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llvm-project/llvm/lib/Transforms/Vectorize/LoopIdiomVectorize.cpp
David Sherwood dc6c3ba4c4
[NFC][IR] Add CreateCountTrailingZeroElems helper (#106711) 
The LoopIdiomVectorize pass already creates calls to the intrinsic
experimental_cttz_elts, but PR #88385 will start calling this more
too so I've created a helper for it.
2024-09-02 13:40:14 +01:00

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//===-------- LoopIdiomVectorize.cpp - Loop idiom vectorization -----------===//
//
// 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 a pass that recognizes certain loop idioms and
// transforms them into more optimized versions of the same loop. In cases
// where this happens, it can be a significant performance win.
//
// We currently only recognize one loop that finds the first mismatched byte
// in an array and returns the index, i.e. something like:
//
// while (++i != n) {
// if (a[i] != b[i])
// break;
// }
//
// In this example we can actually vectorize the loop despite the early exit,
// although the loop vectorizer does not support it. It requires some extra
// checks to deal with the possibility of faulting loads when crossing page
// boundaries. However, even with these checks it is still profitable to do the
// transformation.
//
//===----------------------------------------------------------------------===//
//
// NOTE: This Pass matches a really specific loop pattern because it's only
// supposed to be a temporary solution until our LoopVectorizer is powerful
// enought to vectorize it automatically.
//
// TODO List:
//
// * Add support for the inverse case where we scan for a matching element.
// * Permit 64-bit induction variable types.
// * Recognize loops that increment the IV *after* comparing bytes.
// * Allow 32-bit sign-extends of the IV used by the GEP.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Vectorize/LoopIdiomVectorize.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "loop-idiom-vectorize"
static cl::opt<bool> DisableAll("disable-loop-idiom-vectorize-all", cl::Hidden,
cl::init(false),
cl::desc("Disable Loop Idiom Vectorize Pass."));
static cl::opt<LoopIdiomVectorizeStyle>
LITVecStyle("loop-idiom-vectorize-style", cl::Hidden,
cl::desc("The vectorization style for loop idiom transform."),
cl::values(clEnumValN(LoopIdiomVectorizeStyle::Masked, "masked",
"Use masked vector intrinsics"),
clEnumValN(LoopIdiomVectorizeStyle::Predicated,
"predicated", "Use VP intrinsics")),
cl::init(LoopIdiomVectorizeStyle::Masked));
static cl::opt<bool>
DisableByteCmp("disable-loop-idiom-vectorize-bytecmp", cl::Hidden,
cl::init(false),
cl::desc("Proceed with Loop Idiom Vectorize Pass, but do "
"not convert byte-compare loop(s)."));
static cl::opt<unsigned>
ByteCmpVF("loop-idiom-vectorize-bytecmp-vf", cl::Hidden,
cl::desc("The vectorization factor for byte-compare patterns."),
cl::init(16));
static cl::opt<bool>
VerifyLoops("loop-idiom-vectorize-verify", cl::Hidden, cl::init(false),
cl::desc("Verify loops generated Loop Idiom Vectorize Pass."));
namespace {
class LoopIdiomVectorize {
LoopIdiomVectorizeStyle VectorizeStyle;
unsigned ByteCompareVF;
Loop *CurLoop = nullptr;
DominatorTree *DT;
LoopInfo *LI;
const TargetTransformInfo *TTI;
const DataLayout *DL;
// Blocks that will be used for inserting vectorized code.
BasicBlock *EndBlock = nullptr;
BasicBlock *VectorLoopPreheaderBlock = nullptr;
BasicBlock *VectorLoopStartBlock = nullptr;
BasicBlock *VectorLoopMismatchBlock = nullptr;
BasicBlock *VectorLoopIncBlock = nullptr;
public:
LoopIdiomVectorize(LoopIdiomVectorizeStyle S, unsigned VF, DominatorTree *DT,
LoopInfo *LI, const TargetTransformInfo *TTI,
const DataLayout *DL)
: VectorizeStyle(S), ByteCompareVF(VF), DT(DT), LI(LI), TTI(TTI), DL(DL) {
}
bool run(Loop *L);
private:
/// \name Countable Loop Idiom Handling
/// @{
bool runOnCountableLoop();
bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
SmallVectorImpl<BasicBlock *> &ExitBlocks);
bool recognizeByteCompare();
Value *expandFindMismatch(IRBuilder<> &Builder, DomTreeUpdater &DTU,
GetElementPtrInst *GEPA, GetElementPtrInst *GEPB,
Instruction *Index, Value *Start, Value *MaxLen);
Value *createMaskedFindMismatch(IRBuilder<> &Builder, DomTreeUpdater &DTU,
GetElementPtrInst *GEPA,
GetElementPtrInst *GEPB, Value *ExtStart,
Value *ExtEnd);
Value *createPredicatedFindMismatch(IRBuilder<> &Builder, DomTreeUpdater &DTU,
GetElementPtrInst *GEPA,
GetElementPtrInst *GEPB, Value *ExtStart,
Value *ExtEnd);
void transformByteCompare(GetElementPtrInst *GEPA, GetElementPtrInst *GEPB,
PHINode *IndPhi, Value *MaxLen, Instruction *Index,
Value *Start, bool IncIdx, BasicBlock *FoundBB,
BasicBlock *EndBB);
/// @}
};
} // anonymous namespace
PreservedAnalyses LoopIdiomVectorizePass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &) {
if (DisableAll)
return PreservedAnalyses::all();
const auto *DL = &L.getHeader()->getDataLayout();
LoopIdiomVectorizeStyle VecStyle = VectorizeStyle;
if (LITVecStyle.getNumOccurrences())
VecStyle = LITVecStyle;
unsigned BCVF = ByteCompareVF;
if (ByteCmpVF.getNumOccurrences())
BCVF = ByteCmpVF;
LoopIdiomVectorize LIV(VecStyle, BCVF, &AR.DT, &AR.LI, &AR.TTI, DL);
if (!LIV.run(&L))
return PreservedAnalyses::all();
return PreservedAnalyses::none();
}
//===----------------------------------------------------------------------===//
//
// Implementation of LoopIdiomVectorize
//
//===----------------------------------------------------------------------===//
bool LoopIdiomVectorize::run(Loop *L) {
CurLoop = L;
Function &F = *L->getHeader()->getParent();
if (DisableAll || F.hasOptSize())
return false;
if (F.hasFnAttribute(Attribute::NoImplicitFloat)) {
LLVM_DEBUG(dbgs() << DEBUG_TYPE << " is disabled on " << F.getName()
<< " due to its NoImplicitFloat attribute");
return false;
}
// 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;
LLVM_DEBUG(dbgs() << DEBUG_TYPE " Scanning: F[" << F.getName() << "] Loop %"
<< CurLoop->getHeader()->getName() << "\n");
return recognizeByteCompare();
}
bool LoopIdiomVectorize::recognizeByteCompare() {
// Currently the transformation only works on scalable vector types, although
// there is no fundamental reason why it cannot be made to work for fixed
// width too.
