[Bug target/106012] New: rsqrtss instruction generated even if -mno-recip specified

[Bug target/106012] New: rsqrtss instruction generated even if -mno-recip specified

[vincenzo.innocente at cern dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106012

            Bug ID: 106012
           Summary: rsqrtss instruction generated even if -mno-recip
                    specified
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincenzo.innocente at cern dot ch
  Target Milestone: ---

with option -Ofast -mno-recip rsqrtss instruction is still generated.

https://godbolt.org/z/hGxrG7xPh

inhibiting rsqrtss and rcpss is critical to obtain identical results when
running on INTEL and AMD platforms. Having to inhibit Ofast is clearly a larger
performance penalty.

[Bug tree-optimization/106012] rsqrtss instruction generated even if -mno-recip specified

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106012

Uroš Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-06-17
          Component|target                      |tree-optimization
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
recip pass is converting to .RSQRT, target is just expanding what it gets.

[Bug tree-optimization/106012] rsqrtss instruction generated even if -mno-recip specified

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106012

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|NEW                         |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
You can use -fno-reciprocal-math if you don't want reciprocal instructions
produced.

[Bug tree-optimization/106012] rsqrtps and rcpps instructiona generated even if -fno-reciprocal-math specified

[vincenzo.innocente at cern dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106012

vincenzo Innocente <vincenzo.innocente at cern dot ch> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|rsqrtss instruction         |rsqrtps and rcpps
                   |generated even if           |instructiona generated even
                   |-mno-recip specified        |if -fno-reciprocal-math
                   |                            |specified
             Status|RESOLVED                    |NEW
         Resolution|WONTFIX                     |---

--- Comment #3 from vincenzo Innocente <vincenzo.innocente at cern dot ch> ---
Thanks for the suggestion.

-fno-reciprocal-math does indeed inhibit scalar reciprocal instructions.

NOT in vectorized loop though.

see

https://godbolt.org/z/9eMb4Tjee

[Bug target/106012] rsqrtps and rcpps instructions generated even if -fno-reciprocal-math specified

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106012

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |x86_64-*-*
          Component|tree-optimization           |target

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to vincenzo Innocente from comment #3)
> Thanks for the suggestion.
> 
> -fno-reciprocal-math does indeed inhibit scalar reciprocal instructions.
> 
> NOT in vectorized loop though.
> 
> see
> 
> https://godbolt.org/z/9eMb4Tjee

For that you need -mno-recip as well ... (here it's the target expanding
the division).  Maybe the target should take -fno-reciprocal-math as
enabling -mno-recip.

[Bug target/106012] rsqrtps and rcpps instructions generated even if -fno-reciprocal-math specified

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106012

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Uroš Bizjak from comment #1)
> recip pass is converting to .RSQRT, target is just expanding what it gets.

But it queries the target for support for RSQRT, but yeah, we lack to ask for
"implicit" vs. "explicit" (via xmm intrinsics), so the target cannot possibly
disable RSQRT expanders I guess.

[Bug target/106012] rsqrtps and rcpps instructions generated even if -fno-reciprocal-math specified

[vincenzo.innocente at cern dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106012

--- Comment #6 from vincenzo Innocente <vincenzo.innocente at cern dot ch> ---
just to confirm that
-Ofast  -fno-reciprocal-math -mno-recip
seems to inhibit all reciprocals...
https://godbolt.org/z/f4bccb9GP