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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 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/ScopeExit.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/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/ConstantRange.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("basic-aa-recphi", cl::Hidden,
/// 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().
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)) ||
(PV && Inv.invalidate<PhiValuesAnalysis>(Fn, PA)))
return true;
// Otherwise this analysis result remains valid.
return false;
// Useful predicates
/// 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;
// 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;
// The inttoptr case works because isNonEscapingLocalObject considers all
// means of converting or equating a pointer to an int (ptrtoint, ptr store
// which could be followed by an integer load, ptr<->int compare) as
// escaping, and objects located at well-known addresses via platform-specific
// means cannot be considered non-escaping local objects.
if (isa<IntToPtrInst>(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. We can ignore frees, as an
// access after free would be undefined behavior.
bool CanBeNull, CanBeFreed;
uint64_t DerefBytes =
V.getPointerDereferenceableBytes(DL, CanBeNull, CanBeFreed);
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;
// CaptureInfo implementations
CaptureInfo::~CaptureInfo() = default;
bool SimpleCaptureInfo::isNotCapturedBeforeOrAt(const Value *Object,
const Instruction *I) {
return isNonEscapingLocalObject(Object, &IsCapturedCache);
bool EarliestEscapeInfo::isNotCapturedBeforeOrAt(const Value *Object,
const Instruction *I) {
if (!isIdentifiedFunctionLocal(Object))
return false;
auto Iter = EarliestEscapes.insert({Object, nullptr});
if (Iter.second) {
Instruction *EarliestCapture = FindEarliestCapture(
Object, *const_cast<Function *>(I->getFunction()),
/*ReturnCaptures=*/false, /*StoreCaptures=*/true, DT);
if (EarliestCapture) {
auto Ins = Inst2Obj.insert({EarliestCapture, {}});
Iter.first->second = EarliestCapture;
// No capturing instruction.
if (!Iter.first->second)
return true;
return I != Iter.first->second &&
!isPotentiallyReachable(Iter.first->second, I, nullptr, &DT, &LI);
void EarliestEscapeInfo::removeInstruction(Instruction *I) {
auto Iter = Inst2Obj.find(I);
if (Iter != Inst2Obj.end()) {
for (const Value *Obj : Iter->second)
// GetElementPtr Instruction Decomposition and Analysis
namespace {
/// Represents zext(sext(trunc(V))).
struct CastedValue {
const Value *V;
unsigned ZExtBits = 0;
unsigned SExtBits = 0;
unsigned TruncBits = 0;
explicit CastedValue(const Value *V) : V(V) {}
explicit CastedValue(const Value *V, unsigned ZExtBits, unsigned SExtBits,
unsigned TruncBits)
: V(V), ZExtBits(ZExtBits), SExtBits(SExtBits), TruncBits(TruncBits) {}
unsigned getBitWidth() const {
return V->getType()->getPrimitiveSizeInBits() - TruncBits + ZExtBits +
CastedValue withValue(const Value *NewV) const {
return CastedValue(NewV, ZExtBits, SExtBits, TruncBits);
/// Replace V with zext(NewV)
CastedValue withZExtOfValue(const Value *NewV) const {
unsigned ExtendBy = V->getType()->getPrimitiveSizeInBits() -
if (ExtendBy <= TruncBits)
return CastedValue(NewV, ZExtBits, SExtBits, TruncBits - ExtendBy);
// zext(sext(zext(NewV))) == zext(zext(zext(NewV)))
ExtendBy -= TruncBits;
return CastedValue(NewV, ZExtBits + SExtBits + ExtendBy, 0, 0);
/// Replace V with sext(NewV)
CastedValue withSExtOfValue(const Value *NewV) const {
unsigned ExtendBy = V->getType()->getPrimitiveSizeInBits() -
if (ExtendBy <= TruncBits)
return CastedValue(NewV, ZExtBits, SExtBits, TruncBits - ExtendBy);
// zext(sext(sext(NewV)))
ExtendBy -= TruncBits;
return CastedValue(NewV, ZExtBits, SExtBits + ExtendBy, 0);
APInt evaluateWith(APInt N) const {
assert(N.getBitWidth() == V->getType()->getPrimitiveSizeInBits() &&
"Incompatible bit width");
if (TruncBits) N = N.trunc(N.getBitWidth() - TruncBits);
if (SExtBits) N = N.sext(N.getBitWidth() + SExtBits);
if (ZExtBits) N = N.zext(N.getBitWidth() + ZExtBits);
return N;
ConstantRange evaluateWith(ConstantRange N) const {
assert(N.getBitWidth() == V->getType()->getPrimitiveSizeInBits() &&
"Incompatible bit width");
if (TruncBits) N = N.truncate(N.getBitWidth() - TruncBits);
if (SExtBits) N = N.signExtend(N.getBitWidth() + SExtBits);
if (ZExtBits) N = N.zeroExtend(N.getBitWidth() + ZExtBits);
return N;
bool canDistributeOver(bool NUW, bool NSW) const {
// zext(x op<nuw> y) == zext(x) op<nuw> zext(y)
// sext(x op<nsw> y) == sext(x) op<nsw> sext(y)
// trunc(x op y) == trunc(x) op trunc(y)
return (!ZExtBits || NUW) && (!SExtBits || NSW);
bool hasSameCastsAs(const CastedValue &Other) const {
return ZExtBits == Other.ZExtBits && SExtBits == Other.SExtBits &&
TruncBits == Other.TruncBits;
/// Represents zext(sext(trunc(V))) * Scale + Offset.
