[PATCH] D104854: Introduce intrinsic llvm.isnan

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D104854#2957471 <https://reviews.llvm.org/D104854#2957471>, @sepavloff wrote:

> In D104854#2957423 <https://reviews.llvm.org/D104854#2957423>, @spatel wrote:
>
>> Is it intentional that we are not canonicalizing the intrinsic call back to `fcmp uno` in the default FP environment?
>
> It is lowered to unordered comparison by default. Changing `llvm.isnan` to  `fcmp uno` somewhere in IR would make it possible to optimize out the latter if fast-math mode is on. Preserving semantics of `isnan` when fast-math is in effect was one of the goals of this change.

I understand that the codegen was supposed to be no worse, but the difference in IR causes optimizer regressions like: 
https://llvm.org/PR51556

If we want this intrinsic (and its siblings that haven't been created yet) to survive through IR, then we have to enhance IR passes to recognize the new patterns. 
It would be easier to do this in steps: (1) create the intrinsic only if not in the default FP env, (2) update IR analysis/passes to recognize the intrinsic, (3) create the intrinsic in the default FP env with no FMF, (4) create the intrinsic always.


Repository:
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  https://reviews.llvm.org/D104854/new/

https://reviews.llvm.org/D104854