//===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===//
//
// 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.
//
// FIXME: This could be extended for a very simple form of mod/ref information.
// If a pointer is locally allocated (either malloc or alloca) and never passed
// into a call or stored to memory, then we know that calls will not mod/ref the
// memory. This can be important for tailcallelim, and can support CSE of loads
// and dead store elimination across calls. This is particularly important for
// stack allocated arrays.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Pass.h"
#include "llvm/Argument.h"
#include "llvm/iOther.h"
#include "llvm/iMemory.h"
#include "llvm/Constants.h"
#include "llvm/GlobalVariable.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
using namespace llvm;
// Make sure that anything that uses AliasAnalysis pulls in this file...
void llvm::BasicAAStub() {}
namespace {
struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AliasAnalysis::getAnalysisUsage(AU);
}
virtual void initializePass();
AliasResult alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size);
/// pointsToConstantMemory - Chase pointers until we find a (constant
/// global) or not.
bool pointsToConstantMemory(const Value *P);
private:
// CheckGEPInstructions - Check two GEP instructions with known
// must-aliasing base pointers. This checks to see if the index expressions
// preclude the pointers from aliasing...
AliasResult
CheckGEPInstructions(const Type* BasePtr1Ty, std::vector<Value*> &GEP1Ops,
unsigned G1Size,
const Type *BasePtr2Ty, std::vector<Value*> &GEP2Ops,
unsigned G2Size);
};
// Register this pass...
RegisterOpt<BasicAliasAnalysis>
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() {
InitializeAliasAnalysis(this);
}
// 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;
}
static const User *isGEP(const Value *V) {
if (isa<GetElementPtrInst>(V) ||
(isa<ConstantExpr>(V) &&
cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
return cast<User>(V);
return 0;
}
static const Value *GetGEPOperands(const Value *V, std::vector<Value*> &GEPOps){
assert(GEPOps.empty() && "Expect empty list to populate!");
GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
cast<User>(V)->op_end());
// 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]) ||
!cast<Constant>(GEPOps[0])->isNullValue())
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;
}
/// pointsToConstantMemory - Chase pointers until we find a (constant
/// global) or not.
bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
if (const Value *V = getUnderlyingObject(P))
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
return GV->isConstant();
return false;
}
// 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. :)
//
AliasAnalysis::AliasResult
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->getOpcode() == Instruction::Cast)
V1 = CE->getOperand(0);
if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2))
if (CE->getOpcode() == Instruction::Cast)
V2 = CE->getOperand(0);
// 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 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)) {
// Drill down into the first non-gep value, to test for must-aliasing of
// the base pointers.
const Value *BasePtr1 = V1, *BasePtr2 = V2;
do {
BasePtr1 = cast<User>(BasePtr1)->getOperand(0);
} while (isGEP(BasePtr1) &&
cast<User>(BasePtr1)->getOperand(1) ==
Constant::getNullValue(cast<User>(BasePtr1)->getOperand(1)->getType()));
do {
BasePtr2 = cast<User>(BasePtr2)->getOperand(0);
} while (isGEP(BasePtr2) &&
cast<User>(BasePtr2)->getOperand(1) ==
Constant::getNullValue(cast<User>(BasePtr2)->getOperand(1)->getType()));
// Do the base pointers alias?
AliasResult BaseAlias = alias(BasePtr1, V1Size, BasePtr2, V2Size);
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
std::vector<Value*> GEP1Ops, GEP2Ops;
BasePtr1 = GetGEPOperands(V1, GEP1Ops);
BasePtr2 = GetGEPOperands(V2, GEP2Ops);
AliasResult GAlias =
CheckGEPInstructions(BasePtr1->getType(), GEP1Ops, V1Size,
BasePtr2->getType(), GEP2Ops, 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 (const User *GEP = isGEP(V1)) {
std::vector<Value*> 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;
break;
}
} 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...
for (unsigned i = 0; i != GEPOperands.size(); ++i)
if (!isa<Constant>(GEPOperands[i]) ||
isa<ConstantExpr>(GEPOperands[i]))
GEPOperands[i] =Constant::getNullValue(GEPOperands[i]->getType());
int64_t Offset = getTargetData().getIndexedOffset(BasePtr->getType(),
GEPOperands);
if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
return NoAlias;
}
}
}
return MayAlias;
}
/// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
/// base pointers. This checks to see if the index expressions preclude the
/// pointers from aliasing...
