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llvm-mirror/lib/Analysis/BasicAliasAnalysis.cpp
Alina Sbirlea 1e37b17c69 [BasicAA] Make BasicAA a cfg pass. 
Summary:
Part of the changes in D44564 made BasicAA not CFG only due to it using
PhiAnalysisValues which may have values invalidated.
Subsequent patches (rL340613) appear to have addressed this limitation.

BasicAA should not be invalidated by non-CFG-altering passes.
A concrete example is MemCpyOpt which preserves CFG, but we are testing
it invalidates BasicAA.

llvm-dev RFC: https://groups.google.com/forum/#!topic/llvm-dev/eSPXuWnNfzM

Reviewers: john.brawn, sebpop, hfinkel, brzycki

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74353
2020-02-11 11:30:08 -08:00

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//===- BasicAliasAnalysis.cpp - Stateless Alias Analysis Impl -------------===//
//
// 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 the primary stateless implementation of the
// Alias Analysis interface that implements identities (two different
// globals cannot alias, etc), but does no stateful analysis.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/PhiValues.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/GlobalAlias.h"
#include "llvm/IR/GlobalVariable.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/Metadata.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/KnownBits.h"
#include <cassert>
#include <cstdint>
#include <cstdlib>
#include <utility>
#define DEBUG_TYPE "basicaa"
using namespace llvm;
/// Enable analysis of recursive PHI nodes.
static cl::opt<bool> EnableRecPhiAnalysis("basicaa-recphi", cl::Hidden,
cl::init(false));
/// By default, even on 32-bit architectures we use 64-bit integers for
/// calculations. This will allow us to more-aggressively decompose indexing
/// expressions calculated using i64 values (e.g., long long in C) which is
/// common enough to worry about.
static cl::opt<bool> ForceAtLeast64Bits("basicaa-force-at-least-64b",
cl::Hidden, cl::init(true));
static cl::opt<bool> DoubleCalcBits("basicaa-double-calc-bits",
cl::Hidden, cl::init(false));
/// SearchLimitReached / SearchTimes shows how often the limit of
/// to decompose GEPs is reached. It will affect the precision
/// of basic alias analysis.
STATISTIC(SearchLimitReached, "Number of times the limit to "
"decompose GEPs is reached");
STATISTIC(SearchTimes, "Number of times a GEP is decomposed");
/// Cutoff after which to stop analysing a set of phi nodes potentially involved
/// in a cycle. Because we are analysing 'through' phi nodes, we need to be
/// careful with value equivalence. We use reachability to make sure a value
/// cannot be involved in a cycle.
const unsigned MaxNumPhiBBsValueReachabilityCheck = 20;
// The max limit of the search depth in DecomposeGEPExpression() and
// GetUnderlyingObject(), both functions need to use the same search
// depth otherwise the algorithm in aliasGEP will assert.
static const unsigned MaxLookupSearchDepth = 6;
bool BasicAAResult::invalidate(Function &Fn, const PreservedAnalyses &PA,
FunctionAnalysisManager::Invalidator &Inv) {
// We don't care if this analysis itself is preserved, it has no state. But
// we need to check that the analyses it depends on have been. Note that we
// may be created without handles to some analyses and in that case don't
// depend on them.
if (Inv.invalidate<AssumptionAnalysis>(Fn, PA) ||
(DT && Inv.invalidate<DominatorTreeAnalysis>(Fn, PA)) ||
(LI && Inv.invalidate<LoopAnalysis>(Fn, PA)) ||
(PV && Inv.invalidate<PhiValuesAnalysis>(Fn, PA)))
return true;
// Otherwise this analysis result remains valid.
return false;
}
//===----------------------------------------------------------------------===//
// Useful predicates
//===----------------------------------------------------------------------===//
/// Returns true if the pointer is to a function-local object that never
/// escapes from the function.
static bool isNonEscapingLocalObject(
const Value *V,
SmallDenseMap<const Value *, bool, 8> *IsCapturedCache = nullptr) {
SmallDenseMap<const Value *, bool, 8>::iterator CacheIt;
if (IsCapturedCache) {
bool Inserted;
std::tie(CacheIt, Inserted) = IsCapturedCache->insert({V, false});
if (!Inserted)
// Found cached result, return it!
return CacheIt->second;
}
// If this is a local allocation, check to see if it escapes.
if (isa<AllocaInst>(V) || isNoAliasCall(V)) {
// Set StoreCaptures to True so that we can assume in our callers that the
// pointer is not the result of a load instruction. Currently
// PointerMayBeCaptured doesn't have any special analysis for the
// StoreCaptures=false case; if it did, our callers could be refined to be
// more precise.
auto Ret = !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
if (IsCapturedCache)
CacheIt->second = Ret;
return Ret;
}
// If this is an argument that corresponds to a byval or noalias argument,
// then it has not escaped before entering the function. Check if it escapes
// inside the function.
if (const Argument *A = dyn_cast<Argument>(V))
if (A->hasByValAttr() || A->hasNoAliasAttr()) {
// Note even if the argument is marked nocapture, we still need to check
// for copies made inside the function. The nocapture attribute only
// specifies that there are no copies made that outlive the function.
auto Ret = !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
if (IsCapturedCache)
CacheIt->second = Ret;
return Ret;
}
return false;
}
/// Returns true if the pointer is one which would have been considered an
/// escape by isNonEscapingLocalObject.
static bool isEscapeSource(const Value *V) {
if (isa<CallBase>(V))
return true;
if (isa<Argument>(V))
return true;
// The load case works because isNonEscapingLocalObject considers all
// stores to be escapes (it passes true for the StoreCaptures argument
// to PointerMayBeCaptured).
if (isa<LoadInst>(V))
return true;
return false;
}
/// Returns the size of the object specified by V or UnknownSize if unknown.
static uint64_t getObjectSize(const Value *V, const DataLayout &DL,
const TargetLibraryInfo &TLI,
bool NullIsValidLoc,
bool RoundToAlign = false) {
uint64_t Size;
ObjectSizeOpts Opts;
Opts.RoundToAlign = RoundToAlign;
Opts.NullIsUnknownSize = NullIsValidLoc;
if (getObjectSize(V, Size, DL, &TLI, Opts))
return Size;
return MemoryLocation::UnknownSize;
}
/// Returns true if we can prove that the object specified by V is smaller than
/// Size.
static bool isObjectSmallerThan(const Value *V, uint64_t Size,
const DataLayout &DL,
const TargetLibraryInfo &TLI,
bool NullIsValidLoc) {
// Note that the meanings of the "object" are slightly different in the
// following contexts:
// c1: llvm::getObjectSize()
// c2: llvm.objectsize() intrinsic
// c3: isObjectSmallerThan()
// c1 and c2 share the same meaning; however, the meaning of "object" in c3
// refers to the "entire object".
//
// Consider this example:
// char *p = (char*)malloc(100)
// char *q = p+80;
//
// In the context of c1 and c2, the "object" pointed by q refers to the
// stretch of memory of q[0:19]. So, getObjectSize(q) should return 20.
//
// However, in the context of c3, the "object" refers to the chunk of memory
// being allocated. So, the "object" has 100 bytes, and q points to the middle
// the "object". In case q is passed to isObjectSmallerThan() as the 1st
// parameter, before the llvm::getObjectSize() is called to get the size of
// entire object, we should:
// - either rewind the pointer q to the base-address of the object in
// question (in this case rewind to p), or
// - just give up. It is up to caller to make sure the pointer is pointing
// to the base address the object.
//
// We go for 2nd option for simplicity.
if (!isIdentifiedObject(V))
return false;
// This function needs to use the aligned object size because we allow
// reads a bit past the end given sufficient alignment.
uint64_t ObjectSize = getObjectSize(V, DL, TLI, NullIsValidLoc,
/*RoundToAlign*/ true);
return ObjectSize != MemoryLocation::UnknownSize && ObjectSize < Size;
}
/// Return the minimal extent from \p V to the end of the underlying object,
/// assuming the result is used in an aliasing query. E.g., we do use the query
/// location size and the fact that null pointers cannot alias here.
static uint64_t getMinimalExtentFrom(const Value &V,
const LocationSize &LocSize,
const DataLayout &DL,
bool NullIsValidLoc) {
// If we have dereferenceability information we know a lower bound for the
// extent as accesses for a lower offset would be valid. We need to exclude
// the "or null" part if null is a valid pointer.
bool CanBeNull;
uint64_t DerefBytes = V.getPointerDereferenceableBytes(DL, CanBeNull);
DerefBytes = (CanBeNull && NullIsValidLoc) ? 0 : DerefBytes;
// If queried with a precise location size, we assume that location size to be
// accessed, thus valid.
if (LocSize.isPrecise())
DerefBytes = std::max(DerefBytes, LocSize.getValue());
return DerefBytes;
}
/// Returns true if we can prove that the object specified by V has size Size.
static bool isObjectSize(const Value *V, uint64_t Size, const DataLayout &DL,
const TargetLibraryInfo &TLI, bool NullIsValidLoc) {
uint64_t ObjectSize = getObjectSize(V, DL, TLI, NullIsValidLoc);
return ObjectSize != MemoryLocation::UnknownSize && ObjectSize == Size;
}
//===----------------------------------------------------------------------===//
// GetElementPtr Instruction Decomposition and Analysis
//===----------------------------------------------------------------------===//
/// Analyzes the specified value as a linear expression: "A*V + B", where A and
/// B are constant integers.
///
/// Returns the scale and offset values as APInts and return V as a Value*, and
/// return whether we looked through any sign or zero extends. The incoming
/// Value is known to have IntegerType, and it may already be sign or zero
/// extended.
///
/// Note that this looks through extends, so the high bits may not be
/// represented in the result.
/*static*/ const Value *BasicAAResult::GetLinearExpression(
const Value *V, APInt &Scale, APInt &Offset, unsigned &ZExtBits,
unsigned &SExtBits, const DataLayout &DL, unsigned Depth,
AssumptionCache *AC, DominatorTree *DT, bool &NSW, bool &NUW) {
assert(V->getType()->isIntegerTy() && "Not an integer value");
// Limit our recursion depth.
if (Depth == 6) {
Scale = 1;
Offset = 0;
return V;
}
if (const ConstantInt *Const = dyn_cast<ConstantInt>(V)) {
// If it's a constant, just convert it to an offset and remove the variable.
