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llvm-project/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

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//===- LoopIdiomRecognize.cpp - Loop idiom recognition --------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// 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.
//
// If compiling for code size we avoid idiom recognition if the resulting
// code could be larger than the code for the original loop. One way this could
// happen is if the loop is not removable after idiom recognition due to the
// presence of non-idiom instructions. The initial implementation of the
// heuristics applies to idioms in multi-block loops.
//
//===----------------------------------------------------------------------===//
//
// 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
//
// 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.
//
// This could recognize common matrix multiplies and dot product idioms and
// replace them with calls to BLAS (if linked in??).
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/LoopIdiomRecognize.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CmpInstAnalysis.h"
#include "llvm/Analysis/LoopAccessAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/InstructionCost.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BuildLibCalls.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <utility>
#include <vector>
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");
STATISTIC(
NumShiftUntilBitTest,
"Number of uncountable loops recognized as 'shift until bitttest' idiom");
STATISTIC(NumShiftUntilZero,
"Number of uncountable loops recognized as 'shift until zero' idiom");
bool DisableLIRP::All;
static cl::opt<bool, true>
DisableLIRPAll("disable-" DEBUG_TYPE "-all",
cl::desc("Options to disable Loop Idiom Recognize Pass."),
cl::location(DisableLIRP::All), cl::init(false),
cl::ReallyHidden);
bool DisableLIRP::Memset;
static cl::opt<bool, true>
DisableLIRPMemset("disable-" DEBUG_TYPE "-memset",
cl::desc("Proceed with loop idiom recognize pass, but do "
"not convert loop(s) to memset."),
cl::location(DisableLIRP::Memset), cl::init(false),
cl::ReallyHidden);
bool DisableLIRP::Memcpy;
static cl::opt<bool, true>
DisableLIRPMemcpy("disable-" DEBUG_TYPE "-memcpy",
cl::desc("Proceed with loop idiom recognize pass, but do "
"not convert loop(s) to memcpy."),
cl::location(DisableLIRP::Memcpy), cl::init(false),
cl::ReallyHidden);
static cl::opt<bool> UseLIRCodeSizeHeurs(
"use-lir-code-size-heurs",
cl::desc("Use loop idiom recognition code size heuristics when compiling"
"with -Os/-Oz"),
cl::init(true), cl::Hidden);
namespace {
class LoopIdiomRecognize {
Loop *CurLoop = nullptr;
AliasAnalysis *AA;
DominatorTree *DT;
LoopInfo *LI;
ScalarEvolution *SE;
TargetLibraryInfo *TLI;
const TargetTransformInfo *TTI;
const DataLayout *DL;
OptimizationRemarkEmitter &ORE;
bool ApplyCodeSizeHeuristics;
std::unique_ptr<MemorySSAUpdater> MSSAU;
public:
explicit LoopIdiomRecognize(AliasAnalysis *AA, DominatorTree *DT,
LoopInfo *LI, ScalarEvolution *SE,
TargetLibraryInfo *TLI,
const TargetTransformInfo *TTI, MemorySSA *MSSA,
const DataLayout *DL,
OptimizationRemarkEmitter &ORE)
: AA(AA), DT(DT), LI(LI), SE(SE), TLI(TLI), TTI(TTI), DL(DL), ORE(ORE) {
if (MSSA)
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
}
bool runOnLoop(Loop *L);
private:
using StoreList = SmallVector<StoreInst *, 8>;
using StoreListMap = MapVector<Value *, StoreList>;
StoreListMap StoreRefsForMemset;
StoreListMap StoreRefsForMemsetPattern;
StoreList StoreRefsForMemcpy;
bool HasMemset;
bool HasMemsetPattern;
bool HasMemcpy;
/// Return code for isLegalStore()
enum LegalStoreKind {
None = 0,
Memset,
MemsetPattern,
Memcpy,
UnorderedAtomicMemcpy,
DontUse // Dummy retval never to be used. Allows catching errors in retval
// handling.
};
/// \name Countable Loop Idiom Handling
/// @{
bool runOnCountableLoop();
bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
SmallVectorImpl<BasicBlock *> &ExitBlocks);
void collectStores(BasicBlock *BB);
LegalStoreKind isLegalStore(StoreInst *SI);
enum class ForMemset { No, Yes };
bool processLoopStores(SmallVectorImpl<StoreInst *> &SL, const SCEV *BECount,
ForMemset For);
template <typename MemInst>
bool processLoopMemIntrinsic(
BasicBlock *BB,
bool (LoopIdiomRecognize::*Processor)(MemInst *, const SCEV *),
const SCEV *BECount);
bool processLoopMemCpy(MemCpyInst *MCI, const SCEV *BECount);
bool processLoopMemSet(MemSetInst *MSI, const SCEV *BECount);
bool processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
MaybeAlign StoreAlignment, Value *StoredVal,
Instruction *TheStore,
SmallPtrSetImpl<Instruction *> &Stores,
const SCEVAddRecExpr *Ev, const SCEV *BECount,
bool NegStride, bool IsLoopMemset = false);
bool processLoopStoreOfLoopLoad(StoreInst *SI, const SCEV *BECount);
bool processLoopStoreOfLoopLoad(Value *DestPtr, Value *SourcePtr,
unsigned StoreSize, MaybeAlign StoreAlign,
MaybeAlign LoadAlign, Instruction *TheStore,
Instruction *TheLoad,
const SCEVAddRecExpr *StoreEv,
const SCEVAddRecExpr *LoadEv,
const SCEV *BECount);
bool avoidLIRForMultiBlockLoop(bool IsMemset = false,
bool IsLoopMemset = false);
/// @}
/// \name Noncountable Loop Idiom Handling
/// @{
bool runOnNoncountableLoop();
bool recognizePopcount();
void transformLoopToPopcount(BasicBlock *PreCondBB, Instruction *CntInst,
PHINode *CntPhi, Value *Var);
bool recognizeAndInsertFFS(); /// Find First Set: ctlz or cttz
void transformLoopToCountable(Intrinsic::ID IntrinID, BasicBlock *PreCondBB,
Instruction *CntInst, PHINode *CntPhi,
Value *Var, Instruction *DefX,
const DebugLoc &DL, bool ZeroCheck,
bool IsCntPhiUsedOutsideLoop);
bool recognizeShiftUntilBitTest();
bool recognizeShiftUntilZero();
/// @}
};
class LoopIdiomRecognizeLegacyPass : public LoopPass {
public:
static char ID;
explicit LoopIdiomRecognizeLegacyPass() : LoopPass(ID) {
initializeLoopIdiomRecognizeLegacyPassPass(
*PassRegistry::getPassRegistry());
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override {
if (DisableLIRP::All)
return false;
if (skipLoop(L))
return false;
AliasAnalysis *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
ScalarEvolution *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
TargetLibraryInfo *TLI =
&getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(
*L->getHeader()->getParent());
const TargetTransformInfo *TTI =
&getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
*L->getHeader()->getParent());
const DataLayout *DL = &L->getHeader()->getModule()->getDataLayout();
auto *MSSAAnalysis = getAnalysisIfAvailable<MemorySSAWrapperPass>();
MemorySSA *MSSA = nullptr;
if (MSSAAnalysis)
MSSA = &MSSAAnalysis->getMSSA();
// For the old PM, we can't use OptimizationRemarkEmitter as an analysis
// pass. Function analyses need to be preserved across loop transformations
// but ORE cannot be preserved (see comment before the pass definition).
OptimizationRemarkEmitter ORE(L->getHeader()->getParent());
LoopIdiomRecognize LIR(AA, DT, LI, SE, TLI, TTI, MSSA, DL, ORE);
return LIR.runOnLoop(L);
}
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG.
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
AU.addPreserved<MemorySSAWrapperPass>();
getLoopAnalysisUsage(AU);
}
};
} // end anonymous namespace
char LoopIdiomRecognizeLegacyPass::ID = 0;
PreservedAnalyses LoopIdiomRecognizePass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &) {
if (DisableLIRP::All)
return PreservedAnalyses::all();
const auto *DL = &L.getHeader()->getModule()->getDataLayout();
// For the new PM, we also can't use OptimizationRemarkEmitter as an analysis
// pass. Function analyses need to be preserved across loop transformations
// but ORE cannot be preserved (see comment before the pass definition).
OptimizationRemarkEmitter ORE(L.getHeader()->getParent());
LoopIdiomRecognize LIR(&AR.AA, &AR.DT, &AR.LI, &AR.SE, &AR.TLI, &AR.TTI,
AR.MSSA, DL, ORE);
if (!LIR.runOnLoop(&L))
return PreservedAnalyses::all();
auto PA = getLoopPassPreservedAnalyses();
if (AR.MSSA)
PA.preserve<MemorySSAAnalysis>();
return PA;
}
INITIALIZE_PASS_BEGIN(LoopIdiomRecognizeLegacyPass, "loop-idiom",
"Recognize loop idioms", false, false)
INITIALIZE_PASS_DEPENDENCY(LoopPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_END(LoopIdiomRecognizeLegacyPass, "loop-idiom",
"Recognize loop idioms", false, false)
Pass *llvm::createLoopIdiomPass() { return new LoopIdiomRecognizeLegacyPass(); }
static void deleteDeadInstruction(Instruction *I) {
I->replaceAllUsesWith(UndefValue::get(I->getType()));
I->eraseFromParent();
}
//===----------------------------------------------------------------------===//
//
// Implementation of LoopIdiomRecognize
//
//===----------------------------------------------------------------------===//
bool LoopIdiomRecognize::runOnLoop(Loop *L) {
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;
// Determine if code size heuristics need to be applied.
ApplyCodeSizeHeuristics =
L->getHeader()->getParent()->hasOptSize() && UseLIRCodeSizeHeurs;
HasMemset = TLI->has(LibFunc_memset);
HasMemsetPattern = TLI->has(LibFunc_memset_pattern16);
HasMemcpy = TLI->has(LibFunc_memcpy);
if (HasMemset || HasMemsetPattern || HasMemcpy)
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);
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Scanning: F["
<< CurLoop->getHeader()->getParent()->getName()
<< "] Countable Loop %" << CurLoop->getHeader()->getName()
<< "\n");
// The following transforms hoist stores/memsets into the loop pre-header.
// Give up if the loop has instructions that may throw.
