hint metadata node with the given name (for example, // "llvm.loop.unroll.count"). If no such metadata node exists, then nullptr is // returned. static MDNode *GetUnrollMetadataForLoop(const Loop *L, StringRef Name) { if (MDNode *LoopID = L->getLoopID()) return GetUnrollMetadata(LoopID, Name); return nullptr; } // Returns true if the loop has an unroll(full) pragma. static bool HasUnrollFullPragma(const Loop *L) { return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.full"); } // Returns true if the loop has an unroll(disable) pragma. static bool HasUnrollDisablePragma(const Loop *L) { return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.disable"); } // If loop has an unroll_count pragma return the (necessarily // positive) value from the pragma. Otherwise return 0. static unsigned UnrollCountPragmaValue(const Loop *L) { MDNode *MD = GetUnrollMetadataForLoop(L, "llvm.loop.unroll.count"); if (MD) { assert(MD->getNumOperands() == 2 && "Unroll count hint metadata should have two operands."); unsigned Count = mdconst::extract<ConstantInt>(MD->getOperand(1))->getZExtValue(); assert(Count >= 1 && "Unroll count must be positive."); return Count; } return 0; } // Remove existing unroll metadata and add unroll disable metadata to // indicate the loop has already been unrolled. This prevents a loop // from being unrolled more than is directed by a pragma if the loop // unrolling pass is run more than once (which it generally is). static void SetLoopAlreadyUnrolled(Loop *L) { MDNode *LoopID = L->getLoopID(); if (!LoopID) return; // First remove any existing loop unrolling metadata. SmallVector<Metadata *, 4> MDs; // Reserve first location for self reference to the LoopID metadata node. MDs.push_back(nullptr); for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) { bool IsUnrollMetadata = false; MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i)); if (MD) { const MDString *S = dyn_cast<MDString>(MD->getOperand(0)); IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll."); } if (!IsUnrollMetadata) MDs.push_back(LoopID->getOperand(i)); } // Add unroll(disable) metadata to disable future unrolling. LLVMContext &Context = L->getHeader()->getContext(); SmallVector<Metadata *, 1> DisableOperands; DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.disable")); MDNode *DisableNode = MDNode::get(Context, DisableOperands); MDs.push_back(DisableNode); MDNode *NewLoopID = MDNode::get(Context, MDs); // Set operand 0 to refer to the loop id itself. NewLoopID->replaceOperandWith(0, NewLoopID); L->setLoopID(NewLoopID); } unsigned LoopUnroll::selectUnrollCount( const Loop *L, unsigned TripCount, bool PragmaFullUnroll, unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP, bool &SetExplicitly) { SetExplicitly = true; // User-specified count (either as a command-line option or // constructor parameter) has highest precedence. unsigned Count = UserCount ? CurrentCount : 0; // If there is no user-specified count, unroll pragmas have the next // highest precendence. if (Count == 0) { if (PragmaCount) { Count = PragmaCount; } else if (PragmaFullUnroll) { Count = TripCount; } } if (Count == 0) Count = UP.Count; if (Count == 0) { SetExplicitly = false; if (TripCount == 0) // Runtime trip count. Count = UnrollRuntimeCount; else // Conservative heuristic: if we know the trip count, see if we can // completely unroll (subject to the threshold, checked below); otherwise // try to find greatest modulo of the trip count which is still under // threshold value. Count = TripCount; } if (TripCount && Count > TripCount) return TripCount; return Count; } bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) { if (skipOptnoneFunction(L)) return false; Function &F = *L->getHeader()->getParent(); LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); ScalarEvolution *SE = &getAnalysis<ScalarEvolution>(); const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F); auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F); BasicBlock *Header = L->getHeader(); DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName() << "] Loop %" << Header->getName() << "\n"); if (HasUnrollDisablePragma(L)) { return false; } bool PragmaFullUnroll = HasUnrollFullPragma(L); unsigned PragmaCount = UnrollCountPragmaValue(L); bool HasPragma = PragmaFullUnroll || PragmaCount > 0; TargetTransformInfo::UnrollingPreferences UP; getUnrollingPreferences(L, TTI, UP); // Find trip count and trip multiple if count is not available unsigned TripCount = 0; unsigned TripMultiple = 1; // If there are multiple exiting blocks but one of them is the latch, use the // latch for the trip count estimation. Otherwise insist on a single exiting // block for the trip