OSchip
/
llvm-project
mirror of https://github.com/THU-DSP-LAB/llvm-project.git
13
0
Fork 6
Code Issues Packages Projects Releases Wiki Activity
llvm-project/llvm/lib/Analysis/Loads.cpp

668 lines
27 KiB
C++
Raw Blame History

//===- Loads.cpp - Local load analysis ------------------------------------===//
//
// 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 file defines simple local analyses for load instructions.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumeBundleQueries.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
using namespace llvm;
static bool isAligned(const Value *Base, const APInt &Offset, Align Alignment,
const DataLayout &DL) {
Align BA = Base->getPointerAlignment(DL);
const APInt APAlign(Offset.getBitWidth(), Alignment.value());
assert(APAlign.isPowerOf2() && "must be a power of 2!");
return BA >= Alignment && !(Offset & (APAlign - 1));
}
/// Test if V is always a pointer to allocated and suitably aligned memory for
/// a simple load or store.
static bool isDereferenceableAndAlignedPointer(
const Value *V, Align Alignment, const APInt &Size, const DataLayout &DL,
const Instruction *CtxI, const DominatorTree *DT,
const TargetLibraryInfo *TLI, SmallPtrSetImpl<const Value *> &Visited,
unsigned MaxDepth) {
assert(V->getType()->isPointerTy() && "Base must be pointer");
// Recursion limit.
if (MaxDepth-- == 0)
return false;
// Already visited? Bail out, we've likely hit unreachable code.
if (!Visited.insert(V).second)
return false;
// Note that it is not safe to speculate into a malloc'd region because
// malloc may return null.
// Recurse into both hands of select.
if (const SelectInst *Sel = dyn_cast<SelectInst>(V)) {
return isDereferenceableAndAlignedPointer(Sel->getTrueValue(), Alignment,
Size, DL, CtxI, DT, TLI, Visited,
MaxDepth) &&
isDereferenceableAndAlignedPointer(Sel->getFalseValue(), Alignment,
Size, DL, CtxI, DT, TLI, Visited,
MaxDepth);
}
// bitcast instructions are no-ops as far as dereferenceability is concerned.
if (const BitCastOperator *BC = dyn_cast<BitCastOperator>(V)) {
if (BC->getSrcTy()->isPointerTy())
return isDereferenceableAndAlignedPointer(
BC->getOperand(0), Alignment, Size, DL, CtxI, DT, TLI,
Visited, MaxDepth);
}
bool CheckForNonNull, CheckForFreed;
APInt KnownDerefBytes(Size.getBitWidth(),
V->getPointerDereferenceableBytes(DL, CheckForNonNull,
CheckForFreed));
if (KnownDerefBytes.getBoolValue() && KnownDerefBytes.uge(Size) &&
!CheckForFreed)
if (!CheckForNonNull || isKnownNonZero(V, DL, 0, nullptr, CtxI, DT)) {
// As we recursed through GEPs to get here, we've incrementally checked
// that each step advanced by a multiple of the alignment. If our base is
// properly aligned, then the original offset accessed must also be.
Type *Ty = V->getType();
assert(Ty->isSized() && "must be sized");
APInt Offset(DL.getTypeStoreSizeInBits(Ty), 0);
return isAligned(V, Offset, Alignment, DL);
}
if (CtxI) {
/// Look through assumes to see if both dereferencability and alignment can
/// be provent by an assume
RetainedKnowledge AlignRK;
RetainedKnowledge DerefRK;
if (getKnowledgeForValue(
V, {Attribute::Dereferenceable, Attribute::Alignment}, nullptr,
[&](RetainedKnowledge RK, Instruction *Assume, auto) {
if (!isValidAssumeForContext(Assume, CtxI))
return false;
if (RK.AttrKind == Attribute::Alignment)
AlignRK = std::max(AlignRK, RK);
if (RK.AttrKind == Attribute::Dereferenceable)
DerefRK = std::max(DerefRK, RK);
if (AlignRK && DerefRK && AlignRK.ArgValue >= Alignment.value() &&
DerefRK.ArgValue >= Size.getZExtValue())
return true; // We have found what we needed so we stop looking
return false; // Other assumes may have better information. so
// keep looking
}))
return true;
}
/// TODO refactor this function to be able to search independently for
/// Dereferencability and Alignment requirements.