// We also need to know the minimum page size for the target in order to
// generate runtime memory checks to ensure the vector version won't fault.
if (!TTI->supportsScalableVectors() || !TTI->getMinPageSize().has_value() ||
DisableByteCmp)
return false;
BasicBlock *Header = CurLoop->getHeader();
// In LoopIdiomVectorize::run we have already checked that the loop
// has a preheader so we can assume it's in a canonical form.
if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 2)
return false;
PHINode *PN = dyn_cast<PHINode>(&Header->front());
if (!PN || PN->getNumIncomingValues() != 2)
return false;
auto LoopBlocks = CurLoop->getBlocks();
// The first block in the loop should contain only 4 instructions, e.g.
//
// while.cond:
// %res.phi = phi i32 [ %start, %ph ], [ %inc, %while.body ]
// %inc = add i32 %res.phi, 1
// %cmp.not = icmp eq i32 %inc, %n
// br i1 %cmp.not, label %while.end, label %while.body
//
if (LoopBlocks[0]->sizeWithoutDebug() > 4)
return false;
// The second block should contain 7 instructions, e.g.
//
// while.body:
// %idx = zext i32 %inc to i64
// %idx.a = getelementptr inbounds i8, ptr %a, i64 %idx
// %load.a = load i8, ptr %idx.a
// %idx.b = getelementptr inbounds i8, ptr %b, i64 %idx
// %load.b = load i8, ptr %idx.b
// %cmp.not.ld = icmp eq i8 %load.a, %load.b
// br i1 %cmp.not.ld, label %while.cond, label %while.end
//
if (LoopBlocks[1]->sizeWithoutDebug() > 7)
return false;
// The incoming value to the PHI node from the loop should be an add of 1.
Value *StartIdx = nullptr;
Instruction *Index = nullptr;
if (!CurLoop->contains(PN->getIncomingBlock(0))) {
StartIdx = PN->getIncomingValue(0);
Index = dyn_cast<Instruction>(PN->getIncomingValue(1));
} else {
StartIdx = PN->getIncomingValue(1);
Index = dyn_cast<Instruction>(PN->getIncomingValue(0));
}
// Limit to 32-bit types for now
if (!Index || !Index->getType()->isIntegerTy(32) ||
!match(Index, m_c_Add(m_Specific(PN), m_One())))
return false;
// If we match the pattern, PN and Index will be replaced with the result of
// the cttz.elts intrinsic. If any other instructions are used outside of
// the loop, we cannot replace it.
for (BasicBlock *BB : LoopBlocks)
for (Instruction &I : *BB)
if (&I != PN && &I != Index)
for (User *U : I.users())
if (!CurLoop->contains(cast<Instruction>(U)))
return false;
// Match the branch instruction for the header
Value *MaxLen;
BasicBlock *EndBB, *WhileBB;
if (!match(Header->getTerminator(),
m_Br(m_SpecificICmp(ICmpInst::ICMP_EQ, m_Specific(Index),
m_Value(MaxLen)),
m_BasicBlock(EndBB), m_BasicBlock(WhileBB))) ||
!CurLoop->contains(WhileBB))
return false;
// WhileBB should contain the pattern of load & compare instructions. Match
// the pattern and find the GEP instructions used by the loads.