SimpleLoopSafetyInfo SafetyInfo;
SafetyInfo.computeLoopSafetyInfo(CurLoop);
if (SafetyInfo.anyBlockMayThrow())
return false;
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 APInt getStoreStride(const SCEVAddRecExpr *StoreEv) {
const SCEVConstant *ConstStride = cast<SCEVConstant>(StoreEv->getOperand(1));
return ConstStride->getAPInt();
}
/// 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) {
// FIXME: This could check for UndefValue because it can be merged into any
// other valid pattern.
// 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));
}
LoopIdiomRecognize::LegalStoreKind
LoopIdiomRecognize::isLegalStore(StoreInst *SI) {
// Don't touch volatile stores.
if (SI->isVolatile())
return LegalStoreKind::None;
// We only want simple or unordered-atomic stores.
if (!SI->isUnordered())
return LegalStoreKind::None;
// Avoid merging nontemporal stores.
if (SI->getMetadata(LLVMContext::MD_nontemporal))
return LegalStoreKind::None;
Value *StoredVal = SI->getValueOperand();
Value *StorePtr = SI->getPointerOperand();
// Don't convert stores of non-integral pointer types to memsets (which stores
// integers).
if (DL->isNonIntegralPointerType(StoredVal->getType()->getScalarType()))
return LegalStoreKind::None;
// Reject stores that are so large that they overflow an unsigned.
// When storing out scalable vectors we bail out for now, since the code
// below currently only works for constant strides.
TypeSize SizeInBits = DL->getTypeSizeInBits(StoredVal->getType());
if (SizeInBits.isScalable() || (SizeInBits.getFixedSize() & 7) ||
(SizeInBits.getFixedSize() >> 32) != 0)
return LegalStoreKind::None;
// 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 LegalStoreKind::None;
// Check to see if we have a constant stride.
if (!isa<SCEVConstant>(StoreEv->getOperand(1)))
return LegalStoreKind::None;
// See if the store can be turned into a memset.
// 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, *DL);
// Note: memset and memset_pattern on unordered-atomic is yet not supported
bool UnorderedAtomic = SI->isUnordered() && !SI->isSimple();
// If we're allowed to form a memset, and the stored value would be
// acceptable for memset, use it.
if (!UnorderedAtomic && HasMemset && SplatValue && !DisableLIRP::Memset &&
// Verify that the stored value is loop invariant. If not, we can't
// promote the memset.
CurLoop->isLoopInvariant(SplatValue)) {
// It looks like we can use SplatValue.
return LegalStoreKind::Memset;
}
if (!UnorderedAtomic && HasMemsetPattern && !DisableLIRP::Memset &&
// Don't create memset_pattern16s with address spaces.
StorePtr->getType()->getPointerAddressSpace() == 0 &&
getMemSetPatternValue(StoredVal, DL)) {
// It looks like we can use PatternValue!
return LegalStoreKind::MemsetPattern;
}
// Otherwise, see if the store can be turned into a memcpy.
if (HasMemcpy && !DisableLIRP::Memcpy) {
// 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.
APInt Stride = getStoreStride(StoreEv);
unsigned StoreSize = DL->getTypeStoreSize(SI->getValueOperand()->getType());
if (StoreSize != Stride && StoreSize != -Stride)
return LegalStoreKind::None;
// The store must be feeding a non-volatile load.
LoadInst *LI = dyn_cast<LoadInst>(SI->getValueOperand());
// Only allow non-volatile loads
if (!LI || LI->isVolatile())
return LegalStoreKind::None;
// Only allow simple or unordered-atomic loads
if (!LI->isUnordered())
return LegalStoreKind::None;
// 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 LegalStoreKind::None;
// The store and load must share the same stride.
if (StoreEv->getOperand(1) != LoadEv->getOperand(1))
return LegalStoreKind::None;
// Success. This store can be converted into a memcpy.
UnorderedAtomic = UnorderedAtomic || LI->isAtomic();
return UnorderedAtomic ? LegalStoreKind::UnorderedAtomicMemcpy
: LegalStoreKind::Memcpy;
}
// This store can't be transformed into a memset/memcpy.
return LegalStoreKind::None;
}
void LoopIdiomRecognize::collectStores(BasicBlock *BB) {
StoreRefsForMemset.clear();
StoreRefsForMemsetPattern.clear();
StoreRefsForMemcpy.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.
switch (isLegalStore(SI)) {
case LegalStoreKind::None:
// Nothing to do
break;
case LegalStoreKind::Memset: {
// Find the base pointer.
Value *Ptr = getUnderlyingObject(SI->getPointerOperand());
StoreRefsForMemset[Ptr].push_back(SI);
} break;
case LegalStoreKind::MemsetPattern: {
// Find the base pointer.
Value *Ptr = getUnderlyingObject(SI->getPointerOperand());
StoreRefsForMemsetPattern[Ptr].push_back(SI);
} break;
case LegalStoreKind::Memcpy:
case LegalStoreKind::UnorderedAtomicMemcpy:
StoreRefsForMemcpy.push_back(SI);
break;
default:
assert(false && "unhandled return value");
break;
}
}
}
/// 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);
// Look for a single store or sets of stores with a common base, which can be
// optimized into a memset (memset_pattern). The latter most commonly happens
// with structs and handunrolled loops.
for (auto &SL : StoreRefsForMemset)
MadeChange |= processLoopStores(SL.second, BECount, ForMemset::Yes);
for (auto &SL : StoreRefsForMemsetPattern)
MadeChange |= processLoopStores(SL.second, BECount, ForMemset::No);
// Optimize the store into a memcpy, if it feeds an similarly strided load.
for (auto &SI : StoreRefsForMemcpy)
MadeChange |= processLoopStoreOfLoopLoad(SI, BECount);
MadeChange |= processLoopMemIntrinsic<MemCpyInst>(
BB, &LoopIdiomRecognize::processLoopMemCpy, BECount);
MadeChange |= processLoopMemIntrinsic<MemSetInst>(
BB, &LoopIdiomRecognize::processLoopMemSet, BECount);
return MadeChange;
}
/// See if this store(s) can be promoted to a memset.
bool LoopIdiomRecognize::processLoopStores(SmallVectorImpl<StoreInst *> &SL,
const SCEV *BECount, ForMemset For) {
// Try to find consecutive stores that can be transformed into memsets.
SetVector<StoreInst *> Heads, Tails;
SmallDenseMap<StoreInst *, StoreInst *> ConsecutiveChain;
// Do a quadratic search on all of the given stores and find
// all of the pairs of stores that follow each other.
SmallVector<unsigned, 16> IndexQueue;
for (unsigned i = 0, e = SL.size(); i < e; ++i) {
assert(SL[i]->isSimple() && "Expected only non-volatile stores.");
Value *FirstStoredVal = SL[i]->getValueOperand();
Value *FirstStorePtr = SL[i]->getPointerOperand();
const SCEVAddRecExpr *FirstStoreEv =
cast<SCEVAddRecExpr>(SE->getSCEV(FirstStorePtr));
APInt FirstStride = getStoreStride(FirstStoreEv);
unsigned FirstStoreSize = DL->getTypeStoreSize(SL[i]->getValueOperand()->getType());
// See if we can optimize just this store in isolation.
if (FirstStride == FirstStoreSize || -FirstStride == FirstStoreSize) {
Heads.insert(SL[i]);
continue;
}
Value *FirstSplatValue = nullptr;
Constant *FirstPatternValue = nullptr;
if (For == ForMemset::Yes)
FirstSplatValue = isBytewiseValue(FirstStoredVal, *DL);
else
FirstPatternValue = getMemSetPatternValue(FirstStoredVal, DL);
assert((FirstSplatValue || FirstPatternValue) &&
"Expected either splat value or pattern value.");
IndexQueue.clear();
// If a store has multiple consecutive store candidates, search Stores
// array according to the sequence: from i+1 to e, then from i-1 to 0.
// This is because usually pairing with immediate succeeding or preceding
// candidate create the best chance to find memset opportunity.
unsigned j = 0;
for (j = i + 1; j < e; ++j)
IndexQueue.push_back(j);
for (j = i; j > 0; --j)
IndexQueue.push_back(j - 1);
for (auto &k : IndexQueue) {
assert(SL[k]->isSimple() && "Expected only non-volatile stores.");
Value *SecondStorePtr = SL[k]->getPointerOperand();
const SCEVAddRecExpr *SecondStoreEv =
cast<SCEVAddRecExpr>(SE->getSCEV(SecondStorePtr));
APInt SecondStride = getStoreStride(SecondStoreEv);
if (FirstStride != SecondStride)
continue;
Value *SecondStoredVal = SL[k]->getValueOperand();
Value *SecondSplatValue = nullptr;
Constant *SecondPatternValue = nullptr;
if (For == ForMemset::Yes)
SecondSplatValue = isBytewiseValue(SecondStoredVal, *DL);
else
SecondPatternValue = getMemSetPatternValue(SecondStoredVal, DL);
assert((SecondSplatValue || SecondPatternValue) &&
"Expected either splat value or pattern value.");
if (isConsecutiveAccess(SL[i], SL[k], *DL, *SE, false)) {
if (For == ForMemset::Yes) {
if (isa<UndefValue>(FirstSplatValue))
FirstSplatValue = SecondSplatValue;
if (FirstSplatValue != SecondSplatValue)
continue;
} else {
if (isa<UndefValue>(FirstPatternValue))
FirstPatternValue = SecondPatternValue;
if (FirstPatternValue != SecondPatternValue)
continue;
}
Tails.insert(SL[k]);
Heads.insert(SL[i]);
ConsecutiveChain[SL[i]] = SL[k];
break;
}
}
}
// We may run into multiple chains that merge into a single chain. We mark the
// stores that we transformed so that we don't visit the same store twice.
SmallPtrSet<Value *, 16> TransformedStores;
bool Changed = false;
// For stores that start but don't end a link in the chain:
for (SetVector<StoreInst *>::iterator it = Heads.begin(), e = Heads.end();
it != e; ++it) {
if (Tails.count(*it))
continue;
// We found a store instr that starts a chain. Now follow the chain and try
// to transform it.
SmallPtrSet<Instruction *, 8> AdjacentStores;
StoreInst *I = *it;
StoreInst *HeadStore = I;
unsigned StoreSize = 0;
// Collect the chain into a list.
while (Tails.count(I) || Heads.count(I)) {
if (TransformedStores.count(I))
break;
AdjacentStores.insert(I);
StoreSize += DL->getTypeStoreSize(I->getValueOperand()->getType());
// Move to the next value in the chain.
