// 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.
//
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/Passes.h"
#include "llvm/Pass.h"
-#include "llvm/Support/Compiler.h"
using namespace llvm;
namespace {
/// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
/// implementation that uses ScalarEvolution to answer queries.
- class VISIBILITY_HIDDEN ScalarEvolutionAliasAnalysis : public FunctionPass,
- public AliasAnalysis {
+ class ScalarEvolutionAliasAnalysis : public FunctionPass,
+ public AliasAnalysis {
ScalarEvolution *SE;
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(AnalysisID PI) {
+ if (PI == &AliasAnalysis::ID)
+ return (AliasAnalysis*)this;
+ return this;
+ }
private:
virtual void getAnalysisUsage(AnalysisUsage &AU) const;
virtual AliasResult alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size);
- Value *GetUnderlyingIdentifiedObject(const SCEV *S);
+ Value *GetBaseValue(const SCEV *S);
};
} // End of anonymous namespace
// 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();
return false;
}
+/// GetBaseValue - Given an expression, try to find a
+/// base value. Return null is none was found.
Value *
-ScalarEvolutionAliasAnalysis::GetUnderlyingIdentifiedObject(const SCEV *S) {
+ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
- return GetUnderlyingIdentifiedObject(AR->getStart());
+ // In an addrec, assume that the base will be in the start, rather
+ // than the step.
+ return GetBaseValue(AR->getStart());
} else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
// If there's a pointer operand, it'll be sorted at the end of the list.
const SCEV *Last = A->getOperand(A->getNumOperands()-1);
- if (isa<PointerType>(Last->getType()))
- return GetUnderlyingIdentifiedObject(Last);
+ if (Last->getType()->isPointerTy())
+ return GetBaseValue(Last);
} else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
- // Determine if we've found an Identified object.
- Value *V = U->getValue();
- if (isIdentifiedObject(V))
- return V;
+ // This is a leaf node.
+ return U->getValue();
}
// No Identified object found.
return 0;
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.
// The correctness of this depends on ScalarEvolution not recognizing
// inttoptr and ptrtoint operators.
- Value *AO = GetUnderlyingIdentifiedObject(AS);
- Value *BO = GetUnderlyingIdentifiedObject(BS);
+ 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.