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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/102402] New: Seemingly suboptimal optimization of
 jmp/cmovcc for conditionally loading constants
Date: Sat, 18 Sep 2021 18:17:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102402

            Bug ID: 102402
           Summary: Seemingly suboptimal optimization of jmp/cmovcc for
                    conditionally loading constants
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

#include <stdint.h>

struct MusicPlayerTrack
{
    uint8_t flags;
    uint8_t modT;
};

void ClearModM(struct MusicPlayerTrack *track, uint8_t modT)
{
    if (track->modT =3D=3D 0)
        track->flags |=3D 3;
    else
        track->flags |=3D 12;
}

This is optimized weirdly by GCC. Leaving it as-is gives this AMD64 assembl=
y:

ClearModM:
  movzx edx, BYTE PTR [rdi]
  mov eax, edx
  or eax, 12
  cmp BYTE PTR [rdi+1], 0
  jne .L3
  mov eax, edx
  or eax, 3
.L3:
  mov BYTE PTR [rdi], al
  ret

Whereas changing the `if` to `if (modT =3D=3D 0)` gives this:

ClearModM:
  movzx eax, BYTE PTR [rdi]
  mov edx, eax
  or eax, 12
  or edx, 3
  test sil, sil
  cmove eax, edx
  mov BYTE PTR [rdi], al
  ret

It seems to me that this should be better than the first output, though of
course this could be the other way considering how finicky cmovcc seems to =
be,
but it seems to me like at least one should be preferred above the other.

Note that this also occurs on IA-32, so the issue seems unrelated to whether
modT is in a register or in memory. Perhaps it's about whether it's a funct=
ion
argument ?=