#include "llvm/BasicBlock.h"
#include "llvm/Support/InstVisitor.h"
#include "llvm/iMemory.h"
+#include "llvm/iOther.h"
#include "llvm/Constants.h"
#include "llvm/GlobalValue.h"
#include "llvm/DerivedTypes.h"
-#include "llvm/Pass.h"
// Register the AliasAnalysis interface, providing a nice name to refer to.
static RegisterAnalysisGroup<AliasAnalysis> X("Alias Analysis");
//
namespace {
struct CanModify : public InstVisitor<CanModify, bool> {
- const AliasAnalysis &AA;
+ AliasAnalysis &AA;
const Value *Ptr;
- CanModify(const AliasAnalysis *aa, const Value *ptr)
+ CanModify(AliasAnalysis *aa, const Value *ptr)
: AA(*aa), Ptr(ptr) {}
bool visitInvokeInst(InvokeInst &II) {
/// specified basic block to modify the value pointed to by Ptr.
///
bool AliasAnalysis::canBasicBlockModify(const BasicBlock &bb,
- const Value *Ptr) const {
+ const Value *Ptr) {
CanModify CM(this, Ptr);
BasicBlock &BB = const_cast<BasicBlock&>(bb);
///
bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
const Instruction &I2,
- const Value *Ptr) const {
+ const Value *Ptr) {
assert(I1.getParent() == I2.getParent() &&
"Instructions not in same basic block!");
CanModify CM(this, Ptr);
return isa<GlobalValue>(V) || isa<MallocInst>(V) || isa<AllocaInst>(V);
}
+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)) 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));
+ }
+ 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. :)
+//
AliasAnalysis::Result BasicAliasAnalysis::alias(const Value *V1,
- const Value *V2) const {
+ const Value *V2) {
// Strip off constant pointer refs if they exist
if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
V1 = 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()))
+ if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
+ V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
return NoAlias; // Scalars cannot alias each other
- bool V1Unique = hasUniqueAddress(V1);
- bool V2Unique = hasUniqueAddress(V2);
-
- if (V1Unique && V2Unique)
- return NoAlias; // Can't alias if they are different unique values
-
- if ((V1Unique && isa<ConstantPointerNull>(V2)) ||
- (V2Unique && isa<ConstantPointerNull>(V1)))
- return NoAlias; // Unique values don't alias null
-
- // TODO: Handle getelementptr with nonzero offset
+ // Strip off cast instructions...
+ if (const Instruction *I = dyn_cast<CastInst>(V1))
+ return alias(I->getOperand(0), V2);
+ if (const Instruction *I = dyn_cast<CastInst>(V2))
+ return alias(I->getOperand(0), V1);
+
+ // 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()) {
+ if (std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
+ return alias(GEP1->getOperand(0), GEP2->getOperand(0));
+
+ // If all of the indexes to the getelementptr are constant, but
+ // different (well we already know they are different), then we know
+ // that there cannot be an alias here if the two base pointers DO alias.
+ //
+ bool AllConstant = true;
+ for (unsigned i = 1, e = GEP1->getNumOperands(); i != e; ++i)
+ if (!isa<Constant>(GEP1->getOperand(i)) ||
+ !isa<Constant>(GEP2->getOperand(i))) {
+ AllConstant = false;
+ break;
+ }
+
+ // If we are all constant, then look at where the the base pointers
+ // alias. If they are known not to alias, then we are dealing with two
+ // different arrays or something, so no alias is possible. If they are
+ // known to be the same object, then we cannot alias because we are
+ // indexing into a different part of the object. As usual, MayAlias
+ // doesn't tell us anything.
+ //
+ if (AllConstant &&
+ alias(GEP1->getOperand(0), GEP2->getOperand(1)) != MayAlias)
+ return NoAlias;
+ }
+
+ // 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 discernable object?
+ if (O1 && O2) {
+ // 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<ConstantPointerNull>(V2)) {
+ return NoAlias; // Unique values don't alias null
+ } else if (O2 && isa<ConstantPointerNull>(V1)) {
+ return NoAlias; // Unique values don't alias null
+ }
return MayAlias;
}
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