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Diffstat (limited to 'contrib/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp')
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diff --git a/contrib/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/contrib/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp new file mode 100644 index 0000000..2d577de --- /dev/null +++ b/contrib/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp @@ -0,0 +1,1054 @@ +//===-- 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; } +// +// 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/BasicAliasAnalysis.h" +#include "llvm/Analysis/GlobalsModRef.h" +#include "llvm/Analysis/LoopPass.h" +#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h" +#include "llvm/Analysis/ScalarEvolutionExpander.h" +#include "llvm/Analysis/ScalarEvolutionExpressions.h" +#include "llvm/Analysis/TargetLibraryInfo.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/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 : public LoopPass { + Loop *CurLoop; + AliasAnalysis *AA; + DominatorTree *DT; + LoopInfo *LI; + ScalarEvolution *SE; + TargetLibraryInfo *TLI; + const TargetTransformInfo *TTI; + const DataLayout *DL; + +public: + static char ID; + explicit LoopIdiomRecognize() : LoopPass(ID) { + initializeLoopIdiomRecognizePass(*PassRegistry::getPassRegistry()); + } + + 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<LoopInfoWrapperPass>(); + AU.addPreserved<LoopInfoWrapperPass>(); + AU.addRequiredID(LoopSimplifyID); + AU.addPreservedID(LoopSimplifyID); + AU.addRequiredID(LCSSAID); + AU.addPreservedID(LCSSAID); + AU.addRequired<AAResultsWrapperPass>(); + AU.addPreserved<AAResultsWrapperPass>(); + AU.addRequired<ScalarEvolutionWrapperPass>(); + AU.addPreserved<ScalarEvolutionWrapperPass>(); + AU.addPreserved<SCEVAAWrapperPass>(); + AU.addRequired<DominatorTreeWrapperPass>(); + AU.addPreserved<DominatorTreeWrapperPass>(); + AU.addRequired<TargetLibraryInfoWrapperPass>(); + AU.addRequired<TargetTransformInfoWrapperPass>(); + AU.addPreserved<BasicAAWrapperPass>(); + AU.addPreserved<GlobalsAAWrapperPass>(); + } + +private: + typedef SmallVector<StoreInst *, 8> StoreList; + StoreList StoreRefs; + + /// \name Countable Loop Idiom Handling + /// @{ + + bool runOnCountableLoop(); + bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount, + SmallVectorImpl<BasicBlock *> &ExitBlocks); + + void collectStores(BasicBlock *BB); + bool isLegalStore(StoreInst *SI); + 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 NegStride); + bool processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize, + const SCEVAddRecExpr *StoreEv, + const SCEV *BECount, bool NegStride); + + /// @} + /// \name Noncountable Loop Idiom Handling + /// @{ + + bool runOnNoncountableLoop(); + + bool recognizePopcount(); + void transformLoopToPopcount(BasicBlock *PreCondBB, Instruction *CntInst, + PHINode *CntPhi, Value *Var); + + /// @} +}; + +} // End anonymous namespace. + +char LoopIdiomRecognize::ID = 0; +INITIALIZE_PASS_BEGIN(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms", + false, false) +INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) +INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) +INITIALIZE_PASS_DEPENDENCY(LoopSimplify) +INITIALIZE_PASS_DEPENDENCY(LCSSA) +INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass) +INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) +INITIALIZE_PASS_DEPENDENCY(BasicAAWrapperPass) +INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) +INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass) +INITIALIZE_PASS_DEPENDENCY(SCEVAAWrapperPass) +INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) +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, + const TargetLibraryInfo *TLI) { + SmallVector<Value *, 16> Operands(I->value_op_begin(), I->value_op_end()); + I->replaceAllUsesWith(UndefValue::get(I->getType())); + I->eraseFromParent(); + for (Value *Op : Operands) + RecursivelyDeleteTriviallyDeadInstructions(Op, TLI); +} + +//===----------------------------------------------------------------------===// +// +// Implementation of LoopIdiomRecognize +// +//===----------------------------------------------------------------------===// + +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; + + AA = &getAnalysis<AAResultsWrapperPass>().getAAResults(); + DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); + LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); + SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE(); + TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(); + TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI( + *CurLoop->getHeader()->getParent()); + DL = &CurLoop->getHeader()->getModule()->getDataLayout(); + + if (SE->hasLoopInvariantBackedgeTakenCount(L)) + return runOnCountableLoop(); + + return runOnNoncountableLoop(); +} + +bool LoopIdiomRecognize::runOnCountableLoop() { + const SCEV *BECount = SE->getBackedgeTakenCount(CurLoop); + assert(!isa<SCEVCouldNotCompute>(BECount) && + "runOnCountableLoop() called on a loop without a predictable" + "backedge-taken count"); + + // 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->getAPInt() == 0) + return false; + + 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 (auto *BB : CurLoop->getBlocks()) { + // Ignore blocks in subloops. + if (LI->getLoopFor(BB) != CurLoop) + continue; + + MadeChange |= runOnLoopBlock(BB, BECount, ExitBlocks); + } + return MadeChange; +} + +static unsigned getStoreSizeInBytes(StoreInst *SI, const DataLayout *DL) { + uint64_t SizeInBits = DL->getTypeSizeInBits(SI->getValueOperand()->getType()); + assert(((SizeInBits & 7) || (SizeInBits >> 32) == 0) && + "Don't overflow unsigned."); + return (unsigned)SizeInBits >> 3; +} + +static unsigned getStoreStride(const SCEVAddRecExpr *StoreEv) { + const SCEVConstant *ConstStride = cast<SCEVConstant>(StoreEv->getOperand(1)); + return ConstStride->getAPInt().getZExtValue(); +} + +/// 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)); +} + +bool LoopIdiomRecognize::isLegalStore(StoreInst *SI) { + // Don't touch volatile stores. + 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 we have a constant stride. + if (!isa<SCEVConstant>(StoreEv->getOperand(1))) + return false; + + return true; +} + +void LoopIdiomRecognize::collectStores(BasicBlock *BB) { + StoreRefs.clear(); + for (Instruction &I : *BB) { + StoreInst *SI = dyn_cast<StoreInst>(&I); + if (!SI) + continue; + + // Make sure this is a strided store with a constant stride. + if (!isLegalStore(SI)) + continue; + + // Save the store locations. + StoreRefs.push_back(SI); + } +} + +/// 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; + // Look for store instructions, which may be optimized to memset/memcpy. + collectStores(BB); + for (auto &SI : StoreRefs) + MadeChange |= processLoopStore(SI, BECount); + + for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) { + Instruction *Inst = &*I++; + // 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) { + assert(SI->isSimple() && "Expected only non-volatile stores."); + + Value *StoredVal = SI->getValueOperand(); + Value *StorePtr = SI->getPointerOperand(); + + // 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. + const SCEVAddRecExpr *StoreEv = cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr)); + unsigned Stride = getStoreStride(StoreEv); + unsigned StoreSize = getStoreSizeInBytes(SI, DL); + if (StoreSize != Stride && StoreSize != -Stride) + return false; + + bool NegStride = StoreSize == -Stride; + + // See if we can optimize just this store in isolation. + if (processLoopStridedStore(StorePtr, StoreSize, SI->getAlignment(), + StoredVal, SI, StoreEv, BECount, NegStride)) + return true; + + // Optimize the store into a memcpy, if it feeds an similarly strided load. + return processLoopStoreOfLoopLoad(SI, StoreSize, StoreEv, BECount, NegStride); +} + +/// 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, /*NegStride=*/false); +} + +/// 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, ModRefInfo 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 = MemoryLocation::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]. + MemoryLocation 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; +} + +// If we have a negative stride, Start refers to the end of the memory location +// we're trying to memset. Therefore, we need to recompute the base pointer, +// which is just Start - BECount*Size. +static const SCEV *getStartForNegStride(const SCEV *Start, const SCEV *BECount, + Type *IntPtr, unsigned StoreSize, + ScalarEvolution *SE) { + const SCEV *Index = SE->getTruncateOrZeroExtend(BECount, IntPtr); + if (StoreSize != 1) + Index = SE->getMulExpr(Index, SE->getConstant(IntPtr, StoreSize), + SCEV::FlagNUW); + return SE->getMinusSCEV(Start, Index); +} + +/// 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, bool