// For GEPs, determine if the indexing lands within the allocated object.
if (const GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
const Value *Base = GEP->getPointerOperand();
APInt Offset(DL.getIndexTypeSizeInBits(GEP->getType()), 0);
if (!GEP->accumulateConstantOffset(DL, Offset) || Offset.isNegative() ||
!Offset.urem(APInt(Offset.getBitWidth(), Alignment.value()))
.isMinValue())
return false;
// If the base pointer is dereferenceable for Offset+Size bytes, then the
// GEP (== Base + Offset) is dereferenceable for Size bytes. If the base
// pointer is aligned to Align bytes, and the Offset is divisible by Align
// then the GEP (== Base + Offset == k_0 * Align + k_1 * Align) is also
// aligned to Align bytes.
// Offset and Size may have different bit widths if we have visited an
// addrspacecast, so we can't do arithmetic directly on the APInt values.
return isDereferenceableAndAlignedPointer(
Base, Alignment, Offset + Size.sextOrTrunc(Offset.getBitWidth()), DL,
CtxI, DT, TLI, Visited, MaxDepth);
}
// For gc.relocate, look through relocations
if (const GCRelocateInst *RelocateInst = dyn_cast<GCRelocateInst>(V))
return isDereferenceableAndAlignedPointer(RelocateInst->getDerivedPtr(),
Alignment, Size, DL, CtxI, DT,
TLI, Visited, MaxDepth);
if (const AddrSpaceCastOperator *ASC = dyn_cast<AddrSpaceCastOperator>(V))
return isDereferenceableAndAlignedPointer(ASC->getOperand(0), Alignment,
Size, DL, CtxI, DT, TLI,
Visited, MaxDepth);
if (const auto *Call = dyn_cast<CallBase>(V)) {
if (auto *RP = getArgumentAliasingToReturnedPointer(Call, true))
return isDereferenceableAndAlignedPointer(RP, Alignment, Size, DL, CtxI,
DT, TLI, Visited, MaxDepth);
// If we have a call we can't recurse through, check to see if this is an
// allocation function for which we can establish an minimum object size.
// Such a minimum object size is analogous to a deref_or_null attribute in
// that we still need to prove the result non-null at point of use.
// NOTE: We can only use the object size as a base fact as we a) need to
// prove alignment too, and b) don't want the compile time impact of a
// separate recursive walk.
ObjectSizeOpts Opts;
// TODO: It may be okay to round to align, but that would imply that
// accessing slightly out of bounds was legal, and we're currently
// inconsistent about that. For the moment, be conservative.
Opts.RoundToAlign = false;
Opts.NullIsUnknownSize = true;
uint64_t ObjSize;
if (getObjectSize(V, ObjSize, DL, TLI, Opts)) {
APInt KnownDerefBytes(Size.getBitWidth(), ObjSize);
if (KnownDerefBytes.getBoolValue() && KnownDerefBytes.uge(Size) &&
isKnownNonZero(V, DL, 0, nullptr, CtxI, DT) && !V->canBeFreed()) {
// As we recursed through GEPs to get here, we've incrementally
// checked that each step advanced by a multiple of the alignment. If
// our base is properly aligned, then the original offset accessed
// must also be.
Type *Ty = V->getType();
assert(Ty->isSized() && "must be sized");
APInt Offset(DL.getTypeStoreSizeInBits(Ty), 0);
return isAligned(V, Offset, Alignment, DL);
}
}
}
// If we don't know, assume the worst.
return false;
}
bool llvm::isDereferenceableAndAlignedPointer(const Value *V, Align Alignment,
const APInt &Size,
const DataLayout &DL,
const Instruction *CtxI,
const DominatorTree *DT,
const TargetLibraryInfo *TLI) {
// Note: At the moment, Size can be zero. This ends up being interpreted as
// a query of whether [Base, V] is dereferenceable and V is aligned (since
// that's what the implementation happened to do). It's unclear if this is
// the desired semantic, but at least SelectionDAG does exercise this case.