AliasAnalysis::AliasResult BasicAliasAnalysis::
CheckGEPInstructions(const Type* BasePtr1Ty, std::vector<Value*> &GEP1Ops,
unsigned G1S,
const Type *BasePtr2Ty, std::vector<Value*> &GEP2Ops,
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 Type *GEPPointerTy = BasePtr1Ty;
// Find the (possibly empty) initial sequence of equal values... which are not
// necessarily constants.
unsigned NumGEP1Operands = GEP1Ops.size(), NumGEP2Operands = GEP2Ops.size();
unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
unsigned UnequalOper = 0;
while (UnequalOper != MinOperands &&
GEP1Ops[UnequalOper] == GEP2Ops[UnequalOper]) {
// Advance through the type as we go...
++UnequalOper;
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 (GEP1Ops.size() < GEP2Ops.size()) std::swap(GEP1Ops, GEP2Ops);
bool AllAreZeros = true;
for (unsigned i = UnequalOper; i != MaxOperands; ++i)
if (!isa<Constant>(GEP1Ops[i]) ||
!cast<Constant>(GEP1Ops[i])->isNullValue()) {
AllAreZeros = false;
break;
}
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))
//
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 != 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<Constant>(const_cast<Value*>(G1Oper)))
if (Constant *G2OC = dyn_cast<Constant>(const_cast<Value*>(G2Oper))) {
// Make sure they are comparable (ie, not constant expressions)...
// and make sure the GEP with the smaller leading constant is GEP1.
Constant *Compare = ConstantExpr::get(Instruction::SetGT, G1OC, G2OC);
if (ConstantBool *CV = dyn_cast<ConstantBool>(Compare)) {
if (CV->getValue()) // If they are comparable and G2 > G1
std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
break;
}
}
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. This this now.
if (FirstConstantOper == MinOperands) {
// Make GEP1Ops be the longer one if there is a longer one.
if (GEP1Ops.size() < GEP2Ops.size())
std::swap(GEP1Ops, GEP2Ops);
// Is there anything to check?
if (GEP1Ops.size() > MinOperands) {
for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
if (isa<Constant>(GEP1Ops[i]) && !isa<ConstantExpr>(GEP1Ops[i]) &&
!cast<Constant>(GEP1Ops[i])->isNullValue()) {
// Yup, there's a constant in the tail. Set all variables to
// constants in the GEP instruction to make it suiteable for
// TargetData::getIndexedOffset.
for (i = 0; i != MaxOperands; ++i)
if (!isa<Constant>(GEP1Ops[i]) || isa<ConstantExpr>(GEP1Ops[i]))
GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
// Okay, now get the offset. This is the relative offset for the full
// instruction.
const TargetData &TD = getTargetData();
int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops);
// Now crop off any constants from the end...
GEP1Ops.resize(MinOperands);
int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops);
// If the tail provided a bit enough offset, return noalias!
if ((uint64_t)(Offset2-Offset1) >= SizeMax)
return NoAlias;
}
}
// 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)->getTypeAtIndex(GEP2Ops[FirstConstantOper]);
BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(GEP1Ops[FirstConstantOper]);
// 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 equal sequence of variables into constant zeros to start with.
for (unsigned i = 0; i != FirstConstantOper; ++i) {
if (!isa<Constant>(GEP1Ops[i]) || isa<ConstantExpr>(GEP1Ops[i]) ||
!isa<Constant>(GEP2Ops[i]) || isa<ConstantExpr>(GEP2Ops[i])) {
GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
GEP2Ops[i] = Constant::getNullValue(GEP2Ops[i]->getType());
}
}
// 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 < GEP1Ops.size() ? GEP1Ops[i] : 0;
const Value *Op2 = i < GEP2Ops.size() ? GEP2Ops[i] : 0;
// If they are equal, use a zero index...
if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
if (!isa<Constant>(Op1) || isa<ConstantExpr>(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->getRawValue() >= AT->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] = ConstantSInt::get(Type::LongTy,AT->getNumElements()-1);
}
}
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>(BasePtr1Ty))
if (Op2C->getRawValue() >= AT->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]);
else
BasePtr1Ty = 0;
}
if (BasePtr2Ty && Op2) {
if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
else
BasePtr2Ty = 0;
}
}
int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops);
int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops);
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;
}