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llvm-mirror/lib/Analysis/VectorUtils.cpp
Matt Arsenault 2da3e37d8b Fix vectorization of canonicalize 
llvm-svn: 342390
2018-09-17 13:24:30 +00:00

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//===----------- VectorUtils.cpp - Vectorizer utility functions -----------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines vectorizer utilities.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/VectorUtils.h"
#include "llvm/ADT/EquivalenceClasses.h"
#include "llvm/Analysis/DemandedBits.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Value.h"
#define DEBUG_TYPE "vectorutils"
using namespace llvm;
using namespace llvm::PatternMatch;
/// Maximum factor for an interleaved memory access.
static cl::opt<unsigned> MaxInterleaveGroupFactor(
"max-interleave-group-factor", cl::Hidden,
cl::desc("Maximum factor for an interleaved access group (default = 8)"),
cl::init(8));
/// Identify if the intrinsic is trivially vectorizable.
/// This method returns true if the intrinsic's argument types are all
/// scalars for the scalar form of the intrinsic and all vectors for
/// the vector form of the intrinsic.
bool llvm::isTriviallyVectorizable(Intrinsic::ID ID) {
switch (ID) {
case Intrinsic::sqrt:
case Intrinsic::sin:
case Intrinsic::cos:
case Intrinsic::exp:
case Intrinsic::exp2:
case Intrinsic::log:
case Intrinsic::log10:
case Intrinsic::log2:
case Intrinsic::fabs:
case Intrinsic::minnum:
case Intrinsic::maxnum:
case Intrinsic::copysign:
case Intrinsic::floor:
case Intrinsic::ceil:
case Intrinsic::trunc:
case Intrinsic::rint:
case Intrinsic::nearbyint:
case Intrinsic::round:
case Intrinsic::bswap:
case Intrinsic::bitreverse:
case Intrinsic::ctpop:
case Intrinsic::pow:
case Intrinsic::fma:
case Intrinsic::fmuladd:
case Intrinsic::ctlz:
case Intrinsic::cttz:
case Intrinsic::powi:
case Intrinsic::canonicalize:
return true;
default:
return false;
}
}
/// Identifies if the intrinsic has a scalar operand. It check for
/// ctlz,cttz and powi special intrinsics whose argument is scalar.
bool llvm::hasVectorInstrinsicScalarOpd(Intrinsic::ID ID,
unsigned ScalarOpdIdx) {
switch (ID) {
case Intrinsic::ctlz:
case Intrinsic::cttz:
case Intrinsic::powi:
return (ScalarOpdIdx == 1);
default:
return false;
}
}
/// Returns intrinsic ID for call.
/// For the input call instruction it finds mapping intrinsic and returns
/// its ID, in case it does not found it return not_intrinsic.
Intrinsic::ID llvm::getVectorIntrinsicIDForCall(const CallInst *CI,
const TargetLibraryInfo *TLI) {
Intrinsic::ID ID = getIntrinsicForCallSite(CI, TLI);
if (ID == Intrinsic::not_intrinsic)
return Intrinsic::not_intrinsic;
if (isTriviallyVectorizable(ID) || ID == Intrinsic::lifetime_start ||
ID == Intrinsic::lifetime_end || ID == Intrinsic::assume ||
ID == Intrinsic::sideeffect)
return ID;
return Intrinsic::not_intrinsic;
}
/// Find the operand of the GEP that should be checked for consecutive
/// stores. This ignores trailing indices that have no effect on the final
/// pointer.
unsigned llvm::getGEPInductionOperand(const GetElementPtrInst *Gep) {
const DataLayout &DL = Gep->getModule()->getDataLayout();
unsigned LastOperand = Gep->getNumOperands() - 1;
unsigned GEPAllocSize = DL.getTypeAllocSize(Gep->getResultElementType());
// Walk backwards and try to peel off zeros.
while (LastOperand > 1 && match(Gep->getOperand(LastOperand), m_Zero())) {
// Find the type we're currently indexing into.
gep_type_iterator GEPTI = gep_type_begin(Gep);
std::advance(GEPTI, LastOperand - 2);
// If it's a type with the same allocation size as the result of the GEP we
// can peel off the zero index.
if (DL.getTypeAllocSize(GEPTI.getIndexedType()) != GEPAllocSize)
break;
--LastOperand;
}
return LastOperand;
}
/// If the argument is a GEP, then returns the operand identified by
/// getGEPInductionOperand. However, if there is some other non-loop-invariant
/// operand, it returns that instead.