I = ConsecutiveChain[I];
}
Value *StoredVal = HeadStore->getValueOperand();
Value *StorePtr = HeadStore->getPointerOperand();
const SCEVAddRecExpr *StoreEv = cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
APInt Stride = getStoreStride(StoreEv);
// Check to see if the stride matches the size of the stores. If so, then
// we know that every byte is touched in the loop.
if (StoreSize != Stride && StoreSize != -Stride)
continue;
bool NegStride = StoreSize == -Stride;
if (processLoopStridedStore(StorePtr, StoreSize,
MaybeAlign(HeadStore->getAlignment()),
StoredVal, HeadStore, AdjacentStores, StoreEv,
BECount, NegStride)) {
TransformedStores.insert(AdjacentStores.begin(), AdjacentStores.end());
Changed = true;
}
}
return Changed;
}
/// processLoopMemIntrinsic - Template function for calling different processor
/// functions based on mem instrinsic type.
template <typename MemInst>
bool LoopIdiomRecognize::processLoopMemIntrinsic(
BasicBlock *BB,
bool (LoopIdiomRecognize::*Processor)(MemInst *, const SCEV *),
const SCEV *BECount) {
bool MadeChange = false;
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
Instruction *Inst = &*I++;
// Look for memory instructions, which may be optimized to a larger one.
if (MemInst *MI = dyn_cast<MemInst>(Inst)) {
WeakTrackingVH InstPtr(&*I);
if (!(this->*Processor)(MI, BECount))
continue;
MadeChange = true;
// If processing the instruction invalidated our iterator, start over from
// the top of the block.
if (!InstPtr)
I = BB->begin();
}
}
return MadeChange;
}
/// processLoopMemCpy - See if this memcpy can be promoted to a large memcpy
bool LoopIdiomRecognize::processLoopMemCpy(MemCpyInst *MCI,
const SCEV *BECount) {
// We can only handle non-volatile memcpys with a constant size.
if (MCI->isVolatile() || !isa<ConstantInt>(MCI->getLength()))
return false;
// If we're not allowed to hack on memcpy, we fail.
if (!HasMemcpy || DisableLIRP::Memcpy)
return false;
Value *Dest = MCI->getDest();
Value *Source = MCI->getSource();
if (!Dest || !Source)
return false;
// See if the load and store pointer expressions are AddRec like {base,+,1} on
// the current loop, which indicates a strided load and store. If we have
// something else, it's a random load or store we can't handle.
const SCEVAddRecExpr *StoreEv = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Dest));
if (!StoreEv || StoreEv->getLoop() != CurLoop || !StoreEv->isAffine())
return false;
const SCEVAddRecExpr *LoadEv = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Source));
if (!LoadEv || LoadEv->getLoop() != CurLoop || !LoadEv->isAffine())
return false;
// Reject memcpys that are so large that they overflow an unsigned.
uint64_t SizeInBytes = cast<ConstantInt>(MCI->getLength())->getZExtValue();
if ((SizeInBytes >> 32) != 0)
return false;
// Check if the stride matches the size of the memcpy. If so, then we know
// that every byte is touched in the loop.
const SCEVConstant *StoreStride =
dyn_cast<SCEVConstant>(StoreEv->getOperand(1));
const SCEVConstant *LoadStride =
dyn_cast<SCEVConstant>(LoadEv->getOperand(1));
if (!StoreStride || !LoadStride)
return false;
APInt StoreStrideValue = StoreStride->getAPInt();
APInt LoadStrideValue = LoadStride->getAPInt();
// Huge stride value - give up
if (StoreStrideValue.getBitWidth() > 64 || LoadStrideValue.getBitWidth() > 64)
return false;
if (SizeInBytes != StoreStrideValue && SizeInBytes != -StoreStrideValue) {
ORE.emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "SizeStrideUnequal", MCI)
<< ore::NV("Inst", "memcpy") << " in "
<< ore::NV("Function", MCI->getFunction())
<< " function will not be hoised: "
<< ore::NV("Reason", "memcpy size is not equal to stride");
});
return false;
}
int64_t StoreStrideInt = StoreStrideValue.getSExtValue();
int64_t LoadStrideInt = LoadStrideValue.getSExtValue();
// Check if the load stride matches the store stride.
if (StoreStrideInt != LoadStrideInt)
return false;
return processLoopStoreOfLoopLoad(Dest, Source, (unsigned)SizeInBytes,
MCI->getDestAlign(), MCI->getSourceAlign(),
MCI, MCI, StoreEv, LoadEv, BECount);
}
/// 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 (!HasMemset || DisableLIRP::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 *ConstStride = dyn_cast<SCEVConstant>(Ev->getOperand(1));
if (!ConstStride)
return false;
APInt Stride = ConstStride->getAPInt();
if (SizeInBytes != Stride && SizeInBytes != -Stride)
return false;
// Verify that the memset value is loop invariant. If not, we can't promote
// the memset.
Value *SplatValue = MSI->getValue();
if (!SplatValue || !CurLoop->isLoopInvariant(SplatValue))
return false;
SmallPtrSet<Instruction *, 1> MSIs;
MSIs.insert(MSI);
bool NegStride = SizeInBytes == -Stride;
return processLoopStridedStore(
Pointer, (unsigned)SizeInBytes, MaybeAlign(MSI->getDestAlignment()),
SplatValue, MSI, MSIs, Ev, BECount, NegStride, /*IsLoopMemset=*/true);
}
/// 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,
SmallPtrSetImpl<Instruction *> &IgnoredStores) {
// 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.
LocationSize AccessSize = LocationSize::afterPointer();
// 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 = LocationSize::precise((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 (Instruction &I : **BI)
if (IgnoredStores.count(&I) == 0 &&
isModOrRefSet(
intersectModRef(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);
}
/// Compute the number of bytes as a SCEV from the backedge taken count.
///
/// This also maps the SCEV into the provided type and tries to handle the
/// computation in a way that will fold cleanly.
static const SCEV *getNumBytes(const SCEV *BECount, Type *IntPtr,
unsigned StoreSize, Loop *CurLoop,
const DataLayout *DL, ScalarEvolution *SE) {
const SCEV *NumBytesS;
// The # stored bytes is (BECount+1)*Size. Expand the trip count out to
// pointer size if it isn't already.
//
// If we're going to need to zero extend the BE count, check if we can add
// one to it prior to zero extending without overflow. Provided this is safe,
// it allows better simplification of the +1.
if (DL->getTypeSizeInBits(BECount->getType()).getFixedSize() <
DL->getTypeSizeInBits(IntPtr).getFixedSize() &&
SE->isLoopEntryGuardedByCond(
CurLoop, ICmpInst::ICMP_NE, BECount,
SE->getNegativeSCEV(SE->getOne(BECount->getType())))) {
NumBytesS = SE->getZeroExtendExpr(
SE->getAddExpr(BECount, SE->getOne(BECount->getType()), SCEV::FlagNUW),
IntPtr);
} else {
NumBytesS = SE->getAddExpr(SE->getTruncateOrZeroExtend(BECount, IntPtr),
SE->getOne(IntPtr), SCEV::FlagNUW);
}
// And scale it based on the store size.
if (StoreSize != 1) {
NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize),
SCEV::FlagNUW);
}
return NumBytesS;
}
/// 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, MaybeAlign StoreAlignment,
Value *StoredVal, Instruction *TheStore,
SmallPtrSetImpl<Instruction *> &Stores, const SCEVAddRecExpr *Ev,
const SCEV *BECount, bool NegStride, bool IsLoopMemset) {
Value *SplatValue = isBytewiseValue(StoredVal, *DL);
Constant *PatternValue = nullptr;
if (!SplatValue)
PatternValue = getMemSetPatternValue(StoredVal, DL);
assert((SplatValue || PatternValue) &&
"Expected either splat value or pattern value.");
// 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.
unsigned DestAS = DestPtr->getType()->getPointerAddressSpace();
BasicBlock *Preheader = CurLoop->getLoopPreheader();
IRBuilder<> Builder(Preheader->getTerminator());
SCEVExpander Expander(*SE, *DL, "loop-idiom");
SCEVExpanderCleaner ExpCleaner(Expander, *DT);
Type *DestInt8PtrTy = Builder.getInt8PtrTy(DestAS);
Type *IntIdxTy = DL->getIndexType(DestPtr->getType());
bool Changed = false;
const SCEV *Start = Ev->getStart();
// Handle negative strided loops.
if (NegStride)
Start = getStartForNegStride(Start, BECount, IntIdxTy, StoreSize, SE);
// TODO: ideally we should still be able to generate memset if SCEV expander
// is taught to generate the dependencies at the latest point.
if (!isSafeToExpand(Start, *SE))
return Changed;
// 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());
// From here on out, conservatively report to the pass manager that we've
// changed the IR, even if we later clean up these added instructions. There
// may be structural differences e.g. in the order of use lists not accounted
// for in just a textual dump of the IR. This is written as a variable, even
// though statically all the places this dominates could be replaced with
// 'true', with the hope that anyone trying to be clever / "more precise" with
// the return value will read this comment, and leave them alone.
Changed = true;
if (mayLoopAccessLocation(BasePtr, ModRefInfo::ModRef, CurLoop, BECount,
StoreSize, *AA, Stores))
return Changed;
if (avoidLIRForMultiBlockLoop(/*IsMemset=*/true, IsLoopMemset))
return Changed;
// Okay, everything looks good, insert the memset.
const SCEV *NumBytesS =
getNumBytes(BECount, IntIdxTy, StoreSize, CurLoop, DL, SE);
// TODO: ideally we should still be able to generate memset if SCEV expander
// is taught to generate the dependencies at the latest point.
if (!isSafeToExpand(NumBytesS, *SE))
return Changed;
Value *NumBytes =
Expander.expandCodeFor(NumBytesS, IntIdxTy, Preheader->getTerminator());
CallInst *NewCall;
if (SplatValue) {
NewCall = Builder.CreateMemSet(BasePtr, SplatValue, NumBytes,
MaybeAlign(StoreAlignment));
} else {
// Everything is emitted in default address space
Type *Int8PtrTy = DestInt8PtrTy;
Module *M = TheStore->getModule();
StringRef FuncName = "memset_pattern16";
FunctionCallee MSP = M->getOrInsertFunction(FuncName, Builder.getVoidTy(),
Int8PtrTy, Int8PtrTy, IntIdxTy);
inferLibFuncAttributes(M, FuncName, *TLI);
// 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(GlobalValue::UnnamedAddr::Global); // Ok to merge these.