struct LinearExpression {
CastedValue Val;
APInt Scale;
APInt Offset;
/// True if all operations in this expression are NSW.
bool IsNSW;
LinearExpression(const CastedValue &Val, const APInt &Scale,
const APInt &Offset, bool IsNSW)
: Val(Val), Scale(Scale), Offset(Offset), IsNSW(IsNSW) {}
LinearExpression(const CastedValue &Val) : Val(Val), IsNSW(true) {
unsigned BitWidth = Val.getBitWidth();
Scale = APInt(BitWidth, 1);
Offset = APInt(BitWidth, 0);
LinearExpression mul(const APInt &Other, bool MulIsNSW) const {
// The check for zero offset is necessary, because generally
// (X +nsw Y) *nsw Z does not imply (X *nsw Z) +nsw (Y *nsw Z).
bool NSW = IsNSW && (Other.isOne() || (MulIsNSW && Offset.isZero()));
return LinearExpression(Val, Scale * Other, Offset * Other, NSW);
/// Analyzes the specified value as a linear expression: "A*V + B", where A and
/// B are constant integers.
static LinearExpression GetLinearExpression(
const CastedValue &Val, const DataLayout &DL, unsigned Depth,
AssumptionCache *AC, DominatorTree *DT) {
// Limit our recursion depth.
if (Depth == 6)
return Val;
if (const ConstantInt *Const = dyn_cast<ConstantInt>(Val.V))
return LinearExpression(Val, APInt(Val.getBitWidth(), 0),
Val.evaluateWith(Const->getValue()), true);
if (const BinaryOperator *BOp = dyn_cast<BinaryOperator>(Val.V)) {
if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
APInt RHS = Val.evaluateWith(RHSC->getValue());
// The only non-OBO case we deal with is or, and only limited to the
// case where it is both nuw and nsw.
bool NUW = true, NSW = true;
if (isa<OverflowingBinaryOperator>(BOp)) {
NUW &= BOp->hasNoUnsignedWrap();
NSW &= BOp->hasNoSignedWrap();
if (!Val.canDistributeOver(NUW, NSW))
return Val;
// While we can distribute over trunc, we cannot preserve nowrap flags
// in that case.
if (Val.TruncBits)
NUW = NSW = false;
LinearExpression E(Val);
switch (BOp->getOpcode()) {
// We don't understand this instruction, so we can't decompose it any
// further.
return Val;
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))
return Val;
case Instruction::Add: {
E = GetLinearExpression(Val.withValue(BOp->getOperand(0)), DL,
Depth + 1, AC, DT);
E.Offset += RHS;
case Instruction::Sub: {
E = GetLinearExpression(Val.withValue(BOp->getOperand(0)), DL,
Depth + 1, AC, DT);
E.Offset -= RHS;
case Instruction::Mul:
E = GetLinearExpression(Val.withValue(BOp->getOperand(0)), DL,
Depth + 1, AC, DT)
.mul(RHS, NSW);
case Instruction::Shl:
// 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 (RHS.getLimitedValue() > Val.getBitWidth())
return Val;
E = GetLinearExpression(Val.withValue(BOp->getOperand(0)), DL,
Depth + 1, AC, DT);
E.Offset <<= RHS.getLimitedValue();
E.Scale <<= RHS.getLimitedValue();
return E;
if (isa<ZExtInst>(Val.V))
return GetLinearExpression(
DL, Depth + 1, AC, DT);
if (isa<SExtInst>(Val.V))
return GetLinearExpression(
DL, Depth + 1, AC, DT);
return Val;
/// To ensure a pointer offset fits in an integer of size IndexSize
/// (in bits) when that size is smaller than the maximum index 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 index size is 64b.
static APInt adjustToIndexSize(const APInt &Offset, unsigned IndexSize) {
assert(IndexSize <= Offset.getBitWidth() && "Invalid IndexSize!");
unsigned ShiftBits = Offset.getBitWidth() - IndexSize;
return (Offset << ShiftBits).ashr(ShiftBits);
namespace {
// A linear transformation of a Value; this class represents
// ZExt(SExt(Trunc(V, TruncBits), SExtBits), ZExtBits) * Scale.
struct VariableGEPIndex {
CastedValue Val;
APInt Scale;
// Context instruction to use when querying information about this index.
const Instruction *CxtI;
/// True if all operations in this expression are NSW.
bool IsNSW;
void dump() const {
dbgs() << "\n";
void print(raw_ostream &OS) const {
OS << "(V=" << Val.V->getName()
<< ", zextbits=" << Val.ZExtBits
<< ", sextbits=" << Val.SExtBits
<< ", truncbits=" << Val.TruncBits
<< ", scale=" << Scale << ")";
// Represents the internal structure of a GEP, decomposed into a base pointer,
// constant offsets, and variable scaled indices.
struct BasicAAResult::DecomposedGEP {
// Base pointer of the GEP
const Value *Base;
// Total constant offset from base.
APInt Offset;
// Scaled variable (non-constant) indices.
SmallVector<VariableGEPIndex, 4> VarIndices;
// Are all operations inbounds GEPs or non-indexing operations?
