[llvm-commits] [llvm] r150962 - in /llvm/trunk: lib/Transforms/InstCombine/InstCombineCompares.cpp test/Transforms/InstCombine/icmp.ll

[Duncan Sands]

Hi Benjamin,

> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp Mon Feb 20 09:07:47 2012
> @@ -607,6 +607,20 @@
>           return new ICmpInst(ICmpInst::getSignedPredicate(Cond),
>                               GEPLHS->getOperand(0), GEPRHS->getOperand(0));
>
> +      // If we're comparing GEPs with two base pointers that only differ in type
> +      // and both GEPs have only constant indices or just one use, then fold
> +      // the compare with the adjusted indices.
> +      if (TD&&
> +          (GEPLHS->hasAllConstantIndices() || GEPLHS->hasOneUse())&&
> +          (GEPRHS->hasAllConstantIndices() || GEPRHS->hasOneUse())&&
> +          PtrBase->stripPointerCasts() ==
> +            GEPRHS->getOperand(0)->stripPointerCasts()) {
> +        Value *Cmp = Builder->CreateICmp(ICmpInst::getSignedPredicate(Cond),
> +                                         EmitGEPOffset(GEPLHS),
> +                                         EmitGEPOffset(GEPRHS));

for non-equality comparisons, isn't this only safe if the GEP is inbounds?
Otherwise, by removing the base, you may change an overflowing (non-overflowing)
GEP into a non-overflowing (overflowing) one, which can completely change the
result of the comparison.

Ciao, Duncan.

> +        return ReplaceInstUsesWith(I, Cmp);
> +      }
> +