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//===- llvm/Analysis/BasicAliasAnalysis.h - Alias Analysis Impl -*- C++ -*-===//
// The LLVM Compiler Infrastructure
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
// This file defines the default implementation of the Alias Analysis interface
// that simply implements a few identities (two different globals cannot alias,
// etc), but otherwise does no analysis.
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Pass.h"
#include "llvm/Argument.h"
#include "llvm/iMemory.h"
#include "llvm/iOther.h"
#include "llvm/ConstantHandling.h"
#include "llvm/GlobalValue.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Target/TargetData.h"
// Make sure that anything that uses AliasAnalysis pulls in this file...
void BasicAAStub() {}
namespace {
struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
virtual void initializePass();
// alias - This is the only method here that does anything interesting...
AliasResult alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size);
// CheckGEPInstructions - Check two GEP instructions of compatible types and
// equal number of arguments. This checks to see if the index expressions
// preclude the pointers from aliasing...
AliasResult CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1Size,
GetElementPtrInst *GEP2, unsigned G2Size);
// Register this pass...
X("basicaa", "Basic Alias Analysis (default AA impl)");
// Declare that we implement the AliasAnalysis interface
RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y;
} // End of anonymous namespace
void BasicAliasAnalysis::initializePass() {
// hasUniqueAddress - Return true if the specified value points to something
// with a unique, discernable, address.
static inline bool hasUniqueAddress(const Value *V) {
return isa<GlobalValue>(V) || isa<AllocationInst>(V);
// getUnderlyingObject - This traverses the use chain to figure out what object
// the specified value points to. If the value points to, or is derived from, a
// unique object or an argument, return it.
static const Value *getUnderlyingObject(const Value *V) {
if (!isa<PointerType>(V->getType())) return 0;
// If we are at some type of object... return it.
if (hasUniqueAddress(V) || isa<Argument>(V)) return V;
// Traverse through different addressing mechanisms...
if (const Instruction *I = dyn_cast<Instruction>(V)) {
if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
return getUnderlyingObject(I->getOperand(0));
} else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
if (CE->getOpcode() == Instruction::Cast ||
CE->getOpcode() == Instruction::GetElementPtr)
return getUnderlyingObject(CE->getOperand(0));
} else if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V)) {
return CPR->getValue();
return 0;
// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
// as array references. Note that this function is heavily tail recursive.
// Hopefully we have a smart C++ compiler. :)
BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size) {
// Strip off constant pointer refs if they exist
if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
V1 = CPR->getValue();
if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
V2 = CPR->getValue();
// Are we checking for alias of the same value?
if (V1 == V2) return MustAlias;
if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
return NoAlias; // Scalars cannot alias each other
// Strip off cast instructions...
if (const Instruction *I = dyn_cast<CastInst>(V1))
return alias(I->getOperand(0), V1Size, V2, V2Size);
if (const Instruction *I = dyn_cast<CastInst>(V2))
return alias(V1, V1Size, I->getOperand(0), V2Size);
// Figure out what objects these things are pointing to if we can...
const Value *O1 = getUnderlyingObject(V1);
const Value *O2 = getUnderlyingObject(V2);
// Pointing at a discernible object?
if (O1 && O2) {
if (isa<Argument>(O1)) {
// Incoming argument cannot alias locally allocated object!
if (isa<AllocationInst>(O2)) return NoAlias;
// Otherwise, nothing is known...
} else if (isa<Argument>(O2)) {
// Incoming argument cannot alias locally allocated object!
if (isa<AllocationInst>(O1)) return NoAlias;
// Otherwise, nothing is known...
} else {
// If they are two different objects, we know that we have no alias...
if (O1 != O2) return NoAlias;
// If they are the same object, they we can look at the indexes. If they
// index off of the object is the same for both pointers, they must alias.
// If they are provably different, they must not alias. Otherwise, we can't
// tell anything.
} else if (O1 && !isa<Argument>(O1) && isa<ConstantPointerNull>(V2)) {
return NoAlias; // Unique values don't alias null
} else if (O2 && !isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) {
return NoAlias; // Unique values don't alias null
// If we have two gep instructions with identical indices, return an alias
// result equal to the alias result of the original pointer...
if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(V1))
if (const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(V2))
if (GEP1->getNumOperands() == GEP2->getNumOperands() &&
GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType()) {
AliasResult GAlias =
CheckGEPInstructions((GetElementPtrInst*)GEP1, V1Size,
(GetElementPtrInst*)GEP2, V2Size);
if (GAlias != MayAlias)
return GAlias;
// Check to see if these two pointers are related by a getelementptr
// instruction. If one pointer is a GEP with a non-zero index of the other
// pointer, we know they cannot alias.
if (isa<GetElementPtrInst>(V2)) {
std::swap(V1, V2);
std::swap(V1Size, V2Size);
if (V1Size != ~0U && V2Size != ~0U)
if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V1)) {
AliasResult R = alias(GEP->getOperand(0), V1Size, V2, V2Size);
if (R == MustAlias) {
// If there is at least one non-zero constant index, we know they cannot
// alias.
bool ConstantFound = false;
for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i)))
if (!C->isNullValue()) {
ConstantFound = true;
if (ConstantFound) {
if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
return NoAlias;
// Otherwise we have to check to see that the distance is more than
// the size of the argument... build an index vector that is equal to
// the arguments provided, except substitute 0's for any variable
// indexes we find...
