[Bug target/105328] New: [x86] Failure to optimize out test instruction after add

[Bug target/105328] New: [x86] Failure to optimize out test instruction after add

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105328

            Bug ID: 105328
           Summary: [x86] Failure to optimize out test instruction after
                    add
           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: ---

void f1();
void f2();
void f3();

void g(int b, int c)
{
        int a = b + c;
        if (a > 0)
                f1();
        else if (a < 0)
                f2();
        else
                f3();
}

With -O3, GCC outputs this:

g(int, int):
  add edi, esi
  test edi, edi
  jg .L5
  je .L3
  jmp f2()
.L3:
  jmp f3()
.L5:
  jmp f1()

LLVM instead outputs this:

g(int, int):
  add edi, esi
  jle .LBB0_1
  jmp f1()@PLT # TAILCALL
.LBB0_1:
  js .LBB0_4
  jmp f3()@PLT # TAILCALL
.LBB0_4:
  jmp f2()@PLT # TAILCALL

It appears like the `test` instruction can be removed (I assume without having
to change anything about which functions get called on branchless paths and
things like that). I'm not completely sure about this, considering how complex
x86 performance can sometimes be, but I'd think removing that instruction
should be beneficial everywhere.

[Bug target/105328] [x86] Failure to optimize out test instruction after add

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105328

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=3507

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Most likely a dup of bug 3507.