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//===- BasicAliasAnalysis.cpp - Local 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 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/Analysis/CaptureTracking.h"
#include "llvm/Analysis/Passes.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include <algorithm>
using namespace llvm;
// Useful predicates
static const GEPOperator *isGEP(const Value *V) {
return dyn_cast<GEPOperator>(V);
static const Value *GetGEPOperands(const Value *V,
SmallVector<Value*, 16> &GEPOps) {
assert(GEPOps.empty() && "Expect empty list to populate!");
GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
// Accumulate all of the chained indexes into the operand array
V = cast<User>(V)->getOperand(0);
while (const User *G = isGEP(V)) {
if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
break; // Don't handle folding arbitrary pointer offsets yet...
GEPOps.erase(GEPOps.begin()); // Drop the zero index
GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
V = G->getOperand(0);
return V;
/// isKnownNonNull - Return true if we know that the specified value is never
/// null.
static bool isKnownNonNull(const Value *V) {
// Alloca never returns null, malloc might.
if (isa<AllocaInst>(V)) return true;
// A byval argument is never null.
if (const Argument *A = dyn_cast<Argument>(V))
return A->hasByValAttr();
// Global values are not null unless extern weak.
if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
return !GV->hasExternalWeakLinkage();
return false;
/// isNonEscapingLocalObject - Return 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<AllocationInst>(V) || isNoAliasCall(V))
return !PointerMayBeCaptured(V, false);
// 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()) {
// Don't bother analyzing arguments already known not to escape.
if (A->hasNoCaptureAttr())
return true;
return !PointerMayBeCaptured(V, false);
return false;
/// isObjectSmallerThan - Return true if we can prove that the object specified
/// by V is smaller than Size.
static bool isObjectSmallerThan(const Value *V, unsigned Size,
const TargetData &TD) {
const Type *AccessTy;
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
AccessTy = GV->getType()->getElementType();
} else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) {
if (!AI->isArrayAllocation())
AccessTy = AI->getType()->getElementType();
return false;
} else if (const Argument *A = dyn_cast<Argument>(V)) {
if (A->hasByValAttr())
AccessTy = cast<PointerType>(A->getType())->getElementType();
return false;
} else {
return false;
if (AccessTy->isSized())
return TD.getTypeAllocSize(AccessTy) < Size;
return false;
// NoAA Pass
namespace {
/// NoAA - This class implements the -no-aa pass, which always returns "I
/// don't know" for alias queries. NoAA is unlike other alias analysis
/// implementations, in that it does not chain to a previous analysis. As
/// such it doesn't follow many of the rules that other alias analyses must.
struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis {
static char ID; // Class identification, replacement for typeinfo
NoAA() : ImmutablePass(&ID) {}
explicit NoAA(void *PID) : ImmutablePass(PID) { }
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
virtual void initializePass() {
TD = getAnalysisIfAvailable<TargetData>();
virtual AliasResult alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size) {
return MayAlias;
virtual void getArgumentAccesses(Function *F, CallSite CS,
std::vector<PointerAccessInfo> &Info) {
llvm_unreachable("This method may not be called on this function!");
virtual void getMustAliases(Value *P, std::vector<Value*> &RetVals) { }
virtual bool pointsToConstantMemory(const Value *P) { return false; }
virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) {
return ModRef;
virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
return ModRef;
virtual bool hasNoModRefInfoForCalls() const { return true; }
virtual void deleteValue(Value *V) {}
virtual void copyValue(Value *From, Value *To) {}
} // End of anonymous namespace
// Register this pass...
char NoAA::ID = 0;
static RegisterPass<NoAA>
U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
// Declare that we implement the AliasAnalysis interface
static RegisterAnalysisGroup<AliasAnalysis> V(U);
ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
// BasicAA Pass
namespace {
/// BasicAliasAnalysis - This is the default alias analysis implementation.
/// Because it doesn't chain to a previous alias analysis (like -no-aa), it
/// derives from the NoAA class.
struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA {
static char ID; // Class identification, replacement for typeinfo
BasicAliasAnalysis() : NoAA(&ID) {}
AliasResult alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size);
ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
/// hasNoModRefInfoForCalls - We can provide mod/ref information against
/// non-escaping allocations.
virtual bool hasNoModRefInfoForCalls() const { return false; }
/// pointsToConstantMemory - Chase pointers until we find a (constant
/// global) or not.
bool pointsToConstantMemory(const Value *P);
// CheckGEPInstructions - Check two GEP instructions with known
// must-aliasing base pointers. This checks to see if the index expressions
// preclude the pointers from aliasing...