NegStride) { + + // 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, *DL, "loop-idiom"); + + Type *DestInt8PtrTy = Builder.getInt8PtrTy(DestAS); + Type *IntPtr = Builder.getIntPtrTy(*DL, DestAS); + + const SCEV *Start = Ev->getStart(); + // Handle negative strided loops. + if (NegStride) + Start = getStartForNegStride(Start, BECount, IntPtr, StoreSize, SE); + + // 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(Start, DestInt8PtrTy, Preheader->getTerminator()); + if (mayLoopAccessLocation(BasePtr, MRI_ModRef, CurLoop, BECount, StoreSize, + *AA, TheStore)) { + Expander.clear(); + // If we generated new code for the base pointer, clean up. + RecursivelyDeleteTriviallyDeadInstructions(BasePtr, 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. + BECount = SE->getTruncateOrZeroExtend(BECount, IntPtr); + + const SCEV *NumBytesS = + SE->getAddExpr(BECount, SE->getOne(IntPtr), 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->getModule(); + 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::PrivateLinkage, + PatternValue, ".memset_pattern"); + GV->setUnnamedAddr(true); // Ok to merge these. + GV->setAlignment(16); + Value *PatternPtr = ConstantExpr::getBitCast(GV, Int8PtrTy); + NewCall = Builder.CreateCall(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, TLI); + ++NumMemSet; + return true; +} + +/// If the stored value is a strided load in the same loop with the same stride +/// this may be transformable into a memcpy. This kicks in for stuff like +/// for (i) A[i] = B[i]; +bool LoopIdiomRecognize::processLoopStoreOfLoopLoad( + StoreInst *SI, unsigned StoreSize, const SCEVAddRecExpr *StoreEv, + const SCEV *BECount, bool NegStride) { + // If we're not allowed to form memcpy, we fail. + if (!TLI->has(LibFunc::memcpy)) + return false; + + // The store must be feeding a non-volatile load. + LoadInst *LI = dyn_cast<LoadInst>(SI->getValueOperand()); + if (!LI || !LI->isSimple()) + return false; + + // See if the pointer expression is an AddRec like {base,+,1} on the current + // loop, which indicates a strided load. If we have something else, it's a + // random load we can't handle. + const SCEVAddRecExpr *LoadEv = + dyn_cast<SCEVAddRecExpr>(SE->getSCEV(LI->getPointerOperand())); + if (!LoadEv || LoadEv->getLoop() != CurLoop || !LoadEv->isAffine()) + return false; + + // The store and load must share the same stride. + if (StoreEv->getOperand(1) != LoadEv->getOperand(1)) + 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, *DL, "loop-idiom"); + + const SCEV *StrStart = StoreEv->getStart(); + unsigned StrAS = SI->getPointerAddressSpace(); + Type *IntPtrTy = Builder.getIntPtrTy(*DL, StrAS); + + // Handle negative strided loops. + if (NegStride) + StrStart = getStartForNegStride(StrStart, BECount, IntPtrTy, StoreSize, SE); + + // 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( + StrStart, Builder.getInt8PtrTy(StrAS), Preheader->getTerminator()); + + if (mayLoopAccessLocation(StoreBasePtr, MRI_ModRef, CurLoop, BECount, + StoreSize, *AA, SI)) { + Expander.clear(); + // If we generated new code for the base pointer, clean up. + RecursivelyDeleteTriviallyDeadInstructions(StoreBasePtr, TLI); + return false; + } + + const SCEV *LdStart = LoadEv->getStart(); + unsigned LdAS = LI->getPointerAddressSpace(); + + // Handle negative strided loops. + if (NegStride) + LdStart = getStartForNegStride(LdStart, BECount, IntPtrTy, StoreSize, SE); + + // For a memcpy, we have to make sure that the input array is not being + // mutated by the loop. + Value *LoadBasePtr = Expander.expandCodeFor( + LdStart, Builder.getInt8PtrTy(LdAS), Preheader->getTerminator()); + + if (mayLoopAccessLocation(LoadBasePtr, MRI_Mod, CurLoop, BECount, StoreSize, + *AA, SI)) { + Expander.clear(); + // If we generated new code for the base pointer, clean up. + RecursivelyDeleteTriviallyDeadInstructions(LoadBasePtr, TLI); + RecursivelyDeleteTriviallyDeadInstructions(StoreBasePtr, 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. + BECount = SE->getTruncateOrZeroExtend(BECount, IntPtrTy); + + const SCEV *NumBytesS = + SE->getAddExpr(BECount, SE->getOne(IntPtrTy), 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 memcpy has been formed. Zap the original store and anything that + // feeds into it. + deleteDeadInstruction(SI, TLI); + ++NumMemCpy; + return true; +} + +bool LoopIdiomRecognize::runOnNoncountableLoop() { + return recognizePopcount(); +} + +/// 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. +static