// If we've been called recursively, the Offset bit width will be greater
// than the constant's (the Offset's always as wide as the outermost call),
// so we'll zext here and process any extension in the isa<SExtInst> &
// isa<ZExtInst> cases below.
Offset += Const->getValue().zextOrSelf(Offset.getBitWidth());
assert(Scale == 0 && "Constant values don't have a scale");
return V;
}
if (const BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
// If we've been called recursively, then Offset and Scale will be wider
// than the BOp operands. We'll always zext it here as we'll process sign
// extensions below (see the isa<SExtInst> / isa<ZExtInst> cases).
APInt RHS = RHSC->getValue().zextOrSelf(Offset.getBitWidth());
switch (BOp->getOpcode()) {
default:
// We don't understand this instruction, so we can't decompose it any
// further.
Scale = 1;
Offset = 0;
return V;
case Instruction::Or:
// X|C == X+C if all the bits in C are unset in X. Otherwise we can't
// analyze it.
if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), DL, 0, AC,
BOp, DT)) {
Scale = 1;
Offset = 0;
return V;
}
LLVM_FALLTHROUGH;
case Instruction::Add:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset += RHS;
break;
case Instruction::Sub:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset -= RHS;
break;
case Instruction::Mul:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset *= RHS;
Scale *= RHS;
break;
case Instruction::Shl:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
// We're trying to linearize an expression of the kind:
// shl i8 -128, 36
// where the shift count exceeds the bitwidth of the type.
// We can't decompose this further (the expression would return
// a poison value).
if (Offset.getBitWidth() < RHS.getLimitedValue() ||
Scale.getBitWidth() < RHS.getLimitedValue()) {
Scale = 1;
Offset = 0;
return V;
}
Offset <<= RHS.getLimitedValue();
Scale <<= RHS.getLimitedValue();
// the semantics of nsw and nuw for left shifts don't match those of
// multiplications, so we won't propagate them.
NSW = NUW = false;
return V;
}
if (isa<OverflowingBinaryOperator>(BOp)) {
NUW &= BOp->hasNoUnsignedWrap();
NSW &= BOp->hasNoSignedWrap();
}
return V;
}
}
// Since GEP indices are sign extended anyway, we don't care about the high
// bits of a sign or zero extended value - just scales and offsets. The
// extensions have to be consistent though.
if (isa<SExtInst>(V) || isa<ZExtInst>(V)) {
Value *CastOp = cast<CastInst>(V)->getOperand(0);
unsigned NewWidth = V->getType()->getPrimitiveSizeInBits();
unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
unsigned OldZExtBits = ZExtBits, OldSExtBits = SExtBits;
const Value *Result =
GetLinearExpression(CastOp, Scale, Offset, ZExtBits, SExtBits, DL,
Depth + 1, AC, DT, NSW, NUW);
// zext(zext(%x)) == zext(%x), and similarly for sext; we'll handle this
// by just incrementing the number of bits we've extended by.
unsigned ExtendedBy = NewWidth - SmallWidth;
if (isa<SExtInst>(V) && ZExtBits == 0) {
// sext(sext(%x, a), b) == sext(%x, a + b)
if (NSW) {
// We haven't sign-wrapped, so it's valid to decompose sext(%x + c)
// into sext(%x) + sext(c). We'll sext the Offset ourselves:
unsigned OldWidth = Offset.getBitWidth();
Offset = Offset.trunc(SmallWidth).sext(NewWidth).zextOrSelf(OldWidth);
} else {
// We may have signed-wrapped, so don't decompose sext(%x + c) into
// sext(%x) + sext(c)
Scale = 1;
Offset = 0;
Result = CastOp;
ZExtBits = OldZExtBits;
SExtBits = OldSExtBits;
}
SExtBits += ExtendedBy;
} else {
// sext(zext(%x, a), b) = zext(zext(%x, a), b) = zext(%x, a + b)
if (!NUW) {
// We may have unsigned-wrapped, so don't decompose zext(%x + c) into
// zext(%x) + zext(c)
Scale = 1;
Offset = 0;
Result = CastOp;
ZExtBits = OldZExtBits;
SExtBits = OldSExtBits;
}
ZExtBits += ExtendedBy;
}
return Result;
}
Scale = 1;
Offset = 0;
return V;
}
/// To ensure a pointer offset fits in an integer of size PointerSize
/// (in bits) when that size is smaller than the maximum pointer size. This is
/// an issue, for example, in particular for 32b pointers with negative indices
/// that rely on two's complement wrap-arounds for precise alias information
/// where the maximum pointer size is 64b.
static APInt adjustToPointerSize(APInt Offset, unsigned PointerSize) {
assert(PointerSize <= Offset.getBitWidth() && "Invalid PointerSize!");
unsigned ShiftBits = Offset.getBitWidth() - PointerSize;
return (Offset << ShiftBits).ashr(ShiftBits);
}
static unsigned getMaxPointerSize(const DataLayout &DL) {
unsigned MaxPointerSize = DL.getMaxPointerSizeInBits();
if (MaxPointerSize < 64 && ForceAtLeast64Bits) MaxPointerSize = 64;
if (DoubleCalcBits) MaxPointerSize *= 2;
return MaxPointerSize;
}
/// If V is a symbolic pointer expression, decompose it into a base pointer
/// with a constant offset and a number of scaled symbolic offsets.
///
/// The scaled symbolic offsets (represented by pairs of a Value* and a scale
/// in the VarIndices vector) are Value*'s that are known to be scaled by the
/// specified amount, but which may have other unrepresented high bits. As
/// such, the gep cannot necessarily be reconstructed from its decomposed form.
///
/// When DataLayout is around, this function is capable of analyzing everything
/// that GetUnderlyingObject can look through. To be able to do that
/// GetUnderlyingObject and DecomposeGEPExpression must use the same search
/// depth (MaxLookupSearchDepth). When DataLayout not is around, it just looks
/// through pointer casts.
bool BasicAAResult::DecomposeGEPExpression(const Value *V,
DecomposedGEP &Decomposed, const DataLayout &DL, AssumptionCache *AC,
DominatorTree *DT) {
// Limit recursion depth to limit compile time in crazy cases.
unsigned MaxLookup = MaxLookupSearchDepth;
SearchTimes++;
unsigned MaxPointerSize = getMaxPointerSize(DL);
Decomposed.VarIndices.clear();
do {
// See if this is a bitcast or GEP.
const Operator *Op = dyn_cast<Operator>(V);
if (!Op) {
// The only non-operator case we can handle are GlobalAliases.
if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
if (!GA->isInterposable()) {
V = GA->getAliasee();
continue;
}
}
Decomposed.Base = V;
return false;
}
if (Op->getOpcode() == Instruction::BitCast ||
Op->getOpcode() == Instruction::AddrSpaceCast) {
V = Op->getOperand(0);
continue;
}
const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
if (!GEPOp) {
if (const auto *Call = dyn_cast<CallBase>(V)) {
// CaptureTracking can know about special capturing properties of some
// intrinsics like launder.invariant.group, that can't be expressed with
// the attributes, but have properties like returning aliasing pointer.
// Because some analysis may assume that nocaptured pointer is not
// returned from some special intrinsic (because function would have to
// be marked with returns attribute), it is crucial to use this function
// because it should be in sync with CaptureTracking. Not using it may
// cause weird miscompilations where 2 aliasing pointers are assumed to
// noalias.
if (auto *RP = getArgumentAliasingToReturnedPointer(Call, false)) {
V = RP;
continue;
}
}
// If it's not a GEP, hand it off to SimplifyInstruction to see if it
// can come up with something. This matches what GetUnderlyingObject does.
if (const Instruction *I = dyn_cast<Instruction>(V))
// TODO: Get a DominatorTree and AssumptionCache and use them here
// (these are both now available in this function, but this should be
// updated when GetUnderlyingObject is updated). TLI should be
// provided also.
if (const Value *Simplified =
SimplifyInstruction(const_cast<Instruction *>(I), DL)) {
V = Simplified;
continue;
}
Decomposed.Base = V;
return false;
}
// Don't attempt to analyze GEPs over unsized objects.
if (!GEPOp->getSourceElementType()->isSized()) {
Decomposed.Base = V;
return false;
}
unsigned AS = GEPOp->getPointerAddressSpace();
// Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
gep_type_iterator GTI = gep_type_begin(GEPOp);
unsigned PointerSize = DL.getPointerSizeInBits(AS);
// Assume all GEP operands are constants until proven otherwise.
bool GepHasConstantOffset = true;
for (User::const_op_iterator I = GEPOp->op_begin() + 1, E = GEPOp->op_end();
I != E; ++I, ++GTI) {
const Value *Index = *I;
// Compute the (potentially symbolic) offset in bytes for this index.
if (StructType *STy = GTI.getStructTypeOrNull()) {
// For a struct, add the member offset.
unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
if (FieldNo == 0)
continue;
Decomposed.StructOffset +=
DL.getStructLayout(STy)->getElementOffset(FieldNo);
continue;
}
// For an array/pointer, add the element offset, explicitly scaled.
if (const ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
if (CIdx->isZero())
continue;
Decomposed.OtherOffset +=
(DL.getTypeAllocSize(GTI.getIndexedType()) *
CIdx->getValue().sextOrSelf(MaxPointerSize))
.sextOrTrunc(MaxPointerSize);
continue;
}
GepHasConstantOffset = false;
APInt Scale(MaxPointerSize, DL.getTypeAllocSize(GTI.getIndexedType()));
unsigned ZExtBits = 0, SExtBits = 0;
// If the integer type is smaller than the pointer size, it is implicitly
// sign extended to pointer size.
unsigned Width = Index->getType()->getIntegerBitWidth();
if (PointerSize > Width)
SExtBits += PointerSize - Width;
// Use GetLinearExpression to decompose the index into a C1*V+C2 form.