SmallPtrSet<const Value *, 32> Visited;
return ::isDereferenceableAndAlignedPointer(V, Alignment, Size, DL, CtxI, DT,
TLI, Visited, 16);
}
bool llvm::isDereferenceableAndAlignedPointer(const Value *V, Type *Ty,
Align Alignment,
const DataLayout &DL,
const Instruction *CtxI,
const DominatorTree *DT,
const TargetLibraryInfo *TLI) {
// For unsized types or scalable vectors we don't know exactly how many bytes
// are dereferenced, so bail out.
if (!Ty->isSized() || isa<ScalableVectorType>(Ty))
return false;
// When dereferenceability information is provided by a dereferenceable
// attribute, we know exactly how many bytes are dereferenceable. If we can
// determine the exact offset to the attributed variable, we can use that
// information here.
APInt AccessSize(DL.getPointerTypeSizeInBits(V->getType()),
DL.getTypeStoreSize(Ty));
return isDereferenceableAndAlignedPointer(V, Alignment, AccessSize, DL, CtxI,
DT, TLI);
}
bool llvm::isDereferenceablePointer(const Value *V, Type *Ty,
const DataLayout &DL,
const Instruction *CtxI,
const DominatorTree *DT,
const TargetLibraryInfo *TLI) {
return isDereferenceableAndAlignedPointer(V, Ty, Align(1), DL, CtxI, DT, TLI);
}
/// Test if A and B will obviously have the same value.
///
/// This includes recognizing that %t0 and %t1 will have the same
/// value in code like this:
/// \code
/// %t0 = getelementptr \@a, 0, 3
/// store i32 0, i32* %t0
/// %t1 = getelementptr \@a, 0, 3
/// %t2 = load i32* %t1
/// \endcode
///
static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
// Test if the values are trivially equivalent.
if (A == B)
return true;
// Test if the values come from identical arithmetic instructions.
// Use isIdenticalToWhenDefined instead of isIdenticalTo because
// this function is only used when one address use dominates the
// other, which means that they'll always either have the same
// value or one of them will have an undefined value.
if (isa<BinaryOperator>(A) || isa<CastInst>(A) || isa<PHINode>(A) ||
isa<GetElementPtrInst>(A))
if (const Instruction *BI = dyn_cast<Instruction>(B))
if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
return true;
// Otherwise they may not be equivalent.
return false;
}
bool llvm::isDereferenceableAndAlignedInLoop(LoadInst *LI, Loop *L,
ScalarEvolution &SE,
DominatorTree &DT) {
auto &DL = LI->getModule()->getDataLayout();
Value *Ptr = LI->getPointerOperand();
APInt EltSize(DL.getIndexTypeSizeInBits(Ptr->getType()),
DL.getTypeStoreSize(LI->getType()).getFixedSize());
const Align Alignment = LI->getAlign();
Instruction *HeaderFirstNonPHI = L->getHeader()->getFirstNonPHI();
// If given a uniform (i.e. non-varying) address, see if we can prove the
// access is safe within the loop w/o needing predication.
if (L->isLoopInvariant(Ptr))
return isDereferenceableAndAlignedPointer(Ptr, Alignment, EltSize, DL,
HeaderFirstNonPHI, &DT);
// Otherwise, check to see if we have a repeating access pattern where we can
// prove that all accesses are well aligned and dereferenceable.
auto *AddRec = dyn_cast<SCEVAddRecExpr>(SE.getSCEV(Ptr));
if (!AddRec || AddRec->getLoop() != L || !AddRec->isAffine())
return false;
auto* Step = dyn_cast<SCEVConstant>(AddRec->getStepRecurrence(SE));
if (!Step)
return false;
// TODO: generalize to access patterns which have gaps
if (Step->getAPInt() != EltSize)
return false;
auto TC = SE.getSmallConstantMaxTripCount(L);
if (!TC)
return false;
const APInt AccessSize = TC * EltSize;
auto *StartS = dyn_cast<SCEVUnknown>(AddRec->getStart());
if (!StartS)
return false;
assert(SE.isLoopInvariant(StartS, L) && "implied by addrec definition");
Value *Base = StartS->getValue();
// For the moment, restrict ourselves to the case where the access size is a
// multiple of the requested alignment and the base is aligned.
// TODO: generalize if a case found which warrants
if (EltSize.urem(Alignment.value()) != 0)
return false;
return isDereferenceableAndAlignedPointer(Base, Alignment, AccessSize, DL,
HeaderFirstNonPHI, &DT);
}
/// Check if executing a load of this pointer value cannot trap.