BasicBlock *FoundBB;
BasicBlock *TrueBB;
Value *LoadA, *LoadB;
if (!match(WhileBB->getTerminator(),
m_Br(m_SpecificICmp(ICmpInst::ICMP_EQ, m_Value(LoadA),
m_Value(LoadB)),
m_BasicBlock(TrueBB), m_BasicBlock(FoundBB))) ||
!CurLoop->contains(TrueBB))
return false;
Value *A, *B;
if (!match(LoadA, m_Load(m_Value(A))) || !match(LoadB, m_Load(m_Value(B))))
return false;
LoadInst *LoadAI = cast<LoadInst>(LoadA);
LoadInst *LoadBI = cast<LoadInst>(LoadB);
if (!LoadAI->isSimple() || !LoadBI->isSimple())
return false;
GetElementPtrInst *GEPA = dyn_cast<GetElementPtrInst>(A);
GetElementPtrInst *GEPB = dyn_cast<GetElementPtrInst>(B);
if (!GEPA || !GEPB)
return false;
Value *PtrA = GEPA->getPointerOperand();
Value *PtrB = GEPB->getPointerOperand();
// Check we are loading i8 values from two loop invariant pointers
if (!CurLoop->isLoopInvariant(PtrA) || !CurLoop->isLoopInvariant(PtrB) ||
!GEPA->getResultElementType()->isIntegerTy(8) ||
!GEPB->getResultElementType()->isIntegerTy(8) ||
!LoadAI->getType()->isIntegerTy(8) ||
!LoadBI->getType()->isIntegerTy(8) || PtrA == PtrB)
return false;
// Check that the index to the GEPs is the index we found earlier
if (GEPA->getNumIndices() > 1 || GEPB->getNumIndices() > 1)
return false;
Value *IdxA = GEPA->getOperand(GEPA->getNumIndices());
Value *IdxB = GEPB->getOperand(GEPB->getNumIndices());
if (IdxA != IdxB || !match(IdxA, m_ZExt(m_Specific(Index))))
return false;
// We only ever expect the pre-incremented index value to be used inside the
// loop.
if (!PN->hasOneUse())
return false;
// Ensure that when the Found and End blocks are identical the PHIs have the
// supported format. We don't currently allow cases like this:
// while.cond:
// ...
// br i1 %cmp.not, label %while.end, label %while.body
//
// while.body:
// ...
// br i1 %cmp.not2, label %while.cond, label %while.end
//
// while.end:
// %final_ptr = phi ptr [ %c, %while.body ], [ %d, %while.cond ]
//
// Where the incoming values for %final_ptr are unique and from each of the
// loop blocks, but not actually defined in the loop. This requires extra
// work setting up the byte.compare block, i.e. by introducing a select to
// choose the correct value.
// TODO: We could add support for this in future.
if (FoundBB == EndBB) {
for (PHINode &EndPN : EndBB->phis()) {
Value *WhileCondVal = EndPN.getIncomingValueForBlock(Header);
Value *WhileBodyVal = EndPN.getIncomingValueForBlock(WhileBB);
// The value of the index when leaving the while.cond block is always the
// same as the end value (MaxLen) so we permit either. The value when
// leaving the while.body block should only be the index. Otherwise for
// any other values we only allow ones that are same for both blocks.
if (WhileCondVal != WhileBodyVal &&
((WhileCondVal != Index && WhileCondVal != MaxLen) ||
(WhileBodyVal != Index)))
return false;
}
}
LLVM_DEBUG(dbgs() << "FOUND IDIOM IN LOOP: \n"
<< *(EndBB->getParent()) << "\n\n");
// The index is incremented before the GEP/Load pair so we need to
// add 1 to the start value.
transformByteCompare(GEPA, GEPB, PN, MaxLen, Index, StartIdx, /*IncIdx=*/true,
FoundBB, EndBB);
return true;
}
Value *LoopIdiomVectorize::createMaskedFindMismatch(
IRBuilder<> &Builder, DomTreeUpdater &DTU, GetElementPtrInst *GEPA,
GetElementPtrInst *GEPB, Value *ExtStart, Value *ExtEnd) {
Type *I64Type = Builder.getInt64Ty();
Type *ResType = Builder.getInt32Ty();
Type *LoadType = Builder.getInt8Ty();
Value *PtrA = GEPA->getPointerOperand();
Value *PtrB = GEPB->getPointerOperand();
ScalableVectorType *PredVTy =
ScalableVectorType::get(Builder.getInt1Ty(), ByteCompareVF);
Value *InitialPred = Builder.CreateIntrinsic(
Intrinsic::get_active_lane_mask, {PredVTy, I64Type}, {ExtStart, ExtEnd});
Value *VecLen = Builder.CreateIntrinsic(Intrinsic::vscale, {I64Type}, {});
VecLen =
Builder.CreateMul(VecLen, ConstantInt::get(I64Type, ByteCompareVF), "",
/*HasNUW=*/true, /*HasNSW=*/true);
Value *PFalse = Builder.CreateVectorSplat(PredVTy->getElementCount(),
Builder.getInt1(false));
BranchInst *JumpToVectorLoop = BranchInst::Create(VectorLoopStartBlock);
Builder.Insert(JumpToVectorLoop);
DTU.applyUpdates({{DominatorTree::Insert, VectorLoopPreheaderBlock,
VectorLoopStartBlock}});
// Set up the first vector loop block by creating the PHIs, doing the vector
// loads and comparing the vectors.