APInt IndexScale(Width, 0), IndexOffset(Width, 0);
bool NSW = true, NUW = true;
const Value *OrigIndex = Index;
Index = GetLinearExpression(Index, IndexScale, IndexOffset, ZExtBits,
SExtBits, DL, 0, AC, DT, NSW, NUW);
// The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
// This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
// It can be the case that, even through C1*V+C2 does not overflow for
// relevant values of V, (C2*Scale) can overflow. In that case, we cannot
// decompose the expression in this way.
//
// FIXME: C1*Scale and the other operations in the decomposed
// (C1*Scale)*V+C2*Scale can also overflow. We should check for this
// possibility.
APInt WideScaledOffset = IndexOffset.sextOrTrunc(MaxPointerSize*2) *
Scale.sext(MaxPointerSize*2);
if (WideScaledOffset.getMinSignedBits() > MaxPointerSize) {
Index = OrigIndex;
IndexScale = 1;
IndexOffset = 0;
ZExtBits = SExtBits = 0;
if (PointerSize > Width)
SExtBits += PointerSize - Width;
} else {
Decomposed.OtherOffset += IndexOffset.sextOrTrunc(MaxPointerSize) * Scale;
Scale *= IndexScale.sextOrTrunc(MaxPointerSize);
}
// If we already had an occurrence of this index variable, merge this
// scale into it. For example, we want to handle:
// A[x][x] -> x*16 + x*4 -> x*20
// This also ensures that 'x' only appears in the index list once.
for (unsigned i = 0, e = Decomposed.VarIndices.size(); i != e; ++i) {
if (Decomposed.VarIndices[i].V == Index &&
Decomposed.VarIndices[i].ZExtBits == ZExtBits &&
Decomposed.VarIndices[i].SExtBits == SExtBits) {
Scale += Decomposed.VarIndices[i].Scale;
Decomposed.VarIndices.erase(Decomposed.VarIndices.begin() + i);
break;
}
}
// Make sure that we have a scale that makes sense for this target's
// pointer size.
Scale = adjustToPointerSize(Scale, PointerSize);
if (!!Scale) {
VariableGEPIndex Entry = {Index, ZExtBits, SExtBits, Scale};
Decomposed.VarIndices.push_back(Entry);
}
}
// Take care of wrap-arounds
if (GepHasConstantOffset) {
Decomposed.StructOffset =
adjustToPointerSize(Decomposed.StructOffset, PointerSize);
Decomposed.OtherOffset =
adjustToPointerSize(Decomposed.OtherOffset, PointerSize);
}
// Analyze the base pointer next.
V = GEPOp->getOperand(0);
} while (--MaxLookup);
// If the chain of expressions is too deep, just return early.
Decomposed.Base = V;
SearchLimitReached++;
return true;
}
/// Returns whether the given pointer value points to memory that is local to
/// the function, with global constants being considered local to all
/// functions.
bool BasicAAResult::pointsToConstantMemory(const MemoryLocation &Loc,
AAQueryInfo &AAQI, bool OrLocal) {
assert(Visited.empty() && "Visited must be cleared after use!");
unsigned MaxLookup = 8;
SmallVector<const Value *, 16> Worklist;
Worklist.push_back(Loc.Ptr);
do {
const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), DL);
if (!Visited.insert(V).second) {
Visited.clear();
return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
}
// An alloca instruction defines local memory.
if (OrLocal && isa<AllocaInst>(V))
continue;
// A global constant counts as local memory for our purposes.
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
// Note: this doesn't require GV to be "ODR" because it isn't legal for a
// global to be marked constant in some modules and non-constant in
// others. GV may even be a declaration, not a definition.
if (!GV->isConstant()) {
Visited.clear();
return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
}
continue;
}
// If both select values point to local memory, then so does the select.
if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
Worklist.push_back(SI->getTrueValue());
Worklist.push_back(SI->getFalseValue());
continue;
}
// If all values incoming to a phi node point to local memory, then so does
// the phi.
if (const PHINode *PN = dyn_cast<PHINode>(V)) {
// Don't bother inspecting phi nodes with many operands.
if (PN->getNumIncomingValues() > MaxLookup) {
Visited.clear();
return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
}
for (Value *IncValue : PN->incoming_values())
Worklist.push_back(IncValue);
continue;
}
// Otherwise be conservative.
Visited.clear();
return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
} while (!Worklist.empty() && --MaxLookup);
Visited.clear();
return Worklist.empty();
}
/// Returns the behavior when calling the given call site.
FunctionModRefBehavior BasicAAResult::getModRefBehavior(const CallBase *Call) {
if (Call->doesNotAccessMemory())
// Can't do better than this.
return FMRB_DoesNotAccessMemory;
FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
// If the callsite knows it only reads memory, don't return worse
// than that.
if (Call->onlyReadsMemory())
Min = FMRB_OnlyReadsMemory;
else if (Call->doesNotReadMemory())
Min = FMRB_OnlyWritesMemory;
if (Call->onlyAccessesArgMemory())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesArgumentPointees);
else if (Call->onlyAccessesInaccessibleMemory())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesInaccessibleMem);
else if (Call->onlyAccessesInaccessibleMemOrArgMem())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesInaccessibleOrArgMem);
// If the call has operand bundles then aliasing attributes from the function
// it calls do not directly apply to the call. This can be made more precise
// in the future.
if (!Call->hasOperandBundles())
if (const Function *F = Call->getCalledFunction())
Min =
FunctionModRefBehavior(Min & getBestAAResults().getModRefBehavior(F));
return Min;
}
/// Returns the behavior when calling the given function. For use when the call
/// site is not known.
FunctionModRefBehavior BasicAAResult::getModRefBehavior(const Function *F) {
// If the function declares it doesn't access memory, we can't do better.
if (F->doesNotAccessMemory())
return FMRB_DoesNotAccessMemory;
FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
// If the function declares it only reads memory, go with that.
if (F->onlyReadsMemory())
Min = FMRB_OnlyReadsMemory;
else if (F->doesNotReadMemory())
Min = FMRB_OnlyWritesMemory;
if (F->onlyAccessesArgMemory())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesArgumentPointees);
else if (F->onlyAccessesInaccessibleMemory())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesInaccessibleMem);
else if (F->onlyAccessesInaccessibleMemOrArgMem())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesInaccessibleOrArgMem);
return Min;
}
/// Returns true if this is a writeonly (i.e Mod only) parameter.
static bool isWriteOnlyParam(const CallBase *Call, unsigned ArgIdx,
const TargetLibraryInfo &TLI) {
if (Call->paramHasAttr(ArgIdx, Attribute::WriteOnly))
return true;
// We can bound the aliasing properties of memset_pattern16 just as we can
// for memcpy/memset. This is particularly important because the
// LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
// whenever possible.
// FIXME Consider handling this in InferFunctionAttr.cpp together with other
// attributes.
LibFunc F;
if (Call->getCalledFunction() &&
TLI.getLibFunc(*Call->getCalledFunction(), F) &&
F == LibFunc_memset_pattern16 && TLI.has(F))
if (ArgIdx == 0)
return true;
// TODO: memset_pattern4, memset_pattern8
// TODO: _chk variants
// TODO: strcmp, strcpy
return false;
}
ModRefInfo BasicAAResult::getArgModRefInfo(const CallBase *Call,
unsigned ArgIdx) {
// Checking for known builtin intrinsics and target library functions.
if (isWriteOnlyParam(Call, ArgIdx, TLI))
return ModRefInfo::Mod;
if (Call->paramHasAttr(ArgIdx, Attribute::ReadOnly))
return ModRefInfo::Ref;
if (Call->paramHasAttr(ArgIdx, Attribute::ReadNone))
return ModRefInfo::NoModRef;
return AAResultBase::getArgModRefInfo(Call, ArgIdx);
}
static bool isIntrinsicCall(const CallBase *Call, Intrinsic::ID IID) {
const IntrinsicInst *II = dyn_cast<IntrinsicInst>(Call);
return II && II->getIntrinsicID() == IID;
}
#ifndef NDEBUG
static const Function *getParent(const Value *V) {
if (const Instruction *inst = dyn_cast<Instruction>(V)) {
if (!inst->getParent())
return nullptr;
return inst->getParent()->getParent();
}
if (const Argument *arg = dyn_cast<Argument>(V))
return arg->getParent();
return nullptr;
}
static bool notDifferentParent(const Value *O1, const Value *O2) {
const Function *F1 = getParent(O1);
const Function *F2 = getParent(O2);
return !F1 || !F2 || F1 == F2;
}
#endif
AliasResult BasicAAResult::alias(const MemoryLocation &LocA,
const MemoryLocation &LocB,
AAQueryInfo &AAQI) {
assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
"BasicAliasAnalysis doesn't support interprocedural queries.");
// If we have a directly cached entry for these locations, we have recursed
// through this once, so just return the cached results. Notably, when this
// happens, we don't clear the cache.
auto CacheIt = AAQI.AliasCache.find(AAQueryInfo::LocPair(LocA, LocB));
if (CacheIt != AAQI.AliasCache.end())
return CacheIt->second;
CacheIt = AAQI.AliasCache.find(AAQueryInfo::LocPair(LocB, LocA));
if (CacheIt != AAQI.AliasCache.end())
return CacheIt->second;
AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.AATags, LocB.Ptr,
LocB.Size, LocB.AATags, AAQI);
VisitedPhiBBs.clear();
return Alias;
}
/// Checks to see if the specified callsite can clobber the specified memory
/// object.
///
/// Since we only look at local properties of this function, we really can't
/// say much about this query. We do, however, use simple "address taken"
/// analysis on local objects.