Value *llvm::stripGetElementPtr(Value *Ptr, ScalarEvolution *SE, Loop *Lp) {
GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr);
if (!GEP)
return Ptr;
unsigned InductionOperand = getGEPInductionOperand(GEP);
// Check that all of the gep indices are uniform except for our induction
// operand.
for (unsigned i = 0, e = GEP->getNumOperands(); i != e; ++i)
if (i != InductionOperand &&
!SE->isLoopInvariant(SE->getSCEV(GEP->getOperand(i)), Lp))
return Ptr;
return GEP->getOperand(InductionOperand);
}
/// If a value has only one user that is a CastInst, return it.
Value *llvm::getUniqueCastUse(Value *Ptr, Loop *Lp, Type *Ty) {
Value *UniqueCast = nullptr;
for (User *U : Ptr->users()) {
CastInst *CI = dyn_cast<CastInst>(U);
if (CI && CI->getType() == Ty) {
if (!UniqueCast)
UniqueCast = CI;
else
return nullptr;
}
}
return UniqueCast;
}
/// Get the stride of a pointer access in a loop. Looks for symbolic
/// strides "a[i*stride]". Returns the symbolic stride, or null otherwise.
Value *llvm::getStrideFromPointer(Value *Ptr, ScalarEvolution *SE, Loop *Lp) {
auto *PtrTy = dyn_cast<PointerType>(Ptr->getType());
if (!PtrTy || PtrTy->isAggregateType())
return nullptr;
// Try to remove a gep instruction to make the pointer (actually index at this
// point) easier analyzable. If OrigPtr is equal to Ptr we are analyzing the
// pointer, otherwise, we are analyzing the index.
Value *OrigPtr = Ptr;
// The size of the pointer access.
int64_t PtrAccessSize = 1;
Ptr = stripGetElementPtr(Ptr, SE, Lp);
const SCEV *V = SE->getSCEV(Ptr);
if (Ptr != OrigPtr)
// Strip off casts.
while (const SCEVCastExpr *C = dyn_cast<SCEVCastExpr>(V))
V = C->getOperand();
const SCEVAddRecExpr *S = dyn_cast<SCEVAddRecExpr>(V);
if (!S)
return nullptr;
V = S->getStepRecurrence(*SE);
if (!V)
return nullptr;
// Strip off the size of access multiplication if we are still analyzing the
// pointer.
if (OrigPtr == Ptr) {
if (const SCEVMulExpr *M = dyn_cast<SCEVMulExpr>(V)) {
if (M->getOperand(0)->getSCEVType() != scConstant)
return nullptr;
const APInt &APStepVal = cast<SCEVConstant>(M->getOperand(0))->getAPInt();
// Huge step value - give up.
if (APStepVal.getBitWidth() > 64)
return nullptr;
int64_t StepVal = APStepVal.getSExtValue();
if (PtrAccessSize != StepVal)
return nullptr;
V = M->getOperand(1);
}
}
// Strip off casts.
Type *StripedOffRecurrenceCast = nullptr;
if (const SCEVCastExpr *C = dyn_cast<SCEVCastExpr>(V)) {
StripedOffRecurrenceCast = C->getType();
V = C->getOperand();
}
// Look for the loop invariant symbolic value.
const SCEVUnknown *U = dyn_cast<SCEVUnknown>(V);
if (!U)
return nullptr;
Value *Stride = U->getValue();
if (!Lp->isLoopInvariant(Stride))
return nullptr;
// If we have stripped off the recurrence cast we have to make sure that we
// return the value that is used in this loop so that we can replace it later.
if (StripedOffRecurrenceCast)
Stride = getUniqueCastUse(Stride, Lp, StripedOffRecurrenceCast);
return Stride;
}
/// Given a vector and an element number, see if the scalar value is
/// already around as a register, for example if it were inserted then extracted
/// from the vector.