GV->setAlignment(Align(16));
Value *PatternPtr = ConstantExpr::getBitCast(GV, Int8PtrTy);
NewCall = Builder.CreateCall(MSP, {BasePtr, PatternPtr, NumBytes});
}
NewCall->setDebugLoc(TheStore->getDebugLoc());
if (MSSAU) {
MemoryAccess *NewMemAcc = MSSAU->createMemoryAccessInBB(
NewCall, nullptr, NewCall->getParent(), MemorySSA::BeforeTerminator);
MSSAU->insertDef(cast<MemoryDef>(NewMemAcc), true);
}
LLVM_DEBUG(dbgs() << " Formed memset: " << *NewCall << "\n"
<< " from store to: " << *Ev << " at: " << *TheStore
<< "\n");
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "ProcessLoopStridedStore",
NewCall->getDebugLoc(), Preheader)
<< "Transformed loop-strided store in "
<< ore::NV("Function", TheStore->getFunction())
<< " function into a call to "
<< ore::NV("NewFunction", NewCall->getCalledFunction())
<< "() intrinsic";
});
// Okay, the memset has been formed. Zap the original store and anything that
// feeds into it.
for (auto *I : Stores) {
if (MSSAU)
MSSAU->removeMemoryAccess(I, true);
deleteDeadInstruction(I);
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
++NumMemSet;
ExpCleaner.markResultUsed();
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,
const SCEV *BECount) {
assert(SI->isUnordered() && "Expected only non-volatile non-ordered stores.");
Value *StorePtr = SI->getPointerOperand();
const SCEVAddRecExpr *StoreEv = cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
unsigned StoreSize = DL->getTypeStoreSize(SI->getValueOperand()->getType());
// The store must be feeding a non-volatile load.
LoadInst *LI = cast<LoadInst>(SI->getValueOperand());
assert(LI->isUnordered() && "Expected only non-volatile non-ordered loads.");
// 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.
Value *LoadPtr = LI->getPointerOperand();
const SCEVAddRecExpr *LoadEv = cast<SCEVAddRecExpr>(SE->getSCEV(LoadPtr));
return processLoopStoreOfLoopLoad(StorePtr, LoadPtr, StoreSize,
SI->getAlign(), LI->getAlign(), SI, LI,
StoreEv, LoadEv, BECount);
}
bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
Value *DestPtr, Value *SourcePtr, unsigned StoreSize, MaybeAlign StoreAlign,
MaybeAlign LoadAlign, Instruction *TheStore, Instruction *TheLoad,
const SCEVAddRecExpr *StoreEv, const SCEVAddRecExpr *LoadEv,
const SCEV *BECount) {
// 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");
SCEVExpanderCleaner ExpCleaner(Expander, *DT);
bool Changed = false;
const SCEV *StrStart = StoreEv->getStart();
unsigned StrAS = DestPtr->getType()->getPointerAddressSpace();
Type *IntIdxTy = Builder.getIntNTy(DL->getIndexSizeInBits(StrAS));
APInt Stride = getStoreStride(StoreEv);
bool NegStride = StoreSize == -Stride;
// Handle negative strided loops.
if (NegStride)
StrStart = getStartForNegStride(StrStart, BECount, IntIdxTy, 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());
// From here on out, conservatively report to the pass manager that we've
// changed the IR, even if we later clean up these added instructions. There
// may be structural differences e.g. in the order of use lists not accounted
// for in just a textual dump of the IR. This is written as a variable, even
// though statically all the places this dominates could be replaced with
// 'true', with the hope that anyone trying to be clever / "more precise" with
// the return value will read this comment, and leave them alone.
Changed = true;
SmallPtrSet<Instruction *, 1> Stores;
Stores.insert(TheStore);
bool IsMemCpy = isa<MemCpyInst>(TheStore);
const StringRef InstRemark = IsMemCpy ? "memcpy" : "load and store";
if (mayLoopAccessLocation(StoreBasePtr, ModRefInfo::ModRef, CurLoop, BECount,
StoreSize, *AA, Stores)) {
ORE.emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "LoopMayAccessStore",
TheStore)
<< ore::NV("Inst", InstRemark) << " in "
<< ore::NV("Function", TheStore->getFunction())
<< " function will not be hoisted: "
<< ore::NV("Reason", "The loop may access store location");
});
return Changed;
}
const SCEV *LdStart = LoadEv->getStart();
unsigned LdAS = SourcePtr->getType()->getPointerAddressSpace();
// Handle negative strided loops.
if (NegStride)
LdStart = getStartForNegStride(LdStart, BECount, IntIdxTy, 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 the store is a memcpy instruction, we must check if it will write to
// the load memory locations. So remove it from the ignored stores.
if (IsMemCpy)
Stores.erase(TheStore);
if (mayLoopAccessLocation(LoadBasePtr, ModRefInfo::Mod, CurLoop, BECount,
StoreSize, *AA, Stores)) {
ORE.emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "LoopMayAccessLoad", TheLoad)
<< ore::NV("Inst", InstRemark) << " in "
<< ore::NV("Function", TheStore->getFunction())
<< " function will not be hoisted: "
<< ore::NV("Reason", "The loop may access load location");
});
return Changed;
}
if (avoidLIRForMultiBlockLoop())
return Changed;
// Okay, everything is safe, we can transform this!
const SCEV *NumBytesS =
getNumBytes(BECount, IntIdxTy, StoreSize, CurLoop, DL, SE);
Value *NumBytes =
Expander.expandCodeFor(NumBytesS, IntIdxTy, Preheader->getTerminator());
CallInst *NewCall = nullptr;
// Check whether to generate an unordered atomic memcpy:
// If the load or store are atomic, then they must necessarily be unordered
// by previous checks.
if (!TheStore->isAtomic() && !TheLoad->isAtomic())
NewCall = Builder.CreateMemCpy(StoreBasePtr, StoreAlign, LoadBasePtr,
LoadAlign, NumBytes);
else {
// We cannot allow unaligned ops for unordered load/store, so reject
// anything where the alignment isn't at least the element size.
assert((StoreAlign.hasValue() && LoadAlign.hasValue()) &&
"Expect unordered load/store to have align.");
if (StoreAlign.getValue() < StoreSize || LoadAlign.getValue() < StoreSize)
return Changed;
// If the element.atomic memcpy is not lowered into explicit
// loads/stores later, then it will be lowered into an element-size
// specific lib call. If the lib call doesn't exist for our store size, then
// we shouldn't generate the memcpy.
if (StoreSize > TTI->getAtomicMemIntrinsicMaxElementSize())
return Changed;
// Create the call.
// Note that unordered atomic loads/stores are *required* by the spec to
// have an alignment but non-atomic loads/stores may not.
NewCall = Builder.CreateElementUnorderedAtomicMemCpy(
StoreBasePtr, StoreAlign.getValue(), LoadBasePtr, LoadAlign.getValue(),
NumBytes, StoreSize);
}
NewCall->setDebugLoc(TheStore->getDebugLoc());
if (MSSAU) {
MemoryAccess *NewMemAcc = MSSAU->createMemoryAccessInBB(
NewCall, nullptr, NewCall->getParent(), MemorySSA::BeforeTerminator);
MSSAU->insertDef(cast<MemoryDef>(NewMemAcc), true);
}
LLVM_DEBUG(dbgs() << " Formed memcpy: " << *NewCall << "\n"
<< " from load ptr=" << *LoadEv << " at: " << *TheLoad
<< "\n"
<< " from store ptr=" << *StoreEv << " at: " << *TheStore
<< "\n");
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "ProcessLoopStoreOfLoopLoad",
NewCall->getDebugLoc(), Preheader)
<< "Formed a call to "
<< ore::NV("NewFunction", NewCall->getCalledFunction())
<< "() intrinsic from " << ore::NV("Inst", InstRemark)
<< " instruction in " << ore::NV("Function", TheStore->getFunction())
<< " function";
});
// Okay, the memcpy has been formed. Zap the original store and anything that
// feeds into it.
if (MSSAU)
MSSAU->removeMemoryAccess(TheStore, true);
deleteDeadInstruction(TheStore);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
++NumMemCpy;
ExpCleaner.markResultUsed();
return true;
}
// When compiling for codesize we avoid idiom recognition for a multi-block loop
// unless it is a loop_memset idiom or a memset/memcpy idiom in a nested loop.
//
bool LoopIdiomRecognize::avoidLIRForMultiBlockLoop(bool IsMemset,
bool IsLoopMemset) {
if (ApplyCodeSizeHeuristics && CurLoop->getNumBlocks() > 1) {
if (CurLoop->isOutermost() && (!IsMemset || !IsLoopMemset)) {
LLVM_DEBUG(dbgs() << " " << CurLoop->getHeader()->getParent()->getName()
<< " : LIR " << (IsMemset ? "Memset" : "Memcpy")
<< " avoided: multi-block top-level loop\n");
return true;
}
}
return false;
}
bool LoopIdiomRecognize::runOnNoncountableLoop() {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Scanning: F["
<< CurLoop->getHeader()->getParent()->getName()
<< "] Noncountable Loop %"
<< CurLoop->getHeader()->getName() << "\n");
return recognizePopcount() || recognizeAndInsertFFS() ||
recognizeShiftUntilBitTest() || recognizeShiftUntilZero();
}
/// Check if the given conditional branch is based on the comparison between
/// a variable and zero, and if the variable is non-zero or zero (JmpOnZero is
/// true), the control yields to the loop entry. If the branch matches the
/// behavior, the variable involved in the comparison 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,
bool JmpOnZero = false) {
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;
BasicBlock *TrueSucc = BI->getSuccessor(0);
BasicBlock *FalseSucc = BI->getSuccessor(1);
if (JmpOnZero)
std::swap(TrueSucc, FalseSucc);
ICmpInst::Predicate Pred = Cond->getPredicate();
if ((Pred == ICmpInst::ICMP_NE && TrueSucc == LoopEntry) ||
(Pred == ICmpInst::ICMP_EQ && FalseSucc == LoopEntry))
return Cond->getOperand(0);
return nullptr;
}
// Check if the recurrence variable `VarX` is in the right form to create
// the idiom. Returns the value coerced to a PHINode if so.
static PHINode *getRecurrenceVar(Value *VarX, Instruction *DefX,
BasicBlock *LoopEntry) {
auto *PhiX = dyn_cast<PHINode>(VarX);
if (PhiX && PhiX->getParent() == LoopEntry &&
(PhiX->getOperand(0) == DefX || PhiX->getOperand(1) == DefX))
return PhiX;
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 || SubOneOp->getOperand(0) != VarX1)
return false;
ConstantInt *Dec = dyn_cast<ConstantInt>(SubOneOp->getOperand(1));
if (!Dec ||
!((SubOneOp->getOpcode() == Instruction::Sub && Dec->isOne()) ||
(SubOneOp->getOpcode() == Instruction::Add &&
Dec->isMinusOne()))) {
return false;
}
}
// step 3: Check the recurrence of variable X
PhiX = getRecurrenceVar(VarX1, DefX2, LoopEntry);
if (!PhiX)
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 = getRecurrenceVar(Inst->getOperand(0), Inst, LoopEntry);
if (!Phi)
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;
}
/// Return true if the idiom is detected in the loop.
