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[LAA] Use computeConstantDifference() #103725

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Merged
merged 2 commits into from
Aug 16, 2024
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[LAA] Use computeConstantDifference() #103725

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a83f00e
[LAA] Use computeConstantDifference()
nikic Jul 29, 2024
fd99984
also modify getMinFromExprs
nikic Aug 14, 2024
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14 changes: 6 additions & 8 deletions llvm/lib/Analysis/LoopAccessAnalysis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -412,12 +412,10 @@ bool RuntimePointerChecking::needsChecking(
/// Return nullptr in case we couldn't find an answer.
static const SCEV *getMinFromExprs(const SCEV *I, const SCEV *J,
ScalarEvolution *SE) {
const SCEV *Diff = SE->getMinusSCEV(J, I);
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Off topic - but this is essentially simplifying a three way compare. We've recently added three way compares to IR, and we've got analogous simplification logic inside SCEV for min/max. Should we maybe have an interface on SCEV which does the 3 way compare, and returns either the result or a SCEVCouldNotCompute?

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Off topic, but in a slightly different way... It looks like the sole user of this is addPointer. I can't see any reason that the Min and Max can't simply be the SCEV expressions for the min and max of all pointers. We'd need to cost whether generating such a group is profitable, but that's distinct from legality. One concern is that I think the current logic is relying on the min/max simplification to fail when two pointers are based on different underlying objects. We'd need to explicitly handle that. There may also be a profitability concern about forming "too large" a group if the offsets are runtime values with non-overlapping ranges.

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I think taking a normal SCEV min/max would be fine in terms of correctness, but as you say, profitability is less clear. I also suspect that the logic used here may be stronger than what SCEV does for this particular case. SCEV only uses isKnownViaNonRecursiveReasoning to simplify min/max expression. (Of course, it can also be weaker in other respects...)

const SCEVConstant *C = dyn_cast<const SCEVConstant>(Diff);

if (!C)
std::optional<APInt> Diff = SE->computeConstantDifference(J, I);
if (!Diff)
return nullptr;
return C->getValue()->isNegative() ? J : I;
return Diff->isNegative() ? J : I;
}

bool RuntimeCheckingPtrGroup::addPointer(
Expand Down Expand Up @@ -1599,11 +1597,11 @@ std::optional<int> llvm::getPointersDiff(Type *ElemTyA, Value *PtrA,
// Otherwise compute the distance with SCEV between the base pointers.
const SCEV *PtrSCEVA = SE.getSCEV(PtrA);
const SCEV *PtrSCEVB = SE.getSCEV(PtrB);
const auto *Diff =
dyn_cast<SCEVConstant>(SE.getMinusSCEV(PtrSCEVB, PtrSCEVA));
std::optional<APInt> Diff =
SE.computeConstantDifference(PtrSCEVB, PtrSCEVA);
if (!Diff)
return std::nullopt;
Val = Diff->getAPInt().getSExtValue();
Val = Diff->getSExtValue();
}
int Size = DL.getTypeStoreSize(ElemTyA);
int Dist = Val / Size;
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2 changes: 1 addition & 1 deletion llvm/lib/Analysis/ScalarEvolution.cpp
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Expand Up @@ -11956,7 +11956,7 @@ ScalarEvolution::computeConstantDifference(const SCEV *More, const SCEV *Less) {
APInt DiffMul(BW, 1);
// Try various simplifications to reduce the difference to a constant. Limit
// the number of allowed simplifications to keep compile-time low.
for (unsigned I = 0; I < 5; ++I) {
for (unsigned I = 0; I < 8; ++I) {
if (More == Less)
return Diff;

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