Value *llvm::findScalarElement(Value *V, unsigned EltNo) {
assert(V->getType()->isVectorTy() && "Not looking at a vector?");
VectorType *VTy = cast<VectorType>(V->getType());
unsigned Width = VTy->getNumElements();
if (EltNo >= Width) // Out of range access.
return UndefValue::get(VTy->getElementType());
if (Constant *C = dyn_cast<Constant>(V))
return C->getAggregateElement(EltNo);
if (InsertElementInst *III = dyn_cast<InsertElementInst>(V)) {
// If this is an insert to a variable element, we don't know what it is.
if (!isa<ConstantInt>(III->getOperand(2)))
return nullptr;
unsigned IIElt = cast<ConstantInt>(III->getOperand(2))->getZExtValue();
// If this is an insert to the element we are looking for, return the
// inserted value.
if (EltNo == IIElt)
return III->getOperand(1);
// Otherwise, the insertelement doesn't modify the value, recurse on its
// vector input.
return findScalarElement(III->getOperand(0), EltNo);
}
if (ShuffleVectorInst *SVI = dyn_cast<ShuffleVectorInst>(V)) {
unsigned LHSWidth = SVI->getOperand(0)->getType()->getVectorNumElements();
int InEl = SVI->getMaskValue(EltNo);
if (InEl < 0)
return UndefValue::get(VTy->getElementType());
if (InEl < (int)LHSWidth)
return findScalarElement(SVI->getOperand(0), InEl);
return findScalarElement(SVI->getOperand(1), InEl - LHSWidth);
}
// Extract a value from a vector add operation with a constant zero.
Value *Val = nullptr; Constant *Con = nullptr;
if (match(V, m_Add(m_Value(Val), m_Constant(Con))))
if (Constant *Elt = Con->getAggregateElement(EltNo))
if (Elt->isNullValue())
return findScalarElement(Val, EltNo);
// Otherwise, we don't know.
return nullptr;
}
/// Get splat value if the input is a splat vector or return nullptr.
/// This function is not fully general. It checks only 2 cases:
/// the input value is (1) a splat constants vector or (2) a sequence
/// of instructions that broadcast a single value into a vector.
///
const llvm::Value *llvm::getSplatValue(const Value *V) {
if (auto *C = dyn_cast<Constant>(V))
if (isa<VectorType>(V->getType()))
return C->getSplatValue();
auto *ShuffleInst = dyn_cast<ShuffleVectorInst>(V);
if (!ShuffleInst)
return nullptr;
// All-zero (or undef) shuffle mask elements.
for (int MaskElt : ShuffleInst->getShuffleMask())
if (MaskElt != 0 && MaskElt != -1)
return nullptr;
// The first shuffle source is 'insertelement' with index 0.
auto *InsertEltInst =
dyn_cast<InsertElementInst>(ShuffleInst->getOperand(0));
if (!InsertEltInst || !isa<ConstantInt>(InsertEltInst->getOperand(2)) ||
!cast<ConstantInt>(InsertEltInst->getOperand(2))->isZero())
return nullptr;
return InsertEltInst->getOperand(1);
}
MapVector<Instruction *, uint64_t>
llvm::computeMinimumValueSizes(ArrayRef<BasicBlock *> Blocks, DemandedBits &DB,
const TargetTransformInfo *TTI) {
// DemandedBits will give us every value's live-out bits. But we want
// to ensure no extra casts would need to be inserted, so every DAG
// of connected values must have the same minimum bitwidth.
EquivalenceClasses<Value *> ECs;
SmallVector<Value *, 16> Worklist;
SmallPtrSet<Value *, 4> Roots;
SmallPtrSet<Value *, 16> Visited;
DenseMap<Value *, uint64_t> DBits;
SmallPtrSet<Instruction *, 4> InstructionSet;
MapVector<Instruction *, uint64_t> MinBWs;
// Determine the roots. We work bottom-up, from truncs or icmps.
bool SeenExtFromIllegalType = false;
for (auto *BB : Blocks)
for (auto &I : *BB) {
InstructionSet.insert(&I);
if (TTI && (isa<ZExtInst>(&I) || isa<SExtInst>(&I)) &&
!TTI->isTypeLegal(I.getOperand(0)->getType()))
SeenExtFromIllegalType = true;
// Only deal with non-vector integers up to 64-bits wide.
if ((isa<TruncInst>(&I) || isa<ICmpInst>(&I)) &&
!I.getType()->isVectorTy() &&
I.getOperand(0)->getType()->getScalarSizeInBits() <= 64) {
// Don't make work for ourselves. If we know the loaded type is legal,
// don't add it to the worklist.
if (TTI && isa<TruncInst>(&I) && TTI->isTypeLegal(I.getType()))
continue;
Worklist.push_back(&I);
Roots.insert(&I);
}
}
// Early exit.
if (Worklist.empty() || (TTI && !SeenExtFromIllegalType))
return MinBWs;
// Now proceed breadth-first, unioning values together.