Builder.SetInsertPoint(VectorLoopStartBlock);
PHINode *LoopPred = Builder.CreatePHI(PredVTy, 2, "mismatch_vec_loop_pred");
LoopPred->addIncoming(InitialPred, VectorLoopPreheaderBlock);
PHINode *VectorIndexPhi = Builder.CreatePHI(I64Type, 2, "mismatch_vec_index");
VectorIndexPhi->addIncoming(ExtStart, VectorLoopPreheaderBlock);
Type *VectorLoadType =
ScalableVectorType::get(Builder.getInt8Ty(), ByteCompareVF);
Value *Passthru = ConstantInt::getNullValue(VectorLoadType);
Value *VectorLhsGep =
Builder.CreateGEP(LoadType, PtrA, VectorIndexPhi, "", GEPA->isInBounds());
Value *VectorLhsLoad = Builder.CreateMaskedLoad(VectorLoadType, VectorLhsGep,
Align(1), LoopPred, Passthru);
Value *VectorRhsGep =
Builder.CreateGEP(LoadType, PtrB, VectorIndexPhi, "", GEPB->isInBounds());
Value *VectorRhsLoad = Builder.CreateMaskedLoad(VectorLoadType, VectorRhsGep,
Align(1), LoopPred, Passthru);
Value *VectorMatchCmp = Builder.CreateICmpNE(VectorLhsLoad, VectorRhsLoad);
VectorMatchCmp = Builder.CreateSelect(LoopPred, VectorMatchCmp, PFalse);
Value *VectorMatchHasActiveLanes = Builder.CreateOrReduce(VectorMatchCmp);
BranchInst *VectorEarlyExit = BranchInst::Create(
VectorLoopMismatchBlock, VectorLoopIncBlock, VectorMatchHasActiveLanes);
Builder.Insert(VectorEarlyExit);
DTU.applyUpdates(
{{DominatorTree::Insert, VectorLoopStartBlock, VectorLoopMismatchBlock},
{DominatorTree::Insert, VectorLoopStartBlock, VectorLoopIncBlock}});
// Increment the index counter and calculate the predicate for the next
// iteration of the loop. We branch back to the start of the loop if there
// is at least one active lane.
Builder.SetInsertPoint(VectorLoopIncBlock);
Value *NewVectorIndexPhi =
Builder.CreateAdd(VectorIndexPhi, VecLen, "",
/*HasNUW=*/true, /*HasNSW=*/true);
VectorIndexPhi->addIncoming(NewVectorIndexPhi, VectorLoopIncBlock);
Value *NewPred =
Builder.CreateIntrinsic(Intrinsic::get_active_lane_mask,
{PredVTy, I64Type}, {NewVectorIndexPhi, ExtEnd});
LoopPred->addIncoming(NewPred, VectorLoopIncBlock);
Value *PredHasActiveLanes =
Builder.CreateExtractElement(NewPred, uint64_t(0));
BranchInst *VectorLoopBranchBack =
BranchInst::Create(VectorLoopStartBlock, EndBlock, PredHasActiveLanes);
Builder.Insert(VectorLoopBranchBack);
DTU.applyUpdates(
{{DominatorTree::Insert, VectorLoopIncBlock, VectorLoopStartBlock},
{DominatorTree::Insert, VectorLoopIncBlock, EndBlock}});
// If we found a mismatch then we need to calculate which lane in the vector
// had a mismatch and add that on to the current loop index.
Builder.SetInsertPoint(VectorLoopMismatchBlock);
PHINode *FoundPred = Builder.CreatePHI(PredVTy, 1, "mismatch_vec_found_pred");
FoundPred->addIncoming(VectorMatchCmp, VectorLoopStartBlock);
PHINode *LastLoopPred =
Builder.CreatePHI(PredVTy, 1, "mismatch_vec_last_loop_pred");
LastLoopPred->addIncoming(LoopPred, VectorLoopStartBlock);
PHINode *VectorFoundIndex =
Builder.CreatePHI(I64Type, 1, "mismatch_vec_found_index");
VectorFoundIndex->addIncoming(VectorIndexPhi, VectorLoopStartBlock);
Value *PredMatchCmp = Builder.CreateAnd(LastLoopPred, FoundPred);
Value *Ctz = Builder.CreateCountTrailingZeroElems(ResType, PredMatchCmp);
Ctz = Builder.CreateZExt(Ctz, I64Type);
Value *VectorLoopRes64 = Builder.CreateAdd(VectorFoundIndex, Ctz, "",
/*HasNUW=*/true, /*HasNSW=*/true);
return Builder.CreateTrunc(VectorLoopRes64, ResType);
}
Value *LoopIdiomVectorize::createPredicatedFindMismatch(
IRBuilder<> &Builder, DomTreeUpdater &DTU, GetElementPtrInst *GEPA,
GetElementPtrInst *GEPB, Value *ExtStart, Value *ExtEnd) {
Type *I64Type = Builder.getInt64Ty();
Type *I32Type = Builder.getInt32Ty();
Type *ResType = I32Type;
Type *LoadType = Builder.getInt8Ty();
Value *PtrA = GEPA->getPointerOperand();
Value *PtrB = GEPB->getPointerOperand();
auto *JumpToVectorLoop = BranchInst::Create(VectorLoopStartBlock);
Builder.Insert(JumpToVectorLoop);
DTU.applyUpdates({{DominatorTree::Insert, VectorLoopPreheaderBlock,
VectorLoopStartBlock}});
// Set up the first Vector loop block by creating the PHIs, doing the vector
// loads and comparing the vectors.