// (None iff expression doesn't involve any geps)
Optional<bool> InBounds;
void dump() const {
dbgs() << "\n";
void print(raw_ostream &OS) const {
OS << "(DecomposedGEP Base=" << Base->getName()
<< ", Offset=" << Offset
<< ", VarIndices=[";
for (size_t i = 0; i < VarIndices.size(); i++) {
if (i != 0)
OS << ", ";
OS << "])";
/// 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.
BasicAAResult::DecomposeGEPExpression(const Value *V, const DataLayout &DL,
AssumptionCache *AC, DominatorTree *DT) {
// Limit recursion depth to limit compile time in crazy cases.
unsigned MaxLookup = MaxLookupSearchDepth;
const Instruction *CxtI = dyn_cast<Instruction>(V);
unsigned MaxIndexSize = DL.getMaxIndexSizeInBits();
DecomposedGEP Decomposed;
Decomposed.Offset = APInt(MaxIndexSize, 0);
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();
Decomposed.Base = V;
return Decomposed;
if (Op->getOpcode() == Instruction::BitCast ||
Op->getOpcode() == Instruction::AddrSpaceCast) {
V = Op->getOperand(0);
const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
if (!GEPOp) {
if (const auto *PHI = dyn_cast<PHINode>(V)) {
// Look through single-arg phi nodes created by LCSSA.
if (PHI->getNumIncomingValues() == 1) {
V = PHI->getIncomingValue(0);
} else if (const auto *Call = dyn_cast<CallBase>(V)) {
// CaptureTracking can know about special capturing properties of some
// intrinsics like, 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;
Decomposed.Base = V;
return Decomposed;
// Track whether we've seen at least one in bounds gep, and if so, whether
// all geps parsed were in bounds.
if (Decomposed.InBounds == None)
Decomposed.InBounds = GEPOp->isInBounds();
else if (!GEPOp->isInBounds())
Decomposed.InBounds = false;
assert(GEPOp->getSourceElementType()->isSized() && "GEP must be sized");
// Don't attempt to analyze GEPs if index scale is not a compile-time
// constant.
if (isa<ScalableVectorType>(GEPOp->getSourceElementType())) {
Decomposed.Base = V;
return Decomposed;
unsigned AS = GEPOp->getPointerAddressSpace();
// Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
gep_type_iterator GTI = gep_type_begin(GEPOp);
unsigned IndexSize = DL.getIndexSizeInBits(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)
Decomposed.Offset += DL.getStructLayout(STy)->getElementOffset(FieldNo);
// For an array/pointer, add the element offset, explicitly scaled.
if (const ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
if (CIdx->isZero())
Decomposed.Offset +=
DL.getTypeAllocSize(GTI.getIndexedType()).getFixedSize() *
GepHasConstantOffset = false;
// If the integer type is smaller than the index size, it is implicitly
// sign extended or truncated to index size.
unsigned Width = Index->getType()->getIntegerBitWidth();
unsigned SExtBits = IndexSize > Width ? IndexSize - Width : 0;
unsigned TruncBits = IndexSize < Width ? Width - IndexSize : 0;
LinearExpression LE = GetLinearExpression(
CastedValue(Index, 0, SExtBits, TruncBits), DL, 0, AC, DT);
// Scale by the type size.
unsigned TypeSize =
LE = LE.mul(APInt(IndexSize, TypeSize), GEPOp->isInBounds());
Decomposed.Offset += LE.Offset.sextOrSelf(MaxIndexSize);
APInt Scale = LE.Scale.sextOrSelf(MaxIndexSize);
// 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].Val.V == LE.Val.V &&
Decomposed.VarIndices[i].Val.hasSameCastsAs(LE.Val)) {
Scale += Decomposed.VarIndices[i].Scale;
Decomposed.VarIndices.erase(Decomposed.VarIndices.begin() + i);
// Make sure that we have a scale that makes sense for this target's
// index size.
Scale = adjustToIndexSize(Scale, IndexSize);
if (!!Scale) {
VariableGEPIndex Entry = {LE.Val, Scale, CxtI, LE.IsNSW};
// Take care of wrap-arounds
if (GepHasConstantOffset)
Decomposed.Offset = adjustToIndexSize(Decomposed.Offset, IndexSize);
// 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;
return Decomposed;
/// 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;
do {
const Value *V = getUnderlyingObject(Worklist.pop_back_val());
if (!Visited.insert(V).second) {
return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
// An alloca instruction defines local memory.
if (OrLocal && isa<AllocaInst>(V))
// 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()) {
return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
// If both select values point to local memory, then so does the select.
if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
// 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) {
return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
append_range(Worklist, PN->incoming_values());
// Otherwise be conservative.
return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
} while (!Worklist.empty() && --MaxLookup);
return Worklist.empty();
static bool isIntrinsicCall(const CallBase *Call, Intrinsic::ID IID) {
const IntrinsicInst *II = dyn_cast<IntrinsicInst>(Call);
return II && II->getIntrinsicID() == IID;
/// 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);
#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;
AliasResult BasicAAResult::alias(const MemoryLocation &LocA,
const MemoryLocation &LocB,
AAQueryInfo &AAQI) {
assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
"BasicAliasAnalysis doesn't support interprocedural queries.");
return aliasCheck(LocA.Ptr, LocA.Size, LocB.Ptr, LocB.Size, AAQI);
/// 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);
// 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() &&
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 &&
AAQI.CI->isNotCapturedBeforeOrAt(Object, Call)) {
// 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->arg_size() &&
// Call doesn't access memory through this operand, so we don't care
// if it aliases with Object.
