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//===- BasicAliasAnalysis.cpp - Stateless Alias Analysis Impl -------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// 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/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/CallSite.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/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,
/// 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 &F, 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>(F, PA) ||
(DT && Inv.invalidate<DominatorTreeAnalysis>(F, PA)) ||
(LI && Inv.invalidate<LoopAnalysis>(F, 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) {
// 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.
return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
// 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.
return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
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<CallInst>(V) || isa<InvokeInst>(V) || 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 RoundToAlign = false) {
uint64_t Size;
ObjectSizeOpts Opts;
Opts.RoundToAlign = RoundToAlign;
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) {
// 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, /*RoundToAlign*/ true);
return ObjectSize != MemoryLocation::UnknownSize && ObjectSize < Size;
/// 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) {
uint64_t ObjectSize = getObjectSize(V, DL, TLI);
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()) {
// 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;
case Instruction::Add:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset += RHS;
case Instruction::Sub:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset -= RHS;
case Instruction::Mul:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset *= RHS;
Scale *= RHS;
case Instruction::Shl:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
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 64. This is an issue in
/// particular for 32b programs with negative indices that rely on two's
/// complement wrap-arounds for precise alias information.
static int64_t adjustToPointerSize(int64_t Offset, unsigned PointerSize) {
assert(PointerSize <= 64 && "Invalid PointerSize!");
unsigned ShiftBits = 64 - PointerSize;
return (int64_t)((uint64_t)Offset << ShiftBits) >> ShiftBits;
/// 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;
Decomposed.StructOffset = 0;
Decomposed.OtherOffset = 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 false;
if (Op->getOpcode() == Instruction::BitCast ||
Op->getOpcode() == Instruction::AddrSpaceCast) {
V = Op->getOperand(0);
const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
if (!GEPOp) {
if (auto CS = ImmutableCallSite(V))
if (const Value *RV = CS.getReturnedArgOperand()) {
V = RV;
// 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;
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)
Decomposed.StructOffset +=
// For an array/pointer, add the element offset, explicitly scaled.
if (const ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
if (CIdx->isZero())
Decomposed.OtherOffset +=
DL.getTypeAllocSize(GTI.getIndexedType()) * CIdx->getSExtValue();
GepHasConstantOffset = false;
uint64_t Scale = 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;
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.
Decomposed.OtherOffset += IndexOffset.getSExtValue() * Scale;
Scale *= IndexScale.getSExtValue();
// 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);
// 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,
// 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;
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,
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(), DL);
if (!Visited.insert(V).second) {
return AAResultBase::pointsToConstantMemory(Loc, 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, 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, OrLocal);
for (Value *IncValue : PN->incoming_values())
// Otherwise be conservative.
return AAResultBase::pointsToConstantMemory(Loc, OrLocal);
} while (!Worklist.empty() && --MaxLookup);
return Worklist.empty();
/// Returns the behavior when calling the given call site.
FunctionModRefBehavior BasicAAResult::getModRefBehavior(ImmutableCallSite CS) {
if (CS.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 (CS.onlyReadsMemory())
Min = FMRB_OnlyReadsMemory;
else if (CS.doesNotReadMemory())
Min = FMRB_DoesNotReadMemory;
if (CS.onlyAccessesArgMemory())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesArgumentPointees);
else if (CS.onlyAccessesInaccessibleMemory())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesInaccessibleMem);
else if (CS.onlyAccessesInaccessibleMemOrArgMem())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesInaccessibleOrArgMem);
// If CS has operand bundles then aliasing attributes from the function it
// calls do not directly apply to the CallSite. This can be made more
// precise in the future.
if (!CS.hasOperandBundles())
if (const Function *F = CS.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_DoesNotReadMemory;
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(ImmutableCallSite CS, unsigned ArgIdx,
const TargetLibraryInfo &TLI) {
if (CS.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 (CS.getCalledFunction() && TLI.getLibFunc(*CS.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(ImmutableCallSite CS,
unsigned ArgIdx) {
// Checking for known builtin intrinsics and target library functions.
if (isWriteOnlyParam(CS, ArgIdx, TLI))
return ModRefInfo::Mod;
if (CS.paramHasAttr(ArgIdx, Attribute::ReadOnly))
return ModRefInfo::Ref;
if (CS.paramHasAttr(ArgIdx, Attribute::ReadNone))
return ModRefInfo::NoModRef;
return AAResultBase::getArgModRefInfo(CS, ArgIdx);
static bool isIntrinsicCall(ImmutableCallSite CS, Intrinsic::ID IID) {
const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
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;
AliasResult BasicAAResult::alias(const MemoryLocation &LocA,
const MemoryLocation &LocB) {
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 = AliasCache.find(LocPair(LocA, LocB));
if (CacheIt != AliasCache.end())
return CacheIt->second;
AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.AATags, LocB.Ptr,
LocB.Size, LocB.AATags);
// AliasCache rarely has more than 1 or 2 elements, always use
// shrink_and_clear so it quickly returns to the inline capacity of the
// SmallDenseMap if it ever grows larger.