Builder.SetInsertPoint(VectorLoopStartBlock);
auto *VectorIndexPhi = Builder.CreatePHI(I64Type, 2, "mismatch_vector_index");
VectorIndexPhi->addIncoming(ExtStart, VectorLoopPreheaderBlock);
// Calculate AVL by subtracting the vector loop index from the trip count
Value *AVL = Builder.CreateSub(ExtEnd, VectorIndexPhi, "avl", /*HasNUW=*/true,
/*HasNSW=*/true);
auto *VectorLoadType = ScalableVectorType::get(LoadType, ByteCompareVF);
auto *VF = ConstantInt::get(I32Type, ByteCompareVF);
Value *VL = Builder.CreateIntrinsic(Intrinsic::experimental_get_vector_length,
{I64Type}, {AVL, VF, Builder.getTrue()});
Value *GepOffset = VectorIndexPhi;
Value *VectorLhsGep =
Builder.CreateGEP(LoadType, PtrA, GepOffset, "", GEPA->isInBounds());
VectorType *TrueMaskTy =
VectorType::get(Builder.getInt1Ty(), VectorLoadType->getElementCount());
Value *AllTrueMask = Constant::getAllOnesValue(TrueMaskTy);
Value *VectorLhsLoad = Builder.CreateIntrinsic(
Intrinsic::vp_load, {VectorLoadType, VectorLhsGep->getType()},
{VectorLhsGep, AllTrueMask, VL}, nullptr, "lhs.load");
Value *VectorRhsGep =
Builder.CreateGEP(LoadType, PtrB, GepOffset, "", GEPB->isInBounds());
Value *VectorRhsLoad = Builder.CreateIntrinsic(
Intrinsic::vp_load, {VectorLoadType, VectorLhsGep->getType()},
{VectorRhsGep, AllTrueMask, VL}, nullptr, "rhs.load");
StringRef PredicateStr = CmpInst::getPredicateName(CmpInst::ICMP_NE);
auto *PredicateMDS = MDString::get(VectorLhsLoad->getContext(), PredicateStr);
Value *Pred = MetadataAsValue::get(VectorLhsLoad->getContext(), PredicateMDS);
Value *VectorMatchCmp = Builder.CreateIntrinsic(
Intrinsic::vp_icmp, {VectorLhsLoad->getType()},
{VectorLhsLoad, VectorRhsLoad, Pred, AllTrueMask, VL}, nullptr,
"mismatch.cmp");
Value *CTZ = Builder.CreateIntrinsic(
Intrinsic::vp_cttz_elts, {ResType, VectorMatchCmp->getType()},
{VectorMatchCmp, /*ZeroIsPoison=*/Builder.getInt1(false), AllTrueMask,
VL});
Value *MismatchFound = Builder.CreateICmpNE(CTZ, VL);
auto *VectorEarlyExit = BranchInst::Create(VectorLoopMismatchBlock,
VectorLoopIncBlock, MismatchFound);
Builder.Insert(VectorEarlyExit);
DTU.applyUpdates(
{{DominatorTree::Insert, VectorLoopStartBlock, VectorLoopMismatchBlock},
{DominatorTree::Insert, VectorLoopStartBlock, VectorLoopIncBlock}});
// Increment the index counter and calculate the predicate for the next
// iteration of the loop. We branch back to the start of the loop if there
// is at least one active lane.
Builder.SetInsertPoint(VectorLoopIncBlock);
Value *VL64 = Builder.CreateZExt(VL, I64Type);
Value *NewVectorIndexPhi =
Builder.CreateAdd(VectorIndexPhi, VL64, "",
/*HasNUW=*/true, /*HasNSW=*/true);
VectorIndexPhi->addIncoming(NewVectorIndexPhi, VectorLoopIncBlock);
Value *ExitCond = Builder.CreateICmpNE(NewVectorIndexPhi, ExtEnd);
auto *VectorLoopBranchBack =
BranchInst::Create(VectorLoopStartBlock, EndBlock, ExitCond);
Builder.Insert(VectorLoopBranchBack);
DTU.applyUpdates(
{{DominatorTree::Insert, VectorLoopIncBlock, VectorLoopStartBlock},
{DominatorTree::Insert, VectorLoopIncBlock, EndBlock}});
// If we found a mismatch then we need to calculate which lane in the vector
// had a mismatch and add that on to the current loop index.
Builder.SetInsertPoint(VectorLoopMismatchBlock);
// Add LCSSA phis for CTZ and VectorIndexPhi.
auto *CTZLCSSAPhi = Builder.CreatePHI(CTZ->getType(), 1, "ctz");
CTZLCSSAPhi->addIncoming(CTZ, VectorLoopStartBlock);
auto *VectorIndexLCSSAPhi =
Builder.CreatePHI(VectorIndexPhi->getType(), 1, "mismatch_vector_index");
VectorIndexLCSSAPhi->addIncoming(VectorIndexPhi, VectorLoopStartBlock);
Value *CTZI64 = Builder.CreateZExt(CTZLCSSAPhi, I64Type);
Value *VectorLoopRes64 = Builder.CreateAdd(VectorIndexLCSSAPhi, CTZI64, "",
/*HasNUW=*/true, /*HasNSW=*/true);
return Builder.CreateTrunc(VectorLoopRes64, ResType);
}
Value *LoopIdiomVectorize::expandFindMismatch(
IRBuilder<> &Builder, DomTreeUpdater &DTU, GetElementPtrInst *GEPA,
GetElementPtrInst *GEPB, Instruction *Index, Value *Start, Value *MaxLen) {
Value *PtrA = GEPA->getPointerOperand();
Value *PtrB = GEPB->getPointerOperand();
// Get the arguments and types for the intrinsic.
BasicBlock *Preheader = CurLoop->getLoopPreheader();
BranchInst *PHBranch = cast<BranchInst>(Preheader->getTerminator());
LLVMContext &Ctx = PHBranch->getContext();
Type *LoadType = Type::getInt8Ty(Ctx);
Type *ResType = Builder.getInt32Ty();
// Split block in the original loop preheader.
EndBlock = SplitBlock(Preheader, PHBranch, DT, LI, nullptr, "mismatch_end");
// Create the blocks that we're going to need:
// 1. A block for checking the zero-extended length exceeds 0
// 2. A block to check that the start and end addresses of a given array
// lie on the same page.
// 3. The vector loop preheader.
// 4. The first vector loop block.
// 5. The vector loop increment block.
// 6. A block we can jump to from the vector loop when a mismatch is found.
// 7. The first block of the scalar loop itself, containing PHIs , loads
// and cmp.
// 8. A scalar loop increment block to increment the PHIs and go back
// around the loop.