ModRefInfo BasicAAResult::getModRefInfo(const CallBase *Call,
const MemoryLocation &Loc,
AAQueryInfo &AAQI) {
assert(notDifferentParent(Call, Loc.Ptr) &&
"AliasAnalysis query involving multiple functions!");
const Value *Object = GetUnderlyingObject(Loc.Ptr, DL);
// Calls marked 'tail' cannot read or write allocas from the current frame
// because the current frame might be destroyed by the time they run. However,
// a tail call may use an alloca with byval. Calling with byval copies the
// contents of the alloca into argument registers or stack slots, so there is
// no lifetime issue.
if (isa<AllocaInst>(Object))
if (const CallInst *CI = dyn_cast<CallInst>(Call))
if (CI->isTailCall() &&
!CI->getAttributes().hasAttrSomewhere(Attribute::ByVal))
return ModRefInfo::NoModRef;
// Stack restore is able to modify unescaped dynamic allocas. Assume it may
// modify them even though the alloca is not escaped.
if (auto *AI = dyn_cast<AllocaInst>(Object))
if (!AI->isStaticAlloca() && isIntrinsicCall(Call, Intrinsic::stackrestore))
return ModRefInfo::Mod;
// If the pointer is to a locally allocated object that does not escape,
// then the call can not mod/ref the pointer unless the call takes the pointer
// as an argument, and itself doesn't capture it.
if (!isa<Constant>(Object) && Call != Object &&
isNonEscapingLocalObject(Object, &AAQI.IsCapturedCache)) {
// Optimistically assume that call doesn't touch Object and check this
// assumption in the following loop.
ModRefInfo Result = ModRefInfo::NoModRef;
bool IsMustAlias = true;
unsigned OperandNo = 0;
for (auto CI = Call->data_operands_begin(), CE = Call->data_operands_end();
CI != CE; ++CI, ++OperandNo) {
// Only look at the no-capture or byval pointer arguments. If this
// pointer were passed to arguments that were neither of these, then it
// couldn't be no-capture.
if (!(*CI)->getType()->isPointerTy() ||
(!Call->doesNotCapture(OperandNo) &&
OperandNo < Call->getNumArgOperands() &&
!Call->isByValArgument(OperandNo)))
continue;
// Call doesn't access memory through this operand, so we don't care
// if it aliases with Object.
if (Call->doesNotAccessMemory(OperandNo))
continue;
// If this is a no-capture pointer argument, see if we can tell that it
// is impossible to alias the pointer we're checking.
AliasResult AR = getBestAAResults().alias(MemoryLocation(*CI),
MemoryLocation(Object), AAQI);
if (AR != MustAlias)
IsMustAlias = false;
// Operand doesn't alias 'Object', continue looking for other aliases
if (AR == NoAlias)
continue;
// Operand aliases 'Object', but call doesn't modify it. Strengthen
// initial assumption and keep looking in case if there are more aliases.
if (Call->onlyReadsMemory(OperandNo)) {
Result = setRef(Result);
continue;
}
// Operand aliases 'Object' but call only writes into it.
if (Call->doesNotReadMemory(OperandNo)) {
Result = setMod(Result);
continue;
}
// This operand aliases 'Object' and call reads and writes into it.
// Setting ModRef will not yield an early return below, MustAlias is not
// used further.
Result = ModRefInfo::ModRef;
break;
}
// No operand aliases, reset Must bit. Add below if at least one aliases
// and all aliases found are MustAlias.
if (isNoModRef(Result))
IsMustAlias = false;
// Early return if we improved mod ref information
if (!isModAndRefSet(Result)) {
if (isNoModRef(Result))
return ModRefInfo::NoModRef;
return IsMustAlias ? setMust(Result) : clearMust(Result);
}
}
// If the call is to malloc or calloc, we can assume that it doesn't
// modify any IR visible value. This is only valid because we assume these
// routines do not read values visible in the IR. TODO: Consider special
// casing realloc and strdup routines which access only their arguments as
// well. Or alternatively, replace all of this with inaccessiblememonly once
// that's implemented fully.
if (isMallocOrCallocLikeFn(Call, &TLI)) {
// Be conservative if the accessed pointer may alias the allocation -
// fallback to the generic handling below.
if (getBestAAResults().alias(MemoryLocation(Call), Loc, AAQI) == NoAlias)
return ModRefInfo::NoModRef;
}
// The semantics of memcpy intrinsics forbid overlap between their respective
// operands, i.e., source and destination of any given memcpy must no-alias.
// If Loc must-aliases either one of these two locations, then it necessarily
// no-aliases the other.
if (auto *Inst = dyn_cast<AnyMemCpyInst>(Call)) {
AliasResult SrcAA, DestAA;
if ((SrcAA = getBestAAResults().alias(MemoryLocation::getForSource(Inst),
Loc, AAQI)) == MustAlias)
// Loc is exactly the memcpy source thus disjoint from memcpy dest.
return ModRefInfo::Ref;
if ((DestAA = getBestAAResults().alias(MemoryLocation::getForDest(Inst),
Loc, AAQI)) == MustAlias)
// The converse case.
return ModRefInfo::Mod;
// It's also possible for Loc to alias both src and dest, or neither.
ModRefInfo rv = ModRefInfo::NoModRef;
if (SrcAA != NoAlias)
rv = setRef(rv);
if (DestAA != NoAlias)
rv = setMod(rv);
return rv;
}
// While the assume intrinsic is marked as arbitrarily writing so that
// proper control dependencies will be maintained, it never aliases any
// particular memory location.
if (isIntrinsicCall(Call, Intrinsic::assume))
return ModRefInfo::NoModRef;
// Like assumes, guard intrinsics are also marked as arbitrarily writing so
// that proper control dependencies are maintained but they never mods any
// particular memory location.
//
// *Unlike* assumes, guard intrinsics are modeled as reading memory since the
// heap state at the point the guard is issued needs to be consistent in case
// the guard invokes the "deopt" continuation.
if (isIntrinsicCall(Call, Intrinsic::experimental_guard))
return ModRefInfo::Ref;
// Like assumes, invariant.start intrinsics were also marked as arbitrarily
// writing so that proper control dependencies are maintained but they never
// mod any particular memory location visible to the IR.
// *Unlike* assumes (which are now modeled as NoModRef), invariant.start
// intrinsic is now modeled as reading memory. This prevents hoisting the
// invariant.start intrinsic over stores. Consider:
// *ptr = 40;
// *ptr = 50;
// invariant_start(ptr)
// int val = *ptr;
// print(val);
//
// This cannot be transformed to:
//
// *ptr = 40;
// invariant_start(ptr)
// *ptr = 50;
// int val = *ptr;
// print(val);
//
// The transformation will cause the second store to be ignored (based on
// rules of invariant.start) and print 40, while the first program always
// prints 50.
if (isIntrinsicCall(Call, Intrinsic::invariant_start))
return ModRefInfo::Ref;
// The AAResultBase base class has some smarts, lets use them.
return AAResultBase::getModRefInfo(Call, Loc, AAQI);
}
ModRefInfo BasicAAResult::getModRefInfo(const CallBase *Call1,
const CallBase *Call2,
AAQueryInfo &AAQI) {
// While the assume intrinsic is marked as arbitrarily writing so that
// proper control dependencies will be maintained, it never aliases any
// particular memory location.
if (isIntrinsicCall(Call1, Intrinsic::assume) ||
isIntrinsicCall(Call2, Intrinsic::assume))
return ModRefInfo::NoModRef;
// Like assumes, guard intrinsics are also marked as arbitrarily writing so
// that proper control dependencies are maintained but they never mod any
// particular memory location.
//
// *Unlike* assumes, guard intrinsics are modeled as reading memory since the
// heap state at the point the guard is issued needs to be consistent in case
// the guard invokes the "deopt" continuation.
// NB! This function is *not* commutative, so we special case two
// possibilities for guard intrinsics.
if (isIntrinsicCall(Call1, Intrinsic::experimental_guard))
return isModSet(createModRefInfo(getModRefBehavior(Call2)))
? ModRefInfo::Ref
: ModRefInfo::NoModRef;
if (isIntrinsicCall(Call2, Intrinsic::experimental_guard))
return isModSet(createModRefInfo(getModRefBehavior(Call1)))
? ModRefInfo::Mod
: ModRefInfo::NoModRef;
// The AAResultBase base class has some smarts, lets use them.
return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
}
/// Provide ad-hoc rules to disambiguate accesses through two GEP operators,
/// both having the exact same pointer operand.
static AliasResult aliasSameBasePointerGEPs(const GEPOperator *GEP1,
LocationSize MaybeV1Size,
const GEPOperator *GEP2,
LocationSize MaybeV2Size,
const DataLayout &DL) {
assert(GEP1->getPointerOperand()->stripPointerCastsAndInvariantGroups() ==
GEP2->getPointerOperand()->stripPointerCastsAndInvariantGroups() &&
GEP1->getPointerOperandType() == GEP2->getPointerOperandType() &&
"Expected GEPs with the same pointer operand");
// Try to determine whether GEP1 and GEP2 index through arrays, into structs,
// such that the struct field accesses provably cannot alias.
// We also need at least two indices (the pointer, and the struct field).
if (GEP1->getNumIndices() != GEP2->getNumIndices() ||
GEP1->getNumIndices() < 2)
return MayAlias;
// If we don't know the size of the accesses through both GEPs, we can't
// determine whether the struct fields accessed can't alias.
if (MaybeV1Size == LocationSize::unknown() ||
MaybeV2Size == LocationSize::unknown())
return MayAlias;
const uint64_t V1Size = MaybeV1Size.getValue();
const uint64_t V2Size = MaybeV2Size.getValue();
ConstantInt *C1 =
dyn_cast<ConstantInt>(GEP1->getOperand(GEP1->getNumOperands() - 1));
ConstantInt *C2 =
dyn_cast<ConstantInt>(GEP2->getOperand(GEP2->getNumOperands() - 1));
// If the last (struct) indices are constants and are equal, the other indices
// might be also be dynamically equal, so the GEPs can alias.
if (C1 && C2) {
unsigned BitWidth = std::max(C1->getBitWidth(), C2->getBitWidth());
if (C1->getValue().sextOrSelf(BitWidth) ==
C2->getValue().sextOrSelf(BitWidth))
return MayAlias;
}
// Find the last-indexed type of the GEP, i.e., the type you'd get if
// you stripped the last index.
// On the way, look at each indexed type. If there's something other
// than an array, different indices can lead to different final types.