///
/// Additionally:
/// 1) \p CntInst is set to the instruction Counting Leading Zeros (CTLZ)
/// or nullptr if there is no such.
/// 2) \p CntPhi is set to the corresponding phi node
/// or nullptr if there is no such.
/// 3) \p Var is set to the value whose CTLZ could be used.
/// 4) \p DefX is set to the instruction calculating Loop exit condition.
///
/// 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 {
/// x = phi (x0, x.next); //PhiX
/// cnt = phi(cnt0, cnt.next);
///
/// cnt.next = cnt + 1;
/// ...
/// x.next = x >> 1; // DefX
/// ...
/// } while(x.next != 0);
///
/// loop-exit:
/// \endcode
static bool detectShiftUntilZeroIdiom(Loop *CurLoop, const DataLayout &DL,
Intrinsic::ID &IntrinID, Value *&InitX,
Instruction *&CntInst, PHINode *&CntPhi,
Instruction *&DefX) {
BasicBlock *LoopEntry;
Value *VarX = nullptr;
DefX = nullptr;
CntInst = nullptr;
CntPhi = 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))
DefX = dyn_cast<Instruction>(T);
else
return false;
// step 2: detect instructions corresponding to "x.next = x >> 1 or x << 1"
if (!DefX || !DefX->isShift())
return false;
IntrinID = DefX->getOpcode() == Instruction::Shl ? Intrinsic::cttz :
Intrinsic::ctlz;
ConstantInt *Shft = dyn_cast<ConstantInt>(DefX->getOperand(1));
if (!Shft || !Shft->isOne())
return false;
VarX = DefX->getOperand(0);
// step 3: Check the recurrence of variable X
PHINode *PhiX = getRecurrenceVar(VarX, DefX, LoopEntry);
if (!PhiX)
return false;
InitX = PhiX->getIncomingValueForBlock(CurLoop->getLoopPreheader());
// Make sure the initial value can't be negative otherwise the ashr in the
// loop might never reach zero which would make the loop infinite.
if (DefX->getOpcode() == Instruction::AShr && !isKnownNonNegative(InitX, DL))
return false;
// step 4: Find the instruction which count the CTLZ: cnt.next = cnt + 1
// or cnt.next = cnt + -1.
// TODO: We can skip the step. If loop trip count is known (CTLZ),
// then all uses of "cnt.next" could be optimized to the trip count
// plus "cnt0". Currently it is not optimized.
// This step could be used to detect POPCNT instruction:
// cnt.next = cnt + (x.next & 1)
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() && !Inc->isMinusOne()))
continue;
PHINode *Phi = getRecurrenceVar(Inst->getOperand(0), Inst, LoopEntry);
if (!Phi)
continue;
CntInst = Inst;
CntPhi = Phi;
break;
}
if (!CntInst)
return false;
return true;
}
/// Recognize CTLZ or CTTZ idiom in a non-countable loop and convert the loop
/// to countable (with CTLZ / CTTZ trip count). If CTLZ / CTTZ inserted as a new
/// trip count returns true; otherwise, returns false.
bool LoopIdiomRecognize::recognizeAndInsertFFS() {
// Give up if the loop has multiple blocks or multiple backedges.
if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
return false;
Intrinsic::ID IntrinID;
Value *InitX;
Instruction *DefX = nullptr;
PHINode *CntPhi = nullptr;
Instruction *CntInst = nullptr;
// Help decide if transformation is profitable. For ShiftUntilZero idiom,
// this is always 6.
size_t IdiomCanonicalSize = 6;
if (!detectShiftUntilZeroIdiom(CurLoop, *DL, IntrinID, InitX,
CntInst, CntPhi, DefX))
return false;
bool IsCntPhiUsedOutsideLoop = false;
for (User *U : CntPhi->users())
if (!CurLoop->contains(cast<Instruction>(U))) {
IsCntPhiUsedOutsideLoop = true;
break;
}
bool IsCntInstUsedOutsideLoop = false;
for (User *U : CntInst->users())
if (!CurLoop->contains(cast<Instruction>(U))) {
IsCntInstUsedOutsideLoop = true;
break;
}
// If both CntInst and CntPhi are used outside the loop the profitability
// is questionable.
if (IsCntInstUsedOutsideLoop && IsCntPhiUsedOutsideLoop)
return false;
// For some CPUs result of CTLZ(X) intrinsic is undefined
// when X is 0. If we can not guarantee X != 0, we need to check this
// when expand.
bool ZeroCheck = false;
// It is safe to assume Preheader exist as it was checked in
// parent function RunOnLoop.
BasicBlock *PH = CurLoop->getLoopPreheader();
// If we are using the count instruction outside the loop, make sure we
// have a zero check as a precondition. Without the check the loop would run
// one iteration for before any check of the input value. This means 0 and 1
// would have identical behavior in the original loop and thus
if (!IsCntPhiUsedOutsideLoop) {
auto *PreCondBB = PH->getSinglePredecessor();
if (!PreCondBB)
return false;
auto *PreCondBI = dyn_cast<BranchInst>(PreCondBB->getTerminator());
if (!PreCondBI)
return false;
if (matchCondition(PreCondBI, PH) != InitX)
return false;
ZeroCheck = true;
}
// Check if CTLZ / CTTZ intrinsic is profitable. Assume it is always
// profitable if we delete the loop.
// the loop has only 6 instructions:
// %n.addr.0 = phi [ %n, %entry ], [ %shr, %while.cond ]
// %i.0 = phi [ %i0, %entry ], [ %inc, %while.cond ]
// %shr = ashr %n.addr.0, 1
// %tobool = icmp eq %shr, 0
// %inc = add nsw %i.0, 1
// br i1 %tobool
const Value *Args[] = {InitX,
ConstantInt::getBool(InitX->getContext(), ZeroCheck)};
// @llvm.dbg doesn't count as they have no semantic effect.
auto InstWithoutDebugIt = CurLoop->getHeader()->instructionsWithoutDebug();
uint32_t HeaderSize =
std::distance(InstWithoutDebugIt.begin(), InstWithoutDebugIt.end());
IntrinsicCostAttributes Attrs(IntrinID, InitX->getType(), Args);
InstructionCost Cost =
TTI->getIntrinsicInstrCost(Attrs, TargetTransformInfo::TCK_SizeAndLatency);
if (HeaderSize != IdiomCanonicalSize &&
Cost > TargetTransformInfo::TCC_Basic)
return false;
transformLoopToCountable(IntrinID, PH, CntInst, CntPhi, InitX, DefX,
DefX->getDebugLoc(), ZeroCheck,
IsCntPhiUsedOutsideLoop);
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 || &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 intrinsic
// 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,
const DebugLoc &DL) {
Value *Ops[] = {Val};
Type *Tys[] = {Val->getType()};
Module *M = IRBuilder.GetInsertBlock()->getParent()->getParent();
Function *Func = Intrinsic::getDeclaration(M, Intrinsic::ctpop, Tys);
CallInst *CI = IRBuilder.CreateCall(Func, Ops);
CI->setDebugLoc(DL);
return CI;
}
static CallInst *createFFSIntrinsic(IRBuilder<> &IRBuilder, Value *Val,
const DebugLoc &DL, bool ZeroCheck,
Intrinsic::ID IID) {
Value *Ops[] = {Val, IRBuilder.getInt1(ZeroCheck)};
Type *Tys[] = {Val->getType()};
Module *M = IRBuilder.GetInsertBlock()->getParent()->getParent();
Function *Func = Intrinsic::getDeclaration(M, IID, Tys);
CallInst *CI = IRBuilder.CreateCall(Func, Ops);
CI->setDebugLoc(DL);
return CI;
}
/// Transform the following loop (Using CTLZ, CTTZ is similar):
/// loop:
/// CntPhi = PHI [Cnt0, CntInst]
/// PhiX = PHI [InitX, DefX]
/// CntInst = CntPhi + 1
/// DefX = PhiX >> 1
/// LOOP_BODY
/// Br: loop if (DefX != 0)
/// Use(CntPhi) or Use(CntInst)
///
/// Into:
/// If CntPhi used outside the loop:
/// CountPrev = BitWidth(InitX) - CTLZ(InitX >> 1)
/// Count = CountPrev + 1
/// else
/// Count = BitWidth(InitX) - CTLZ(InitX)
/// loop:
/// CntPhi = PHI [Cnt0, CntInst]
/// PhiX = PHI [InitX, DefX]
/// PhiCount = PHI [Count, Dec]
/// CntInst = CntPhi + 1
/// DefX = PhiX >> 1
/// Dec = PhiCount - 1
/// LOOP_BODY
/// Br: loop if (Dec != 0)
/// Use(CountPrev + Cnt0) // Use(CntPhi)
/// or
/// Use(Count + Cnt0) // Use(CntInst)
///
/// If LOOP_BODY is empty the loop will be deleted.
