//===-- LoopIdiomRecognize.cpp - Loop idiom recognition -------------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This pass implements an idiom recognizer that transforms simple loops into a // non-loop form. In cases that this kicks in, it can be a significant // performance win. // //===----------------------------------------------------------------------===// // // TODO List: // // Future loop memory idioms to recognize: // memcmp, memmove, strlen, etc. // Future floating point idioms to recognize in -ffast-math mode: // fpowi // Future integer operation idioms to recognize: // ctpop, ctlz, cttz // // Beware that isel's default lowering for ctpop is highly inefficient for // i64 and larger types when i64 is legal and the value has few bits set. It // would be good to enhance isel to emit a loop for ctpop in this case. // // We should enhance the memset/memcpy recognition to handle multiple stores in // the loop. This would handle things like: // void foo(_Complex float *P) // for (i) { __real__(*P) = 0; __imag__(*P) = 0; } // // We should enhance this to handle negative strides through memory. // Alternatively (and perhaps better) we could rely on an earlier pass to force // forward iteration through memory, which is generally better for cache // behavior. Negative strides *do* happen for memset/memcpy loops. // // This could recognize common matrix multiplies and dot product idioms and // replace them with calls to BLAS (if linked in??). // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/ADT/Statistic.h" #include "llvm/Analysis/AliasAnalysis.h" #include "llvm/Analysis/LoopPass.h" #include "llvm/Analysis/ScalarEvolutionExpander.h" #include "llvm/Analysis/ScalarEvolutionExpressions.h" #include "llvm/Analysis/TargetTransformInfo.h" #include "llvm/Analysis/ValueTracking.h" #include "llvm/IR/DataLayout.h" #include "llvm/IR/Dominators.h" #include "llvm/IR/IRBuilder.h" #include "llvm/IR/IntrinsicInst.h" #include "llvm/IR/Module.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" #include "llvm/Target/TargetLibraryInfo.h" #include "llvm/Transforms/Utils/Local.h" using namespace llvm; #define DEBUG_TYPE "loop-idiom" STATISTIC(NumMemSet, "Number of memset's formed from loop stores"); STATISTIC(NumMemCpy, "Number of memcpy's formed from loop load+stores"); namespace { class LoopIdiomRecognize; /// This class defines some utility functions for loop idiom recognization. class LIRUtil { public: /// Return true iff the block contains nothing but an uncondition branch /// (aka goto instruction). static bool isAlmostEmpty(BasicBlock *); static BranchInst *getBranch(BasicBlock *BB) { return dyn_cast<BranchInst>(BB->getTerminator()); } /// Derive the precondition block (i.e the block that guards the loop /// preheader) from the given preheader. static BasicBlock *getPrecondBb(BasicBlock *PreHead); }; /// This class is to recoginize idioms of population-count conducted in /// a noncountable loop. Currently it only recognizes this pattern: /// \code /// while(x) {cnt++; ...; x &= x - 1; ...} /// \endcode class NclPopcountRecognize { LoopIdiomRecognize &LIR; Loop *CurLoop; BasicBlock *PreCondBB; typedef IRBuilder<> IRBuilderTy; public: explicit NclPopcountRecognize(LoopIdiomRecognize &TheLIR); bool recognize(); private: /// Take a glimpse of the loop to see if we need to go ahead recoginizing /// the idiom. bool preliminaryScreen(); /// Check if the given conditional branch is based on the comparison /// between a variable and zero, and if the variable is non-zero, the /// control yields to the loop entry. If the branch matches the behavior, /// the variable involved in the comparion is returned. This function will /// be called to see if the precondition and postcondition of the loop /// are in desirable form. Value *matchCondition(BranchInst *Br, BasicBlock *NonZeroTarget) const; /// Return true iff the idiom is detected in the loop. and 1) \p CntInst /// is set to the instruction counting the population bit. 2) \p CntPhi /// is set to the corresponding phi node. 3) \p Var is set to the value /// whose population bits are being counted. bool detectIdiom (Instruction *&CntInst, PHINode *&CntPhi, Value *&Var) const; /// Insert ctpop intrinsic function and some obviously dead instructions. void transform(Instruction *CntInst, PHINode *CntPhi, Value *Var); /// Create llvm.ctpop.