[Bug target/95737] PPC: Unnecessary extsw after negative less than

[wschmidt at linux dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95737

--- Comment #4 from wschmidt at linux dot ibm.com ---
On 6/19/20 12:43 PM, jens.seifert at de dot ibm.com wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95737
>
> Jens Seifert <jens.seifert at de dot ibm.com> changed:
>
>             What    |Removed                     |Added
> ----------------------------------------------------------------------------
>               Status|RESOLVED                    |UNCONFIRMED
>           Resolution|DUPLICATE                   |---
>
> --- Comment #3 from Jens Seifert <jens.seifert at de dot ibm.com> ---
> This is different as the extsw also happens if the result gets used e.g.
> followed by a andc, which is my case. I obviously oversimplified the sample. It
> has nothing to do with function result and ABI requirements. gcc assume that
> the result of -(a < b) implemented by subfc, subfe is signed 32-bit. But the
> result is already 64-bit.
>
> unsigned long long branchlesconditional(unsigned long long a, unsigned long
> long b, unsigned long long c)
> {
>     unsigned long long mask = -(a < b);
>     return c &~ mask;
> }
>
> results in
>
> _Z20branchlesconditionalyyy:
> .LFB1:
>          .cfi_startproc
>          subfc 4,4,3
>          subfe 3,3,3
>          not 3,3
>          extsw 3,3
>          and 3,3,5
>          blr
>
> expected
> subfc
> subfe
> andc
>
Thanks for verifying, Jens!