SmallVector<Value *, 8> IntermediateIndices;
// Insert the first index; we don't need to check the type indexed
// through it as it only drops the pointer indirection.
assert(GEP1->getNumIndices() > 1 && "Not enough GEP indices to examine");
IntermediateIndices.push_back(GEP1->getOperand(1));
// Insert all the remaining indices but the last one.
// Also, check that they all index through arrays.
for (unsigned i = 1, e = GEP1->getNumIndices() - 1; i != e; ++i) {
if (!isa<ArrayType>(GetElementPtrInst::getIndexedType(
GEP1->getSourceElementType(), IntermediateIndices)))
return MayAlias;
IntermediateIndices.push_back(GEP1->getOperand(i + 1));
}
auto *Ty = GetElementPtrInst::getIndexedType(
GEP1->getSourceElementType(), IntermediateIndices);
StructType *LastIndexedStruct = dyn_cast<StructType>(Ty);
if (isa<SequentialType>(Ty)) {
// We know that:
// - both GEPs begin indexing from the exact same pointer;
// - the last indices in both GEPs are constants, indexing into a sequential
// type (array or pointer);
// - both GEPs only index through arrays prior to that.
//
// Because array indices greater than the number of elements are valid in
// GEPs, unless we know the intermediate indices are identical between
// GEP1 and GEP2 we cannot guarantee that the last indexed arrays don't
// partially overlap. We also need to check that the loaded size matches
// the element size, otherwise we could still have overlap.
const uint64_t ElementSize =
DL.getTypeStoreSize(cast<SequentialType>(Ty)->getElementType());
if (V1Size != ElementSize || V2Size != ElementSize)
return MayAlias;
for (unsigned i = 0, e = GEP1->getNumIndices() - 1; i != e; ++i)
if (GEP1->getOperand(i + 1) != GEP2->getOperand(i + 1))
return MayAlias;
// Now we know that the array/pointer that GEP1 indexes into and that
// that GEP2 indexes into must either precisely overlap or be disjoint.
// Because they cannot partially overlap and because fields in an array
// cannot overlap, if we can prove the final indices are different between
// GEP1 and GEP2, we can conclude GEP1 and GEP2 don't alias.
// If the last indices are constants, we've already checked they don't
// equal each other so we can exit early.
if (C1 && C2)
return NoAlias;
{
Value *GEP1LastIdx = GEP1->getOperand(GEP1->getNumOperands() - 1);
Value *GEP2LastIdx = GEP2->getOperand(GEP2->getNumOperands() - 1);
if (isa<PHINode>(GEP1LastIdx) || isa<PHINode>(GEP2LastIdx)) {
// If one of the indices is a PHI node, be safe and only use
// computeKnownBits so we don't make any assumptions about the
// relationships between the two indices. This is important if we're
// asking about values from different loop iterations. See PR32314.
// TODO: We may be able to change the check so we only do this when
// we definitely looked through a PHINode.
if (GEP1LastIdx != GEP2LastIdx &&
GEP1LastIdx->getType() == GEP2LastIdx->getType()) {
KnownBits Known1 = computeKnownBits(GEP1LastIdx, DL);
KnownBits Known2 = computeKnownBits(GEP2LastIdx, DL);
if (Known1.Zero.intersects(Known2.One) ||
Known1.One.intersects(Known2.Zero))
return NoAlias;
}
} else if (isKnownNonEqual(GEP1LastIdx, GEP2LastIdx, DL))
return NoAlias;
}
return MayAlias;
} else if (!LastIndexedStruct || !C1 || !C2) {
return MayAlias;
}
if (C1->getValue().getActiveBits() > 64 ||
C2->getValue().getActiveBits() > 64)
return MayAlias;
// We know that:
// - both GEPs begin indexing from the exact same pointer;
// - the last indices in both GEPs are constants, indexing into a struct;
// - said indices are different, hence, the pointed-to fields are different;
// - both GEPs only index through arrays prior to that.
//
// This lets us determine that the struct that GEP1 indexes into and the
// struct that GEP2 indexes into must either precisely overlap or be
// completely disjoint. Because they cannot partially overlap, indexing into
// different non-overlapping fields of the struct will never alias.
// Therefore, the only remaining thing needed to show that both GEPs can't
// alias is that the fields are not overlapping.
const StructLayout *SL = DL.getStructLayout(LastIndexedStruct);
const uint64_t StructSize = SL->getSizeInBytes();
const uint64_t V1Off = SL->getElementOffset(C1->getZExtValue());
const uint64_t V2Off = SL->getElementOffset(C2->getZExtValue());
auto EltsDontOverlap = [StructSize](uint64_t V1Off, uint64_t V1Size,
uint64_t V2Off, uint64_t V2Size) {
return V1Off < V2Off && V1Off + V1Size <= V2Off &&
((V2Off + V2Size <= StructSize) ||
(V2Off + V2Size - StructSize <= V1Off));
};
if (EltsDontOverlap(V1Off, V1Size, V2Off, V2Size) ||
EltsDontOverlap(V2Off, V2Size, V1Off, V1Size))
return NoAlias;
return MayAlias;
}
// If a we have (a) a GEP and (b) a pointer based on an alloca, and the
// beginning of the object the GEP points would have a negative offset with
// repsect to the alloca, that means the GEP can not alias pointer (b).
// Note that the pointer based on the alloca may not be a GEP. For
// example, it may be the alloca itself.
// The same applies if (b) is based on a GlobalVariable. Note that just being
// based on isIdentifiedObject() is not enough - we need an identified object
// that does not permit access to negative offsets. For example, a negative
// offset from a noalias argument or call can be inbounds w.r.t the actual
// underlying object.
//
// For example, consider:
//
// struct { int f0, int f1, ...} foo;
// foo alloca;
// foo* random = bar(alloca);
// int *f0 = &alloca.f0
// int *f1 = &random->f1;
//
// Which is lowered, approximately, to:
//
// %alloca = alloca %struct.foo
// %random = call %struct.foo* @random(%struct.foo* %alloca)
// %f0 = getelementptr inbounds %struct, %struct.foo* %alloca, i32 0, i32 0
// %f1 = getelementptr inbounds %struct, %struct.foo* %random, i32 0, i32 1
//
// Assume %f1 and %f0 alias. Then %f1 would point into the object allocated
// by %alloca. Since the %f1 GEP is inbounds, that means %random must also
// point into the same object. But since %f0 points to the beginning of %alloca,
// the highest %f1 can be is (%alloca + 3). This means %random can not be higher
// than (%alloca - 1), and so is not inbounds, a contradiction.
bool BasicAAResult::isGEPBaseAtNegativeOffset(const GEPOperator *GEPOp,
const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompObject,
LocationSize MaybeObjectAccessSize) {
// If the object access size is unknown, or the GEP isn't inbounds, bail.
if (MaybeObjectAccessSize == LocationSize::unknown() || !GEPOp->isInBounds())
return false;
const uint64_t ObjectAccessSize = MaybeObjectAccessSize.getValue();
// We need the object to be an alloca or a globalvariable, and want to know
// the offset of the pointer from the object precisely, so no variable
// indices are allowed.
if (!(isa<AllocaInst>(DecompObject.Base) ||
isa<GlobalVariable>(DecompObject.Base)) ||
!DecompObject.VarIndices.empty())
return false;
APInt ObjectBaseOffset = DecompObject.StructOffset +
DecompObject.OtherOffset;
// If the GEP has no variable indices, we know the precise offset
// from the base, then use it. If the GEP has variable indices,
// we can't get exact GEP offset to identify pointer alias. So return
// false in that case.
if (!DecompGEP.VarIndices.empty())
return false;
APInt GEPBaseOffset = DecompGEP.StructOffset;
GEPBaseOffset += DecompGEP.OtherOffset;
return GEPBaseOffset.sge(ObjectBaseOffset + (int64_t)ObjectAccessSize);
}
/// Provides a bunch of ad-hoc rules to disambiguate a GEP instruction against
/// another pointer.
///
/// We know that V1 is a GEP, but we don't know anything about V2.
/// UnderlyingV1 is GetUnderlyingObject(GEP1, DL), UnderlyingV2 is the same for
/// V2.
AliasResult BasicAAResult::aliasGEP(
const GEPOperator *GEP1, LocationSize V1Size, const AAMDNodes &V1AAInfo,
const Value *V2, LocationSize V2Size, const AAMDNodes &V2AAInfo,
const Value *UnderlyingV1, const Value *UnderlyingV2, AAQueryInfo &AAQI) {
DecomposedGEP DecompGEP1, DecompGEP2;
unsigned MaxPointerSize = getMaxPointerSize(DL);
DecompGEP1.StructOffset = DecompGEP1.OtherOffset = APInt(MaxPointerSize, 0);
DecompGEP2.StructOffset = DecompGEP2.OtherOffset = APInt(MaxPointerSize, 0);
bool GEP1MaxLookupReached =
DecomposeGEPExpression(GEP1, DecompGEP1, DL, &AC, DT);
bool GEP2MaxLookupReached =
DecomposeGEPExpression(V2, DecompGEP2, DL, &AC, DT);
APInt GEP1BaseOffset = DecompGEP1.StructOffset + DecompGEP1.OtherOffset;
APInt GEP2BaseOffset = DecompGEP2.StructOffset + DecompGEP2.OtherOffset;
assert(DecompGEP1.Base == UnderlyingV1 && DecompGEP2.Base == UnderlyingV2 &&
"DecomposeGEPExpression returned a result different from "
"GetUnderlyingObject");
// If the GEP's offset relative to its base is such that the base would
// fall below the start of the object underlying V2, then the GEP and V2
// cannot alias.
if (!GEP1MaxLookupReached && !GEP2MaxLookupReached &&
isGEPBaseAtNegativeOffset(GEP1, DecompGEP1, DecompGEP2, V2Size))
return NoAlias;
// If we have two gep instructions with must-alias or not-alias'ing base
// pointers, figure out if the indexes to the GEP tell us anything about the
// derived pointer.
if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
// Check for the GEP base being at a negative offset, this time in the other
// direction.
if (!GEP1MaxLookupReached && !GEP2MaxLookupReached &&
isGEPBaseAtNegativeOffset(GEP2, DecompGEP2, DecompGEP1, V1Size))
return NoAlias;
// Do the base pointers alias?