while (!Worklist.empty()) {
Value *Val = Worklist.pop_back_val();
Value *Leader = ECs.getOrInsertLeaderValue(Val);
if (Visited.count(Val))
continue;
Visited.insert(Val);
// Non-instructions terminate a chain successfully.
if (!isa<Instruction>(Val))
continue;
Instruction *I = cast<Instruction>(Val);
// If we encounter a type that is larger than 64 bits, we can't represent
// it so bail out.
if (DB.getDemandedBits(I).getBitWidth() > 64)
return MapVector<Instruction *, uint64_t>();
uint64_t V = DB.getDemandedBits(I).getZExtValue();
DBits[Leader] |= V;
DBits[I] = V;
// Casts, loads and instructions outside of our range terminate a chain
// successfully.
if (isa<SExtInst>(I) || isa<ZExtInst>(I) || isa<LoadInst>(I) ||
!InstructionSet.count(I))
continue;
// Unsafe casts terminate a chain unsuccessfully. We can't do anything
// useful with bitcasts, ptrtoints or inttoptrs and it'd be unsafe to
// transform anything that relies on them.
if (isa<BitCastInst>(I) || isa<PtrToIntInst>(I) || isa<IntToPtrInst>(I) ||
!I->getType()->isIntegerTy()) {
DBits[Leader] |= ~0ULL;
continue;
}
// We don't modify the types of PHIs. Reductions will already have been
// truncated if possible, and inductions' sizes will have been chosen by
// indvars.
if (isa<PHINode>(I))
continue;
if (DBits[Leader] == ~0ULL)
// All bits demanded, no point continuing.
continue;
for (Value *O : cast<User>(I)->operands()) {
ECs.unionSets(Leader, O);
Worklist.push_back(O);
}
}
// Now we've discovered all values, walk them to see if there are
// any users we didn't see. If there are, we can't optimize that
// chain.
for (auto &I : DBits)
for (auto *U : I.first->users())
if (U->getType()->isIntegerTy() && DBits.count(U) == 0)
DBits[ECs.getOrInsertLeaderValue(I.first)] |= ~0ULL;
for (auto I = ECs.begin(), E = ECs.end(); I != E; ++I) {
uint64_t LeaderDemandedBits = 0;
for (auto MI = ECs.member_begin(I), ME = ECs.member_end(); MI != ME; ++MI)
LeaderDemandedBits |= DBits[*MI];
uint64_t MinBW = (sizeof(LeaderDemandedBits) * 8) -
llvm::countLeadingZeros(LeaderDemandedBits);
// Round up to a power of 2
if (!isPowerOf2_64((uint64_t)MinBW))
MinBW = NextPowerOf2(MinBW);
// We don't modify the types of PHIs. Reductions will already have been
// truncated if possible, and inductions' sizes will have been chosen by
// indvars.
// If we are required to shrink a PHI, abandon this entire equivalence class.
bool Abort = false;
for (auto MI = ECs.member_begin(I), ME = ECs.member_end(); MI != ME; ++MI)
if (isa<PHINode>(*MI) && MinBW < (*MI)->getType()->getScalarSizeInBits()) {
Abort = true;
break;
}
if (Abort)
continue;
for (auto MI = ECs.member_begin(I), ME = ECs.member_end(); MI != ME; ++MI) {
if (!isa<Instruction>(*MI))
continue;
Type *Ty = (*MI)->getType();
if (Roots.count(*MI))
Ty = cast<Instruction>(*MI)->getOperand(0)->getType();
if (MinBW < Ty->getScalarSizeInBits())
MinBWs[cast<Instruction>(*MI)] = MinBW;
}
}
return MinBWs;
}
/// \returns \p I after propagating metadata from \p VL.