CheckGEPInstructions(const Type* BasePtr1Ty,
Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1Size,
const Type *BasePtr2Ty,
Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
} // End of anonymous namespace
// Register this pass...
char BasicAliasAnalysis::ID = 0;
static RegisterPass<BasicAliasAnalysis>
X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
// Declare that we implement the AliasAnalysis interface
static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
ImmutablePass *llvm::createBasicAliasAnalysisPass() {
return new BasicAliasAnalysis();
/// pointsToConstantMemory - Chase pointers until we find a (constant
/// global) or not.
bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
if (const GlobalVariable *GV =
return GV->isConstant();
return false;
// getModRefInfo - Check 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.
BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
if (!isa<Constant>(P)) {
const Value *Object = P->getUnderlyingObject();
// If this is a tail call and P 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 (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
if (CI->isTailCall())
return 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
// argument without capturing it.
if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) {
bool passedAsArg = false;
// TODO: Eventually only check 'nocapture' arguments.
for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
CI != CE; ++CI)
if (isa<PointerType>((*CI)->getType()) &&
alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
passedAsArg = true;
if (!passedAsArg)
return NoModRef;
// The AliasAnalysis base class has some smarts, lets use them.
return AliasAnalysis::getModRefInfo(CS, P, Size);
BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
// If CS1 or CS2 are readnone, they don't interact.
ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
if (CS1B == DoesNotAccessMemory) return NoModRef;
ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
if (CS2B == DoesNotAccessMemory) return NoModRef;
// If they both only read from memory, just return ref.
if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
return Ref;
// Otherwise, fall back to NoAA (mod+ref).
return NoAA::getModRefInfo(CS1, CS2);
// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
// as array references.
BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size) {
// Strip off any constant expression casts if they exist
if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1))
if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
V1 = CE->getOperand(0);
if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2))
if (CE->isCast() && isa<PointerType>(CE->getOperand(0)->getType()))
V2 = CE->getOperand(0);
// Are we checking for alias of the same value?
if (V1 == V2) return MustAlias;
if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
return NoAlias; // Scalars cannot alias each other
// Strip off cast instructions. Since V1 and V2 are pointers, they must be
// pointer<->pointer bitcasts.
if (const BitCastInst *I = dyn_cast<BitCastInst>(V1))
return alias(I->getOperand(0), V1Size, V2, V2Size);
if (const BitCastInst *I = dyn_cast<BitCastInst>(V2))
return alias(V1, V1Size, I->getOperand(0), V2Size);
// Figure out what objects these things are pointing to if we can.
const Value *O1 = V1->getUnderlyingObject();
const Value *O2 = V2->getUnderlyingObject();
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;
// Arguments can't alias with local allocations or noalias calls.
if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) ||
(isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1))))
return NoAlias;
// Most objects can't alias null.
if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
(isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
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 (TD)
if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) ||
(V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD)))
return NoAlias;
// If one pointer is the result of a call/invoke and the other is a
// non-escaping local object, then we know the object couldn't escape to a
// point where the call could return it.
if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
isNonEscapingLocalObject(O2) && O1 != O2)
return NoAlias;
if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
isNonEscapingLocalObject(O1) && O1 != O2)
return NoAlias;
// If we have two gep instructions with must-alias'ing base pointers, figure
// out if the indexes to the GEP tell us anything about the derived pointer.
// Note that we also handle chains of getelementptr instructions as well as
// constant expression getelementptrs here.
if (isGEP(V1) && isGEP(V2)) {
const User *GEP1 = cast<User>(V1);
const User *GEP2 = cast<User>(V2);
// If V1 and V2 are identical GEPs, just recurse down on both of them.