AliasResult BaseAlias =
aliasCheck(UnderlyingV1, LocationSize::unknown(), AAMDNodes(),
UnderlyingV2, LocationSize::unknown(), AAMDNodes(), AAQI);
// Check for geps of non-aliasing underlying pointers where the offsets are
// identical.
if ((BaseAlias == MayAlias) && V1Size == V2Size) {
// Do the base pointers alias assuming type and size.
AliasResult PreciseBaseAlias = aliasCheck(
UnderlyingV1, V1Size, V1AAInfo, UnderlyingV2, V2Size, V2AAInfo, AAQI);
if (PreciseBaseAlias == NoAlias) {
// See if the computed offset from the common pointer tells us about the
// relation of the resulting pointer.
// If the max search depth is reached the result is undefined
if (GEP2MaxLookupReached || GEP1MaxLookupReached)
return MayAlias;
// Same offsets.
if (GEP1BaseOffset == GEP2BaseOffset &&
DecompGEP1.VarIndices == DecompGEP2.VarIndices)
return NoAlias;
}
}
// If we get a No or May, then return it immediately, no amount of analysis
// will improve this situation.
if (BaseAlias != MustAlias) {
assert(BaseAlias == NoAlias || BaseAlias == MayAlias);
return BaseAlias;
}
// Otherwise, we have a MustAlias. Since the base pointers alias each other
// exactly, see if the computed offset from the common pointer tells us
// about the relation of the resulting pointer.
// If we know the two GEPs are based off of the exact same pointer (and not
// just the same underlying object), see if that tells us anything about
// the resulting pointers.
if (GEP1->getPointerOperand()->stripPointerCastsAndInvariantGroups() ==
GEP2->getPointerOperand()->stripPointerCastsAndInvariantGroups() &&
GEP1->getPointerOperandType() == GEP2->getPointerOperandType()) {
AliasResult R = aliasSameBasePointerGEPs(GEP1, V1Size, GEP2, V2Size, DL);
// If we couldn't find anything interesting, don't abandon just yet.
if (R != MayAlias)
return R;
}
// If the max search depth is reached, the result is undefined
if (GEP2MaxLookupReached || GEP1MaxLookupReached)
return MayAlias;
// Subtract the GEP2 pointer from the GEP1 pointer to find out their
// symbolic difference.
GEP1BaseOffset -= GEP2BaseOffset;
GetIndexDifference(DecompGEP1.VarIndices, DecompGEP2.VarIndices);
} else {
// Check to see if these two pointers are related by the getelementptr
// instruction. If one pointer is a GEP with a non-zero index of the other
// pointer, we know they cannot alias.
// If both accesses are unknown size, we can't do anything useful here.
if (V1Size == LocationSize::unknown() && V2Size == LocationSize::unknown())
return MayAlias;
AliasResult R = aliasCheck(UnderlyingV1, LocationSize::unknown(),
AAMDNodes(), V2, LocationSize::unknown(),
V2AAInfo, AAQI, nullptr, UnderlyingV2);
if (R != MustAlias) {
// If V2 may alias GEP base pointer, conservatively returns MayAlias.
// If V2 is known not to alias GEP base pointer, then the two values
// cannot alias per GEP semantics: "Any memory access must be done through
// a pointer value associated with an address range of the memory access,
// otherwise the behavior is undefined.".
assert(R == NoAlias || R == MayAlias);
return R;
}
// If the max search depth is reached the result is undefined
if (GEP1MaxLookupReached)
return MayAlias;
}
// In the two GEP Case, if there is no difference in the offsets of the
// computed pointers, the resultant pointers are a must alias. This
// happens when we have two lexically identical GEP's (for example).
//
// In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
// must aliases the GEP, the end result is a must alias also.
if (GEP1BaseOffset == 0 && DecompGEP1.VarIndices.empty())
return MustAlias;
// If there is a constant difference between the pointers, but the difference
// is less than the size of the associated memory object, then we know
// that the objects are partially overlapping. If the difference is
// greater, we know they do not overlap.
if (GEP1BaseOffset != 0 && DecompGEP1.VarIndices.empty()) {
if (GEP1BaseOffset.sge(0)) {
if (V2Size != LocationSize::unknown()) {
if (GEP1BaseOffset.ult(V2Size.getValue()))
return PartialAlias;
return NoAlias;
}
} else {
// We have the situation where:
// + +
// | BaseOffset |
// ---------------->|
// |-->V1Size |-------> V2Size
// GEP1 V2
// We need to know that V2Size is not unknown, otherwise we might have
// stripped a gep with negative index ('gep <ptr>, -1, ...).
if (V1Size != LocationSize::unknown() &&
V2Size != LocationSize::unknown()) {
if ((-GEP1BaseOffset).ult(V1Size.getValue()))
return PartialAlias;
return NoAlias;
}
}
}
if (!DecompGEP1.VarIndices.empty()) {
APInt Modulo(MaxPointerSize, 0);
bool AllPositive = true;
for (unsigned i = 0, e = DecompGEP1.VarIndices.size(); i != e; ++i) {
// Try to distinguish something like &A[i][1] against &A[42][0].
// Grab the least significant bit set in any of the scales. We
// don't need std::abs here (even if the scale's negative) as we'll
// be ^'ing Modulo with itself later.
Modulo |= DecompGEP1.VarIndices[i].Scale;
if (AllPositive) {
// If the Value could change between cycles, then any reasoning about
// the Value this cycle may not hold in the next cycle. We'll just
// give up if we can't determine conditions that hold for every cycle:
const Value *V = DecompGEP1.VarIndices[i].V;
KnownBits Known =
computeKnownBits(V, DL, 0, &AC, dyn_cast<Instruction>(GEP1), DT);
bool SignKnownZero = Known.isNonNegative();
bool SignKnownOne = Known.isNegative();
// Zero-extension widens the variable, and so forces the sign
// bit to zero.
bool IsZExt = DecompGEP1.VarIndices[i].ZExtBits > 0 || isa<ZExtInst>(V);
SignKnownZero |= IsZExt;
SignKnownOne &= !IsZExt;
// If the variable begins with a zero then we know it's
// positive, regardless of whether the value is signed or
// unsigned.
APInt Scale = DecompGEP1.VarIndices[i].Scale;
AllPositive =
(SignKnownZero && Scale.sge(0)) || (SignKnownOne && Scale.slt(0));
}
}
Modulo = Modulo ^ (Modulo & (Modulo - 1));
// We can compute the difference between the two addresses
// mod Modulo. Check whether that difference guarantees that the
// two locations do not alias.
APInt ModOffset = GEP1BaseOffset & (Modulo - 1);
if (V1Size != LocationSize::unknown() &&
V2Size != LocationSize::unknown() && ModOffset.uge(V2Size.getValue()) &&
(Modulo - ModOffset).uge(V1Size.getValue()))
return NoAlias;
// If we know all the variables are positive, then GEP1 >= GEP1BasePtr.
// If GEP1BasePtr > V2 (GEP1BaseOffset > 0) then we know the pointers
// don't alias if V2Size can fit in the gap between V2 and GEP1BasePtr.
if (AllPositive && GEP1BaseOffset.sgt(0) &&
V2Size != LocationSize::unknown() &&
GEP1BaseOffset.uge(V2Size.getValue()))
return NoAlias;
if (constantOffsetHeuristic(DecompGEP1.VarIndices, V1Size, V2Size,
GEP1BaseOffset, &AC, DT))
return NoAlias;
}
// Statically, we can see that the base objects are the same, but the
// pointers have dynamic offsets which we can't resolve. And none of our
// little tricks above worked.
return MayAlias;
}
static AliasResult MergeAliasResults(AliasResult A, AliasResult B) {
// If the results agree, take it.
if (A == B)
return A;
// A mix of PartialAlias and MustAlias is PartialAlias.
if ((A == PartialAlias && B == MustAlias) ||
(B == PartialAlias && A == MustAlias))
return PartialAlias;
// Otherwise, we don't know anything.
return MayAlias;
}
/// Provides a bunch of ad-hoc rules to disambiguate a Select instruction
/// against another.
AliasResult
BasicAAResult::aliasSelect(const SelectInst *SI, LocationSize SISize,
const AAMDNodes &SIAAInfo, const Value *V2,
LocationSize V2Size, const AAMDNodes &V2AAInfo,
const Value *UnderV2, AAQueryInfo &AAQI) {
// If the values are Selects with the same condition, we can do a more precise
// check: just check for aliases between the values on corresponding arms.
if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
if (SI->getCondition() == SI2->getCondition()) {
AliasResult Alias =
aliasCheck(SI->getTrueValue(), SISize, SIAAInfo, SI2->getTrueValue(),
V2Size, V2AAInfo, AAQI);
if (Alias == MayAlias)
return MayAlias;
AliasResult ThisAlias =
aliasCheck(SI->getFalseValue(), SISize, SIAAInfo,
SI2->getFalseValue(), V2Size, V2AAInfo, AAQI);
return MergeAliasResults(ThisAlias, Alias);
}
// If both arms of the Select node NoAlias or MustAlias V2, then returns
// NoAlias / MustAlias. Otherwise, returns MayAlias.
AliasResult Alias = aliasCheck(V2, V2Size, V2AAInfo, SI->getTrueValue(),
SISize, SIAAInfo, AAQI, UnderV2);
if (Alias == MayAlias)
return MayAlias;
AliasResult ThisAlias = aliasCheck(V2, V2Size, V2AAInfo, SI->getFalseValue(),
SISize, SIAAInfo, AAQI, UnderV2);
return MergeAliasResults(ThisAlias, Alias);
}
/// Provide a bunch of ad-hoc rules to disambiguate a PHI instruction against
/// another.