Instruction *llvm::propagateMetadata(Instruction *Inst, ArrayRef<Value *> VL) {
Instruction *I0 = cast<Instruction>(VL[0]);
SmallVector<std::pair<unsigned, MDNode *>, 4> Metadata;
I0->getAllMetadataOtherThanDebugLoc(Metadata);
for (auto Kind :
{LLVMContext::MD_tbaa, LLVMContext::MD_alias_scope,
LLVMContext::MD_noalias, LLVMContext::MD_fpmath,
LLVMContext::MD_nontemporal, LLVMContext::MD_invariant_load}) {
MDNode *MD = I0->getMetadata(Kind);
for (int J = 1, E = VL.size(); MD && J != E; ++J) {
const Instruction *IJ = cast<Instruction>(VL[J]);
MDNode *IMD = IJ->getMetadata(Kind);
switch (Kind) {
case LLVMContext::MD_tbaa:
MD = MDNode::getMostGenericTBAA(MD, IMD);
break;
case LLVMContext::MD_alias_scope:
MD = MDNode::getMostGenericAliasScope(MD, IMD);
break;
case LLVMContext::MD_fpmath:
MD = MDNode::getMostGenericFPMath(MD, IMD);
break;
case LLVMContext::MD_noalias:
case LLVMContext::MD_nontemporal:
case LLVMContext::MD_invariant_load:
MD = MDNode::intersect(MD, IMD);
break;
default:
llvm_unreachable("unhandled metadata");
}
}
Inst->setMetadata(Kind, MD);
}
return Inst;
}
Constant *llvm::createInterleaveMask(IRBuilder<> &Builder, unsigned VF,
unsigned NumVecs) {
SmallVector<Constant *, 16> Mask;
for (unsigned i = 0; i < VF; i++)
for (unsigned j = 0; j < NumVecs; j++)
Mask.push_back(Builder.getInt32(j * VF + i));
return ConstantVector::get(Mask);
}
Constant *llvm::createStrideMask(IRBuilder<> &Builder, unsigned Start,
unsigned Stride, unsigned VF) {
SmallVector<Constant *, 16> Mask;
for (unsigned i = 0; i < VF; i++)
Mask.push_back(Builder.getInt32(Start + i * Stride));
return ConstantVector::get(Mask);
}
Constant *llvm::createSequentialMask(IRBuilder<> &Builder, unsigned Start,
unsigned NumInts, unsigned NumUndefs) {
SmallVector<Constant *, 16> Mask;
for (unsigned i = 0; i < NumInts; i++)
Mask.push_back(Builder.getInt32(Start + i));
Constant *Undef = UndefValue::get(Builder.getInt32Ty());
for (unsigned i = 0; i < NumUndefs; i++)
Mask.push_back(Undef);
return ConstantVector::get(Mask);
}
/// A helper function for concatenating vectors. This function concatenates two
/// vectors having the same element type. If the second vector has fewer
/// elements than the first, it is padded with undefs.
static Value *concatenateTwoVectors(IRBuilder<> &Builder, Value *V1,
Value *V2) {
VectorType *VecTy1 = dyn_cast<VectorType>(V1->getType());
VectorType *VecTy2 = dyn_cast<VectorType>(V2->getType());
assert(VecTy1 && VecTy2 &&
VecTy1->getScalarType() == VecTy2->getScalarType() &&
"Expect two vectors with the same element type");
unsigned NumElts1 = VecTy1->getNumElements();
unsigned NumElts2 = VecTy2->getNumElements();
assert(NumElts1 >= NumElts2 && "Unexpect the first vector has less elements");
if (NumElts1 > NumElts2) {
// Extend with UNDEFs.
Constant *ExtMask =
createSequentialMask(Builder, 0, NumElts2, NumElts1 - NumElts2);
V2 = Builder.CreateShuffleVector(V2, UndefValue::get(VecTy2), ExtMask);
}
Constant *Mask = createSequentialMask(Builder, 0, NumElts1 + NumElts2, 0);
return Builder.CreateShuffleVector(V1, V2, Mask);
}
Value *llvm::concatenateVectors(IRBuilder<> &Builder, ArrayRef<Value *> Vecs) {
unsigned NumVecs = Vecs.size();
assert(NumVecs > 1 && "Should be at least two vectors");
SmallVector<Value *, 8> ResList;
ResList.append(Vecs.begin(), Vecs.end());
do {
SmallVector<Value *, 8> TmpList;
for (unsigned i = 0; i < NumVecs - 1; i += 2) {
Value *V0 = ResList[i], *V1 = ResList[i + 1];
assert((V0->getType() == V1->getType() || i == NumVecs - 2) &&
"Only the last vector may have a different type");
TmpList.push_back(concatenateTwoVectors(Builder, V0, V1));
}
// Push the last vector if the total number of vectors is odd.
if (NumVecs % 2 != 0)
TmpList.push_back(ResList[NumVecs - 1]);
ResList = TmpList;
NumVecs = ResList.size();
} while (NumVecs > 1);
return ResList[0];
}
bool InterleavedAccessInfo::isStrided(int Stride) {
unsigned Factor = std::abs(Stride);
return Factor >= 2 && Factor <= MaxInterleaveGroupFactor;
}
void InterleavedAccessInfo::collectConstStrideAccesses(
MapVector<Instruction *, StrideDescriptor> &AccessStrideInfo,
const ValueToValueMap &Strides) {
auto &DL = TheLoop->getHeader()->getModule()->getDataLayout();
// Since it's desired that the load/store instructions be maintained in
// "program order" for the interleaved access analysis, we have to visit the
// blocks in the loop in reverse postorder (i.e., in a topological order).
