[PATCH] D117912: [IR] document and update ctlz/cttz intrinsics to optionally return poison rather than undef

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

In D117912#3263773 <https://reviews.llvm.org/D117912#3263773>, @xbolva00 wrote:

>>> __builtin_unpredictable
>
> Backend (SDAG) ignores this hint sadly, it would be really cool to have it.

Does the hint even survive through the middle end? I think the pattern will be turned into llvm.cttz.i64(i64 %x, i1 false). Then, CodeGenPrepare turns it back into the control flow version based on isCheapToSpeculateCTTZ.


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