[PATCH] D138814: [InstCombine] Combine a/lshr of add -> uadd.with.overflow

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp:917-918
+  // Replace the use of `Op0` by `I` with `Overflow`.
+  if (IsAShr)
+    return new SExtInst(Overflow, Ty);
+  return new ZExtInst(Overflow, Ty);
----------------
Pierre-vh wrote:
> lebedev.ri wrote:
> > `mul i16 255, 2` is `i16 510`, which is non-negative,
> > so the signbit of the original result is never set.
> > IOW, just zero-extend.
> Is it normal that Alive2 reports the transformation as incorrect for i2 then (see @foad's comment above) ?
> Maybe we just need to SExt if it's i2 + ashr?
Ok, good point, please add that as a comment.


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  https://reviews.llvm.org/D138814/new/

https://reviews.llvm.org/D138814