if (Call->doesNotAccessMemory(OperandNo))
// 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::getBeforeOrAfter(Object), AAQI);
if (AR != AliasResult::MustAlias)
IsMustAlias = false;
// Operand doesn't alias 'Object', continue looking for other aliases
if (AR == AliasResult::NoAlias)
// 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);
// Operand aliases 'Object' but call only writes into it.
if (Call->doesNotReadMemory(OperandNo)) {
Result = setMod(Result);
// 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;
// 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 malloc/calloc like, 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::getBeforeOrAfter(Call), Loc,
AAQI) == AliasResult::NoAlias)
return ModRefInfo::NoModRef;
// The semantics of memcpy intrinsics either exactly overlap or do not
// overlap, i.e., source and destination of any given memcpy are either
// no-alias or must-alias.
if (auto *Inst = dyn_cast<AnyMemCpyInst>(Call)) {
AliasResult SrcAA =
getBestAAResults().alias(MemoryLocation::getForSource(Inst), Loc, AAQI);
AliasResult DestAA =
getBestAAResults().alias(MemoryLocation::getForDest(Inst), Loc, AAQI);
// It's also possible for Loc to alias both src and dest, or neither.
ModRefInfo rv = ModRefInfo::NoModRef;
if (SrcAA != AliasResult::NoAlias)
rv = setRef(rv);
if (DestAA != AliasResult::NoAlias)
rv = setMod(rv);
return rv;
// Guard intrinsics are 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;
// The same applies to deoptimize which is essentially a guard(false).
if (isIntrinsicCall(Call, Intrinsic::experimental_deoptimize))
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) {
// Guard intrinsics are 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.
// 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);
/// Return true if we know V to the base address of the corresponding memory
/// object. This implies that any address less than V must be out of bounds
/// for the underlying object. Note that just being isIdentifiedObject() is
/// not enough - For example, a negative offset from a noalias argument or call
/// can be inbounds w.r.t the actual underlying object.
static bool isBaseOfObject(const Value *V) {
// TODO: We can handle other cases here
// 1) For GC languages, arguments to functions are often required to be
// base pointers.
// 2) Result of allocation routines are often base pointers. Leverage TLI.
return (isa<AllocaInst>(V) || isa<GlobalVariable>(V));
/// 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), UnderlyingV2 is the same for
/// V2.
AliasResult BasicAAResult::aliasGEP(
const GEPOperator *GEP1, LocationSize V1Size,
const Value *V2, LocationSize V2Size,
const Value *UnderlyingV1, const Value *UnderlyingV2, AAQueryInfo &AAQI) {
if (!V1Size.hasValue() && !V2Size.hasValue()) {
// TODO: This limitation exists for compile-time reasons. Relax it if we
// can avoid exponential pathological cases.
if (!isa<GEPOperator>(V2))
return AliasResult::MayAlias;
// If both accesses have unknown size, we can only check whether the base
// objects don't alias.
AliasResult BaseAlias = getBestAAResults().alias(
MemoryLocation::getBeforeOrAfter(UnderlyingV2), AAQI);
return BaseAlias == AliasResult::NoAlias ? AliasResult::NoAlias
: AliasResult::MayAlias;
DecomposedGEP DecompGEP1 = DecomposeGEPExpression(GEP1, DL, &AC, DT);
DecomposedGEP DecompGEP2 = DecomposeGEPExpression(V2, DL, &AC, DT);
// Bail if we were not able to decompose anything.
if (DecompGEP1.Base == GEP1 && DecompGEP2.Base == V2)
return AliasResult::MayAlias;
// Subtract the GEP2 pointer from the GEP1 pointer to find out their
// symbolic difference.
subtractDecomposedGEPs(DecompGEP1, DecompGEP2);
// If an inbounds GEP would have to start from an out of bounds address
// for the two to alias, then we can assume noalias.
if (*DecompGEP1.InBounds && DecompGEP1.VarIndices.empty() &&
V2Size.hasValue() && DecompGEP1.Offset.sge(V2Size.getValue()) &&
return AliasResult::NoAlias;
if (isa<GEPOperator>(V2)) {
// Symmetric case to above.
if (*DecompGEP2.InBounds && DecompGEP1.VarIndices.empty() &&
V1Size.hasValue() && DecompGEP1.Offset.sle(-V1Size.getValue()) &&
return AliasResult::NoAlias;
// For GEPs with identical offsets, we can preserve the size and AAInfo
// when performing the alias check on the underlying objects.
if (DecompGEP1.Offset == 0 && DecompGEP1.VarIndices.empty())
return getBestAAResults().alias(MemoryLocation(DecompGEP1.Base, V1Size),
MemoryLocation(DecompGEP2.Base, V2Size),
// Do the base pointers alias?