BasicBlock *MinItCheckBlock = BasicBlock::Create(
Ctx, "mismatch_min_it_check", EndBlock->getParent(), EndBlock);
// Update the terminator added by SplitBlock to branch to the first block
Preheader->getTerminator()->setSuccessor(0, MinItCheckBlock);
BasicBlock *MemCheckBlock = BasicBlock::Create(
Ctx, "mismatch_mem_check", EndBlock->getParent(), EndBlock);
VectorLoopPreheaderBlock = BasicBlock::Create(
Ctx, "mismatch_vec_loop_preheader", EndBlock->getParent(), EndBlock);
VectorLoopStartBlock = BasicBlock::Create(Ctx, "mismatch_vec_loop",
EndBlock->getParent(), EndBlock);
VectorLoopIncBlock = BasicBlock::Create(Ctx, "mismatch_vec_loop_inc",
EndBlock->getParent(), EndBlock);
VectorLoopMismatchBlock = BasicBlock::Create(Ctx, "mismatch_vec_loop_found",
EndBlock->getParent(), EndBlock);
BasicBlock *LoopPreHeaderBlock = BasicBlock::Create(
Ctx, "mismatch_loop_pre", EndBlock->getParent(), EndBlock);
BasicBlock *LoopStartBlock =
BasicBlock::Create(Ctx, "mismatch_loop", EndBlock->getParent(), EndBlock);
BasicBlock *LoopIncBlock = BasicBlock::Create(
Ctx, "mismatch_loop_inc", EndBlock->getParent(), EndBlock);
DTU.applyUpdates({{DominatorTree::Insert, Preheader, MinItCheckBlock},
{DominatorTree::Delete, Preheader, EndBlock}});
// Update LoopInfo with the new vector & scalar loops.
auto VectorLoop = LI->AllocateLoop();
auto ScalarLoop = LI->AllocateLoop();
if (CurLoop->getParentLoop()) {
CurLoop->getParentLoop()->addBasicBlockToLoop(MinItCheckBlock, *LI);
CurLoop->getParentLoop()->addBasicBlockToLoop(MemCheckBlock, *LI);
CurLoop->getParentLoop()->addBasicBlockToLoop(VectorLoopPreheaderBlock,
*LI);
CurLoop->getParentLoop()->addChildLoop(VectorLoop);
CurLoop->getParentLoop()->addBasicBlockToLoop(VectorLoopMismatchBlock, *LI);
CurLoop->getParentLoop()->addBasicBlockToLoop(LoopPreHeaderBlock, *LI);
CurLoop->getParentLoop()->addChildLoop(ScalarLoop);
} else {
LI->addTopLevelLoop(VectorLoop);
LI->addTopLevelLoop(ScalarLoop);
}
// Add the new basic blocks to their associated loops.
VectorLoop->addBasicBlockToLoop(VectorLoopStartBlock, *LI);
VectorLoop->addBasicBlockToLoop(VectorLoopIncBlock, *LI);
ScalarLoop->addBasicBlockToLoop(LoopStartBlock, *LI);
ScalarLoop->addBasicBlockToLoop(LoopIncBlock, *LI);
// Set up some types and constants that we intend to reuse.
Type *I64Type = Builder.getInt64Ty();
// Check the zero-extended iteration count > 0
Builder.SetInsertPoint(MinItCheckBlock);
Value *ExtStart = Builder.CreateZExt(Start, I64Type);
Value *ExtEnd = Builder.CreateZExt(MaxLen, I64Type);
// This check doesn't really cost us very much.
Value *LimitCheck = Builder.CreateICmpULE(Start, MaxLen);
BranchInst *MinItCheckBr =
BranchInst::Create(MemCheckBlock, LoopPreHeaderBlock, LimitCheck);
MinItCheckBr->setMetadata(
LLVMContext::MD_prof,
MDBuilder(MinItCheckBr->getContext()).createBranchWeights(99, 1));
Builder.Insert(MinItCheckBr);
DTU.applyUpdates(
{{DominatorTree::Insert, MinItCheckBlock, MemCheckBlock},
{DominatorTree::Insert, MinItCheckBlock, LoopPreHeaderBlock}});
// For each of the arrays, check the start/end addresses are on the same
// page.
Builder.SetInsertPoint(MemCheckBlock);
// The early exit in the original loop means that when performing vector
// loads we are potentially reading ahead of the early exit. So we could
// fault if crossing a page boundary. Therefore, we create runtime memory
// checks based on the minimum page size as follows:
// 1. Calculate the addresses of the first memory accesses in the loop,
// i.e. LhsStart and RhsStart.
// 2. Get the last accessed addresses in the loop, i.e. LhsEnd and RhsEnd.
// 3. Determine which pages correspond to all the memory accesses, i.e
// LhsStartPage, LhsEndPage, RhsStartPage, RhsEndPage.
// 4. If LhsStartPage == LhsEndPage and RhsStartPage == RhsEndPage, then
// we know we won't cross any page boundaries in the loop so we can
// enter the vector loop! Otherwise we fall back on the scalar loop.
