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InstSimplify: improve computePointerICmp (NFC) #126255

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merged 2 commits into from
Feb 10, 2025
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InstSimplify: improve computePointerICmp (NFC) #126255

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InstSimplify: improve computePointerICmp (NFC)
artagnon Feb 7, 2025
b7fc260
IS: fix thinko
artagnon Feb 7, 2025
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27 changes: 7 additions & 20 deletions llvm/lib/Analysis/InstructionSimplify.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2686,27 +2686,14 @@ static Constant *computePointerICmp(CmpPredicate Pred, Value *LHS, Value *RHS,
const DataLayout &DL = Q.DL;
const TargetLibraryInfo *TLI = Q.TLI;

// We can only fold certain predicates on pointer comparisons.
switch (Pred) {
default:
// We fold equality and unsigned predicates on pointer comparisons, but forbid
// signed predicates since a GEP with inbounds could cross the sign boundary.
if (CmpInst::isSigned(Pred))
return nullptr;

// Equality comparisons are easy to fold.
case CmpInst::ICMP_EQ:
case CmpInst::ICMP_NE:
break;

// We can only handle unsigned relational comparisons because 'inbounds' on
// a GEP only protects against unsigned wrapping.
case CmpInst::ICMP_UGT:
case CmpInst::ICMP_UGE:
case CmpInst::ICMP_ULT:
case CmpInst::ICMP_ULE:
// However, we have to switch them to their signed variants to handle
// negative indices from the base pointer.
Pred = ICmpInst::getSignedPredicate(Pred);
break;
}
// We have to switch to a signed predicate to handle negative indices from
// the base pointer.
Pred = ICmpInst::getSignedPredicate(Pred);

// Strip off any constant offsets so that we can reason about them.
// It's tempting to use getUnderlyingObject or even just stripInBoundsOffsets
Expand All @@ -2730,7 +2717,7 @@ static Constant *computePointerICmp(CmpPredicate Pred, Value *LHS, Value *RHS,
ICmpInst::compare(LHSOffset, RHSOffset, Pred));

// Various optimizations for (in)equality comparisons.
if (Pred == CmpInst::ICMP_EQ || Pred == CmpInst::ICMP_NE) {
if (ICmpInst::isEquality(Pred)) {
// Different non-empty allocations that exist at the same time have
// different addresses (if the program can tell). If the offsets are
// within the bounds of their allocations (and not one-past-the-end!
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