///
/// If DT and ScanFrom are specified this method performs context-sensitive
/// analysis and returns true if it is safe to load immediately before ScanFrom.
///
/// If it is not obviously safe to load from the specified pointer, we do
/// a quick local scan of the basic block containing \c ScanFrom, to determine
/// if the address is already accessed.
///
/// This uses the pointee type to determine how many bytes need to be safe to
/// load from the pointer.
bool llvm::isSafeToLoadUnconditionally(Value *V, Align Alignment, APInt &Size,
const DataLayout &DL,
Instruction *ScanFrom,
const DominatorTree *DT,
const TargetLibraryInfo *TLI) {
// If DT is not specified we can't make context-sensitive query
const Instruction* CtxI = DT ? ScanFrom : nullptr;
if (isDereferenceableAndAlignedPointer(V, Alignment, Size, DL, CtxI, DT, TLI))
return true;
if (!ScanFrom)
return false;
if (Size.getBitWidth() > 64)
return false;
const uint64_t LoadSize = Size.getZExtValue();
// Otherwise, be a little bit aggressive by scanning the local block where we
// want to check to see if the pointer is already being loaded or stored
// from/to. If so, the previous load or store would have already trapped,
// so there is no harm doing an extra load (also, CSE will later eliminate
// the load entirely).
BasicBlock::iterator BBI = ScanFrom->getIterator(),
E = ScanFrom->getParent()->begin();
// We can at least always strip pointer casts even though we can't use the
// base here.
V = V->stripPointerCasts();
while (BBI != E) {
--BBI;
// If we see a free or a call which may write to memory (i.e. which might do
// a free) the pointer could be marked invalid.
if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
!isa<DbgInfoIntrinsic>(BBI))
return false;
Value *AccessedPtr;
Type *AccessedTy;
Align AccessedAlign;
if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
// Ignore volatile loads. The execution of a volatile load cannot
// be used to prove an address is backed by regular memory; it can,
// for example, point to an MMIO register.
if (LI->isVolatile())
continue;
AccessedPtr = LI->getPointerOperand();
AccessedTy = LI->getType();
AccessedAlign = LI->getAlign();
} else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
// Ignore volatile stores (see comment for loads).
if (SI->isVolatile())
continue;
AccessedPtr = SI->getPointerOperand();
AccessedTy = SI->getValueOperand()->getType();
AccessedAlign = SI->getAlign();
} else
continue;
if (AccessedAlign < Alignment)
continue;
// Handle trivial cases.
if (AccessedPtr == V &&
LoadSize <= DL.getTypeStoreSize(AccessedTy))
return true;
if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) &&
LoadSize <= DL.getTypeStoreSize(AccessedTy))
return true;
}
return false;
}
bool llvm::isSafeToLoadUnconditionally(Value *V, Type *Ty, Align Alignment,
const DataLayout &DL,
Instruction *ScanFrom,
const DominatorTree *DT,
const TargetLibraryInfo *TLI) {
TypeSize TySize = DL.getTypeStoreSize(Ty);
if (TySize.isScalable())
return false;
APInt Size(DL.getIndexTypeSizeInBits(V->getType()), TySize.getFixedValue());
return isSafeToLoadUnconditionally(V, Alignment, Size, DL, ScanFrom, DT, TLI);
}
/// DefMaxInstsToScan - the default number of maximum instructions
/// to scan in the block, used by FindAvailableLoadedValue().
/// FindAvailableLoadedValue() was introduced in r60148, to improve jump
/// threading in part by eliminating partially redundant loads.
/// At that point, the value of MaxInstsToScan was already set to '6'
/// without documented explanation.
cl::opt<unsigned>
llvm::DefMaxInstsToScan("available-load-scan-limit", cl::init(6), cl::Hidden,
cl::desc("Use this to specify the default maximum number of instructions "
"to scan backward from a given instruction, when searching for "
"available loaded value"));
Value *llvm::FindAvailableLoadedValue(LoadInst *Load,
BasicBlock *ScanBB,
BasicBlock::iterator &ScanFrom,
unsigned MaxInstsToScan,
AAResults *AA, bool *IsLoad,
unsigned *NumScanedInst) {
// Don't CSE load that is volatile or anything stronger than unordered.