Value *LhsStartGEP = Builder.CreateGEP(LoadType, PtrA, ExtStart);
Value *RhsStartGEP = Builder.CreateGEP(LoadType, PtrB, ExtStart);
Value *RhsStart = Builder.CreatePtrToInt(RhsStartGEP, I64Type);
Value *LhsStart = Builder.CreatePtrToInt(LhsStartGEP, I64Type);
Value *LhsEndGEP = Builder.CreateGEP(LoadType, PtrA, ExtEnd);
Value *RhsEndGEP = Builder.CreateGEP(LoadType, PtrB, ExtEnd);
Value *LhsEnd = Builder.CreatePtrToInt(LhsEndGEP, I64Type);
Value *RhsEnd = Builder.CreatePtrToInt(RhsEndGEP, I64Type);
const uint64_t MinPageSize = TTI->getMinPageSize().value();
const uint64_t AddrShiftAmt = llvm::Log2_64(MinPageSize);
Value *LhsStartPage = Builder.CreateLShr(LhsStart, AddrShiftAmt);
Value *LhsEndPage = Builder.CreateLShr(LhsEnd, AddrShiftAmt);
Value *RhsStartPage = Builder.CreateLShr(RhsStart, AddrShiftAmt);
Value *RhsEndPage = Builder.CreateLShr(RhsEnd, AddrShiftAmt);
Value *LhsPageCmp = Builder.CreateICmpNE(LhsStartPage, LhsEndPage);
Value *RhsPageCmp = Builder.CreateICmpNE(RhsStartPage, RhsEndPage);
Value *CombinedPageCmp = Builder.CreateOr(LhsPageCmp, RhsPageCmp);
BranchInst *CombinedPageCmpCmpBr = BranchInst::Create(
LoopPreHeaderBlock, VectorLoopPreheaderBlock, CombinedPageCmp);
CombinedPageCmpCmpBr->setMetadata(
LLVMContext::MD_prof, MDBuilder(CombinedPageCmpCmpBr->getContext())
.createBranchWeights(10, 90));
Builder.Insert(CombinedPageCmpCmpBr);
DTU.applyUpdates(
{{DominatorTree::Insert, MemCheckBlock, LoopPreHeaderBlock},
{DominatorTree::Insert, MemCheckBlock, VectorLoopPreheaderBlock}});
// Set up the vector loop preheader, i.e. calculate initial loop predicate,
// zero-extend MaxLen to 64-bits, determine the number of vector elements
// processed in each iteration, etc.
Builder.SetInsertPoint(VectorLoopPreheaderBlock);
// At this point we know two things must be true:
// 1. Start <= End
// 2. ExtMaxLen <= MinPageSize due to the page checks.
// Therefore, we know that we can use a 64-bit induction variable that
// starts from 0 -> ExtMaxLen and it will not overflow.
Value *VectorLoopRes = nullptr;
switch (VectorizeStyle) {
case LoopIdiomVectorizeStyle::Masked:
VectorLoopRes =
createMaskedFindMismatch(Builder, DTU, GEPA, GEPB, ExtStart, ExtEnd);
break;
case LoopIdiomVectorizeStyle::Predicated:
VectorLoopRes = createPredicatedFindMismatch(Builder, DTU, GEPA, GEPB,
ExtStart, ExtEnd);
break;
}
Builder.Insert(BranchInst::Create(EndBlock));
DTU.applyUpdates(
{{DominatorTree::Insert, VectorLoopMismatchBlock, EndBlock}});
// Generate code for scalar loop.
Builder.SetInsertPoint(LoopPreHeaderBlock);
Builder.Insert(BranchInst::Create(LoopStartBlock));
DTU.applyUpdates(
{{DominatorTree::Insert, LoopPreHeaderBlock, LoopStartBlock}});
Builder.SetInsertPoint(LoopStartBlock);
PHINode *IndexPhi = Builder.CreatePHI(ResType, 2, "mismatch_index");
IndexPhi->addIncoming(Start, LoopPreHeaderBlock);
// Otherwise compare the values
// Load bytes from each array and compare them.
Value *GepOffset = Builder.CreateZExt(IndexPhi, I64Type);
Value *LhsGep =
Builder.CreateGEP(LoadType, PtrA, GepOffset, "", GEPA->isInBounds());
Value *LhsLoad = Builder.CreateLoad(LoadType, LhsGep);
Value *RhsGep =
Builder.CreateGEP(LoadType, PtrB, GepOffset, "", GEPB->isInBounds());
Value *RhsLoad = Builder.CreateLoad(LoadType, RhsGep);
Value *MatchCmp = Builder.CreateICmpEQ(LhsLoad, RhsLoad);
// If we have a mismatch then exit the loop ...
BranchInst *MatchCmpBr = BranchInst::Create(LoopIncBlock, EndBlock, MatchCmp);
Builder.Insert(MatchCmpBr);
DTU.applyUpdates({{DominatorTree::Insert, LoopStartBlock, LoopIncBlock},
{DominatorTree::Insert, LoopStartBlock, EndBlock}});
// Have we reached the maximum permitted length for the loop?
Builder.SetInsertPoint(LoopIncBlock);
Value *PhiInc = Builder.CreateAdd(IndexPhi, ConstantInt::get(ResType, 1), "",
/*HasNUW=*/Index->hasNoUnsignedWrap(),
/*HasNSW=*/Index->hasNoSignedWrap());
IndexPhi->addIncoming(PhiInc, LoopIncBlock);
Value *IVCmp = Builder.CreateICmpEQ(PhiInc, MaxLen);
BranchInst *IVCmpBr = BranchInst::Create(EndBlock, LoopStartBlock, IVCmp);
Builder.Insert(IVCmpBr);
DTU.applyUpdates({{DominatorTree::Insert, LoopIncBlock, EndBlock},
{DominatorTree::Insert, LoopIncBlock, LoopStartBlock}});
// In the end block we need to insert a PHI node to deal with three cases:
// 1. We didn't find a mismatch in the scalar loop, so we return MaxLen.
// 2. We exitted the scalar loop early due to a mismatch and need to return
// the index that we found.
// 3. We didn't find a mismatch in the vector loop, so we return MaxLen.