AliasResult BaseAlias = getBestAAResults().alias(
MemoryLocation::getBeforeOrAfter(DecompGEP2.Base), AAQI);
// If we get a No or May, then return it immediately, no amount of analysis
// will improve this situation.
if (BaseAlias != AliasResult::MustAlias) {
assert(BaseAlias == AliasResult::NoAlias ||
BaseAlias == AliasResult::MayAlias);
return BaseAlias;
// 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 (DecompGEP1.VarIndices.empty()) {
APInt &Off = DecompGEP1.Offset;
// Initialize for Off >= 0 (V2 <= GEP1) case.
const Value *LeftPtr = V2;
const Value *RightPtr = GEP1;
LocationSize VLeftSize = V2Size;
LocationSize VRightSize = V1Size;
const bool Swapped = Off.isNegative();
if (Swapped) {
// Swap if we have the situation where:
// + +
// | BaseOffset |
// ---------------->|
// |-->V1Size |-------> V2Size
// GEP1 V2
std::swap(LeftPtr, RightPtr);
std::swap(VLeftSize, VRightSize);
Off = -Off;
if (!VLeftSize.hasValue())
return AliasResult::MayAlias;
const uint64_t LSize = VLeftSize.getValue();
if (Off.ult(LSize)) {
// Conservatively drop processing if a phi was visited and/or offset is
// too big.
AliasResult AR = AliasResult::PartialAlias;
if (VRightSize.hasValue() && Off.ule(INT32_MAX) &&
(Off + VRightSize.getValue()).ule(LSize)) {
// Memory referenced by right pointer is nested. Save the offset in
// cache. Note that originally offset estimated as GEP1-V2, but
// AliasResult contains the shift that represents GEP1+Offset=V2.
return AR;
return AliasResult::NoAlias;
// We need to know both acess sizes for all the following heuristics.
if (!V1Size.hasValue() || !V2Size.hasValue())
return AliasResult::MayAlias;
ConstantRange OffsetRange = ConstantRange(DecompGEP1.Offset);
for (unsigned i = 0, e = DecompGEP1.VarIndices.size(); i != e; ++i) {
const VariableGEPIndex &Index = DecompGEP1.VarIndices[i];
const APInt &Scale = Index.Scale;
APInt ScaleForGCD = Scale;
if (!Index.IsNSW)
ScaleForGCD = APInt::getOneBitSet(Scale.getBitWidth(),
if (i == 0)
GCD = ScaleForGCD.abs();
GCD = APIntOps::GreatestCommonDivisor(GCD, ScaleForGCD.abs());
ConstantRange CR =
computeConstantRange(Index.Val.V, true, &AC, Index.CxtI);
KnownBits Known =
computeKnownBits(Index.Val.V, DL, 0, &AC, Index.CxtI, DT);
CR = CR.intersectWith(
ConstantRange::fromKnownBits(Known, /* Signed */ true),
CR = Index.Val.evaluateWith(CR).sextOrTrunc(OffsetRange.getBitWidth());
assert(OffsetRange.getBitWidth() == Scale.getBitWidth() &&
"Bit widths are normalized to MaxIndexSize");
if (Index.IsNSW)
OffsetRange = OffsetRange.add(CR.smul_sat(ConstantRange(Scale)));
OffsetRange = OffsetRange.add(CR.smul_fast(ConstantRange(Scale)));
// We now have accesses at two offsets from the same base:
// 1. (...)*GCD + DecompGEP1.Offset with size V1Size
// 2. 0 with size V2Size
// Using arithmetic modulo GCD, the accesses are at
// [ModOffset..ModOffset+V1Size) and [0..V2Size). If the first access fits
// into the range [V2Size..GCD), then we know they cannot overlap.
APInt ModOffset = DecompGEP1.Offset.srem(GCD);
if (ModOffset.isNegative())
ModOffset += GCD; // We want mod, not rem.
if (ModOffset.uge(V2Size.getValue()) &&
(GCD - ModOffset).uge(V1Size.getValue()))
return AliasResult::NoAlias;
// Compute ranges of potentially accessed bytes for both accesses. If the
// interseciton is empty, there can be no overlap.
unsigned BW = OffsetRange.getBitWidth();
ConstantRange Range1 = OffsetRange.add(
ConstantRange(APInt(BW, 0), APInt(BW, V1Size.getValue())));
ConstantRange Range2 =
ConstantRange(APInt(BW, 0), APInt(BW, V2Size.getValue()));
if (Range1.intersectWith(Range2).isEmptySet())
return AliasResult::NoAlias;
// Try to determine the range of values for VarIndex such that
// VarIndex <= -MinAbsVarIndex || MinAbsVarIndex <= VarIndex.
Optional<APInt> MinAbsVarIndex;
if (DecompGEP1.VarIndices.size() == 1) {
// VarIndex = Scale*V.
const VariableGEPIndex &Var = DecompGEP1.VarIndices[0];
if (Var.Val.TruncBits == 0 &&
isKnownNonZero(Var.Val.V, DL, 0, &AC, Var.CxtI, DT)) {
// If V != 0 then abs(VarIndex) >= abs(Scale).
MinAbsVarIndex = Var.Scale.abs();
} else if (DecompGEP1.VarIndices.size() == 2) {
// VarIndex = Scale*V0 + (-Scale)*V1.
// If V0 != V1 then abs(VarIndex) >= abs(Scale).
// Check that VisitedPhiBBs is empty, to avoid reasoning about
// inequality of values across loop iterations.
const VariableGEPIndex &Var0 = DecompGEP1.VarIndices[0];
const VariableGEPIndex &Var1 = DecompGEP1.VarIndices[1];
if (Var0.Scale == -Var1.Scale && Var0.Val.TruncBits == 0 &&
Var0.Val.hasSameCastsAs(Var1.Val) && VisitedPhiBBs.empty() &&
isKnownNonEqual(Var0.Val.V, Var1.Val.V, DL, &AC, /* CxtI */ nullptr,
MinAbsVarIndex = Var0.Scale.abs();
if (MinAbsVarIndex) {
// The constant offset will have added at least +/-MinAbsVarIndex to it.