if (!Load->isUnordered())
return nullptr;
MemoryLocation Loc = MemoryLocation::get(Load);
return findAvailablePtrLoadStore(Loc, Load->getType(), Load->isAtomic(),
ScanBB, ScanFrom, MaxInstsToScan, AA, IsLoad,
NumScanedInst);
}
// Check if the load and the store have the same base, constant offsets and
// non-overlapping access ranges.
static bool areNonOverlapSameBaseLoadAndStore(const Value *LoadPtr,
Type *LoadTy,
const Value *StorePtr,
Type *StoreTy,
const DataLayout &DL) {
APInt LoadOffset(DL.getIndexTypeSizeInBits(LoadPtr->getType()), 0);
APInt StoreOffset(DL.getIndexTypeSizeInBits(StorePtr->getType()), 0);
const Value *LoadBase = LoadPtr->stripAndAccumulateConstantOffsets(
DL, LoadOffset, /* AllowNonInbounds */ false);
const Value *StoreBase = StorePtr->stripAndAccumulateConstantOffsets(
DL, StoreOffset, /* AllowNonInbounds */ false);
if (LoadBase != StoreBase)
return false;
auto LoadAccessSize = LocationSize::precise(DL.getTypeStoreSize(LoadTy));
auto StoreAccessSize = LocationSize::precise(DL.getTypeStoreSize(StoreTy));
ConstantRange LoadRange(LoadOffset,
LoadOffset + LoadAccessSize.toRaw());
ConstantRange StoreRange(StoreOffset,
StoreOffset + StoreAccessSize.toRaw());
return LoadRange.intersectWith(StoreRange).isEmptySet();
}
static Value *getAvailableLoadStore(Instruction *Inst, const Value *Ptr,
Type *AccessTy, bool AtLeastAtomic,
const DataLayout &DL, bool *IsLoadCSE) {
// If this is a load of Ptr, the loaded value is available.
// (This is true even if the load is volatile or atomic, although
// those cases are unlikely.)
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
// We can value forward from an atomic to a non-atomic, but not the
// other way around.
if (LI->isAtomic() < AtLeastAtomic)
return nullptr;
Value *LoadPtr = LI->getPointerOperand()->stripPointerCasts();
if (!AreEquivalentAddressValues(LoadPtr, Ptr))
return nullptr;
if (CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) {
if (IsLoadCSE)
*IsLoadCSE = true;
return LI;
}
}
// If this is a store through Ptr, the value is available!
// (This is true even if the store is volatile or atomic, although
// those cases are unlikely.)
if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
// We can value forward from an atomic to a non-atomic, but not the
// other way around.
if (SI->isAtomic() < AtLeastAtomic)
return nullptr;
Value *StorePtr = SI->getPointerOperand()->stripPointerCasts();
if (!AreEquivalentAddressValues(StorePtr, Ptr))
return nullptr;
if (IsLoadCSE)
*IsLoadCSE = false;
Value *Val = SI->getValueOperand();
if (CastInst::isBitOrNoopPointerCastable(Val->getType(), AccessTy, DL))
return Val;
TypeSize StoreSize = DL.getTypeSizeInBits(Val->getType());
TypeSize LoadSize = DL.getTypeSizeInBits(AccessTy);
if (TypeSize::isKnownLE(LoadSize, StoreSize))
if (auto *C = dyn_cast<Constant>(Val))
return ConstantFoldLoadFromConst(C, AccessTy, DL);
}
return nullptr;
}
Value *llvm::findAvailablePtrLoadStore(
const MemoryLocation &Loc, Type *AccessTy, bool AtLeastAtomic,
BasicBlock *ScanBB, BasicBlock::iterator &ScanFrom, unsigned MaxInstsToScan,
AAResults *AA, bool *IsLoadCSE, unsigned *NumScanedInst) {
if (MaxInstsToScan == 0)
MaxInstsToScan = ~0U;
const DataLayout &DL = ScanBB->getModule()->getDataLayout();
const Value *StrippedPtr = Loc.Ptr->stripPointerCasts();
while (ScanFrom != ScanBB->begin()) {
// We must ignore debug info directives when counting (otherwise they
// would affect codegen).