// 4. We exitted the vector loop early due to a mismatch and need to return
// the index that we found.
Builder.SetInsertPoint(EndBlock, EndBlock->getFirstInsertionPt());
PHINode *ResPhi = Builder.CreatePHI(ResType, 4, "mismatch_result");
ResPhi->addIncoming(MaxLen, LoopIncBlock);
ResPhi->addIncoming(IndexPhi, LoopStartBlock);
ResPhi->addIncoming(MaxLen, VectorLoopIncBlock);
ResPhi->addIncoming(VectorLoopRes, VectorLoopMismatchBlock);
Value *FinalRes = Builder.CreateTrunc(ResPhi, ResType);
if (VerifyLoops) {
ScalarLoop->verifyLoop();
VectorLoop->verifyLoop();
if (!VectorLoop->isRecursivelyLCSSAForm(*DT, *LI))
report_fatal_error("Loops must remain in LCSSA form!");
if (!ScalarLoop->isRecursivelyLCSSAForm(*DT, *LI))
report_fatal_error("Loops must remain in LCSSA form!");
}
return FinalRes;
}
void LoopIdiomVectorize::transformByteCompare(GetElementPtrInst *GEPA,
GetElementPtrInst *GEPB,
PHINode *IndPhi, Value *MaxLen,
Instruction *Index, Value *Start,
bool IncIdx, BasicBlock *FoundBB,
BasicBlock *EndBB) {
// Insert the byte compare code at the end of the preheader block
BasicBlock *Preheader = CurLoop->getLoopPreheader();
BasicBlock *Header = CurLoop->getHeader();
BranchInst *PHBranch = cast<BranchInst>(Preheader->getTerminator());
IRBuilder<> Builder(PHBranch);
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
Builder.SetCurrentDebugLocation(PHBranch->getDebugLoc());
// Increment the pointer if this was done before the loads in the loop.
if (IncIdx)
Start = Builder.CreateAdd(Start, ConstantInt::get(Start->getType(), 1));
Value *ByteCmpRes =
expandFindMismatch(Builder, DTU, GEPA, GEPB, Index, Start, MaxLen);
// Replaces uses of index & induction Phi with intrinsic (we already
// checked that the the first instruction of Header is the Phi above).
assert(IndPhi->hasOneUse() && "Index phi node has more than one use!");
Index->replaceAllUsesWith(ByteCmpRes);
assert(PHBranch->isUnconditional() &&
"Expected preheader to terminate with an unconditional branch.");
// If no mismatch was found, we can jump to the end block. Create a
// new basic block for the compare instruction.
auto *CmpBB = BasicBlock::Create(Preheader->getContext(), "byte.compare",
Preheader->getParent());
CmpBB->moveBefore(EndBB);
// Replace the branch in the preheader with an always-true conditional branch.
// This ensures there is still a reference to the original loop.
Builder.CreateCondBr(Builder.getTrue(), CmpBB, Header);
PHBranch->eraseFromParent();
BasicBlock *MismatchEnd = cast<Instruction>(ByteCmpRes)->getParent();
DTU.applyUpdates({{DominatorTree::Insert, MismatchEnd, CmpBB}});
// Create the branch to either the end or found block depending on the value
// returned by the intrinsic.
Builder.SetInsertPoint(CmpBB);
if (FoundBB != EndBB) {
Value *FoundCmp = Builder.CreateICmpEQ(ByteCmpRes, MaxLen);
Builder.CreateCondBr(FoundCmp, EndBB, FoundBB);
DTU.applyUpdates({{DominatorTree::Insert, CmpBB, FoundBB},
{DominatorTree::Insert, CmpBB, EndBB}});
} else {
Builder.CreateBr(FoundBB);
DTU.applyUpdates({{DominatorTree::Insert, CmpBB, FoundBB}});
}
auto fixSuccessorPhis = [&](BasicBlock *SuccBB) {
for (PHINode &PN : SuccBB->phis()) {
// At this point we've already replaced all uses of the result from the
// loop with ByteCmp. Look through the incoming values to find ByteCmp,
// meaning this is a Phi collecting the results of the byte compare.
bool ResPhi = false;
for (Value *Op : PN.incoming_values())
if (Op == ByteCmpRes) {
ResPhi = true;
break;
}
// Any PHI that depended upon the result of the byte compare needs a new
// incoming value from CmpBB. This is because the original loop will get
// deleted.
if (ResPhi)
PN.addIncoming(ByteCmpRes, CmpBB);
else {
// There should be no other outside uses of other values in the
// original loop. Any incoming values should either:
// 1. Be for blocks outside the loop, which aren't interesting. Or ..
// 2. These are from blocks in the loop with values defined outside
// the loop. We should a similar incoming value from CmpBB.
for (BasicBlock *BB : PN.blocks())
if (CurLoop->contains(BB)) {
PN.addIncoming(PN.getIncomingValueForBlock(BB), CmpBB);
break;
}
}
}
};
// Ensure all Phis in the successors of CmpBB have an incoming value from it.
fixSuccessorPhis(EndBB);
if (EndBB != FoundBB)
fixSuccessorPhis(FoundBB);
// The new CmpBB block isn't part of the loop, but will need to be added to
// the outer loop if there is one.
if (!CurLoop->isOutermost())
CurLoop->getParentLoop()->addBasicBlockToLoop(CmpBB, *LI);
if (VerifyLoops && CurLoop->getParentLoop()) {
CurLoop->getParentLoop()->verifyLoop();
if (!CurLoop->getParentLoop()->isRecursivelyLCSSAForm(*DT, *LI))
report_fatal_error("Loops must remain in LCSSA form!");
}
}
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