APInt OffsetLo = DecompGEP1.Offset - *MinAbsVarIndex;
APInt OffsetHi = DecompGEP1.Offset + *MinAbsVarIndex;
// We know that Offset <= OffsetLo || Offset >= OffsetHi
if (OffsetLo.isNegative() && (-OffsetLo).uge(V1Size.getValue()) &&
OffsetHi.isNonNegative() && OffsetHi.uge(V2Size.getValue()))
return AliasResult::NoAlias;
if (constantOffsetHeuristic(DecompGEP1, V1Size, V2Size, &AC, DT))
return AliasResult::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 AliasResult::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 == AliasResult::PartialAlias && B == AliasResult::MustAlias) ||
(B == AliasResult::PartialAlias && A == AliasResult::MustAlias))
return AliasResult::PartialAlias;
// Otherwise, we don't know anything.
return AliasResult::MayAlias;
/// Provides a bunch of ad-hoc rules to disambiguate a Select instruction
/// against another.
BasicAAResult::aliasSelect(const SelectInst *SI, LocationSize SISize,
const Value *V2, LocationSize V2Size,
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 = getBestAAResults().alias(
MemoryLocation(SI->getTrueValue(), SISize),
MemoryLocation(SI2->getTrueValue(), V2Size), AAQI);
if (Alias == AliasResult::MayAlias)
return AliasResult::MayAlias;
AliasResult ThisAlias = getBestAAResults().alias(
MemoryLocation(SI->getFalseValue(), SISize),
MemoryLocation(SI2->getFalseValue(), V2Size), 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 = getBestAAResults().alias(
MemoryLocation(V2, V2Size),
MemoryLocation(SI->getTrueValue(), SISize), AAQI);
if (Alias == AliasResult::MayAlias)
return AliasResult::MayAlias;
AliasResult ThisAlias = getBestAAResults().alias(
MemoryLocation(V2, V2Size),
MemoryLocation(SI->getFalseValue(), SISize), AAQI);
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 Value *V2, LocationSize V2Size,
AAQueryInfo &AAQI) {
if (!PN->getNumIncomingValues())
return AliasResult::NoAlias;
// 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()) {
Optional<AliasResult> Alias;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
AliasResult ThisAlias = getBestAAResults().alias(
MemoryLocation(PN->getIncomingValue(i), PNSize),
PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), V2Size),
if (Alias)
*Alias = MergeAliasResults(*Alias, ThisAlias);
Alias = ThisAlias;
if (*Alias == AliasResult::MayAlias)
return *Alias;
SmallVector<Value *, 4> V1Srcs;
// If a phi operand recurses back to the phi, we can still determine NoAlias
// if we don't alias the underlying objects of the other phi operands, as we
// know that the recursive phi needs to be based on them in some way.
bool isRecursive = false;
auto CheckForRecPhi = [&](Value *PV) {
if (!EnableRecPhiAnalysis)
return false;
if (getUnderlyingObject(PV) == PN) {
isRecursive = true;
return true;
return 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 AliasResult::MayAlias;
// Add the values to V1Srcs
for (Value *PV1 : PhiValueSet) {
if (CheckForRecPhi(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;
Value *OnePhi = nullptr;
for (Value *PV1 : PN->incoming_values()) {
if (isa<PHINode>(PV1)) {
if (OnePhi && OnePhi != PV1) {
// To control potential compile time explosion, we choose to be
// conserviate when we have more than one Phi input. It is important
// that we handle the single phi case as that lets us handle LCSSA
// phi nodes and (combined with the recursive phi handling) simple
// pointer induction variable patterns.
return AliasResult::MayAlias;
OnePhi = PV1;
if (CheckForRecPhi(PV1))
if (UniqueSrc.insert(PV1).second)
if (OnePhi && UniqueSrc.size() > 1)
// Out of an abundance of caution, allow only the trivial lcssa and
// recursive phi cases.
return AliasResult::MayAlias;
// 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 AliasResult::MayAlias;
// If this PHI node is recursive, indicate that the pointer may be moved
// across iterations. We can only prove NoAlias if different underlying
// objects are involved.
if (isRecursive)
PNSize = LocationSize::beforeOrAfterPointer();
// In the recursive alias queries below, we may compare values from two
// different loop iterations. Keep track of visited phi blocks, which will
// be used when determining value equivalence.
bool BlockInserted = VisitedPhiBBs.insert(PN->getParent()).second;
auto _ = make_scope_exit([&]() {
if (BlockInserted)
// If we inserted a block into VisitedPhiBBs, alias analysis results that
// have been cached earlier may no longer be valid. Perform recursive queries
// with a new AAQueryInfo.
AAQueryInfo NewAAQI = AAQI.withEmptyCache();
AAQueryInfo *UseAAQI = BlockInserted ? &NewAAQI : &AAQI;
AliasResult Alias = getBestAAResults().alias(
MemoryLocation(V2, V2Size),
MemoryLocation(V1Srcs[0], PNSize), *UseAAQI);
// Early exit if the check of the first PHI source against V2 is MayAlias.