// This allows us to analyze things like:
// P = gep A, 0, i, 1
// Q = gep B, 0, i, 1
// by just analyzing A and B. This is even safe for variable indices.
if (GEP1->getType() == GEP2->getType() &&
GEP1->getNumOperands() == GEP2->getNumOperands() &&
GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
// All operands are the same, ignoring the base.
std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size);
// Drill down into the first non-gep value, to test for must-aliasing of
// the base pointers.
while (isGEP(GEP1->getOperand(0)) &&
GEP1->getOperand(1) ==
GEP1 = cast<User>(GEP1->getOperand(0));
const Value *BasePtr1 = GEP1->getOperand(0);
while (isGEP(GEP2->getOperand(0)) &&
GEP2->getOperand(1) ==
GEP2 = cast<User>(GEP2->getOperand(0));
const Value *BasePtr2 = GEP2->getOperand(0);
// Do the base pointers alias?
AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U);
if (BaseAlias == NoAlias) return NoAlias;
if (BaseAlias == MustAlias) {
// If the base pointers alias each other exactly, check to see if we can
// figure out anything about the resultant pointers, to try to prove
// non-aliasing.
// Collect all of the chained GEP operands together into one simple place
SmallVector<Value*, 16> GEP1Ops, GEP2Ops;
BasePtr1 = GetGEPOperands(V1, GEP1Ops);
BasePtr2 = GetGEPOperands(V2, GEP2Ops);
// If GetGEPOperands were able to fold to the same must-aliased pointer,
// do the comparison.
if (BasePtr1 == BasePtr2) {
AliasResult GAlias =
&GEP1Ops[0], GEP1Ops.size(), V1Size,
&GEP2Ops[0], GEP2Ops.size(), 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 (isGEP(V2)) {
std::swap(V1, V2);
std::swap(V1Size, V2Size);
if (V1Size != ~0U && V2Size != ~0U)
if (isGEP(V1)) {
SmallVector<Value*, 16> GEPOperands;
const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
AliasResult R = alias(BasePtr, V1Size, V2, V2Size);
if (R == MustAlias) {
// If there is at least one non-zero constant index, we know they cannot
// alias.
bool ConstantFound = false;
bool AllZerosFound = true;
for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
if (!C->isNullValue()) {
ConstantFound = true;
AllZerosFound = false;
} else {
AllZerosFound = false;
// If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
// the ptr, the end result is a must alias also.
if (AllZerosFound)
return MustAlias;
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...
if (TD && cast<PointerType>(
BasePtr->getType())->getElementType()->isSized()) {
for (unsigned i = 0; i != GEPOperands.size(); ++i)
if (!isa<ConstantInt>(GEPOperands[i]))
GEPOperands[i] =
int64_t Offset =
if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
return NoAlias;
return MayAlias;
// This function is used to determine if the indices of two GEP instructions are
// equal. V1 and V2 are the indices.
static bool IndexOperandsEqual(Value *V1, Value *V2, LLVMContext &Context) {
if (V1->getType() == V2->getType())
return V1 == V2;
if (Constant *C1 = dyn_cast<Constant>(V1))
if (Constant *C2 = dyn_cast<Constant>(V2)) {
// Sign extend the constants to long types, if necessary
if (C1->getType() != Type::getInt64Ty(Context))
C1 = ConstantExpr::getSExt(C1, Type::getInt64Ty(Context));
if (C2->getType() != Type::getInt64Ty(Context))
C2 = ConstantExpr::getSExt(C2, Type::getInt64Ty(Context));
return C1 == C2;
return false;
/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
/// base pointers. This checks to see if the index expressions preclude the
/// pointers from aliasing...
const Type* BasePtr1Ty, Value **GEP1Ops, unsigned NumGEP1Ops, unsigned G1S,
const Type *BasePtr2Ty, Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2S) {
// We currently can't handle the case when the base pointers have different
// primitive types. Since this is uncommon anyway, we are happy being
// extremely conservative.
if (BasePtr1Ty != BasePtr2Ty)
return MayAlias;
const PointerType *GEPPointerTy = cast<PointerType>(BasePtr1Ty);
LLVMContext &Context = GEPPointerTy->getContext();
// Find the (possibly empty) initial sequence of equal values... which are not
// necessarily constants.
unsigned NumGEP1Operands = NumGEP1Ops, NumGEP2Operands = NumGEP2Ops;
unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
unsigned UnequalOper = 0;
while (UnequalOper != MinOperands &&
IndexOperandsEqual(GEP1Ops[UnequalOper], GEP2Ops[UnequalOper],
Context)) {
// Advance through the type as we go...
if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
else {
// If all operands equal each other, then the derived pointers must
// alias each other...