Instruction *Inst = &*--ScanFrom;
if (Inst->isDebugOrPseudoInst())
continue;
// Restore ScanFrom to expected value in case next test succeeds
ScanFrom++;
if (NumScanedInst)
++(*NumScanedInst);
// Don't scan huge blocks.
if (MaxInstsToScan-- == 0)
return nullptr;
--ScanFrom;
if (Value *Available = getAvailableLoadStore(Inst, StrippedPtr, AccessTy,
AtLeastAtomic, DL, IsLoadCSE))
return Available;
// Try to get the store size for the type.
if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
Value *StorePtr = SI->getPointerOperand()->stripPointerCasts();
// If both StrippedPtr and StorePtr reach all the way to an alloca or
// global and they are different, ignore the store. This is a trivial form
// of alias analysis that is important for reg2mem'd code.
if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) &&
(isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) &&
StrippedPtr != StorePtr)
continue;
if (!AA) {
// When AA isn't available, but if the load and the store have the same
// base, constant offsets and non-overlapping access ranges, ignore the
// store. This is a simple form of alias analysis that is used by the
// inliner. FIXME: use BasicAA if possible.
if (areNonOverlapSameBaseLoadAndStore(
Loc.Ptr, AccessTy, SI->getPointerOperand(),
SI->getValueOperand()->getType(), DL))
continue;
} else {
// If we have alias analysis and it says the store won't modify the
// loaded value, ignore the store.
if (!isModSet(AA->getModRefInfo(SI, Loc)))
continue;
}
// Otherwise the store that may or may not alias the pointer, bail out.
++ScanFrom;
return nullptr;
}
// If this is some other instruction that may clobber Ptr, bail out.
if (Inst->mayWriteToMemory()) {
// If alias analysis claims that it really won't modify the load,
// ignore it.
if (AA && !isModSet(AA->getModRefInfo(Inst, Loc)))
continue;
// May modify the pointer, bail out.
++ScanFrom;
return nullptr;
}
}
// Got to the start of the block, we didn't find it, but are done for this
// block.
return nullptr;
}
Value *llvm::FindAvailableLoadedValue(LoadInst *Load, AAResults &AA,
bool *IsLoadCSE,
unsigned MaxInstsToScan) {
const DataLayout &DL = Load->getModule()->getDataLayout();
Value *StrippedPtr = Load->getPointerOperand()->stripPointerCasts();
BasicBlock *ScanBB = Load->getParent();
Type *AccessTy = Load->getType();
bool AtLeastAtomic = Load->isAtomic();
if (!Load->isUnordered())
return nullptr;
// Try to find an available value first, and delay expensive alias analysis
// queries until later.
Value *Available = nullptr;;
SmallVector<Instruction *> MustNotAliasInsts;
for (Instruction &Inst : make_range(++Load->getReverseIterator(),
ScanBB->rend())) {
if (Inst.isDebugOrPseudoInst())
continue;
if (MaxInstsToScan-- == 0)
return nullptr;
Available = getAvailableLoadStore(&Inst, StrippedPtr, AccessTy,
AtLeastAtomic, DL, IsLoadCSE);
if (Available)
break;
if (Inst.mayWriteToMemory())
MustNotAliasInsts.push_back(&Inst);
}
// If we found an available value, ensure that the instructions in between
// did not modify the memory location.
if (Available) {
MemoryLocation Loc = MemoryLocation::get(Load);
for (Instruction *Inst : MustNotAliasInsts)
if (isModSet(AA.getModRefInfo(Inst, Loc)))
return nullptr;
}
return Available;
}
bool llvm::canReplacePointersIfEqual(Value *A, Value *B, const DataLayout &DL,
Instruction *CtxI) {
Type *Ty = A->getType();
assert(Ty == B->getType() && Ty->isPointerTy() &&
"values must have matching pointer types");
// NOTE: The checks in the function are incomplete and currently miss illegal
// cases! The current implementation is a starting point and the
// implementation should be made stricter over time.
if (auto *C = dyn_cast<Constant>(B)) {
// Do not allow replacing a pointer with a constant pointer, unless it is
// either null or at least one byte is dereferenceable.
APInt OneByte(DL.getPointerTypeSizeInBits(Ty), 1);
return C->isNullValue() ||
isDereferenceableAndAlignedPointer(B, Align(1), OneByte, DL, CtxI);
}
return true;
}
Reference in New Issue View Git Blame Copy Permalink