Value *matchCondition(BranchInst *BI, BasicBlock *LoopEntry) { + if (!BI || !BI->isConditional()) + return nullptr; + + ICmpInst *Cond = dyn_cast<ICmpInst>(BI->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 && BI->getSuccessor(0) == LoopEntry) || + (Pred == ICmpInst::ICMP_EQ && BI->getSuccessor(1) == LoopEntry)) + return Cond->getOperand(0); + + return nullptr; +} + +/// Return true iff the idiom is detected in the loop. +/// +/// Additionally: +/// 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. +/// +/// The core idiom we are trying to detect is: +/// \code +/// 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: +/// \endcode +static bool detectPopcountIdiom(Loop *CurLoop, BasicBlock *PreCondBB, + Instruction *&CntInst, PHINode *&CntPhi, + Value *&Var) { + // 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( + dyn_cast<BranchInst>(LoopEntry->getTerminator()), 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()->getIterator(), + 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;" + { + auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator()); + 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; +} + +/// Recognizes a population count idiom in a non-countable loop. +/// +/// If detected, transforms the relevant code to issue the popcount intrinsic +/// function call, and returns true; otherwise, returns false. +bool LoopIdiomRecognize::recognizePopcount() { + if (TTI->getPopcntSupport(32) != TargetTransformInfo::PSK_FastHardware) + return false; + + // Counting population are usually conducted by few arithmetic instructions. + // Such instructions can be easily "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 an unconditional branch. + BasicBlock *PH = CurLoop->getLoopPreheader(); + if (!PH) + return false; + if (&PH->front() != PH->getTerminator()) + return false; + auto *EntryBI = dyn_cast<BranchInst>(PH->getTerminator()); + if (!EntryBI || EntryBI->isConditional()) + return false; + + // It should have a precondition block where the generated popcount instrinsic + // function can be inserted. + auto *PreCondBB = PH->getSinglePredecessor(); + if (!PreCondBB) + return false; + auto *PreCondBI = dyn_cast<BranchInst>(PreCondBB->getTerminator()); + if (!PreCondBI || PreCondBI->isUnconditional()) + return false; + + Instruction *CntInst; + PHINode *CntPhi; + Value *Val; + if (!detectPopcountIdiom(CurLoop, PreCondBB, CntInst, CntPhi, Val)) + return false; + + transformLoopToPopcount(PreCondBB, CntInst, CntPhi, Val); + return true; +} + +static CallInst *createPopcntIntrinsic(IRBuilder<> &IRBuilder, Value *Val, + DebugLoc DL) { + Value *Ops[] = {Val}; + Type *Tys[] = {Val->getType()}; + + Module *M = IRBuilder.GetInsertBlock()->getParent()->getParent(); + Value *Func = Intrinsic::getDeclaration(M, Intrinsic::ctpop, Tys); + CallInst *CI = IRBuilder.CreateCall(Func, Ops); + CI->setDebugLoc(DL); + + return CI; +} + +void LoopIdiomRecognize::transformLoopToPopcount(BasicBlock *PreCondBB, + Instruction *CntInst, + PHINode *CntPhi, Value *Var) { + BasicBlock *PreHead = CurLoop->getLoopPreheader(); + auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator()); + 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 + IRBuilder<> 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". Without 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)); + PreCondBr->setCondition(NewPreCond); + + RecursivelyDeleteTriviallyDeadInstructions(PreCond, TLI); + } + + // Step 3: Note that the population count is exactly the trip count of the + // loop in question, which enable 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 becomes dead after + // the transformation. It is a lot easier to prove a countable loop dead + // than to prove a noncountable one. (In some C dialects, an infinite 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()); + { + auto *LbBr = dyn_cast<BranchInst>(Body->getTerminator()); + ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition()); + Type *Ty = TripCnt->getType(); + + PHINode *TcPhi = PHINode::Create(Ty, 2, "tcphi", &Body->front()); + + Builder.SetInsertPoint(LbCond); + Instruction *TcDec = cast<Instruction>( + Builder.CreateSub(TcPhi, ConstantInt::get(Ty, 1), + "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, 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(). + CntInst->replaceUsesOutsideBlock(NewCount, Body); + + // 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); +} |