// FIXME: This should really be shrink_to_inline_capacity_and_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(ImmutableCallSite CS,
const MemoryLocation &Loc) {
assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
"AliasAnalysis query involving multiple functions!");
const Value *Object = GetUnderlyingObject(Loc.Ptr, DL);
// If this is a tail call and Loc.Ptr points to a stack location, we know that
// the tail call cannot access or modify the local stack.
// We cannot exclude byval arguments here; these belong to the caller of
// the current function not to the current function, and a tail callee
// may reference them.
if (isa<AllocaInst>(Object))
if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
if (CI->isTailCall())
return ModRefInfo::NoModRef;
// 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) && CS.getInstruction() != Object &&
isNonEscapingLocalObject(Object)) {
// 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 = CS.data_operands_begin(), CE = CS.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() ||
(!CS.doesNotCapture(OperandNo) &&
OperandNo < CS.getNumArgOperands() && !CS.isByValArgument(OperandNo)))
// Call doesn't access memory through this operand, so we don't care
// if it aliases with Object.
if (CS.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(*CI), MemoryLocation(Object));
if (AR != MustAlias)
IsMustAlias = false;
// Operand doesnt alias 'Object', continue looking for other aliases
if (AR == NoAlias)
// Operand aliases 'Object', but call doesn't modify it. Strengthen
// initial assumption and keep looking in case if there are more aliases.
if (CS.onlyReadsMemory(OperandNo)) {
Result = setRef(Result);
// Operand aliases 'Object' but call only writes into it.
if (CS.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))
return IsMustAlias ? setMust(Result) : clearMust(Result);
// If the CallSite 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.
auto *Inst = CS.getInstruction();
if (isMallocOrCallocLikeFn(Inst, &TLI)) {
// Be conservative if the accessed pointer may alias the allocation -
// fallback to the generic handling below.
if (getBestAAResults().alias(MemoryLocation(Inst), Loc) == 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<MemCpyInst>(CS.getInstruction())) {
AliasResult SrcAA, DestAA;
if ((SrcAA = getBestAAResults().alias(MemoryLocation::getForSource(Inst),
Loc)) == MustAlias)
// Loc is exactly the memcpy source thus disjoint from memcpy dest.
return ModRefInfo::Ref;
if ((DestAA = getBestAAResults().alias(MemoryLocation::getForDest(Inst),
Loc)) == 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(CS, 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(CS, 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(CS, Intrinsic::invariant_start))
return ModRefInfo::Ref;
// The AAResultBase base class has some smarts, lets use them.
return AAResultBase::getModRefInfo(CS, Loc);
ModRefInfo BasicAAResult::getModRefInfo(ImmutableCallSite CS1,
ImmutableCallSite CS2) {
// 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(CS1, Intrinsic::assume) ||
isIntrinsicCall(CS2, 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 specical case two
// possibilities for guard intrinsics.
if (isIntrinsicCall(CS1, Intrinsic::experimental_guard))
return isModSet(createModRefInfo(getModRefBehavior(CS2)))
? ModRefInfo::Ref
: ModRefInfo::NoModRef;
if (isIntrinsicCall(CS2, Intrinsic::experimental_guard))
return isModSet(createModRefInfo(getModRefBehavior(CS1)))
? ModRefInfo::Mod
: ModRefInfo::NoModRef;
// The AAResultBase base class has some smarts, lets use them.
return AAResultBase::getModRefInfo(CS1, CS2);
/// Provide ad-hoc rules to disambiguate accesses through two GEP operators,
/// both having the exact same pointer operand.
static AliasResult aliasSameBasePointerGEPs(const GEPOperator *GEP1,
uint64_t V1Size,
const GEPOperator *GEP2,
uint64_t V2Size,
const DataLayout &DL) {
assert(GEP1->getPointerOperand()->stripPointerCastsAndBarriers() ==
GEP2->getPointerOperand()->stripPointerCastsAndBarriers() &&
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 (V1Size == MemoryLocation::UnknownSize ||
V2Size == MemoryLocation::UnknownSize)
return MayAlias;
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 && C1->getSExtValue() == C2->getSExtValue())
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");
// 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 =
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) ||
return NoAlias;
} else if (isKnownNonEqual(GEP1LastIdx, GEP2LastIdx, DL))
return NoAlias;
return MayAlias;
} else if (!LastIndexedStruct || !C1 || !C2) {
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
// %random = call* @random(* %alloca)
// %f0 = getelementptr inbounds %struct,* %alloca, i32 0, i32 0
// %f1 = getelementptr inbounds %struct,* %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,
uint64_t ObjectAccessSize) {
// If the object access size is unknown, or the GEP isn't inbounds, bail.