// Such an ordering will ensure that any load/store that may be executed
// before a second load/store will precede the second load/store in
// AccessStrideInfo.
LoopBlocksDFS DFS(TheLoop);
DFS.perform(LI);
for (BasicBlock *BB : make_range(DFS.beginRPO(), DFS.endRPO()))
for (auto &I : *BB) {
auto *LI = dyn_cast<LoadInst>(&I);
auto *SI = dyn_cast<StoreInst>(&I);
if (!LI && !SI)
continue;
Value *Ptr = getLoadStorePointerOperand(&I);
// We don't check wrapping here because we don't know yet if Ptr will be
// part of a full group or a group with gaps. Checking wrapping for all
// pointers (even those that end up in groups with no gaps) will be overly
// conservative. For full groups, wrapping should be ok since if we would
// wrap around the address space we would do a memory access at nullptr
// even without the transformation. The wrapping checks are therefore
// deferred until after we've formed the interleaved groups.
int64_t Stride = getPtrStride(PSE, Ptr, TheLoop, Strides,
/*Assume=*/true, /*ShouldCheckWrap=*/false);
const SCEV *Scev = replaceSymbolicStrideSCEV(PSE, Strides, Ptr);
PointerType *PtrTy = dyn_cast<PointerType>(Ptr->getType());
uint64_t Size = DL.getTypeAllocSize(PtrTy->getElementType());
// An alignment of 0 means target ABI alignment.
unsigned Align = getLoadStoreAlignment(&I);
if (!Align)
Align = DL.getABITypeAlignment(PtrTy->getElementType());
AccessStrideInfo[&I] = StrideDescriptor(Stride, Scev, Size, Align);
}
}
// Analyze interleaved accesses and collect them into interleaved load and
// store groups.
//
// When generating code for an interleaved load group, we effectively hoist all
// loads in the group to the location of the first load in program order. When
// generating code for an interleaved store group, we sink all stores to the
// location of the last store. This code motion can change the order of load
// and store instructions and may break dependences.
//
// The code generation strategy mentioned above ensures that we won't violate
// any write-after-read (WAR) dependences.
//
// E.g., for the WAR dependence: a = A[i]; // (1)
// A[i] = b; // (2)
//
// The store group of (2) is always inserted at or below (2), and the load
// group of (1) is always inserted at or above (1). Thus, the instructions will
// never be reordered. All other dependences are checked to ensure the
// correctness of the instruction reordering.
//
// The algorithm visits all memory accesses in the loop in bottom-up program
// order. Program order is established by traversing the blocks in the loop in
// reverse postorder when collecting the accesses.
//
// We visit the memory accesses in bottom-up order because it can simplify the
// construction of store groups in the presence of write-after-write (WAW)
// dependences.
//
// E.g., for the WAW dependence: A[i] = a; // (1)
// A[i] = b; // (2)
// A[i + 1] = c; // (3)
//
// We will first create a store group with (3) and (2). (1) can't be added to
// this group because it and (2) are dependent. However, (1) can be grouped
// with other accesses that may precede it in program order. Note that a
// bottom-up order does not imply that WAW dependences should not be checked.
void InterleavedAccessInfo::analyzeInterleaving() {
LLVM_DEBUG(dbgs() << "LV: Analyzing interleaved accesses...\n");
const ValueToValueMap &Strides = LAI->getSymbolicStrides();
// Holds all accesses with a constant stride.
MapVector<Instruction *, StrideDescriptor> AccessStrideInfo;
collectConstStrideAccesses(AccessStrideInfo, Strides);
if (AccessStrideInfo.empty())
return;
// Collect the dependences in the loop.
collectDependences();
// Holds all interleaved store groups temporarily.
SmallSetVector<InterleaveGroup *, 4> StoreGroups;
// Holds all interleaved load groups temporarily.