std::vector<Value*> Indices;
for (unsigned i = 1; i != GEP->getNumOperands(); ++i)
if (const Constant *C = dyn_cast<Constant>(GEP->getOperand(i)))
const Type *Ty = GEP->getOperand(0)->getType();
int Offset = getTargetData().getIndexedOffset(Ty, Indices);
if (Offset >= (int)V2Size || Offset <= -(int)V1Size)
return NoAlias;
return MayAlias;
static Value *CheckArrayIndicesForOverflow(const Type *PtrTy,
const std::vector<Value*> &Indices,
const ConstantInt *Idx) {
if (const ConstantSInt *IdxS = dyn_cast<ConstantSInt>(Idx)) {
if (IdxS->getValue() < 0) // Underflow on the array subscript?
return Constant::getNullValue(Type::LongTy);
else { // Check for overflow
const ArrayType *ATy =
cast<ArrayType>(GetElementPtrInst::getIndexedType(PtrTy, Indices,true));
if (IdxS->getValue() >= (int64_t)ATy->getNumElements())
return ConstantSInt::get(Type::LongTy, ATy->getNumElements()-1);
return (Value*)Idx; // Everything is acceptable.
// CheckGEPInstructions - Check two GEP instructions of compatible types and
// equal number of arguments. This checks to see if the index expressions
// preclude the pointers from aliasing...
BasicAliasAnalysis::CheckGEPInstructions(GetElementPtrInst *GEP1, unsigned G1S,
GetElementPtrInst *GEP2, unsigned G2S){
// Do the base pointers alias?
AliasResult BaseAlias = alias(GEP1->getOperand(0), G1S,
GEP2->getOperand(0), G2S);
if (BaseAlias != MustAlias) // No or May alias: We cannot add anything...
return BaseAlias;
// Find the (possibly empty) initial sequence of equal values...
unsigned NumGEPOperands = GEP1->getNumOperands();
unsigned UnequalOper = 1;
while (UnequalOper != NumGEPOperands &&
GEP1->getOperand(UnequalOper) == GEP2->getOperand(UnequalOper))
// If all operands equal each other, then the derived pointers must
// alias each other...
if (UnequalOper == NumGEPOperands) return MustAlias;
// So now we know that the indexes derived from the base pointers,
// which are known to alias, are different. We can still determine a
// no-alias result if there are differing constant pairs in the index
// chain. For example:
// A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
unsigned SizeMax = std::max(G1S, G2S);
if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work...
// Scan for the first operand that is constant and unequal in the
// two getelemenptrs...
unsigned FirstConstantOper = UnequalOper;
for (; FirstConstantOper != NumGEPOperands; ++FirstConstantOper) {
const Value *G1Oper = GEP1->getOperand(FirstConstantOper);
const Value *G2Oper = GEP2->getOperand(FirstConstantOper);
if (G1Oper != G2Oper && // Found non-equal constant indexes...
isa<Constant>(G1Oper) && isa<Constant>(G2Oper)) {
// Make sure they are comparable... and make sure the GEP with
// the smaller leading constant is GEP1.
ConstantBool *Compare =
*cast<Constant>(GEP1->getOperand(FirstConstantOper)) >
if (Compare) { // If they are comparable...
if (Compare->getValue())
std::swap(GEP1, GEP2); // Make GEP1 < GEP2
// No constant operands, we cannot tell anything...
if (FirstConstantOper == NumGEPOperands) return MayAlias;
// If there are non-equal constants arguments, then we can figure
// out a minimum known delta between the two index expressions... at
// this point we know that the first constant index of GEP1 is less
// than the first constant index of GEP2.
std::vector<Value*> Indices1;
for (unsigned i = 1; i != FirstConstantOper; ++i)
if (GEP1->getOperand(i)->getType() == Type::UByteTy)
std::vector<Value*> Indices2;
Indices2 = Indices1; // Copy the zeros prefix...
// Add the two known constant operands...
const Type *GEPPointerTy = GEP1->getOperand(0)->getType();
// Loop over the rest of the operands...
for (unsigned i = FirstConstantOper+1; i != NumGEPOperands; ++i) {
const Value *Op1 = GEP1->getOperand(i);
const Value *Op2 = GEP2->getOperand(i);
if (Op1 == Op2) { // If they are equal, use a zero index...
if (!isa<Constant>(Op1)) {
} else {
} else {
if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
// If this is an array index, make sure the array element is in range...
if (i != 1) // The pointer index can be "out of range"
Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices1, Op1C);
} else {
// GEP1 is known to produce a value less than GEP2. To be
// conservatively correct, we must assume the largest possible constant
// is used in this position. This cannot be the initial index to the
// GEP instructions (because we know we have at least one element before
// this one with the different constant arguments), so we know that the
// current index must be into either a struct or array. Because we know
// it's not constant, this cannot be a structure index. Because of
// this, we can calculate the maximum value possible.
const ArrayType *ElTy =
cast<ArrayType>(GEP1->getIndexedType(GEPPointerTy, Indices1, true));
if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op2)) {
// If this is an array index, make sure the array element is in range...
if (i != 1) // The pointer index can be "out of range"
Op1 = CheckArrayIndicesForOverflow(GEPPointerTy, Indices2, Op1C);
else // Conservatively assume the minimum value for this index
int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, Indices1);
int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, Indices2);
assert(Offset1 < Offset2 &&"There is at least one different constant here!");
if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
//std::cerr << "Determined that these two GEP's don't alias ["
// << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
return NoAlias;
return MayAlias;