/// If CntInst and DefX are not used in LOOP_BODY they will be removed.
void LoopIdiomRecognize::transformLoopToCountable(
Intrinsic::ID IntrinID, BasicBlock *Preheader, Instruction *CntInst,
PHINode *CntPhi, Value *InitX, Instruction *DefX, const DebugLoc &DL,
bool ZeroCheck, bool IsCntPhiUsedOutsideLoop) {
BranchInst *PreheaderBr = cast<BranchInst>(Preheader->getTerminator());
// Step 1: Insert the CTLZ/CTTZ instruction at the end of the preheader block
IRBuilder<> Builder(PreheaderBr);
Builder.SetCurrentDebugLocation(DL);
// If there are no uses of CntPhi crate:
// Count = BitWidth - CTLZ(InitX);
// NewCount = Count;
// If there are uses of CntPhi create:
// NewCount = BitWidth - CTLZ(InitX >> 1);
// Count = NewCount + 1;
Value *InitXNext;
if (IsCntPhiUsedOutsideLoop) {
if (DefX->getOpcode() == Instruction::AShr)
InitXNext = Builder.CreateAShr(InitX, 1);
else if (DefX->getOpcode() == Instruction::LShr)
InitXNext = Builder.CreateLShr(InitX, 1);
else if (DefX->getOpcode() == Instruction::Shl) // cttz
InitXNext = Builder.CreateShl(InitX, 1);
else
llvm_unreachable("Unexpected opcode!");
} else
InitXNext = InitX;
Value *Count =
createFFSIntrinsic(Builder, InitXNext, DL, ZeroCheck, IntrinID);
Type *CountTy = Count->getType();
Count = Builder.CreateSub(
ConstantInt::get(CountTy, CountTy->getIntegerBitWidth()), Count);
Value *NewCount = Count;
if (IsCntPhiUsedOutsideLoop)
Count = Builder.CreateAdd(Count, ConstantInt::get(CountTy, 1));
NewCount = Builder.CreateZExtOrTrunc(NewCount, CntInst->getType());
Value *CntInitVal = CntPhi->getIncomingValueForBlock(Preheader);
if (cast<ConstantInt>(CntInst->getOperand(1))->isOne()) {
// If the counter was being incremented in the loop, add NewCount to the
// counter's initial value, but only if the initial value is not zero.
ConstantInt *InitConst = dyn_cast<ConstantInt>(CntInitVal);
if (!InitConst || !InitConst->isZero())
NewCount = Builder.CreateAdd(NewCount, CntInitVal);
} else {
// If the count was being decremented in the loop, subtract NewCount from
// the counter's initial value.
NewCount = Builder.CreateSub(CntInitVal, NewCount);
}
// Step 2: Insert new IV and loop condition:
// loop:
// ...
// PhiCount = PHI [Count, Dec]
// ...
// Dec = PhiCount - 1
// ...
// Br: loop if (Dec != 0)
BasicBlock *Body = *(CurLoop->block_begin());
auto *LbBr = cast<BranchInst>(Body->getTerminator());
ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition());
PHINode *TcPhi = PHINode::Create(CountTy, 2, "tcphi", &Body->front());
Builder.SetInsertPoint(LbCond);
Instruction *TcDec = cast<Instruction>(Builder.CreateSub(
TcPhi, ConstantInt::get(CountTy, 1), "tcdec", false, true));
TcPhi->addIncoming(Count, Preheader);
TcPhi->addIncoming(TcDec, Body);
CmpInst::Predicate Pred =
(LbBr->getSuccessor(0) == Body) ? CmpInst::ICMP_NE : CmpInst::ICMP_EQ;
LbCond->setPredicate(Pred);
LbCond->setOperand(0, TcDec);
LbCond->setOperand(1, ConstantInt::get(CountTy, 0));
// Step 3: All the references to the original counter outside
// the loop are replaced with the NewCount
if (IsCntPhiUsedOutsideLoop)
CntPhi->replaceUsesOutsideBlock(NewCount, Body);
else
CntInst->replaceUsesOutsideBlock(NewCount, Body);
// step 4: 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);
}
void LoopIdiomRecognize::transformLoopToPopcount(BasicBlock *PreCondBB,
Instruction *CntInst,
PHINode *CntPhi, Value *Var) {
BasicBlock *PreHead = CurLoop->getLoopPreheader();
auto *PreCondBr = 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 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 = 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);
}
/// Match loop-invariant value.
template <typename SubPattern_t> struct match_LoopInvariant {
SubPattern_t SubPattern;
const Loop *L;
match_LoopInvariant(const SubPattern_t &SP, const Loop *L)
: SubPattern(SP), L(L) {}
template <typename ITy> bool match(ITy *V) {
return L->isLoopInvariant(V) && SubPattern.match(V);
}
};
/// Matches if the value is loop-invariant.
template <typename Ty>
inline match_LoopInvariant<Ty> m_LoopInvariant(const Ty &M, const Loop *L) {
return match_LoopInvariant<Ty>(M, L);
}
/// Return true if the idiom is detected in the loop.
///
/// The core idiom we are trying to detect is:
/// \code
/// entry:
/// <...>
/// %bitmask = shl i32 1, %bitpos
/// br label %loop
///
/// loop:
/// %x.curr = phi i32 [ %x, %entry ], [ %x.next, %loop ]
/// %x.curr.bitmasked = and i32 %x.curr, %bitmask
/// %x.curr.isbitunset = icmp eq i32 %x.curr.bitmasked, 0
/// %x.next = shl i32 %x.curr, 1
/// <...>
/// br i1 %x.curr.isbitunset, label %loop, label %end
///
/// end:
/// %x.curr.res = phi i32 [ %x.curr, %loop ] <...>
/// %x.next.res = phi i32 [ %x.next, %loop ] <...>
/// <...>
/// \endcode
static bool detectShiftUntilBitTestIdiom(Loop *CurLoop, Value *&BaseX,
Value *&BitMask, Value *&BitPos,
Value *&CurrX, Instruction *&NextX) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE
" Performing shift-until-bittest idiom detection.\n");
// Give up if the loop has multiple blocks or multiple backedges.
if (CurLoop->getNumBlocks() != 1 || CurLoop->getNumBackEdges() != 1) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad block/backedge count.\n");
return false;
}
BasicBlock *LoopHeaderBB = CurLoop->getHeader();
BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
assert(LoopPreheaderBB && "There is always a loop preheader.");
using namespace PatternMatch;
// Step 1: Check if the loop backedge is in desirable form.
ICmpInst::Predicate Pred;
Value *CmpLHS, *CmpRHS;
BasicBlock *TrueBB, *FalseBB;
if (!match(LoopHeaderBB->getTerminator(),
m_Br(m_ICmp(Pred, m_Value(CmpLHS), m_Value(CmpRHS)),
m_BasicBlock(TrueBB), m_BasicBlock(FalseBB)))) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge structure.\n");
return false;
}
// Step 2: Check if the backedge's condition is in desirable form.
auto MatchVariableBitMask = [&]() {
return ICmpInst::isEquality(Pred) && match(CmpRHS, m_Zero()) &&
match(CmpLHS,
m_c_And(m_Value(CurrX),
m_CombineAnd(
m_Value(BitMask),
m_LoopInvariant(m_Shl(m_One(), m_Value(BitPos)),
CurLoop))));
};
auto MatchConstantBitMask = [&]() {
return ICmpInst::isEquality(Pred) && match(CmpRHS, m_Zero()) &&
match(CmpLHS, m_And(m_Value(CurrX),
m_CombineAnd(m_Value(BitMask), m_Power2()))) &&
(BitPos = ConstantExpr::getExactLogBase2(cast<Constant>(BitMask)));
};
auto MatchDecomposableConstantBitMask = [&]() {
APInt Mask;
return llvm::decomposeBitTestICmp(CmpLHS, CmpRHS, Pred, CurrX, Mask) &&
ICmpInst::isEquality(Pred) && Mask.isPowerOf2() &&
(BitMask = ConstantInt::get(CurrX->getType(), Mask)) &&
(BitPos = ConstantInt::get(CurrX->getType(), Mask.logBase2()));
};
if (!MatchVariableBitMask() && !MatchConstantBitMask() &&
!MatchDecomposableConstantBitMask()) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge comparison.\n");
return false;
}
// Step 3: Check if the recurrence is in desirable form.
auto *CurrXPN = dyn_cast<PHINode>(CurrX);
if (!CurrXPN || CurrXPN->getParent() != LoopHeaderBB) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Not an expected PHI node.\n");
return false;
}
BaseX = CurrXPN->getIncomingValueForBlock(LoopPreheaderBB);
NextX =
dyn_cast<Instruction>(CurrXPN->getIncomingValueForBlock(LoopHeaderBB));
assert(CurLoop->isLoopInvariant(BaseX) &&
"Expected BaseX to be avaliable in the preheader!");
if (!NextX || !match(NextX, m_Shl(m_Specific(CurrX), m_One()))) {
// FIXME: support right-shift?
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad recurrence.\n");
return false;
}
// Step 4: Check if the backedge's destinations are in desirable form.
assert(ICmpInst::isEquality(Pred) &&
"Should only get equality predicates here.");
// cmp-br is commutative, so canonicalize to a single variant.
if (Pred != ICmpInst::Predicate::ICMP_EQ) {
Pred = ICmpInst::getInversePredicate(Pred);
std::swap(TrueBB, FalseBB);
}
// We expect to exit loop when comparison yields false,
// so when it yields true we should branch back to loop header.
if (TrueBB != LoopHeaderBB) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge flow.\n");
return false;
}
// Okay, idiom checks out.
return true;
}
/// Look for the following loop:
/// \code
/// entry:
/// <...>
/// %bitmask = shl i32 1, %bitpos
/// br label %loop
///
/// loop:
/// %x.curr = phi i32 [ %x, %entry ], [ %x.next, %loop ]
/// %x.curr.bitmasked = and i32 %x.curr, %bitmask
/// %x.curr.isbitunset = icmp eq i32 %x.curr.bitmasked, 0
/// %x.next = shl i32 %x.curr, 1
/// <...>
/// br i1 %x.curr.isbitunset, label %loop, label %end
///
/// end:
/// %x.curr.res = phi i32 [ %x.curr, %loop ] <...>
/// %x.next.res = phi i32 [ %x.next, %loop ] <...>
/// <...>
/// \endcode
///
/// And transform it into:
/// \code
/// entry:
/// %bitmask = shl i32 1, %bitpos
/// %lowbitmask = add i32 %bitmask, -1
/// %mask = or i32 %lowbitmask, %bitmask
/// %x.masked = and i32 %x, %mask
/// %x.masked.numleadingzeros = call i32 @llvm.ctlz.i32(i32 %x.masked,
/// i1 true)
/// %x.masked.numactivebits = sub i32 32, %x.masked.numleadingzeros
/// %x.masked.leadingonepos = add i32 %x.masked.numactivebits, -1
/// %backedgetakencount = sub i32 %bitpos, %x.masked.leadingonepos
/// %tripcount = add i32 %backedgetakencount, 1
/// %x.curr = shl i32 %x, %backedgetakencount
/// %x.next = shl i32 %x, %tripcount
/// br label %loop
///
/// loop:
/// %loop.iv = phi i32 [ 0, %entry ], [ %loop.iv.next, %loop ]
/// %loop.iv.next = add nuw i32 %loop.iv, 1
/// %loop.ivcheck = icmp eq i32 %loop.iv.next, %tripcount
/// <...>
/// br i1 %loop.ivcheck, label %end, label %loop
///
/// end:
/// %x.curr.res = phi i32 [ %x.curr, %loop ] <...>
/// %x.next.res = phi i32 [ %x.next, %loop ] <...>
/// <...>
/// \endcode
bool LoopIdiomRecognize::recognizeShiftUntilBitTest() {
bool MadeChange = false;
Value *X, *BitMask, *BitPos, *XCurr;
Instruction *XNext;
if (!detectShiftUntilBitTestIdiom(CurLoop, X, BitMask, BitPos, XCurr,
XNext)) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE
" shift-until-bittest idiom detection failed.\n");
return MadeChange;
}
LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-bittest idiom detected!\n");
// Ok, it is the idiom we were looking for, we *could* transform this loop,
// but is it profitable to transform?