if (ObjectAccessSize == MemoryLocation::UnknownSize || !GEPOp->isInBounds())
return false;
// 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)) ||
return false;
int64_t ObjectBaseOffset = DecompObject.StructOffset +
// 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're in
// a bit more trouble: we can't count on the constant offsets that come
// from non-struct sources, since these can be "rewound" by a negative
// variable offset. So use only offsets that came from structs.
int64_t GEPBaseOffset = DecompGEP.StructOffset;
if (DecompGEP.VarIndices.empty())
GEPBaseOffset += DecompGEP.OtherOffset;
return (GEPBaseOffset >= 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, uint64_t V1Size,
const AAMDNodes &V1AAInfo, const Value *V2,
uint64_t V2Size, const AAMDNodes &V2AAInfo,
const Value *UnderlyingV1,
const Value *UnderlyingV2) {
DecomposedGEP DecompGEP1, DecompGEP2;
bool GEP1MaxLookupReached =
DecomposeGEPExpression(GEP1, DecompGEP1, DL, &AC, DT);
bool GEP2MaxLookupReached =
DecomposeGEPExpression(V2, DecompGEP2, DL, &AC, DT);
int64_t GEP1BaseOffset = DecompGEP1.StructOffset + DecompGEP1.OtherOffset;
int64_t GEP2BaseOffset = DecompGEP2.StructOffset + DecompGEP2.OtherOffset;
assert(DecompGEP1.Base == UnderlyingV1 && DecompGEP2.Base == UnderlyingV2 &&
"DecomposeGEPExpression returned a result different from "
// 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, MemoryLocation::UnknownSize, AAMDNodes(),
UnderlyingV2, MemoryLocation::UnknownSize, AAMDNodes());
// 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);
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()->stripPointerCastsAndBarriers() ==
GEP2->getPointerOperand()->stripPointerCastsAndBarriers() &&
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 == MemoryLocation::UnknownSize &&
V2Size == MemoryLocation::UnknownSize)
return MayAlias;
AliasResult R = aliasCheck(UnderlyingV1, MemoryLocation::UnknownSize,
AAMDNodes(), V2, MemoryLocation::UnknownSize,
V2AAInfo, 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 >= 0) {
if (V2Size != MemoryLocation::UnknownSize) {
if ((uint64_t)GEP1BaseOffset < V2Size)
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 != MemoryLocation::UnknownSize &&
V2Size != MemoryLocation::UnknownSize) {
if (-(uint64_t)GEP1BaseOffset < V1Size)
return PartialAlias;
return NoAlias;
if (!DecompGEP1.VarIndices.empty()) {
uint64_t Modulo = 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 |= (uint64_t)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, nullptr, 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.
int64_t Scale = DecompGEP1.VarIndices[i].Scale;
AllPositive =
(SignKnownZero && Scale >= 0) || (SignKnownOne && Scale < 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.
uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1);
if (V1Size != MemoryLocation::UnknownSize &&
V2Size != MemoryLocation::UnknownSize && ModOffset >= V2Size &&
V1Size <= Modulo - ModOffset)
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 > 0 && V2Size <= (uint64_t)GEP1BaseOffset)
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, uint64_t SISize,
const AAMDNodes &SIAAInfo,
const Value *V2, uint64_t V2Size,
const AAMDNodes &V2AAInfo,
const Value *UnderV2) {
// 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);
if (Alias == MayAlias)
return MayAlias;
AliasResult ThisAlias =
aliasCheck(SI->getFalseValue(), SISize, SIAAInfo,
SI2->getFalseValue(), V2Size, V2AAInfo);
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, UnderV2);
if (Alias == MayAlias)
return MayAlias;
AliasResult ThisAlias =
aliasCheck(V2, V2Size, V2AAInfo, SI->getFalseValue(), SISize, SIAAInfo,
return MergeAliasResults(ThisAlias, Alias);
/// Provide a bunch of ad-hoc rules to disambiguate a PHI instruction against
/// another.
AliasResult BasicAAResult::aliasPHI(const PHINode *PN, uint64_t PNSize,
const AAMDNodes &PNAAInfo, const Value *V2,
uint64_t V2Size, const AAMDNodes &V2AAInfo,
const Value *UnderV2) {
// Track phi nodes we have visited. We use this information when we determine
// value equivalence.