* intrinsic function. CallInst *createPopcntIntrinsic(IRBuilderTy &IRB, Value *Val, DebugLoc DL); }; class LoopIdiomRecognize : public LoopPass { Loop *CurLoop; const DataLayout *DL; DominatorTree *DT; ScalarEvolution *SE; TargetLibraryInfo *TLI; const TargetTransformInfo *TTI; public: static char ID; explicit LoopIdiomRecognize() : LoopPass(ID) { initializeLoopIdiomRecognizePass(*PassRegistry::getPassRegistry()); DL = nullptr; DT = nullptr; SE = nullptr; TLI = nullptr; TTI = nullptr; } bool runOnLoop(Loop *L, LPPassManager &LPM) override; bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount, SmallVectorImpl<BasicBlock*> &ExitBlocks); bool processLoopStore(StoreInst *SI, const SCEV *BECount); bool processLoopMemSet(MemSetInst *MSI, const SCEV *BECount); bool processLoopStridedStore(Value *DestPtr, unsigned StoreSize, unsigned StoreAlignment, Value *SplatValue, Instruction *TheStore, const SCEVAddRecExpr *Ev, const SCEV *BECount); bool processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize, const SCEVAddRecExpr *StoreEv, const SCEVAddRecExpr *LoadEv, const SCEV *BECount); /// This transformation requires natural loop information & requires that /// loop preheaders be inserted into the CFG. /// void getAnalysisUsage(AnalysisUsage &AU) const override { AU.addRequired<LoopInfo>(); AU.addPreserved<LoopInfo>(); AU.addRequiredID(LoopSimplifyID); AU.addPreservedID(LoopSimplifyID); AU.addRequiredID(LCSSAID); AU.addPreservedID(LCSSAID); AU.addRequired<AliasAnalysis>(); AU.addPreserved<AliasAnalysis>(); AU.addRequired<ScalarEvolution>(); AU.addPreserved<ScalarEvolution>(); AU.addPreserved<DominatorTreeWrapperPass>(); AU.addRequired<DominatorTreeWrapperPass>(); AU.addRequired<TargetLibraryInfo>(); AU.addRequired<TargetTransformInfo>(); } const DataLayout *getDataLayout() { if (DL) return DL; DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>(); DL = DLP ? &DLP->getDataLayout() : nullptr; return DL; } DominatorTree *getDominatorTree() { return DT ? DT : (DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree()); } ScalarEvolution *getScalarEvolution() { return SE ? SE : (SE = &getAnalysis<ScalarEvolution>()); } TargetLibraryInfo *getTargetLibraryInfo() { return TLI ? TLI : (TLI = &getAnalysis<TargetLibraryInfo>()); } const TargetTransformInfo *getTargetTransformInfo() { return TTI ? TTI : (TTI = &getAnalysis<TargetTransformInfo>()); } Loop *getLoop() const { return CurLoop; } private: bool runOnNoncountableLoop(); bool runOnCountableLoop(); }; } char LoopIdiomRecognize::ID = 0; INITIALIZE_PASS_BEGIN(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms", false, false) INITIALIZE_PASS_DEPENDENCY(LoopInfo) INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) INITIALIZE_PASS_DEPENDENCY(LoopSimplify) INITIALIZE_PASS_DEPENDENCY(LCSSA) INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo) INITIALIZE_AG_DEPENDENCY(AliasAnalysis) INITIALIZE_AG_DEPENDENCY(TargetTransformInfo) INITIALIZE_PASS_END(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms", false, false) Pass *llvm::createLoopIdiomPass() { return new LoopIdiomRecognize(); } /// deleteDeadInstruction - Delete this instruction. Before we do, go through /// and zero out all the operands of this instruction. If any of them become /// dead, delete them and the computation tree that feeds them. /// static void deleteDeadInstruction(Instruction *I, ScalarEvolution &SE, const TargetLibraryInfo *TLI) { SmallVector<Instruction*, 32> NowDeadInsts; NowDeadInsts.push_back(I); // Before we touch this instruction, remove it from SE! do { Instruction *DeadInst = NowDeadInsts.pop_back_val(); // This instruction is dead, zap it, in stages. Start by removing it from // SCEV. SE.forgetValue(DeadInst); for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) { Value *Op = DeadInst->getOperand(op); DeadInst->setOperand(op, nullptr); // If this operand just became dead, add it to the NowDeadInsts list. if (!Op->use_empty()) continue; if (Instruction *OpI = dyn_cast<Instruction>(Op)) if (isInstructionTriviallyDead(OpI, TLI)) NowDeadInsts.push_back(OpI); } DeadInst->eraseFromParent(); } while (!NowDeadInsts.empty()); } /// deleteIfDeadInstruction - If the specified value is a dead instruction, /// delete it and any recursively used instructions. static void deleteIfDeadInstruction(Value *V, ScalarEvolution &SE, const TargetLibraryInfo *TLI) { if (Instruction *I = dyn_cast<Instruction>(V)) if (isInstructionTriviallyDead(I, TLI)) deleteDeadInstruction(I, SE, TLI); } //===----------------------------------------------------------------------===// // // Implementation of LIRUtil // //===----------------------------------------------------------------------===// // This function will return true iff the given block contains nothing but goto. // A typical usage of this function is to check if the preheader function is // "almost" empty such that generated intrinsic functions can be moved across // the preheader and be placed at the end of the precondition block without // the concern of breaking data dependence. bool LIRUtil::isAlmostEmpty(BasicBlock *BB) { if (BranchInst *Br = getBranch(BB)) { return Br->isUnconditional() && BB->size() == 1; } return false; } BasicBlock *LIRUtil::getPrecondBb(BasicBlock *PreHead) { if (BasicBlock *BB = PreHead->getSinglePredecessor()) { BranchInst *Br = getBranch(BB); return Br && Br->isConditional() ? BB : nullptr; } return nullptr; } //===----------------------------------------------------------------------===// // // Implementation of NclPopcountRecognize // //===----------------------------------------------------------------------===// NclPopcountRecognize::NclPopcountRecognize(LoopIdiomRecognize &TheLIR): LIR(TheLIR), CurLoop(TheLIR.getLoop()), PreCondBB(nullptr) { } bool NclPopcountRecognize::preliminaryScreen() { const TargetTransformInfo *TTI = LIR.getTargetTransformInfo(); if (TTI->getPopcntSupport(32) != TargetTransformInfo::PSK_FastHardware) return false; // Counting population are usually conducted by few arithmetic instructions. // Such instructions can be easilly "absorbed" by vacant slots in a // non-compact loop. Therefore, recognizing popcount idiom only makes sense // in a compact loop. // Give up if the loop has multiple blocks or multiple backedges. if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1) return false; BasicBlock *LoopBody = *(CurLoop->block_begin()); if (LoopBody->size() >= 20) { // The loop is too big, bail out. return false; } // It should have a preheader containing nothing but a goto instruction. BasicBlock *PreHead = CurLoop->getLoopPreheader(); if (!PreHead || !LIRUtil::isAlmostEmpty(PreHead)) return false; // It should have a precondition block where the generated popcount instrinsic // function will be inserted. PreCondBB = LIRUtil::getPrecondBb(PreHead); if (!PreCondBB) return false; return true; } Value *NclPopcountRecognize::matchCondition(BranchInst *Br, BasicBlock *LoopEntry) const { if (!Br || !Br->isConditional()) return nullptr; ICmpInst *Cond = dyn_cast<ICmpInst>(Br->getCondition()); if (!Cond) return nullptr; ConstantInt *CmpZero = dyn_cast<ConstantInt>(Cond->getOperand(1)); if (!CmpZero || !CmpZero->isZero()) return nullptr; ICmpInst::Predicate Pred = Cond->getPredicate(); if ((Pred == ICmpInst::ICMP_NE && Br->getSuccessor(0) == LoopEntry) || (Pred == ICmpInst::ICMP_EQ && Br->getSuccessor(1) == LoopEntry)) return Cond->getOperand(0); return nullptr; } bool NclPopcountRecognize::detectIdiom(Instruction *&CntInst, PHINode *&CntPhi, Value *&Var) const { // Following code tries to detect this idiom: // // if (x0 != 0) // goto loop-exit // the precondition of the loop // cnt0 = init-val; // do { // x1 = phi (x0, x2); // cnt1 = phi(cnt0, cnt2); // // cnt2 = cnt1 + 1; // ... // x2 = x1 & (x1 - 1); // ... // } while(x != 0); // // loop-exit: // // step 1: Check to see if the look-back branch match this pattern: // "if (a!=0) goto loop-entry". BasicBlock *LoopEntry; Instruction *DefX2, *CountInst; Value *VarX1, *VarX0; PHINode *PhiX, *CountPhi; DefX2 = CountInst = nullptr; VarX1 = VarX0 = nullptr; PhiX = CountPhi = nullptr; LoopEntry = *(CurLoop->block_begin()); // step 1: Check if the loop-back branch is in desirable form. { if (Value *T = matchCondition (LIRUtil::getBranch(LoopEntry), LoopEntry)) DefX2 = dyn_cast<Instruction>(T); else return false; } // step 2: detect instructions corresponding to "x2 = x1 & (x1 - 1)" { if (!DefX2 || DefX2->getOpcode() != Instruction::And) return false; BinaryOperator *SubOneOp; if ((SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(0)))) VarX1 = DefX2->getOperand(1); else { VarX1 = DefX2->getOperand(0); SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(1)); } if (!SubOneOp) return false; Instruction *SubInst = cast<Instruction>(SubOneOp); ConstantInt *Dec = dyn_cast<ConstantInt>(SubInst->getOperand(1)); if (!Dec || !((SubInst->getOpcode() == Instruction::Sub && Dec->isOne()) || (SubInst->getOpcode() == Instruction::Add && Dec->isAllOnesValue()))) { return false; } } // step 3: Check the recurrence of variable X { PhiX = dyn_cast<PHINode>(VarX1); if (!PhiX || (PhiX->getOperand(0) != DefX2 && PhiX->getOperand(1) != DefX2)) { return false; } } // step 4: Find the instruction which count the population: cnt2 = cnt1 + 1 { CountInst = nullptr; for (BasicBlock::iterator Iter = LoopEntry->getFirstNonPHI(), IterE = LoopEntry->end(); Iter != IterE; Iter++) { Instruction *Inst = Iter; if (Inst->getOpcode() != Instruction::Add) continue; ConstantInt *Inc = dyn_cast<ConstantInt>(Inst->getOperand(1)); if (!Inc || !Inc->isOne()) continue; PHINode *Phi = dyn_cast<PHINode>(Inst->getOperand(0)); if (!Phi || Phi->getParent() != LoopEntry) continue; // Check if the result of the instruction is live of the loop. bool LiveOutLoop = false; for (User *U : Inst->users()) { if ((cast<Instruction>(U))->getParent() != LoopEntry) { LiveOutLoop = true; break; } } if (LiveOutLoop) { CountInst = Inst; CountPhi = Phi; break; } } if (!CountInst) return false; } // step 5: check if the precondition is in this form: // "if (x != 0) goto loop-head ; else goto somewhere-we-don't-care;" { BranchInst *PreCondBr = LIRUtil::getBranch(PreCondBB); Value *T = matchCondition (PreCondBr, CurLoop->getLoopPreheader()); if (T != PhiX->getOperand(0) && T != PhiX->getOperand(1)) return false; CntInst = CountInst; CntPhi = CountPhi; Var = T; } return true; } void NclPopcountRecognize::transform(Instruction *CntInst, PHINode *CntPhi, Value *Var) { ScalarEvolution *SE = LIR.getScalarEvolution(); TargetLibraryInfo *TLI = LIR.getTargetLibraryInfo(); BasicBlock *PreHead = CurLoop->getLoopPreheader(); BranchInst *PreCondBr = LIRUtil::getBranch(PreCondBB); const DebugLoc DL = CntInst->getDebugLoc(); // Assuming before transformation, the loop is following: // if (x) // the precondition // do { cnt++; x &= x - 1; } while(x); // Step 1: Insert the ctpop instruction at the end of the precondition block IRBuilderTy Builder(PreCondBr); Value *PopCnt, *PopCntZext, *NewCount, *TripCnt; { PopCnt = createPopcntIntrinsic(Builder, Var, DL); NewCount = PopCntZext = Builder.CreateZExtOrTrunc(PopCnt, cast<IntegerType>(CntPhi->getType())); if (NewCount != PopCnt) (cast<Instruction>(NewCount))->setDebugLoc(DL); // TripCnt is exactly the number of iterations the loop has TripCnt = NewCount; // If the population counter's initial value is not zero, insert Add Inst. Value *CntInitVal = CntPhi->getIncomingValueForBlock(PreHead); ConstantInt *InitConst = dyn_cast<ConstantInt>(CntInitVal); if (!InitConst || !InitConst->isZero()) { NewCount = Builder.CreateAdd(NewCount, CntInitVal); (cast<Instruction>(NewCount))->setDebugLoc(DL); } } // Step 2: Replace the precondition from "if(x == 0) goto loop-exit" to // "if(NewCount == 0) loop-exit". Withtout this change, the intrinsic // function would be partial dead code, and downstream passes will drag // it back from the precondition block to the preheader. { ICmpInst *PreCond = cast<ICmpInst>(PreCondBr->getCondition()); Value *Opnd0 = PopCntZext; Value *Opnd1 = ConstantInt::get(PopCntZext->getType(), 0); if (PreCond->getOperand(0) != Var) std::swap(Opnd0, Opnd1); ICmpInst *NewPreCond = cast<ICmpInst>(Builder.CreateICmp(PreCond->getPredicate(), Opnd0, Opnd1)); PreCond->replaceAllUsesWith(NewPreCond); deleteDeadInstruction(PreCond, *SE, TLI); } // Step 3: Note that the population count is exactly the trip count of the // loop in question, which enble us to to convert the loop from noncountable // loop into a countable one. The benefit is twofold: // // - If the loop only counts population, the entire loop become dead after // the transformation. It is lots easier to prove a countable loop dead // than to prove a noncountable one. (In some C dialects, a infite loop // isn't dead even if it computes nothing useful. In general, DCE needs // to prove a noncountable loop finite before safely delete it.) // // - If the loop also performs something else, it remains alive. // Since it is transformed to countable form, it can be aggressively // optimized by some optimizations which are in general not applicable // to a noncountable loop. // // After this step, this loop (conceptually) would look like following: // newcnt = __builtin_ctpop(x); // t = newcnt; // if (x) // do { cnt++; x &= x-1; t--) } while (t > 0); BasicBlock *Body = *(CurLoop->block_begin()); { BranchInst *LbBr = LIRUtil::getBranch(Body); ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition()); Type *Ty = TripCnt->getType(); PHINode *TcPhi = PHINode::Create(Ty, 2, "tcphi", Body->begin()); Builder.SetInsertPoint(LbCond); Value *Opnd1 = cast<Value>(TcPhi); Value *Opnd2 = cast<Value>(ConstantInt::get(Ty, 1)); Instruction *TcDec = cast<Instruction>(Builder.CreateSub(Opnd1, Opnd2, "tcdec", false, true)); TcPhi->addIncoming(TripCnt, PreHead); TcPhi->addIncoming(TcDec, Body); CmpInst::Predicate Pred = (LbBr->getSuccessor(0) == Body) ? CmpInst::ICMP_UGT : CmpInst::ICMP_SLE; LbCond->setPredicate(Pred); LbCond->setOperand(0, TcDec); LbCond->setOperand(1, cast<Value>(ConstantInt::get(Ty, 0))); } // Step 4: All the references to the original population counter outside // the loop are replaced with the NewCount -- the value returned from // __builtin_ctpop(). { SmallVector<Value *, 4> CntUses; for (User *U : CntInst->users()) if (cast<Instruction>(U)->getParent() != Body) CntUses.push_back(U); for (unsigned Idx = 0; Idx < CntUses.size(); Idx++) { (cast<Instruction>(CntUses[Idx]))->replaceUsesOfWith(CntInst, NewCount); } } // step 5: Forget the "non-computable" trip-count SCEV associated with the // loop. The loop would otherwise not