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Rework scev-aa's basic computation so that it doesn't depend

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on ScalarEvolution successfully folding and preserving
range information for both A-B and B-A. Now, if it gets
either one, it's sufficient.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107249 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2010-06-30 06:12:16 +00:00
parent 204b7cad7a
commit 49bda917db
1 changed files with 31 additions and 7 deletions
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38
lib/Analysis/ScalarEvolutionAliasAnalysis.cpp
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@ -106,6 +106,12 @@ ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
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));
@ -118,14 +124,32 @@ ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
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.