AliasResult BasicAAResult::aliasPHI(const PHINode *PN, LocationSize PNSize,
const AAMDNodes &PNAAInfo, const Value *V2,
LocationSize V2Size,
const AAMDNodes &V2AAInfo,
const Value *UnderV2, AAQueryInfo &AAQI) {
// Track phi nodes we have visited. We use this information when we determine
// value equivalence.
VisitedPhiBBs.insert(PN->getParent());
// If the values are PHIs in the same block, we can do a more precise
// as well as efficient check: just check for aliases between the values
// on corresponding edges.
if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
if (PN2->getParent() == PN->getParent()) {
AAQueryInfo::LocPair Locs(MemoryLocation(PN, PNSize, PNAAInfo),
MemoryLocation(V2, V2Size, V2AAInfo));
if (PN > V2)
std::swap(Locs.first, Locs.second);
// Analyse the PHIs' inputs under the assumption that the PHIs are
// NoAlias.
// If the PHIs are May/MustAlias there must be (recursively) an input
// operand from outside the PHIs' cycle that is MayAlias/MustAlias or
// there must be an operation on the PHIs within the PHIs' value cycle
// that causes a MayAlias.
// Pretend the phis do not alias.
AliasResult Alias = NoAlias;
AliasResult OrigAliasResult;
{
// Limited lifetime iterator invalidated by the aliasCheck call below.
auto CacheIt = AAQI.AliasCache.find(Locs);
assert((CacheIt != AAQI.AliasCache.end()) &&
"There must exist an entry for the phi node");
OrigAliasResult = CacheIt->second;
CacheIt->second = NoAlias;
}
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
AliasResult ThisAlias =
aliasCheck(PN->getIncomingValue(i), PNSize, PNAAInfo,
PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
V2Size, V2AAInfo, AAQI);
Alias = MergeAliasResults(ThisAlias, Alias);
if (Alias == MayAlias)
break;
}
// Reset if speculation failed.
if (Alias != NoAlias) {
auto Pair =
AAQI.AliasCache.insert(std::make_pair(Locs, OrigAliasResult));
assert(!Pair.second && "Entry must have existed");
Pair.first->second = OrigAliasResult;
}
return Alias;
}
SmallVector<Value *, 4> V1Srcs;
bool isRecursive = false;
if (PV) {
// If we have PhiValues then use it to get the underlying phi values.
const PhiValues::ValueSet &PhiValueSet = PV->getValuesForPhi(PN);
// If we have more phi values than the search depth then return MayAlias
// conservatively to avoid compile time explosion. The worst possible case
// is if both sides are PHI nodes. In which case, this is O(m x n) time
// where 'm' and 'n' are the number of PHI sources.
if (PhiValueSet.size() > MaxLookupSearchDepth)
return MayAlias;
// Add the values to V1Srcs
for (Value *PV1 : PhiValueSet) {
if (EnableRecPhiAnalysis) {
if (GEPOperator *PV1GEP = dyn_cast<GEPOperator>(PV1)) {
// Check whether the incoming value is a GEP that advances the pointer
// result of this PHI node (e.g. in a loop). If this is the case, we
// would recurse and always get a MayAlias. Handle this case specially
// below.
if (PV1GEP->getPointerOperand() == PN && PV1GEP->getNumIndices() == 1 &&
isa<ConstantInt>(PV1GEP->idx_begin())) {
isRecursive = true;
continue;
}
}
}
V1Srcs.push_back(PV1);
}
} else {
// If we don't have PhiInfo then just look at the operands of the phi itself
// FIXME: Remove this once we can guarantee that we have PhiInfo always
SmallPtrSet<Value *, 4> UniqueSrc;
for (Value *PV1 : PN->incoming_values()) {
if (isa<PHINode>(PV1))
// If any of the source itself is a PHI, return MayAlias conservatively
// to avoid compile time explosion. The worst possible case is if both
// sides are PHI nodes. In which case, this is O(m x n) time where 'm'
// and 'n' are the number of PHI sources.
return MayAlias;
if (EnableRecPhiAnalysis)
if (GEPOperator *PV1GEP = dyn_cast<GEPOperator>(PV1)) {
// Check whether the incoming value is a GEP that advances the pointer
// result of this PHI node (e.g. in a loop). If this is the case, we
// would recurse and always get a MayAlias. Handle this case specially
// below.
if (PV1GEP->getPointerOperand() == PN && PV1GEP->getNumIndices() == 1 &&
isa<ConstantInt>(PV1GEP->idx_begin())) {
isRecursive = true;
continue;
}
}
if (UniqueSrc.insert(PV1).second)
V1Srcs.push_back(PV1);
}
}
// If V1Srcs is empty then that means that the phi has no underlying non-phi
// value. This should only be possible in blocks unreachable from the entry
// block, but return MayAlias just in case.
if (V1Srcs.empty())
return MayAlias;
// If this PHI node is recursive, set the size of the accessed memory to
// unknown to represent all the possible values the GEP could advance the
// pointer to.
if (isRecursive)
PNSize = LocationSize::unknown();
AliasResult Alias = aliasCheck(V2, V2Size, V2AAInfo, V1Srcs[0], PNSize,
PNAAInfo, AAQI, UnderV2);
// Early exit if the check of the first PHI source against V2 is MayAlias.
// Other results are not possible.
if (Alias == MayAlias)
return MayAlias;
// If all sources of the PHI node NoAlias or MustAlias V2, then returns
// NoAlias / MustAlias. Otherwise, returns MayAlias.
for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
Value *V = V1Srcs[i];
AliasResult ThisAlias =
aliasCheck(V2, V2Size, V2AAInfo, V, PNSize, PNAAInfo, AAQI, UnderV2);
Alias = MergeAliasResults(ThisAlias, Alias);
if (Alias == MayAlias)
break;
}
return Alias;
}
/// Provides a bunch of ad-hoc rules to disambiguate in common cases, such as
/// array references.
AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
AAMDNodes V1AAInfo, const Value *V2,
LocationSize V2Size, AAMDNodes V2AAInfo,
AAQueryInfo &AAQI, const Value *O1,
const Value *O2) {
// If either of the memory references is empty, it doesn't matter what the
// pointer values are.
if (V1Size.isZero() || V2Size.isZero())
return NoAlias;
// Strip off any casts if they exist.
V1 = V1->stripPointerCastsAndInvariantGroups();
V2 = V2->stripPointerCastsAndInvariantGroups();
// If V1 or V2 is undef, the result is NoAlias because we can always pick a
// value for undef that aliases nothing in the program.
if (isa<UndefValue>(V1) || isa<UndefValue>(V2))
return NoAlias;
// Are we checking for alias of the same value?
// Because we look 'through' phi nodes, we could look at "Value" pointers from
// different iterations. We must therefore make sure that this is not the
// case. The function isValueEqualInPotentialCycles ensures that this cannot
// happen by looking at the visited phi nodes and making sure they cannot
// reach the value.
if (isValueEqualInPotentialCycles(V1, V2))
return MustAlias;
if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
return NoAlias; // Scalars cannot alias each other
// Figure out what objects these things are pointing to if we can.
if (O1 == nullptr)
O1 = GetUnderlyingObject(V1, DL, MaxLookupSearchDepth);
if (O2 == nullptr)
O2 = GetUnderlyingObject(V2, DL, MaxLookupSearchDepth);
// Null values in the default address space don't point to any object, so they
// don't alias any other pointer.
if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
if (!NullPointerIsDefined(&F, CPN->getType()->getAddressSpace()))
return NoAlias;
if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
if (!NullPointerIsDefined(&F, CPN->getType()->getAddressSpace()))
return NoAlias;
if (O1 != O2) {
// If V1/V2 point to two different objects, we know that we have no alias.
if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
return NoAlias;
// Constant pointers can't alias with non-const isIdentifiedObject objects.
if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
(isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
return NoAlias;
// Function arguments can't alias with things that are known to be
// unambigously identified at the function level.
if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
(isa<Argument>(O2) && isIdentifiedFunctionLocal(O1)))
return NoAlias;
// If one pointer is the result of a call/invoke or load and the other is a
// non-escaping local object within the same function, then we know the
// object couldn't escape to a point where the call could return it.
//
// Note that if the pointers are in different functions, there are a
// variety of complications. A call with a nocapture argument may still
// temporary store the nocapture argument's value in a temporary memory
// location if that memory location doesn't escape. Or it may pass a
// nocapture value to other functions as long as they don't capture it.
if (isEscapeSource(O1) &&
isNonEscapingLocalObject(O2, &AAQI.IsCapturedCache))
return NoAlias;
if (isEscapeSource(O2) &&
isNonEscapingLocalObject(O1, &AAQI.IsCapturedCache))
return NoAlias;
}
// If the size of one access is larger than the entire object on the other
// side, then we know such behavior is undefined and can assume no alias.
bool NullIsValidLocation = NullPointerIsDefined(&F);
if ((isObjectSmallerThan(
O2, getMinimalExtentFrom(*V1, V1Size, DL, NullIsValidLocation), DL,
TLI, NullIsValidLocation)) ||
(isObjectSmallerThan(
O1, getMinimalExtentFrom(*V2, V2Size, DL, NullIsValidLocation), DL,
TLI, NullIsValidLocation)))
return NoAlias;
// Check the cache before climbing up use-def chains. This also terminates
// otherwise infinitely recursive queries.