be deleted even if it becomes empty. SE->forgetLoop(CurLoop); } CallInst *NclPopcountRecognize::createPopcntIntrinsic(IRBuilderTy &IRBuilder, Value *Val, DebugLoc DL) { Value *Ops[] = { Val }; Type *Tys[] = { Val->getType() }; Module *M = (*(CurLoop->block_begin()))->getParent()->getParent(); Value *Func = Intrinsic::getDeclaration(M, Intrinsic::ctpop, Tys); CallInst *CI = IRBuilder.CreateCall(Func, Ops); CI->setDebugLoc(DL); return CI; } /// recognize - detect population count idiom in a non-countable loop. If /// detected, transform the relevant code to popcount intrinsic function /// call, and return true; otherwise, return false. bool NclPopcountRecognize::recognize() { if (!LIR.getTargetTransformInfo()) return false; LIR.getScalarEvolution(); if (!preliminaryScreen()) return false; Instruction *CntInst; PHINode *CntPhi; Value *Val; if (!detectIdiom(CntInst, CntPhi, Val)) return false; transform(CntInst, CntPhi, Val); return true; } //===----------------------------------------------------------------------===// // // Implementation of LoopIdiomRecognize // //===----------------------------------------------------------------------===// bool LoopIdiomRecognize::runOnCountableLoop() { const SCEV *BECount = SE->getBackedgeTakenCount(CurLoop); if (isa<SCEVCouldNotCompute>(BECount)) return false; // If this loop executes exactly one time, then it should be peeled, not // optimized by this pass. if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount)) if (BECst->getValue()->getValue() == 0) return false; // We require target data for now. if (!getDataLayout()) return false; // set DT (void)getDominatorTree(); LoopInfo &LI = getAnalysis<LoopInfo>(); TLI = &getAnalysis<TargetLibraryInfo>(); // set TLI (void)getTargetLibraryInfo(); SmallVector<BasicBlock*, 8> ExitBlocks; CurLoop->getUniqueExitBlocks(ExitBlocks); DEBUG(dbgs() << "loop-idiom Scanning: F[" << CurLoop->getHeader()->getParent()->getName() << "] Loop %" << CurLoop->getHeader()->getName() << "\n"); bool MadeChange = false; // Scan all the blocks in the loop that are not in subloops. for (Loop::block_iterator BI = CurLoop->block_begin(), E = CurLoop->block_end(); BI != E; ++BI) { // Ignore blocks in subloops. if (LI.getLoopFor(*BI) != CurLoop) continue; MadeChange |= runOnLoopBlock(*BI, BECount, ExitBlocks); } return MadeChange; } bool LoopIdiomRecognize::runOnNoncountableLoop() { NclPopcountRecognize Popcount(*this); if (Popcount.recognize()) return true; return false; } bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) { if (skipOptnoneFunction(L)) return false; CurLoop = L; // If the loop could not be converted to canonical form, it must have an // indirectbr in it, just give up. if (!L->getLoopPreheader()) return false; // Disable loop idiom recognition if the function's name is a common idiom. StringRef Name = L->getHeader()->getParent()->getName(); if (Name == "memset" || Name == "memcpy") return false; SE = &getAnalysis<ScalarEvolution>(); if (SE->hasLoopInvariantBackedgeTakenCount(L)) return runOnCountableLoop(); return runOnNoncountableLoop(); } /// runOnLoopBlock - Process the specified block, which lives in a counted loop /// with the specified backedge count. This block is known to be in the current /// loop and not in any subloops. bool LoopIdiomRecognize::runOnLoopBlock(BasicBlock *BB, const SCEV *BECount, SmallVectorImpl<BasicBlock*> &ExitBlocks) { // We can only promote stores in this block if they are unconditionally // executed in the loop. For a block to be unconditionally executed, it has // to dominate all the exit blocks of the loop. Verify this now. for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) if (!DT->dominates(BB, ExitBlocks[i])) return false; bool MadeChange = false; for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { Instruction *Inst = I++; // Look for store instructions, which may be optimized to memset/memcpy. if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { WeakVH InstPtr(I); if (!processLoopStore(SI, BECount)) continue; MadeChange = true; // If processing the store invalidated our iterator, start over from the // top of the block. if (!InstPtr) I = BB->begin(); continue; } // Look for memset instructions, which may be optimized to a larger memset. if (MemSetInst *MSI = dyn_cast<MemSetInst>(Inst)) { WeakVH InstPtr(I); if (!processLoopMemSet(MSI, BECount)) continue; MadeChange = true; // If processing the memset invalidated our iterator, start over from the // top of the block. if (!InstPtr) I = BB->begin(); continue; } } return MadeChange; } /// processLoopStore - See if this store can be promoted to a memset or memcpy. bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) { if (!SI->isSimple()) return false; Value *StoredVal = SI->getValueOperand(); Value *StorePtr = SI->getPointerOperand(); // Reject stores that are so large that they overflow an unsigned. uint64_t SizeInBits = DL->getTypeSizeInBits(StoredVal->getType()); if ((SizeInBits & 7) || (SizeInBits >> 32) != 0) return false; // See if the pointer expression is an AddRec like {base,+,1} on the current // loop, which indicates a strided store. If we have something else, it's a // random store we can't handle. const SCEVAddRecExpr *StoreEv = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr)); if (!StoreEv || StoreEv->getLoop() != CurLoop || !StoreEv->isAffine()) return false; // Check to see if the stride matches the size of the store. If so, then we // know that every byte is touched in the loop. unsigned StoreSize = (unsigned)SizeInBits >> 3; const SCEVConstant *Stride = dyn_cast<SCEVConstant>(StoreEv->getOperand(1)); if (!Stride || StoreSize != Stride->getValue()->getValue()) { // TODO: Could also handle negative stride here someday, that will require // the validity check in mayLoopAccessLocation to be updated though. // Enable this to print exact negative strides. if (0 && Stride && StoreSize == -Stride->getValue()->getValue()) { dbgs() << "NEGATIVE STRIDE: " << *SI << "\n"; dbgs() << "BB: " << *SI->getParent(); } return false; } // See if we can optimize just this store in isolation. if (processLoopStridedStore(StorePtr, StoreSize, SI->getAlignment(), StoredVal, SI, StoreEv, BECount)) return true; // If the stored value is a strided load in the same loop with the same stride // this this may be transformable into a memcpy. This kicks in for stuff like // for (i) A[i] = B[i]; if (LoadInst *LI = dyn_cast<LoadInst>(StoredVal)) { const SCEVAddRecExpr *LoadEv = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(LI->getOperand(0))); if (LoadEv && LoadEv->getLoop() == CurLoop && LoadEv->isAffine() && StoreEv->getOperand(1) == LoadEv->getOperand(1) && LI->isSimple()) if (processLoopStoreOfLoopLoad(SI, StoreSize, StoreEv, LoadEv, BECount)) return true; } //errs() << "UNHANDLED strided store: " << *StoreEv << " - " << *SI << "\n"; return false; } /// processLoopMemSet - See if this memset can be promoted to a large memset. bool LoopIdiomRecognize:: processLoopMemSet(MemSetInst *MSI, const SCEV *BECount) { // We can only handle non-volatile memsets with a constant size. if (MSI->isVolatile() || !isa<ConstantInt>(MSI->getLength())) return false; // If we're not allowed to hack on memset, we fail. if (!TLI->has(LibFunc::memset)) return false; Value *Pointer = MSI->getDest(); // See if the pointer expression is an AddRec like {base,+,1} on the current // loop, which indicates a strided store. If we have something else, it's a // random store we can't handle. const SCEVAddRecExpr *Ev = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Pointer)); if (!Ev || Ev->getLoop() != CurLoop || !Ev->isAffine()) return false; // Reject memsets that are so large that they overflow an unsigned. uint64_t SizeInBytes = cast<ConstantInt>(MSI->getLength())->getZExtValue(); if ((SizeInBytes >> 32) != 0) return false; // Check to see if the stride matches the size of the memset. If so, then we // know that every byte is touched in the loop. const SCEVConstant *Stride = dyn_cast<SCEVConstant>(Ev->getOperand(1)); // TODO: Could also handle negative stride here someday, that will require the // validity check in mayLoopAccessLocation to be updated though. if (!Stride || MSI->getLength() != Stride->getValue()) return false; return processLoopStridedStore(Pointer, (unsigned)SizeInBytes, MSI->getAlignment(), MSI->getValue(), MSI, Ev, BECount); } /// mayLoopAccessLocation - Return true if the specified loop might access the /// specified pointer location, which is a loop-strided access. The 'Access' /// argument specifies what the verboten forms of access are (read or write). static bool mayLoopAccessLocation(Value *Ptr,AliasAnalysis::ModRefResult Access, Loop *L, const SCEV *BECount, unsigned StoreSize, AliasAnalysis &AA, Instruction *IgnoredStore) { // Get the location that may be stored across the loop. Since the access is // strided positively through memory, we say that the modified location starts // at the pointer and has infinite size. uint64_t AccessSize = AliasAnalysis::UnknownSize; // If the loop iterates a fixed number of times, we can refine the access size // to be exactly the size of the memset, which is (BECount+1)*StoreSize if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount)) AccessSize = (BECst->getValue()->getZExtValue()+1)*StoreSize; // TODO: For this to be really effective, we have to dive into the pointer // operand in the store. Store to &A[i] of 100 will always return may alias // with store of &A[100], we need