// This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
// simple alias analysis implemented in terms of ScalarEvolution queries.
//
+// This differs from traditional loop dependence analysis in that it tests
+// for dependencies within a single iteration of a loop, rather than
+// dependencies between different iterations.
+//
// ScalarEvolution has a more complete understanding of pointer arithmetic
// than BasicAliasAnalysis' collection of ad-hoc analyses.
//
public:
static char ID; // Class identification, replacement for typeinfo
- ScalarEvolutionAliasAnalysis() : FunctionPass(&ID), SE(0) {}
+ ScalarEvolutionAliasAnalysis() : FunctionPass(ID), SE(0) {}
/// getAdjustedAnalysisPointer - This method is used when a pass implements
/// an analysis interface through multiple inheritance. If needed, it
/// should override this to adjust the this pointer as needed for the
/// specified pass info.
- virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
- if (PI->isPassID(&AliasAnalysis::ID))
+ virtual void *getAdjustedAnalysisPointer(AnalysisID PI) {
+ if (PI == &AliasAnalysis::ID)
return (AliasAnalysis*)this;
return this;
}
-
+
private:
virtual void getAnalysisUsage(AnalysisUsage &AU) const;
virtual bool runOnFunction(Function &F);
// Register this pass...
char ScalarEvolutionAliasAnalysis::ID = 0;
-static RegisterPass<ScalarEvolutionAliasAnalysis>
-X("scev-aa", "ScalarEvolution-based Alias Analysis", false, true);
-
-// Declare that we implement the AliasAnalysis interface
-static RegisterAnalysisGroup<AliasAnalysis> Y(X);
+INITIALIZE_AG_PASS(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
+ "ScalarEvolution-based Alias Analysis", false, true, false);
FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
return new ScalarEvolutionAliasAnalysis();
AliasAnalysis::AliasResult
ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
const Value *B, unsigned BSize) {
+ // If either of the memory references is empty, it doesn't matter what the
+ // pointer values are. This allows the code below to ignore this special
+ // case.
+ if (ASize == 0 || BSize == 0)
+ return NoAlias;
+
// This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
const SCEV *AS = SE->getSCEV(const_cast<Value *>(A));
const SCEV *BS = SE->getSCEV(const_cast<Value *>(B));
if (SE->getEffectiveSCEVType(AS->getType()) ==
SE->getEffectiveSCEVType(BS->getType())) {
unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
- APInt AI(BitWidth, ASize);
+ APInt ASizeInt(BitWidth, ASize);
+ APInt BSizeInt(BitWidth, BSize);
+
+ // Compute the difference between the two pointers.
const SCEV *BA = SE->getMinusSCEV(BS, AS);
- if (AI.ule(SE->getUnsignedRange(BA).getUnsignedMin())) {
- APInt BI(BitWidth, BSize);
- const SCEV *AB = SE->getMinusSCEV(AS, BS);
- if (BI.ule(SE->getUnsignedRange(AB).getUnsignedMin()))
- return NoAlias;
- }
+
+ // Test whether the difference is known to be great enough that memory of
+ // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
+ // are non-zero, which is special-cased above.
+ if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) &&
+ (-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax()))
+ return NoAlias;
+
+ // Folding the subtraction while preserving range information can be tricky
+ // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
+ // and try again to see if things fold better that way.
+
+ // Compute the difference between the two pointers.
+ const SCEV *AB = SE->getMinusSCEV(AS, BS);
+
+ // Test whether the difference is known to be great enough that memory of
+ // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
+ // are non-zero, which is special-cased above.
+ if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) &&
+ (-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax()))
+ return NoAlias;
}
// If ScalarEvolution can find an underlying object, form a new query.
Value *AO = GetBaseValue(AS);
Value *BO = GetBaseValue(BS);
if ((AO && AO != A) || (BO && BO != B))
- if (alias(AO ? AO : A, AO ? ~0u : ASize,
- BO ? BO : B, BO ? ~0u : BSize) == NoAlias)
+ if (alias(AO ? AO : A, AO ? UnknownSize : ASize,
+ BO ? BO : B, BO ? UnknownSize : BSize) == NoAlias)
return NoAlias;
// Forward the query to the next analysis.