count estimation. BasicBlock *ExitingBlock = L->getLoopLatch(); if (!ExitingBlock || !L->isLoopExiting(ExitingBlock)) ExitingBlock = L->getExitingBlock(); if (ExitingBlock) { TripCount = SE->getSmallConstantTripCount(L, ExitingBlock); TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock); } // Select an initial unroll count. This may be reduced later based // on size thresholds. bool CountSetExplicitly; unsigned Count = selectUnrollCount(L, TripCount, PragmaFullUnroll, PragmaCount, UP, CountSetExplicitly); unsigned NumInlineCandidates; bool notDuplicatable; unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates, notDuplicatable, TTI, &AC); DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n"); // When computing the unrolled size, note that the conditional branch on the // backedge and the comparison feeding it are not replicated like the rest of // the loop body (which is why 2 is subtracted). uint64_t UnrolledSize = (uint64_t)(LoopSize-2) * Count + 2; if (notDuplicatable) { DEBUG(dbgs() << " Not unrolling loop which contains non-duplicatable" << " instructions.\n"); return false; } if (NumInlineCandidates != 0) { DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n"); return false; } unsigned NumberOfOptimizedInstructions = approximateNumberOfOptimizedInstructions(L, *SE, TripCount, TTI); DEBUG(dbgs() << " Complete unrolling could save: " << NumberOfOptimizedInstructions << "\n"); unsigned Threshold, PartialThreshold; selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold, NumberOfOptimizedInstructions); // Given Count, TripCount and thresholds determine the type of // unrolling which is to be performed. enum { Full = 0, Partial = 1, Runtime = 2 }; int Unrolling; if (TripCount && Count == TripCount) { if (Threshold != NoThreshold && UnrolledSize > Threshold) { DEBUG(dbgs() << " Too large to fully unroll with count: " << Count << " because size: " << UnrolledSize << ">" << Threshold << "\n"); Unrolling = Partial; } else { Unrolling = Full; } } else if (TripCount && Count < TripCount) { Unrolling = Partial; } else { Unrolling = Runtime; } // Reduce count based on the type of unrolling and the threshold values. unsigned OriginalCount = Count; bool AllowRuntime = UserRuntime ? CurrentRuntime : UP.Runtime; if (Unrolling == Partial) { bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial; if (!AllowPartial && !CountSetExplicitly) { DEBUG(dbgs() << " will not try to unroll partially because " << "-unroll-allow-partial not given\n"); return false; } if (PartialThreshold != NoThreshold && UnrolledSize > PartialThreshold) { // Reduce unroll count to be modulo of TripCount for partial unrolling. Count = (std::max(PartialThreshold, 3u)-2) / (LoopSize-2); while (Count != 0 && TripCount % Count != 0) Count--; } } else if (Unrolling == Runtime) { if (!AllowRuntime && !CountSetExplicitly) { DEBUG(dbgs() << " will not try to unroll loop with runtime trip count " << "-unroll-runtime not given\n"); return false; } // Reduce unroll count to be the largest power-of-two factor of // the original count which satisfies the threshold limit. while (Count != 0 && UnrolledSize > PartialThreshold) { Count >>= 1; UnrolledSize = (LoopSize-2) * Count + 2; } if (Count > UP.MaxCount) Count = UP.MaxCount; DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n"); } if (HasPragma) { if (PragmaCount != 0) // If loop has an unroll count pragma mark loop as unrolled to prevent // unrolling beyond that requested by the pragma. SetLoopAlreadyUnrolled(L); // Emit optimization remarks if we are unable to unroll the loop // as directed by a pragma. DebugLoc LoopLoc = L->getStartLoc(); Function *F = Header->getParent(); LLVMContext &Ctx = F->getContext(); if (PragmaFullUnroll && PragmaCount == 0) { if (TripCount && Count != TripCount) { emitOptimizationRemarkMissed( Ctx, DEBUG_TYPE, *F, LoopLoc, "Unable to fully unroll loop as directed by unroll(full) pragma " "because unrolled size is too large."); } else if (!TripCount) { emitOptimizationRemarkMissed( Ctx, DEBUG_TYPE, *F, LoopLoc, "Unable to fully unroll loop as directed by unroll(full) pragma " "because loop has a runtime trip count."); } } else if (PragmaCount > 0 && Count != OriginalCount) { emitOptimizationRemarkMissed( Ctx, DEBUG_TYPE, *F, LoopLoc, "Unable to unroll loop the number of times directed by " "unroll_count pragma because unrolled size is too large."); } } if (Unrolling != Full && Count < 2) { // Partial unrolling by 1 is a nop. For full unrolling, a factor // of 1 makes sense because loop control can be eliminated. return false; } // Unroll the loop. if (!UnrollLoop(L, Count, TripCount, AllowRuntime, TripMultiple, LI, this, &LPM, &AC)) return false; return true; }