// Other results are not possible.
if (Alias == AliasResult::MayAlias)
return AliasResult::MayAlias;
// With recursive phis we cannot guarantee that MustAlias/PartialAlias will
// remain valid to all elements and needs to conservatively return MayAlias.
if (isRecursive && Alias != AliasResult::NoAlias)
return AliasResult::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 = getBestAAResults().alias(
MemoryLocation(V2, V2Size), MemoryLocation(V, PNSize), *UseAAQI);
Alias = MergeAliasResults(ThisAlias, Alias);
if (Alias == AliasResult::MayAlias)
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,
const Value *V2, LocationSize V2Size,
AAQueryInfo &AAQI) {
// If either of the memory references is empty, it doesn't matter what the
// pointer values are.
if (V1Size.isZero() || V2Size.isZero())
return AliasResult::NoAlias;
// Strip off any casts if they exist.
V1 = V1->stripPointerCastsForAliasAnalysis();
V2 = V2->stripPointerCastsForAliasAnalysis();
// 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 AliasResult::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 AliasResult::MustAlias;
if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
return AliasResult::NoAlias; // Scalars cannot alias each other
// Figure out what objects these things are pointing to if we can.
const Value *O1 = getUnderlyingObject(V1, MaxLookupSearchDepth);
const Value *O2 = getUnderlyingObject(V2, 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 AliasResult::NoAlias;
if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
if (!NullPointerIsDefined(&F, CPN->getType()->getAddressSpace()))
return AliasResult::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 AliasResult::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 AliasResult::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 AliasResult::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) &&
AAQI.CI->isNotCapturedBeforeOrAt(O2, cast<Instruction>(O1)))
return AliasResult::NoAlias;
if (isEscapeSource(O2) &&
AAQI.CI->isNotCapturedBeforeOrAt(O1, cast<Instruction>(O2)))
return AliasResult::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)) ||
O1, getMinimalExtentFrom(*V2, V2Size, DL, NullIsValidLocation), DL,
TLI, NullIsValidLocation)))
return AliasResult::NoAlias;
// If one the accesses may be before the accessed pointer, canonicalize this
// by using unknown after-pointer sizes for both accesses. This is
// equivalent, because regardless of which pointer is lower, one of them
// will always came after the other, as long as the underlying objects aren't
// disjoint. We do this so that the rest of BasicAA does not have to deal
// with accesses before the base pointer, and to improve cache utilization by
// merging equivalent states.
if (V1Size.mayBeBeforePointer() || V2Size.mayBeBeforePointer()) {
V1Size = LocationSize::afterPointer();
V2Size = LocationSize::afterPointer();
// FIXME: If this depth limit is hit, then we may cache sub-optimal results
// for recursive queries. For this reason, this limit is chosen to be large
// enough to be very rarely hit, while still being small enough to avoid
// stack overflows.
if (AAQI.Depth >= 512)
return AliasResult::MayAlias;
// Check the cache before climbing up use-def chains. This also terminates
// otherwise infinitely recursive queries.
AAQueryInfo::LocPair Locs({V1, V1Size}, {V2, V2Size});
const bool Swapped = V1 > V2;
if (Swapped)
std::swap(Locs.first, Locs.second);
const auto &Pair = AAQI.AliasCache.try_emplace(
Locs, AAQueryInfo::CacheEntry{AliasResult::NoAlias, 0});
if (!Pair.second) {
auto &Entry = Pair.first->second;
if (!Entry.isDefinitive()) {
// Remember that we used an assumption.
// Cache contains sorted {V1,V2} pairs but we should return original order.
auto Result = Entry.Result;
return Result;
int OrigNumAssumptionUses = AAQI.NumAssumptionUses;
unsigned OrigNumAssumptionBasedResults = AAQI.AssumptionBasedResults.size();
AliasResult Result =
aliasCheckRecursive(V1, V1Size, V2, V2Size, AAQI, O1, O2);
auto It = AAQI.AliasCache.find(Locs);
assert(It != AAQI.AliasCache.end() && "Must be in cache");
auto &Entry = It->second;
// Check whether a NoAlias assumption has been used, but disproven.
bool AssumptionDisproven =
Entry.NumAssumptionUses > 0 && Result != AliasResult::NoAlias;
if (AssumptionDisproven)
Result = AliasResult::MayAlias;
// This is a definitive result now, when considered as a root query.
AAQI.NumAssumptionUses -= Entry.NumAssumptionUses;
Entry.Result = Result;
// Cache contains sorted {V1,V2} pairs.
Entry.NumAssumptionUses = -1;
// If the assumption has been disproven, remove any results that may have
// been based on this assumption. Do this after the Entry updates above to
// avoid iterator invalidation.
if (AssumptionDisproven)
while (AAQI.AssumptionBasedResults.size() > OrigNumAssumptionBasedResults)
// The result may still be based on assumptions higher up in the chain.