BasicBlock *LoopHeaderBB = CurLoop->getHeader();
BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
assert(LoopPreheaderBB && "There is always a loop preheader.");
BasicBlock *SuccessorBB = CurLoop->getExitBlock();
assert(SuccessorBB && "There is only a single successor.");
IRBuilder<> Builder(LoopPreheaderBB->getTerminator());
Builder.SetCurrentDebugLocation(cast<Instruction>(XCurr)->getDebugLoc());
Intrinsic::ID IntrID = Intrinsic::ctlz;
Type *Ty = X->getType();
unsigned Bitwidth = Ty->getScalarSizeInBits();
TargetTransformInfo::TargetCostKind CostKind =
TargetTransformInfo::TCK_SizeAndLatency;
// The rewrite is considered to be unprofitable iff and only iff the
// intrinsic/shift we'll use are not cheap. Note that we are okay with *just*
// making the loop countable, even if nothing else changes.
IntrinsicCostAttributes Attrs(
IntrID, Ty, {UndefValue::get(Ty), /*is_zero_undef=*/Builder.getTrue()});
InstructionCost Cost = TTI->getIntrinsicInstrCost(Attrs, CostKind);
if (Cost > TargetTransformInfo::TCC_Basic) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE
" Intrinsic is too costly, not beneficial\n");
return MadeChange;
}
if (TTI->getArithmeticInstrCost(Instruction::Shl, Ty, CostKind) >
TargetTransformInfo::TCC_Basic) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Shift is too costly, not beneficial\n");
return MadeChange;
}
// Ok, transform appears worthwhile.
MadeChange = true;
// Step 1: Compute the loop trip count.
Value *LowBitMask = Builder.CreateAdd(BitMask, Constant::getAllOnesValue(Ty),
BitPos->getName() + ".lowbitmask");
Value *Mask =
Builder.CreateOr(LowBitMask, BitMask, BitPos->getName() + ".mask");
Value *XMasked = Builder.CreateAnd(X, Mask, X->getName() + ".masked");
CallInst *XMaskedNumLeadingZeros = Builder.CreateIntrinsic(
IntrID, Ty, {XMasked, /*is_zero_undef=*/Builder.getTrue()},
/*FMFSource=*/nullptr, XMasked->getName() + ".numleadingzeros");
Value *XMaskedNumActiveBits = Builder.CreateSub(
ConstantInt::get(Ty, Ty->getScalarSizeInBits()), XMaskedNumLeadingZeros,
XMasked->getName() + ".numactivebits", /*HasNUW=*/true,
/*HasNSW=*/Bitwidth != 2);
Value *XMaskedLeadingOnePos =
Builder.CreateAdd(XMaskedNumActiveBits, Constant::getAllOnesValue(Ty),
XMasked->getName() + ".leadingonepos", /*HasNUW=*/false,
/*HasNSW=*/Bitwidth > 2);
Value *LoopBackedgeTakenCount = Builder.CreateSub(
BitPos, XMaskedLeadingOnePos, CurLoop->getName() + ".backedgetakencount",
/*HasNUW=*/true, /*HasNSW=*/true);
// We know loop's backedge-taken count, but what's loop's trip count?
// Note that while NUW is always safe, while NSW is only for bitwidths != 2.
Value *LoopTripCount =
Builder.CreateAdd(LoopBackedgeTakenCount, ConstantInt::get(Ty, 1),
CurLoop->getName() + ".tripcount", /*HasNUW=*/true,
/*HasNSW=*/Bitwidth != 2);
// Step 2: Compute the recurrence's final value without a loop.
// NewX is always safe to compute, because `LoopBackedgeTakenCount`
// will always be smaller than `bitwidth(X)`, i.e. we never get poison.
Value *NewX = Builder.CreateShl(X, LoopBackedgeTakenCount);
NewX->takeName(XCurr);
if (auto *I = dyn_cast<Instruction>(NewX))
I->copyIRFlags(XNext, /*IncludeWrapFlags=*/true);
Value *NewXNext;
// Rewriting XNext is more complicated, however, because `X << LoopTripCount`
// will be poison iff `LoopTripCount == bitwidth(X)` (which will happen
// iff `BitPos` is `bitwidth(x) - 1` and `X` is `1`). So unless we know
// that isn't the case, we'll need to emit an alternative, safe IR.
if (XNext->hasNoSignedWrap() || XNext->hasNoUnsignedWrap() ||
PatternMatch::match(
BitPos, PatternMatch::m_SpecificInt_ICMP(
ICmpInst::ICMP_NE, APInt(Ty->getScalarSizeInBits(),
Ty->getScalarSizeInBits() - 1))))
NewXNext = Builder.CreateShl(X, LoopTripCount);
else {
// Otherwise, just additionally shift by one. It's the smallest solution,
// alternatively, we could check that NewX is INT_MIN (or BitPos is )
// and select 0 instead.
NewXNext = Builder.CreateShl(NewX, ConstantInt::get(Ty, 1));
}
NewXNext->takeName(XNext);
if (auto *I = dyn_cast<Instruction>(NewXNext))
I->copyIRFlags(XNext, /*IncludeWrapFlags=*/true);
// Step 3: Adjust the successor basic block to recieve the computed
// recurrence's final value instead of the recurrence itself.
XCurr->replaceUsesOutsideBlock(NewX, LoopHeaderBB);
XNext->replaceUsesOutsideBlock(NewXNext, LoopHeaderBB);
// Step 4: Rewrite the loop into a countable form, with canonical IV.
// The new canonical induction variable.
Builder.SetInsertPoint(&LoopHeaderBB->front());
auto *IV = Builder.CreatePHI(Ty, 2, CurLoop->getName() + ".iv");
// The induction itself.
// Note that while NUW is always safe, while NSW is only for bitwidths != 2.
Builder.SetInsertPoint(LoopHeaderBB->getTerminator());
auto *IVNext =
Builder.CreateAdd(IV, ConstantInt::get(Ty, 1), IV->getName() + ".next",
/*HasNUW=*/true, /*HasNSW=*/Bitwidth != 2);
// The loop trip count check.
auto *IVCheck = Builder.CreateICmpEQ(IVNext, LoopTripCount,
CurLoop->getName() + ".ivcheck");
Builder.CreateCondBr(IVCheck, SuccessorBB, LoopHeaderBB);
LoopHeaderBB->getTerminator()->eraseFromParent();
// Populate the IV PHI.
IV->addIncoming(ConstantInt::get(Ty, 0), LoopPreheaderBB);
IV->addIncoming(IVNext, LoopHeaderBB);
// 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);
// Other passes will take care of actually deleting the loop if possible.
LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-bittest idiom optimized!\n");
++NumShiftUntilBitTest;
return MadeChange;
}
/// Return true if the idiom is detected in the loop.
///
/// The core idiom we are trying to detect is:
/// \code
/// entry:
/// <...>
/// %start = <...>
/// %extraoffset = <...>
/// <...>
/// br label %for.cond
///
/// loop:
/// %iv = phi i8 [ %start, %entry ], [ %iv.next, %for.cond ]
/// %nbits = add nsw i8 %iv, %extraoffset
/// %val.shifted = lshr i8 %val, %nbits
/// %val.shifted.iszero = icmp eq i8 %val.shifted, 0
/// %iv.next = add i8 %iv, 1
/// <...>
/// br i1 %val.shifted.iszero, label %end, label %loop
///
/// end:
/// %iv.res = phi i8 [ %iv, %loop ] <...>
/// %nbits.res = phi i8 [ %nbits, %loop ] <...>
/// %val.shifted.res = phi i8 [ %val.shifted, %loop ] <...>
/// %val.shifted.iszero.res = phi i1 [ %val.shifted.iszero, %loop ] <...>
/// %iv.next.res = phi i8 [ %iv.next, %loop ] <...>
/// <...>
/// \endcode
static bool detectShiftUntilZeroIdiom(Loop *CurLoop, ScalarEvolution *SE,
Instruction *&ValShiftedIsZero,
Instruction *&IV, Value *&Start,
Value *&Val, const SCEV *&ExtraOffsetExpr,
bool &InvertedCond) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE
" Performing shift-until-zero idiom detection.\n");
// Give up if the loop has multiple blocks or multiple backedges.
if (CurLoop->getNumBlocks() != 1 || CurLoop->getNumBackEdges() != 1) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad block/backedge count.\n");
return false;
}
Instruction *ValShifted, *NBits, *IVNext;
Value *ExtraOffset;
BasicBlock *LoopHeaderBB = CurLoop->getHeader();
BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
assert(LoopPreheaderBB && "There is always a loop preheader.");
using namespace PatternMatch;
// Step 1: Check if the loop backedge, condition is in desirable form.
ICmpInst::Predicate Pred;
BasicBlock *TrueBB, *FalseBB;
if (!match(LoopHeaderBB->getTerminator(),
m_Br(m_Instruction(ValShiftedIsZero), m_BasicBlock(TrueBB),
m_BasicBlock(FalseBB))) ||
!match(ValShiftedIsZero,
m_ICmp(Pred, m_Instruction(ValShifted), m_Zero())) ||
!ICmpInst::isEquality(Pred)) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge structure.\n");
return false;
}
// Step 2: Check if the comparison's operand is in desirable form.