BasePtr1Ty = 0;
assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
"Ran out of type nesting, but not out of operands?");
return MustAlias;
// If we have seen all constant operands, and run out of indexes on one of the
// getelementptrs, check to see if the tail of the leftover one is all zeros.
// If so, return mustalias.
if (UnequalOper == MinOperands) {
if (NumGEP1Ops < NumGEP2Ops) {
std::swap(GEP1Ops, GEP2Ops);
std::swap(NumGEP1Ops, NumGEP2Ops);
bool AllAreZeros = true;
for (unsigned i = UnequalOper; i != MaxOperands; ++i)
if (!isa<Constant>(GEP1Ops[i]) ||
!cast<Constant>(GEP1Ops[i])->isNullValue()) {
AllAreZeros = false;
if (AllAreZeros) 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))
// We have to be careful here about array accesses. In particular, consider:
// A[1][0] vs A[0][i]
// In this case, we don't *know* that the array will be accessed in bounds:
// the index could even be negative. Because of this, we have to
// conservatively *give up* and return may alias. We disregard differing
// array subscripts that are followed by a variable index without going
// through a struct.
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 getelementptrs...
unsigned FirstConstantOper = UnequalOper;
for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
const Value *G1Oper = GEP1Ops[FirstConstantOper];
const Value *G2Oper = GEP2Ops[FirstConstantOper];
if (G1Oper != G2Oper) // Found non-equal constant indexes...
if (Constant *G1OC = dyn_cast<ConstantInt>(const_cast<Value*>(G1Oper)))
if (Constant *G2OC = dyn_cast<ConstantInt>(const_cast<Value*>(G2Oper))){
if (G1OC->getType() != G2OC->getType()) {
// Sign extend both operands to long.
if (G1OC->getType() != Type::getInt64Ty(Context))
G1OC = ConstantExpr::getSExt(G1OC, Type::getInt64Ty(Context));
if (G2OC->getType() != Type::getInt64Ty(Context))
G2OC = ConstantExpr::getSExt(G2OC, Type::getInt64Ty(Context));
GEP1Ops[FirstConstantOper] = G1OC;
GEP2Ops[FirstConstantOper] = G2OC;
if (G1OC != G2OC) {
// Handle the "be careful" case above: if this is an array/vector
// subscript, scan for a subsequent variable array index.
if (const SequentialType *STy =
dyn_cast<SequentialType>(BasePtr1Ty)) {
const Type *NextTy = STy;
bool isBadCase = false;
for (unsigned Idx = FirstConstantOper;
Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
isBadCase = true;
// If the array is indexed beyond the bounds of the static type
// at this level, it will also fall into the "be careful" case.
// It would theoretically be possible to analyze these cases,
// but for now just be conservatively correct.
if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
if (cast<ConstantInt>(G1OC)->getZExtValue() >=
ATy->getNumElements() ||
cast<ConstantInt>(G2OC)->getZExtValue() >=
ATy->getNumElements()) {
isBadCase = true;
if (const VectorType *VTy = dyn_cast<VectorType>(STy))
if (cast<ConstantInt>(G1OC)->getZExtValue() >=
VTy->getNumElements() ||
cast<ConstantInt>(G2OC)->getZExtValue() >=
VTy->getNumElements()) {
isBadCase = true;
STy = cast<SequentialType>(NextTy);
NextTy = cast<SequentialType>(NextTy)->getElementType();
if (isBadCase) G1OC = 0;
// Make sure they are comparable (ie, not constant expressions), and
// make sure the GEP with the smaller leading constant is GEP1.
if (G1OC) {
Constant *Compare = ConstantExpr::getICmp(ICmpInst::ICMP_SGT,
G1OC, G2OC);
if (ConstantInt *CV = dyn_cast<ConstantInt>(Compare)) {
if (CV->getZExtValue()) { // If they are comparable and G2 > G1
std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
std::swap(NumGEP1Ops, NumGEP2Ops);
BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
// No shared constant operands, and we ran out of common operands. At this
// point, the GEP instructions have run through all of their operands, and we
// haven't found evidence that there are any deltas between the GEP's.
