[Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage

[Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977

            Bug ID: 98977
           Summary: [x86] Failure to optimize consecutive sub flags usage
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

extern bool z, c;

uint8_t f(uint8_t dest, uint8_t src)
{
    u8 res = dest - src;
    z = !res;
    c = src > dest;
    return res;
}

With -O3, LLVM outputs this:

f(unsigned char, unsigned char):
  mov eax, edi
  sub al, sil
  sete byte ptr [rip + z]
  setb byte ptr [rip + c]
  ret

GCC outputs this:

f(unsigned char, unsigned char):
  mov eax, edi
  sub al, sil
  sete BYTE PTR z[rip]
  cmp dil, sil
  setb BYTE PTR c[rip]
  ret

It seems desirable to eliminate the `cmp`, unless there's some weird flag stall
thing I'm not aware of.

[Bug target/98977] [x86] Failure to optimize consecutive sub flags usage

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

[Bug target/98977] [x86] Failure to optimize consecutive sub flags usage

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-12-23
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
         Depends on|                            |3507

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, PR 3507 is part of it (maybe all of it) as shown by:
#include <stdbool.h>
#include <stdint.h>

extern bool z, c;

uint8_t f(uint8_t dest, uint8_t src)
{
    uint8_t res = dest - src;
    //z = !res;
    c = src > dest;
    return res;
}


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3507
[Bug 3507] appalling optimisation with sub/cmp on multiple targets

[Bug rtl-optimization/98977] [x86] Failure to optimize consecutive sub flags usage

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|target                      |rtl-optimization
             Target|x86_64-*-* i?86-*-*         |x86_64-*-* i?86-*-*
                   |                            |aarch64*-*-*

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is a testcase which shows the issue on other targets (aarch64) too:
#include <stdbool.h>
#include <stdint.h>

extern bool z, c;

uint32_t f(uint32_t dest, uint32_t src)
{
    uint32_t res = dest - src;
    z = !res;
    c = src > dest;
    return res;
}

[Bug rtl-optimization/98977] Failure to optimize consecutive sub flags usage

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977

--- Comment #3 from Hongtao.liu <crazylht at gmail dot com> ---
LLVM has a separate module to merge sub and cmp, GCC can do similar thing.
Alternative choice is canonicalizing cmp patterns to be same as subs' with a
unused dest(result of sub), then CSE/PRE would be able to do the elimination
work?