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

Fri Jan 21 12:56:43 PST 2022

craig.topper added a comment.

In D117912#3262280 <https://reviews.llvm.org/D117912#3262280>, @spatel wrote:

> In D117912#3262125 <https://reviews.llvm.org/D117912#3262125>, @efriedma wrote:
>
>> You mean, we might reach a point where we could make clang emit is_zero_poison=false without anyone caring?  Don't think we're quite there yet.  x86-64-v2 doesn't include lzcnt.  And not sure about the current state of the compiler-rt/libgcc intrinsics.
>
> Yes, I was wondering what level of default x86 tuning was needed to flip the setting...I suppose that's x86-64-v3 ("Close to Haswell").

Is updating isCheapToSpeculateCTTZ/CTLZ to check CMOV instead of BMI/LZCNT enough?

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https://reviews.llvm.org/D117912