[PATCH] [InstCombine] mark ADD with nuw if no unsigned overflow

[Chandler Carruth]

Cool.


On Sat, Jun 14, 2014 at 10:36 PM, Jingyue Wu <jingyue at google.com> wrote:

> Thanks for your review! I agree with your concerns. computeKnownBits is a
> little expensive. I'm considering merging WillNotOverflowSignedAdd and
> WillNotOverflowUnsignedAdd in a separate patch.
>
> ================
> Comment at: lib/Transforms/InstCombine/InstCombineAddSub.cpp:1264
> @@ -1246,1 +1263,3 @@
>    }
> +  if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedAdd(LHS, RHS)) {
> +    Changed = true;
> ----------------
> Chandler Carruth wrote:
> > This should be an else if, no? If we succeed at adding NSW, why would we
> check for NUW?
> Even with nsw, there's still value of checking for NUW. e.g., "add i16
> 16383, 16383" can be marked with both nsw and nuw, while "add i16 65535,
> 65535" can be marked with only nsw but not nuw.
>
> http://reviews.llvm.org/D4144
>
>
>
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