// Remember it, so it can be purged from the cache later.
if (OrigNumAssumptionUses != AAQI.NumAssumptionUses &&
Result != AliasResult::MayAlias)
return Result;
AliasResult BasicAAResult::aliasCheckRecursive(
const Value *V1, LocationSize V1Size,
const Value *V2, LocationSize V2Size,
AAQueryInfo &AAQI, const Value *O1, const Value *O2) {
if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, O1, O2, AAQI);
if (Result != AliasResult::MayAlias)
return Result;
} else if (const GEPOperator *GV2 = dyn_cast<GEPOperator>(V2)) {
AliasResult Result = aliasGEP(GV2, V2Size, V1, V1Size, O2, O1, AAQI);
if (Result != AliasResult::MayAlias)
return Result;
if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
AliasResult Result = aliasPHI(PN, V1Size, V2, V2Size, AAQI);
if (Result != AliasResult::MayAlias)
return Result;
} else if (const PHINode *PN = dyn_cast<PHINode>(V2)) {
AliasResult Result = aliasPHI(PN, V2Size, V1, V1Size, AAQI);
if (Result != AliasResult::MayAlias)
return Result;
if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
AliasResult Result = aliasSelect(S1, V1Size, V2, V2Size, AAQI);
if (Result != AliasResult::MayAlias)
return Result;
} else if (const SelectInst *S2 = dyn_cast<SelectInst>(V2)) {
AliasResult Result = aliasSelect(S2, V2Size, V1, V1Size, AAQI);
if (Result != AliasResult::MayAlias)
return 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) {
bool NullIsValidLocation = NullPointerIsDefined(&F);
if (V1Size.isPrecise() && V2Size.isPrecise() &&
(isObjectSize(O1, V1Size.getValue(), DL, TLI, NullIsValidLocation) ||
isObjectSize(O2, V2Size.getValue(), DL, TLI, NullIsValidLocation)))
return AliasResult::PartialAlias;
return AliasResult::MayAlias;
/// 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))
return false;
return true;
/// Computes the symbolic difference between two de-composed GEPs.
void BasicAAResult::subtractDecomposedGEPs(DecomposedGEP &DestGEP,
const DecomposedGEP &SrcGEP) {
DestGEP.Offset -= SrcGEP.Offset;
for (const VariableGEPIndex &Src : SrcGEP.VarIndices) {
// Find V in Dest. This is N^2, but pointer indices almost never have more
// than a few variable indexes.
bool Found = false;
for (auto I : enumerate(DestGEP.VarIndices)) {
VariableGEPIndex &Dest = I.value();
if (!isValueEqualInPotentialCycles(Dest.Val.V, Src.Val.V) ||
// If we found it, subtract off Scale V's from the entry in Dest. If it
// goes to zero, remove the entry.
if (Dest.Scale != Src.Scale) {
Dest.Scale -= Src.Scale;
Dest.IsNSW = false;
} else {
DestGEP.VarIndices.erase(DestGEP.VarIndices.begin() + I.index());
Found = true;
// If we didn't consume this entry, add it to the end of the Dest list.
if (!Found) {
VariableGEPIndex Entry = {Src.Val, -Src.Scale, Src.CxtI, Src.IsNSW};
bool BasicAAResult::constantOffsetHeuristic(
const DecomposedGEP &GEP, LocationSize MaybeV1Size,
LocationSize MaybeV2Size, AssumptionCache *AC, DominatorTree *DT) {
if (GEP.VarIndices.size() != 2 || !MaybeV1Size.hasValue() ||
return false;
const uint64_t V1Size = MaybeV1Size.getValue();
const uint64_t V2Size = MaybeV2Size.getValue();
const VariableGEPIndex &Var0 = GEP.VarIndices[0], &Var1 = GEP.VarIndices[1];
if (Var0.Val.TruncBits != 0 || !Var0.Val.hasSameCastsAs(Var1.Val) ||
Var0.Scale != -Var1.Scale ||
Var0.Val.V->getType() != Var1.Val.V->getType())
return false;
// 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.
LinearExpression E0 =
GetLinearExpression(CastedValue(Var0.Val.V), DL, 0, AC, DT);
LinearExpression E1 =
GetLinearExpression(CastedValue(Var1.Val.V), DL, 0, AC, DT);
if (E0.Scale != E1.Scale || !E0.Val.hasSameCastsAs(E1.Val) ||
!isValueEqualInPotentialCycles(E0.Val.V, E1.Val.V))
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 = E0.Offset - E1.Offset, 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 + GEP.Offset.abs()) &&
MinDiffBytes.uge(V2Size + GEP.Offset.abs());
// BasicAliasAnalysis Pass
AnalysisKey BasicAA::Key;
BasicAAResult BasicAA::run(Function &F, FunctionAnalysisManager &AM) {
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
auto &AC = AM.getResult<AssumptionAnalysis>(F);
auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
auto *PV = AM.getCachedResult<PhiValuesAnalysis>(F);
return BasicAAResult(F.getParent()->getDataLayout(), F, TLI, AC, DT, PV);
BasicAAWrapperPass::BasicAAWrapperPass() : FunctionPass(ID) {
char BasicAAWrapperPass::ID = 0;
void BasicAAWrapperPass::anchor() {}
INITIALIZE_PASS_BEGIN(BasicAAWrapperPass, "basic-aa",
"Basic Alias Analysis (stateless AA impl)", true, true)
INITIALIZE_PASS_END(BasicAAWrapperPass, "basic-aa",
"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 *PVWP = getAnalysisIfAvailable<PhiValuesWrapperPass>();
Result.reset(new BasicAAResult(F.getParent()->getDataLayout(), F,
TLIWP.getTLI(F), ACT.getAssumptionCache(F),
PVWP ? &PVWP->getResult() : nullptr));
return false;
void BasicAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
BasicAAResult llvm::createLegacyPMBasicAAResult(Pass &P, Function &F) {
return BasicAAResult(
F.getParent()->getDataLayout(), F,