// 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()) {
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;
assert(AliasCache.count(Locs) &&
"There must exist an entry for the phi node");
AliasResult OrigAliasResult = AliasCache[Locs];
AliasCache[Locs] = NoAlias;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
AliasResult ThisAlias =
aliasCheck(PN->getIncomingValue(i), PNSize, PNAAInfo,
V2Size, V2AAInfo);
Alias = MergeAliasResults(ThisAlias, Alias);
if (Alias == MayAlias)
// Reset if speculation failed.
if (Alias != NoAlias)
AliasCache[Locs] = OrigAliasResult;
return Alias;
SmallPtrSet<Value *, 4> UniqueSrc;
SmallVector<Value *, 4> V1Srcs;
bool isRecursive = false;
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;
if (UniqueSrc.insert(PV1).second)
// 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 = MemoryLocation::UnknownSize;
AliasResult Alias =
aliasCheck(V2, V2Size, V2AAInfo, V1Srcs[0],
PNSize, PNAAInfo, 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, UnderV2);
Alias = MergeAliasResults(ThisAlias, Alias);
if (Alias == 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, uint64_t V1Size,
AAMDNodes V1AAInfo, const Value *V2,
uint64_t V2Size, AAMDNodes V2AAInfo,
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 == 0 || V2Size == 0)
return NoAlias;
// Strip off any casts if they exist.
V1 = V1->stripPointerCastsAndBarriers();
V2 = V2->stripPointerCastsAndBarriers();
// 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 (CPN->getType()->getAddressSpace() == 0)
return NoAlias;
if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
if (CPN->getType()->getAddressSpace() == 0)
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))
return NoAlias;
if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
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.
if ((V1Size != MemoryLocation::UnknownSize &&
isObjectSmallerThan(O2, V1Size, DL, TLI)) ||
(V2Size != MemoryLocation::UnknownSize &&
isObjectSmallerThan(O1, V2Size, DL, TLI)))
return NoAlias;
// Check the cache before climbing up use-def chains. This also terminates
// otherwise infinitely recursive queries.
LocPair Locs(MemoryLocation(V1, V1Size, V1AAInfo),
MemoryLocation(V2, V2Size, V2AAInfo));
if (V1 > V2)
std::swap(Locs.first, Locs.second);
std::pair<AliasCacheTy::iterator, bool> Pair =
AliasCache.insert(std::make_pair(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);
if (Result != MayAlias)
return AliasCache[Locs] = 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);
if (Result != MayAlias)
return AliasCache[Locs] = 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);
if (Result != MayAlias)
return AliasCache[Locs] = 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 != MemoryLocation::UnknownSize &&
V2Size != MemoryLocation::UnknownSize &&
(isObjectSize(O1, V1Size, DL, TLI) ||
isObjectSize(O2, V2Size, DL, TLI)))
return AliasCache[Locs] = 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);
return AliasCache[Locs] = 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, 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())
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;
int64_t 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)
// 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;
Dest.erase(Dest.begin() + j);
Scale = 0;
// If we didn't consume this entry, add it to the end of the Dest list.
if (Scale) {
VariableGEPIndex Entry = {V, ZExtBits, SExtBits, -Scale};
bool BasicAAResult::constantOffsetHeuristic(
const SmallVectorImpl<VariableGEPIndex> &VarIndices, uint64_t V1Size,
uint64_t V2Size, int64_t BaseOffset, AssumptionCache *AC,
DominatorTree *DT) {
if (VarIndices.size() != 2 || V1Size == MemoryLocation::UnknownSize ||
V2Size == MemoryLocation::UnknownSize)
return false;
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);
uint64_t MinDiffBytes = MinDiff.getZExtValue() * std::abs(Var0.Scale);
// 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 V1Size + std::abs(BaseOffset) <= MinDiffBytes &&
V2Size + std::abs(BaseOffset) <= MinDiffBytes;
// BasicAliasAnalysis Pass
AnalysisKey BasicAA::Key;
BasicAAResult BasicAA::run(Function &F, FunctionAnalysisManager &AM) {
return BasicAAResult(F.getParent()->getDataLayout(),
BasicAAWrapperPass::BasicAAWrapperPass() : FunctionPass(ID) {
char BasicAAWrapperPass::ID = 0;
void BasicAAWrapperPass::anchor() {}
INITIALIZE_PASS_BEGIN(BasicAAWrapperPass, "basicaa",
"Basic Alias Analysis (stateless AA impl)", true, true)
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>();
Result.reset(new BasicAAResult(F.getParent()->getDataLayout(), TLIWP.getTLI(),
ACT.getAssumptionCache(F), &DTWP.getDomTree(),
LIWP ? &LIWP->getLoopInfo() : nullptr));
return false;
void BasicAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
BasicAAResult llvm::createLegacyPMBasicAAResult(Pass &P, Function &F) {
return BasicAAResult(