SmallSetVector<InterleaveGroup *, 4> LoadGroups;
// Search in bottom-up program order for pairs of accesses (A and B) that can
// form interleaved load or store groups. In the algorithm below, access A
// precedes access B in program order. We initialize a group for B in the
// outer loop of the algorithm, and then in the inner loop, we attempt to
// insert each A into B's group if:
//
// 1. A and B have the same stride,
// 2. A and B have the same memory object size, and
// 3. A belongs in B's group according to its distance from B.
//
// Special care is taken to ensure group formation will not break any
// dependences.
for (auto BI = AccessStrideInfo.rbegin(), E = AccessStrideInfo.rend();
BI != E; ++BI) {
Instruction *B = BI->first;
StrideDescriptor DesB = BI->second;
// Initialize a group for B if it has an allowable stride. Even if we don't
// create a group for B, we continue with the bottom-up algorithm to ensure
// we don't break any of B's dependences.
InterleaveGroup *Group = nullptr;
if (isStrided(DesB.Stride)) {
Group = getInterleaveGroup(B);
if (!Group) {
LLVM_DEBUG(dbgs() << "LV: Creating an interleave group with:" << *B
<< '\n');
Group = createInterleaveGroup(B, DesB.Stride, DesB.Align);
}
if (B->mayWriteToMemory())
StoreGroups.insert(Group);
else
LoadGroups.insert(Group);
}
for (auto AI = std::next(BI); AI != E; ++AI) {
Instruction *A = AI->first;
StrideDescriptor DesA = AI->second;
// Our code motion strategy implies that we can't have dependences
// between accesses in an interleaved group and other accesses located
// between the first and last member of the group. Note that this also
// means that a group can't have more than one member at a given offset.
// The accesses in a group can have dependences with other accesses, but
// we must ensure we don't extend the boundaries of the group such that
// we encompass those dependent accesses.
//
// For example, assume we have the sequence of accesses shown below in a
// stride-2 loop:
//
// (1, 2) is a group | A[i] = a; // (1)
// | A[i-1] = b; // (2) |
// A[i-3] = c; // (3)
// A[i] = d; // (4) | (2, 4) is not a group
//
// Because accesses (2) and (3) are dependent, we can group (2) with (1)
// but not with (4). If we did, the dependent access (3) would be within
// the boundaries of the (2, 4) group.
if (!canReorderMemAccessesForInterleavedGroups(&*AI, &*BI)) {
// If a dependence exists and A is already in a group, we know that A
// must be a store since A precedes B and WAR dependences are allowed.
// Thus, A would be sunk below B. We release A's group to prevent this
// illegal code motion. A will then be free to form another group with
// instructions that precede it.
if (isInterleaved(A)) {
InterleaveGroup *StoreGroup = getInterleaveGroup(A);
StoreGroups.remove(StoreGroup);
releaseGroup(StoreGroup);
}
// If a dependence exists and A is not already in a group (or it was
// and we just released it), B might be hoisted above A (if B is a
// load) or another store might be sunk below A (if B is a store). In
// either case, we can't add additional instructions to B's group. B
// will only form a group with instructions that it precedes.
break;
}
// At this point, we've checked for illegal code motion. If either A or B
// isn't strided, there's nothing left to do.
if (!isStrided(DesA.Stride) || !isStrided(DesB.Stride))
continue;
// Ignore A if it's already in a group or isn't the same kind of memory
// operation as B.
// Note that mayReadFromMemory() isn't mutually exclusive to
// mayWriteToMemory in the case of atomic loads. We shouldn't see those
// here, canVectorizeMemory() should have returned false - except for the
// case we asked for optimization remarks.
if (isInterleaved(A) ||
(A->mayReadFromMemory() != B->mayReadFromMemory()) ||
(A->mayWriteToMemory() != B->mayWriteToMemory()))
continue;
// Check rules 1 and 2. Ignore A if its stride or size is different from
// that of B.
if (DesA.Stride != DesB.Stride || DesA.Size != DesB.Size)
continue;
// Ignore A if the memory object of A and B don't belong to the same
// address space
if (getLoadStoreAddressSpace(A) != getLoadStoreAddressSpace(B))
continue;
// Calculate the distance from A to B.
const SCEVConstant *DistToB = dyn_cast<SCEVConstant>(
PSE.getSE()->getMinusSCEV(DesA.Scev, DesB.Scev));
if (!DistToB)
continue;
int64_t DistanceToB = DistToB->getAPInt().getSExtValue();
// Check rule 3. Ignore A if its distance to B is not a multiple of the
// size.