AAQueryInfo::LocPair Locs(MemoryLocation(V1, V1Size, V1AAInfo),
MemoryLocation(V2, V2Size, V2AAInfo));
if (V1 > V2)
std::swap(Locs.first, Locs.second);
std::pair<AAQueryInfo::AliasCacheT::iterator, bool> Pair =
AAQI.AliasCache.try_emplace(Locs, MayAlias);
if (!Pair.second)
return Pair.first->second;
// FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
// GEP can't simplify, we don't even look at the PHI cases.
if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
std::swap(V1, V2);
std::swap(V1Size, V2Size);
std::swap(O1, O2);
std::swap(V1AAInfo, V2AAInfo);
}
if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
AliasResult Result =
aliasGEP(GV1, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O1, O2, AAQI);
if (Result != MayAlias) {
auto ItInsPair = AAQI.AliasCache.insert(std::make_pair(Locs, Result));
assert(!ItInsPair.second && "Entry must have existed");
ItInsPair.first->second = Result;
return Result;
}
}
if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
std::swap(V1, V2);
std::swap(O1, O2);
std::swap(V1Size, V2Size);
std::swap(V1AAInfo, V2AAInfo);
}
if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
AliasResult Result =
aliasPHI(PN, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O2, AAQI);
if (Result != MayAlias) {
Pair = AAQI.AliasCache.try_emplace(Locs, Result);
assert(!Pair.second && "Entry must have existed");
return Pair.first->second = Result;
}
}
if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
std::swap(V1, V2);
std::swap(O1, O2);
std::swap(V1Size, V2Size);
std::swap(V1AAInfo, V2AAInfo);
}
if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
AliasResult Result =
aliasSelect(S1, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O2, AAQI);
if (Result != MayAlias) {
Pair = AAQI.AliasCache.try_emplace(Locs, Result);
assert(!Pair.second && "Entry must have existed");
return Pair.first->second = Result;
}
}
// If both pointers are pointing into the same object and one of them
// accesses the entire object, then the accesses must overlap in some way.
if (O1 == O2)
if (V1Size.isPrecise() && V2Size.isPrecise() &&
(isObjectSize(O1, V1Size.getValue(), DL, TLI, NullIsValidLocation) ||
isObjectSize(O2, V2Size.getValue(), DL, TLI, NullIsValidLocation))) {
Pair = AAQI.AliasCache.try_emplace(Locs, PartialAlias);
assert(!Pair.second && "Entry must have existed");
return Pair.first->second = PartialAlias;
}
// Recurse back into the best AA results we have, potentially with refined
// memory locations. We have already ensured that BasicAA has a MayAlias
// cache result for these, so any recursion back into BasicAA won't loop.
AliasResult Result = getBestAAResults().alias(Locs.first, Locs.second, AAQI);
Pair = AAQI.AliasCache.try_emplace(Locs, Result);
assert(!Pair.second && "Entry must have existed");
return Pair.first->second = Result;
}
/// Check whether two Values can be considered equivalent.
///
/// In addition to pointer equivalence of \p V1 and \p V2 this checks whether
/// they can not be part of a cycle in the value graph by looking at all
/// visited phi nodes an making sure that the phis cannot reach the value. We
/// have to do this because we are looking through phi nodes (That is we say
/// noalias(V, phi(VA, VB)) if noalias(V, VA) and noalias(V, VB).
bool BasicAAResult::isValueEqualInPotentialCycles(const Value *V,
const Value *V2) {
if (V != V2)
return false;
const Instruction *Inst = dyn_cast<Instruction>(V);
if (!Inst)
return true;
if (VisitedPhiBBs.empty())
return true;
if (VisitedPhiBBs.size() > MaxNumPhiBBsValueReachabilityCheck)
return false;
// Make sure that the visited phis cannot reach the Value. This ensures that
// the Values cannot come from different iterations of a potential cycle the
// phi nodes could be involved in.
for (auto *P : VisitedPhiBBs)
if (isPotentiallyReachable(&P->front(), Inst, nullptr, DT, LI))
return false;
return true;
}
/// Computes the symbolic difference between two de-composed GEPs.
///
/// Dest and Src are the variable indices from two decomposed GetElementPtr
/// instructions GEP1 and GEP2 which have common base pointers.
void BasicAAResult::GetIndexDifference(
SmallVectorImpl<VariableGEPIndex> &Dest,
const SmallVectorImpl<VariableGEPIndex> &Src) {
if (Src.empty())
return;
for (unsigned i = 0, e = Src.size(); i != e; ++i) {
const Value *V = Src[i].V;
unsigned ZExtBits = Src[i].ZExtBits, SExtBits = Src[i].SExtBits;
APInt Scale = Src[i].Scale;
// Find V in Dest. This is N^2, but pointer indices almost never have more
// than a few variable indexes.
for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
if (!isValueEqualInPotentialCycles(Dest[j].V, V) ||
Dest[j].ZExtBits != ZExtBits || Dest[j].SExtBits != SExtBits)
continue;
// If we found it, subtract off Scale V's from the entry in Dest. If it
// goes to zero, remove the entry.
if (Dest[j].Scale != Scale)
Dest[j].Scale -= Scale;
else
Dest.erase(Dest.begin() + j);
Scale = 0;
break;
}
// If we didn't consume this entry, add it to the end of the Dest list.
if (!!Scale) {
VariableGEPIndex Entry = {V, ZExtBits, SExtBits, -Scale};
Dest.push_back(Entry);
}
}
}
bool BasicAAResult::constantOffsetHeuristic(
const SmallVectorImpl<VariableGEPIndex> &VarIndices,
LocationSize MaybeV1Size, LocationSize MaybeV2Size, APInt BaseOffset,
AssumptionCache *AC, DominatorTree *DT) {
if (VarIndices.size() != 2 || MaybeV1Size == LocationSize::unknown() ||
MaybeV2Size == LocationSize::unknown())
return false;
const uint64_t V1Size = MaybeV1Size.getValue();
const uint64_t V2Size = MaybeV2Size.getValue();
const VariableGEPIndex &Var0 = VarIndices[0], &Var1 = VarIndices[1];
if (Var0.ZExtBits != Var1.ZExtBits || Var0.SExtBits != Var1.SExtBits ||
Var0.Scale != -Var1.Scale)
return false;
unsigned Width = Var1.V->getType()->getIntegerBitWidth();
// We'll strip off the Extensions of Var0 and Var1 and do another round
// of GetLinearExpression decomposition. In the example above, if Var0
// is zext(%x + 1) we should get V1 == %x and V1Offset == 1.
APInt V0Scale(Width, 0), V0Offset(Width, 0), V1Scale(Width, 0),
V1Offset(Width, 0);
bool NSW = true, NUW = true;
unsigned V0ZExtBits = 0, V0SExtBits = 0, V1ZExtBits = 0, V1SExtBits = 0;
const Value *V0 = GetLinearExpression(Var0.V, V0Scale, V0Offset, V0ZExtBits,
V0SExtBits, DL, 0, AC, DT, NSW, NUW);
NSW = true;
NUW = true;
const Value *V1 = GetLinearExpression(Var1.V, V1Scale, V1Offset, V1ZExtBits,
V1SExtBits, DL, 0, AC, DT, NSW, NUW);
if (V0Scale != V1Scale || V0ZExtBits != V1ZExtBits ||
V0SExtBits != V1SExtBits || !isValueEqualInPotentialCycles(V0, V1))
return false;
// We have a hit - Var0 and Var1 only differ by a constant offset!
// If we've been sext'ed then zext'd the maximum difference between Var0 and
// Var1 is possible to calculate, but we're just interested in the absolute
// minimum difference between the two. The minimum distance may occur due to
// wrapping; consider "add i3 %i, 5": if %i == 7 then 7 + 5 mod 8 == 4, and so
// the minimum distance between %i and %i + 5 is 3.
APInt MinDiff = V0Offset - V1Offset, Wrapped = -MinDiff;
MinDiff = APIntOps::umin(MinDiff, Wrapped);
APInt MinDiffBytes =
MinDiff.zextOrTrunc(Var0.Scale.getBitWidth()) * Var0.Scale.abs();
// We can't definitely say whether GEP1 is before or after V2 due to wrapping
// arithmetic (i.e. for some values of GEP1 and V2 GEP1 < V2, and for other
// values GEP1 > V2). We'll therefore only declare NoAlias if both V1Size and
// V2Size can fit in the MinDiffBytes gap.
return MinDiffBytes.uge(V1Size + BaseOffset.abs()) &&
MinDiffBytes.uge(V2Size + BaseOffset.abs());
}
//===----------------------------------------------------------------------===//
// BasicAliasAnalysis Pass
//===----------------------------------------------------------------------===//
AnalysisKey BasicAA::Key;
BasicAAResult BasicAA::run(Function &F, FunctionAnalysisManager &AM) {
return BasicAAResult(F.getParent()->getDataLayout(),
F,
AM.getResult<TargetLibraryAnalysis>(F),
AM.getResult<AssumptionAnalysis>(F),
&AM.getResult<DominatorTreeAnalysis>(F),
AM.getCachedResult<LoopAnalysis>(F),
AM.getCachedResult<PhiValuesAnalysis>(F));
}
BasicAAWrapperPass::BasicAAWrapperPass() : FunctionPass(ID) {
initializeBasicAAWrapperPassPass(*PassRegistry::getPassRegistry());
}
char BasicAAWrapperPass::ID = 0;
void BasicAAWrapperPass::anchor() {}
INITIALIZE_PASS_BEGIN(BasicAAWrapperPass, "basicaa",
"Basic Alias Analysis (stateless AA impl)", true, true)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(PhiValuesWrapperPass)
INITIALIZE_PASS_END(BasicAAWrapperPass, "basicaa",
"Basic Alias Analysis (stateless AA impl)", true, true)
FunctionPass *llvm::createBasicAAWrapperPass() {
return new BasicAAWrapperPass();
}
bool BasicAAWrapperPass::runOnFunction(Function &F) {
auto &ACT = getAnalysis<AssumptionCacheTracker>();
auto &TLIWP = getAnalysis<TargetLibraryInfoWrapperPass>();
auto &DTWP = getAnalysis<DominatorTreeWrapperPass>();
auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
auto *PVWP = getAnalysisIfAvailable<PhiValuesWrapperPass>();
Result.reset(new BasicAAResult(F.getParent()->getDataLayout(), F,
TLIWP.getTLI(F), ACT.getAssumptionCache(F),
&DTWP.getDomTree(),
LIWP ? &LIWP->getLoopInfo() : nullptr,
PVWP ? &PVWP->getResult() : nullptr));
return false;
}
void BasicAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addUsedIfAvailable<PhiValuesWrapperPass>();
}
BasicAAResult llvm::createLegacyPMBasicAAResult(Pass &P, Function &F) {
return BasicAAResult(
F.getParent()->getDataLayout(), F,
P.getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F),
P.getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F));
}
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