to StoreLoc to be "A" with size of 100, // which will then no-alias a store to &A[100]. AliasAnalysis::Location StoreLoc(Ptr, AccessSize); for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E; ++BI) for (BasicBlock::iterator I = (*BI)->begin(), E = (*BI)->end(); I != E; ++I) if (&*I != IgnoredStore && (AA.getModRefInfo(I, StoreLoc) & Access)) return true; return false; } /// getMemSetPatternValue - If a strided store of the specified value is safe to /// turn into a memset_pattern16, return a ConstantArray of 16 bytes that should /// be passed in. Otherwise, return null. /// /// Note that we don't ever attempt to use memset_pattern8 or 4, because these /// just replicate their input array and then pass on to memset_pattern16. static Constant *getMemSetPatternValue(Value *V, const DataLayout &DL) { // If the value isn't a constant, we can't promote it to being in a constant // array. We could theoretically do a store to an alloca or something, but // that doesn't seem worthwhile. Constant *C = dyn_cast<Constant>(V); if (!C) return nullptr; // Only handle simple values that are a power of two bytes in size. uint64_t Size = DL.getTypeSizeInBits(V->getType()); if (Size == 0 || (Size & 7) || (Size & (Size-1))) return nullptr; // Don't care enough about darwin/ppc to implement this. if (DL.isBigEndian()) return nullptr; // Convert to size in bytes. Size /= 8; // TODO: If CI is larger than 16-bytes, we can try slicing it in half to see // if the top and bottom are the same (e.g. for vectors and large integers). if (Size > 16) return nullptr; // If the constant is exactly 16 bytes, just use it. if (Size == 16) return C; // Otherwise, we'll use an array of the constants. unsigned ArraySize = 16/Size; ArrayType *AT = ArrayType::get(V->getType(), ArraySize); return ConstantArray::get(AT, std::vector<Constant*>(ArraySize, C)); } /// processLoopStridedStore - We see a strided store of some value. If we can /// transform this into a memset or memset_pattern in the loop preheader, do so. bool LoopIdiomRecognize:: processLoopStridedStore(Value *DestPtr, unsigned StoreSize, unsigned StoreAlignment, Value *StoredVal, Instruction *TheStore, const SCEVAddRecExpr *Ev, const SCEV *BECount) { // If the stored value is a byte-wise value (like i32 -1), then it may be // turned into a memset of i8 -1, assuming that all the consecutive bytes // are stored. A store of i32 0x01020304 can never be turned into a memset, // but it can be turned into memset_pattern if the target supports it. Value *SplatValue = isBytewiseValue(StoredVal); Constant *PatternValue = nullptr; unsigned DestAS = DestPtr->getType()->getPointerAddressSpace(); // If we're allowed to form a memset, and the stored value would be acceptable // for memset, use it. if (SplatValue && TLI->has(LibFunc::memset) && // Verify that the stored value is loop invariant. If not, we can't // promote the memset. CurLoop->isLoopInvariant(SplatValue)) { // Keep and use SplatValue. PatternValue = nullptr; } else if (DestAS == 0 && TLI->has(LibFunc::memset_pattern16) && (PatternValue = getMemSetPatternValue(StoredVal, *DL))) { // Don't create memset_pattern16s with address spaces. // It looks like we can use PatternValue! SplatValue = nullptr; } else { // Otherwise, this isn't an idiom we can transform. For example, we can't // do anything with a 3-byte store. return false; } // The trip count of the loop and the base pointer of the addrec SCEV is // guaranteed to be loop invariant, which means that it should dominate the // header. This allows us to insert code for it in the preheader. BasicBlock *Preheader = CurLoop->getLoopPreheader(); IRBuilder<> Builder(Preheader->getTerminator()); SCEVExpander Expander(*SE, "loop-idiom"); Type *DestInt8PtrTy = Builder.getInt8PtrTy(DestAS); // Okay, we have a strided store "p[i]" of a splattable value. We can turn // this into a memset in the loop preheader now if we want. However, this // would be unsafe to do if there is anything else in the loop that may read // or write to the aliased location. Check for any overlap by generating the // base pointer and checking the region. Value *BasePtr = Expander.expandCodeFor(Ev->getStart(), DestInt8PtrTy, Preheader->getTerminator()); if (mayLoopAccessLocation(BasePtr, AliasAnalysis::ModRef, CurLoop, BECount, StoreSize, getAnalysis<AliasAnalysis>(), TheStore)) { Expander.clear(); // If we generated new code for the base pointer, clean up. deleteIfDeadInstruction(BasePtr, *SE, TLI); return false; } // Okay, everything looks good, insert the memset. // The # stored bytes is (BECount+1)*Size. Expand the trip count out to // pointer size if it isn't already. Type *IntPtr = Builder.getIntPtrTy(DL, DestAS); BECount = SE->getTruncateOrZeroExtend(BECount, IntPtr); const