if (DistanceToB % static_cast<int64_t>(DesB.Size))
continue;
// Ignore A if either A or B is in a predicated block. Although we
// currently prevent group formation for predicated accesses, we may be
// able to relax this limitation in the future once we handle more
// complicated blocks.
if (isPredicated(A->getParent()) || isPredicated(B->getParent()))
continue;
// The index of A is the index of B plus A's distance to B in multiples
// of the size.
int IndexA =
Group->getIndex(B) + DistanceToB / static_cast<int64_t>(DesB.Size);
// Try to insert A into B's group.
if (Group->insertMember(A, IndexA, DesA.Align)) {
LLVM_DEBUG(dbgs() << "LV: Inserted:" << *A << '\n'
<< " into the interleave group with" << *B
<< '\n');
InterleaveGroupMap[A] = Group;
// Set the first load in program order as the insert position.
if (A->mayReadFromMemory())
Group->setInsertPos(A);
}
} // Iteration over A accesses.
} // Iteration over B accesses.
// Remove interleaved store groups with gaps.
for (InterleaveGroup *Group : StoreGroups)
if (Group->getNumMembers() != Group->getFactor()) {
LLVM_DEBUG(
dbgs() << "LV: Invalidate candidate interleaved store group due "
"to gaps.\n");
releaseGroup(Group);
}
// Remove interleaved groups with gaps (currently only loads) whose memory
// accesses may wrap around. We have to revisit the getPtrStride analysis,
// this time with ShouldCheckWrap=true, since collectConstStrideAccesses does
// not check wrapping (see documentation there).
// FORNOW we use Assume=false;
// TODO: Change to Assume=true but making sure we don't exceed the threshold
// of runtime SCEV assumptions checks (thereby potentially failing to
// vectorize altogether).
// Additional optional optimizations:
// TODO: If we are peeling the loop and we know that the first pointer doesn't
// wrap then we can deduce that all pointers in the group don't wrap.
// This means that we can forcefully peel the loop in order to only have to
// check the first pointer for no-wrap. When we'll change to use Assume=true
// we'll only need at most one runtime check per interleaved group.
for (InterleaveGroup *Group : LoadGroups) {
// Case 1: A full group. Can Skip the checks; For full groups, if the wide
// load would wrap around the address space we would do a memory access at
// nullptr even without the transformation.
if (Group->getNumMembers() == Group->getFactor())
continue;
// Case 2: If first and last members of the group don't wrap this implies
// that all the pointers in the group don't wrap.
// So we check only group member 0 (which is always guaranteed to exist),
// and group member Factor - 1; If the latter doesn't exist we rely on
// peeling (if it is a non-reveresed accsess -- see Case 3).
Value *FirstMemberPtr = getLoadStorePointerOperand(Group->getMember(0));
if (!getPtrStride(PSE, FirstMemberPtr, TheLoop, Strides, /*Assume=*/false,
/*ShouldCheckWrap=*/true)) {
LLVM_DEBUG(
dbgs() << "LV: Invalidate candidate interleaved group due to "
"first group member potentially pointer-wrapping.\n");
releaseGroup(Group);
continue;
}
Instruction *LastMember = Group->getMember(Group->getFactor() - 1);
if (LastMember) {
Value *LastMemberPtr = getLoadStorePointerOperand(LastMember);
if (!getPtrStride(PSE, LastMemberPtr, TheLoop, Strides, /*Assume=*/false,
/*ShouldCheckWrap=*/true)) {
LLVM_DEBUG(
dbgs() << "LV: Invalidate candidate interleaved group due to "
"last group member potentially pointer-wrapping.\n");
releaseGroup(Group);
}
} else {
// Case 3: A non-reversed interleaved load group with gaps: We need
// to execute at least one scalar epilogue iteration. This will ensure
// we don't speculatively access memory out-of-bounds. We only need
// to look for a member at index factor - 1, since every group must have
// a member at index zero.
if (Group->isReverse()) {
LLVM_DEBUG(
dbgs() << "LV: Invalidate candidate interleaved group due to "
"a reverse access with gaps.\n");
releaseGroup(Group);
continue;
}
LLVM_DEBUG(
dbgs() << "LV: Interleaved group requires epilogue iteration.\n");
RequiresScalarEpilogue = true;
}
}
}
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