// However, one GEP may have more operands than the other. If this is the
// case, there may still be hope. Check this now.
if (FirstConstantOper == MinOperands) {
// Without TargetData, we won't know what the offsets are.
if (!TD)
return MayAlias;
// Make GEP1Ops be the longer one if there is a longer one.
if (NumGEP1Ops < NumGEP2Ops) {
std::swap(GEP1Ops, GEP2Ops);
std::swap(NumGEP1Ops, NumGEP2Ops);
// Is there anything to check?
if (NumGEP1Ops > MinOperands) {
for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
if (isa<ConstantInt>(GEP1Ops[i]) &&
!cast<ConstantInt>(GEP1Ops[i])->isZero()) {
// Yup, there's a constant in the tail. Set all variables to
// constants in the GEP instruction to make it suitable for
// TargetData::getIndexedOffset.
for (i = 0; i != MaxOperands; ++i)
if (!isa<ConstantInt>(GEP1Ops[i]))
GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
// Okay, now get the offset. This is the relative offset for the full
// instruction.
int64_t Offset1 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
// Now check without any constants at the end.
int64_t Offset2 = TD->getIndexedOffset(GEPPointerTy, GEP1Ops,
// Make sure we compare the absolute difference.
if (Offset1 > Offset2)
std::swap(Offset1, Offset2);
// If the tail provided a bit enough offset, return noalias!
if ((uint64_t)(Offset2-Offset1) >= SizeMax)
return NoAlias;
// Otherwise break - we don't look for another constant in the tail.
// Couldn't find anything useful.
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.
// Advance BasePtr[12]Ty over this first differing constant operand.
BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->
BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->
// We are going to be using TargetData::getIndexedOffset to determine the
// offset that each of the GEP's is reaching. To do this, we have to convert
// all variable references to constant references. To do this, we convert the
// initial sequence of array subscripts into constant zeros to start with.
const Type *ZeroIdxTy = GEPPointerTy;
for (unsigned i = 0; i != FirstConstantOper; ++i) {
if (!isa<StructType>(ZeroIdxTy))
GEP1Ops[i] = GEP2Ops[i] =
if (const CompositeType *CT = dyn_cast<CompositeType>(ZeroIdxTy))
ZeroIdxTy = CT->getTypeAtIndex(GEP1Ops[i]);
// We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
// Loop over the rest of the operands...
for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
const Value *Op1 = i < NumGEP1Ops ? GEP1Ops[i] : 0;
const Value *Op2 = i < NumGEP2Ops ? GEP2Ops[i] : 0;
// If they are equal, use a zero index...
if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
if (!isa<ConstantInt>(Op1))
GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
// Otherwise, just keep the constants we have.
} else {
if (Op1) {
if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
// If this is an array index, make sure the array element is in range.
if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty)) {
if (Op1C->getZExtValue() >= AT->getNumElements())
return MayAlias; // Be conservative with out-of-range accesses
} else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty)) {
if (Op1C->getZExtValue() >= VT->getNumElements())
return MayAlias; // Be conservative with out-of-range accesses
} 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.
if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
GEP1Ops[i] =
else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr1Ty))
GEP1Ops[i] =
if (Op2) {
if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
// If this is an array index, make sure the array element is in range.
if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr2Ty)) {
if (Op2C->getZExtValue() >= AT->getNumElements())
return MayAlias; // Be conservative with out-of-range accesses
} else if (const VectorType *VT = dyn_cast<VectorType>(BasePtr2Ty)) {
if (Op2C->getZExtValue() >= VT->getNumElements())
return MayAlias; // Be conservative with out-of-range accesses
} else { // Conservatively assume the minimum value for this index
GEP2Ops[i] = Constant::getNullValue(Op2->getType());
if (BasePtr1Ty && Op1) {
if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
BasePtr1Ty = 0;
if (BasePtr2Ty && Op2) {
if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
BasePtr2Ty = 0;
if (TD && GEPPointerTy->getElementType()->isSized()) {
int64_t Offset1 =
TD->getIndexedOffset(GEPPointerTy, GEP1Ops, NumGEP1Ops);
int64_t Offset2 =
TD->getIndexedOffset(GEPPointerTy, GEP2Ops, NumGEP2Ops);
assert(Offset1 != Offset2 &&
"There is at least one different constant here!");
// Make sure we compare the absolute difference.
if (Offset1 > Offset2)
std::swap(Offset1, Offset2);
if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
//cerr << "Determined that these two GEP's don't alias ["
// << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;
return NoAlias;
return MayAlias;
// Make sure that anything that uses AliasAnalysis pulls in this file...