SCEV *NumBytesS = SE->getAddExpr(BECount, SE->getConstant(IntPtr, 1), SCEV::FlagNUW); if (StoreSize != 1) { NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize), SCEV::FlagNUW); } Value *NumBytes = Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator()); CallInst *NewCall; if (SplatValue) { NewCall = Builder.CreateMemSet(BasePtr, SplatValue, NumBytes, StoreAlignment); } else { // Everything is emitted in default address space Type *Int8PtrTy = DestInt8PtrTy; Module *M = TheStore->getParent()->getParent()->getParent(); Value *MSP = M->getOrInsertFunction("memset_pattern16", Builder.getVoidTy(), Int8PtrTy, Int8PtrTy, IntPtr, (void*)nullptr); // Otherwise we should form a memset_pattern16. PatternValue is known to be // an constant array of 16-bytes. Plop the value into a mergable global. GlobalVariable *GV = new GlobalVariable(*M, PatternValue->getType(), true, GlobalValue::InternalLinkage, PatternValue, ".memset_pattern"); GV->setUnnamedAddr(true); // Ok to merge these. GV->setAlignment(16); Value *PatternPtr = ConstantExpr::getBitCast(GV, Int8PtrTy); NewCall = Builder.CreateCall3(MSP, BasePtr, PatternPtr, NumBytes); } DEBUG(dbgs() << " Formed memset: " << *NewCall << "\n" << " from store to: " << *Ev << " at: " << *TheStore << "\n"); NewCall->setDebugLoc(TheStore->getDebugLoc()); // Okay, the memset has been formed. Zap the original store and anything that // feeds into it. deleteDeadInstruction(TheStore, *SE, TLI); ++NumMemSet; return true; } /// processLoopStoreOfLoopLoad - We see a strided store whose value is a /// same-strided load. bool LoopIdiomRecognize:: processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize, const SCEVAddRecExpr *StoreEv, const SCEVAddRecExpr *LoadEv, const SCEV *BECount) { // If we're not allowed to form memcpy, we fail. if (!TLI->has(LibFunc::memcpy)) return false; LoadInst *LI = cast<LoadInst>(SI->getValueOperand()); // The trip count of the loop and the base pointer of the addrec SCEV is // guaranteed to be loop invariant, which means that it should dominate the // header. This allows us to insert code for it in the preheader. BasicBlock *Preheader = CurLoop->getLoopPreheader(); IRBuilder<> Builder(Preheader->getTerminator()); SCEVExpander Expander(*SE, "loop-idiom"); // Okay, we have a strided store "p[i]" of a loaded value. We can turn // this into a memcpy in the loop preheader now if we want. However, this // would be unsafe to do if there is anything else in the loop that may read // or write the memory region we're storing to. This includes the load that // feeds the stores. Check for an alias by generating the base address and // checking everything. Value *StoreBasePtr = Expander.expandCodeFor(StoreEv->getStart(), Builder.getInt8PtrTy(SI->getPointerAddressSpace()), Preheader->getTerminator()); if (mayLoopAccessLocation(StoreBasePtr, AliasAnalysis::ModRef, CurLoop, BECount, StoreSize, getAnalysis<AliasAnalysis>(), SI)) { Expander.clear(); // If we generated new code for the base pointer, clean up. deleteIfDeadInstruction(StoreBasePtr, *SE, TLI); return false; } // For a memcpy, we have to make sure that the input array is not being // mutated by the loop. Value *LoadBasePtr = Expander.expandCodeFor(LoadEv->getStart(), Builder.getInt8PtrTy(LI->getPointerAddressSpace()), Preheader->getTerminator()); if (mayLoopAccessLocation(LoadBasePtr, AliasAnalysis::Mod, CurLoop, BECount, StoreSize, getAnalysis<AliasAnalysis>(), SI)) { Expander.clear(); // If we generated new code for the base pointer, clean up. deleteIfDeadInstruction(LoadBasePtr, *SE, TLI); deleteIfDeadInstruction(StoreBasePtr, *SE, TLI); return false; } // Okay, everything is safe, we can transform this! // The # stored bytes is (BECount+1)*Size. Expand the trip count out to // pointer size if it isn't already. Type *IntPtrTy = Builder.getIntPtrTy(DL, SI->getPointerAddressSpace()); BECount = SE->getTruncateOrZeroExtend(BECount, IntPtrTy); const SCEV *NumBytesS = SE->getAddExpr(BECount, SE->getConstant(IntPtrTy, 1), SCEV::FlagNUW); if (StoreSize != 1) NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtrTy, StoreSize), SCEV::FlagNUW); Value *NumBytes = Expander.expandCodeFor(NumBytesS, IntPtrTy, Preheader->getTerminator()); CallInst *NewCall = Builder.CreateMemCpy(StoreBasePtr, LoadBasePtr, NumBytes, std::min(SI->getAlignment(), LI->getAlignment())); NewCall->setDebugLoc(SI->getDebugLoc()); DEBUG(dbgs() << " Formed memcpy: " << *NewCall << "\n" << " from load ptr=" << *LoadEv << " at: " << *LI << "\n" << " from store ptr=" << *StoreEv << " at: " << *SI << "\n"); // Okay, the memset has been formed. Zap the original store and anything that // feeds into it. deleteDeadInstruction(SI, *SE, TLI); ++NumMemCpy; return true; }