// FIXME: Val could be a one-input PHI node, which we should look past.
if (!match(ValShifted, m_LShr(m_LoopInvariant(m_Value(Val), CurLoop),
m_Instruction(NBits)))) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad comparisons value computation.\n");
return false;
}
// Step 3: Check if the shift amount is in desirable form.
if (match(NBits, m_c_Add(m_Instruction(IV),
m_LoopInvariant(m_Value(ExtraOffset), CurLoop))) &&
(NBits->hasNoSignedWrap() || NBits->hasNoUnsignedWrap()))
ExtraOffsetExpr = SE->getNegativeSCEV(SE->getSCEV(ExtraOffset));
else if (match(NBits,
m_Sub(m_Instruction(IV),
m_LoopInvariant(m_Value(ExtraOffset), CurLoop))) &&
NBits->hasNoSignedWrap())
ExtraOffsetExpr = SE->getSCEV(ExtraOffset);
else {
IV = NBits;
ExtraOffsetExpr = SE->getZero(NBits->getType());
}
// Step 4: Check if the recurrence is in desirable form.
auto *IVPN = dyn_cast<PHINode>(IV);
if (!IVPN || IVPN->getParent() != LoopHeaderBB) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Not an expected PHI node.\n");
return false;
}
Start = IVPN->getIncomingValueForBlock(LoopPreheaderBB);
IVNext = dyn_cast<Instruction>(IVPN->getIncomingValueForBlock(LoopHeaderBB));
if (!IVNext || !match(IVNext, m_Add(m_Specific(IVPN), m_One()))) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad recurrence.\n");
return false;
}
// Step 4: Check if the backedge's destinations are in desirable form.
assert(ICmpInst::isEquality(Pred) &&
"Should only get equality predicates here.");
// cmp-br is commutative, so canonicalize to a single variant.
InvertedCond = Pred != ICmpInst::Predicate::ICMP_EQ;
if (InvertedCond) {
Pred = ICmpInst::getInversePredicate(Pred);
std::swap(TrueBB, FalseBB);
}
// We expect to exit loop when comparison yields true,
// so when it yields false we should branch back to loop header.
if (FalseBB != LoopHeaderBB) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge flow.\n");
return false;
}
// Okay, idiom checks out.
return true;
}
/// Look for the following loop:
/// \code
/// entry:
/// <...>
/// %start = <...>
/// %extraoffset = <...>
/// <...>
/// br label %for.cond
///
/// loop:
/// %iv = phi i8 [ %start, %entry ], [ %iv.next, %for.cond ]
/// %nbits = add nsw i8 %iv, %extraoffset
/// %val.shifted = lshr i8 %val, %nbits
/// %val.shifted.iszero = icmp eq i8 %val.shifted, 0
/// %iv.next = add i8 %iv, 1
/// <...>
/// br i1 %val.shifted.iszero, label %end, label %loop
///
/// end:
/// %iv.res = phi i8 [ %iv, %loop ] <...>
/// %nbits.res = phi i8 [ %nbits, %loop ] <...>
/// %val.shifted.res = phi i8 [ %val.shifted, %loop ] <...>
/// %val.shifted.iszero.res = phi i1 [ %val.shifted.iszero, %loop ] <...>
/// %iv.next.res = phi i8 [ %iv.next, %loop ] <...>
/// <...>
/// \endcode
///
/// And transform it into:
/// \code
/// entry:
/// <...>
/// %start = <...>
/// %extraoffset = <...>
/// <...>
/// %val.numleadingzeros = call i8 @llvm.ctlz.i8(i8 %val, i1 0)
/// %val.numactivebits = sub i8 8, %val.numleadingzeros
/// %extraoffset.neg = sub i8 0, %extraoffset
/// %tmp = add i8 %val.numactivebits, %extraoffset.neg
/// %iv.final = call i8 @llvm.smax.i8(i8 %tmp, i8 %start)
/// %loop.tripcount = sub i8 %iv.final, %start
/// br label %loop
///
/// loop:
/// %loop.iv = phi i8 [ 0, %entry ], [ %loop.iv.next, %loop ]
/// %loop.iv.next = add i8 %loop.iv, 1
/// %loop.ivcheck = icmp eq i8 %loop.iv.next, %loop.tripcount
/// %iv = add i8 %loop.iv, %start
/// <...>
/// br i1 %loop.ivcheck, label %end, label %loop
///
/// end:
/// %iv.res = phi i8 [ %iv.final, %loop ] <...>
/// <...>
/// \endcode
bool LoopIdiomRecognize::recognizeShiftUntilZero() {
bool MadeChange = false;
Instruction *ValShiftedIsZero, *IV;
Value *Start, *Val;
const SCEV *ExtraOffsetExpr;
bool InvertedCond;
if (!detectShiftUntilZeroIdiom(CurLoop, SE, ValShiftedIsZero, IV, Start, Val,
ExtraOffsetExpr, InvertedCond)) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE
" shift-until-zero idiom detection failed.\n");
return MadeChange;
}
LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-zero idiom detected!\n");
// Ok, it is the idiom we were looking for, we *could* transform this loop,
// but is it profitable to transform?
BasicBlock *LoopHeaderBB = CurLoop->getHeader();
BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
assert(LoopPreheaderBB && "There is always a loop preheader.");
BasicBlock *SuccessorBB = CurLoop->getExitBlock();
assert(SuccessorBB && "There is only a single successor.");
IRBuilder<> Builder(LoopPreheaderBB->getTerminator());
Builder.SetCurrentDebugLocation(IV->getDebugLoc());
Intrinsic::ID IntrID = Intrinsic::ctlz;
Type *Ty = Val->getType();
unsigned Bitwidth = Ty->getScalarSizeInBits();
TargetTransformInfo::TargetCostKind CostKind =
TargetTransformInfo::TCK_SizeAndLatency;
// The rewrite is considered to be unprofitable iff and only iff the
// intrinsic we'll use are not cheap. Note that we are okay with *just*
// making the loop countable, even if nothing else changes.
IntrinsicCostAttributes Attrs(
IntrID, Ty, {UndefValue::get(Ty), /*is_zero_undef=*/Builder.getFalse()});
InstructionCost Cost = TTI->getIntrinsicInstrCost(Attrs, CostKind);
if (Cost > TargetTransformInfo::TCC_Basic) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE
" Intrinsic is too costly, not beneficial\n");
return MadeChange;
}
// Ok, transform appears worthwhile.
MadeChange = true;
bool OffsetIsZero = false;
if (auto *ExtraOffsetExprC = dyn_cast<SCEVConstant>(ExtraOffsetExpr))
OffsetIsZero = ExtraOffsetExprC->isZero();
// Step 1: Compute the loop's final IV value / trip count.
CallInst *ValNumLeadingZeros = Builder.CreateIntrinsic(
IntrID, Ty, {Val, /*is_zero_undef=*/Builder.getFalse()},
/*FMFSource=*/nullptr, Val->getName() + ".numleadingzeros");
Value *ValNumActiveBits = Builder.CreateSub(
ConstantInt::get(Ty, Ty->getScalarSizeInBits()), ValNumLeadingZeros,
Val->getName() + ".numactivebits", /*HasNUW=*/true,
/*HasNSW=*/Bitwidth != 2);
SCEVExpander Expander(*SE, *DL, "loop-idiom");
Expander.setInsertPoint(&*Builder.GetInsertPoint());
Value *ExtraOffset = Expander.expandCodeFor(ExtraOffsetExpr);
Value *ValNumActiveBitsOffset = Builder.CreateAdd(
ValNumActiveBits, ExtraOffset, ValNumActiveBits->getName() + ".offset",
/*HasNUW=*/OffsetIsZero, /*HasNSW=*/true);
Value *IVFinal = Builder.CreateIntrinsic(Intrinsic::smax, {Ty},
{ValNumActiveBitsOffset, Start},
/*FMFSource=*/nullptr, "iv.final");
auto *LoopBackedgeTakenCount = cast<Instruction>(Builder.CreateSub(
IVFinal, Start, CurLoop->getName() + ".backedgetakencount",
/*HasNUW=*/OffsetIsZero, /*HasNSW=*/true));
// FIXME: or when the offset was `add nuw`
// We know loop's backedge-taken count, but what's loop's trip count?
Value *LoopTripCount =
Builder.CreateAdd(LoopBackedgeTakenCount, ConstantInt::get(Ty, 1),
CurLoop->getName() + ".tripcount", /*HasNUW=*/true,
/*HasNSW=*/Bitwidth != 2);
// Step 2: Adjust the successor basic block to recieve the original
// induction variable's final value instead of the orig. IV itself.
IV->replaceUsesOutsideBlock(IVFinal, LoopHeaderBB);
// Step 3: Rewrite the loop into a countable form, with canonical IV.
// The new canonical induction variable.
Builder.SetInsertPoint(&LoopHeaderBB->front());
auto *CIV = Builder.CreatePHI(Ty, 2, CurLoop->getName() + ".iv");
// The induction itself.
Builder.SetInsertPoint(LoopHeaderBB->getFirstNonPHI());
auto *CIVNext =
Builder.CreateAdd(CIV, ConstantInt::get(Ty, 1), CIV->getName() + ".next",
/*HasNUW=*/true, /*HasNSW=*/Bitwidth != 2);
// The loop trip count check.
auto *CIVCheck = Builder.CreateICmpEQ(CIVNext, LoopTripCount,
CurLoop->getName() + ".ivcheck");
auto *NewIVCheck = CIVCheck;
if (InvertedCond) {
NewIVCheck = Builder.CreateNot(CIVCheck);
NewIVCheck->takeName(ValShiftedIsZero);
}
// The original IV, but rebased to be an offset to the CIV.
auto *IVDePHId = Builder.CreateAdd(CIV, Start, "", /*HasNUW=*/false,
/*HasNSW=*/true); // FIXME: what about NUW?
IVDePHId->takeName(IV);
// The loop terminator.
Builder.SetInsertPoint(LoopHeaderBB->getTerminator());
Builder.CreateCondBr(CIVCheck, SuccessorBB, LoopHeaderBB);
LoopHeaderBB->getTerminator()->eraseFromParent();
// Populate the IV PHI.
CIV->addIncoming(ConstantInt::get(Ty, 0), LoopPreheaderBB);
CIV->addIncoming(CIVNext, LoopHeaderBB);
// Step 4: 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);
// Step 5: Try to cleanup the loop's body somewhat.
IV->replaceAllUsesWith(IVDePHId);
IV->eraseFromParent();
ValShiftedIsZero->replaceAllUsesWith(NewIVCheck);
ValShiftedIsZero->eraseFromParent();
// Other passes will take care of actually deleting the loop if possible.
LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-zero idiom optimized!\n");
++NumShiftUntilZero;
return MadeChange;
}
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