[Bug middle-end/114547] New: missed-optimization: use sign flag

[Bug middle-end/114547] New: missed-optimization: use sign flag

[goon.pri.low at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114547

            Bug ID: 114547
           Summary: missed-optimization: use sign flag
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: goon.pri.low at gmail dot com
  Target Milestone: ---

Example functions

int s(int *v, int n) {
        *v -= n;
        return *v < 0;
}

int ns(int *v, int n) {
        *v -= n;
        return *v >= 0;
}

Generated assembly

s:
        mov     eax, DWORD PTR [rdi]
        sub     eax, esi
        mov     DWORD PTR [rdi], eax
        shr     eax, 31
        ret
ns:
        mov     eax, DWORD PTR [rdi]
        sub     eax, esi
        mov     DWORD PTR [rdi], eax
        not     eax
        shr     eax, 31
        ret

Optimal assembly

s:
        xor     eax, eax
        sub     DWORD PTR [rdi], esi
        sets    al
        ret
ns:
        xor     eax, eax
        sub     DWORD PTR [rdi], esi
        setns   al
        ret

[Bug middle-end/114547] missed-optimization: use sign flag

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114547

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu.org
           Severity|normal                      |enhancement

[Bug middle-end/114547] comparison than less than 0 (or greater or equal to than 0) after a subtraction does not use the flags regster

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114547

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|missed-optimization: use    |comparison than less than 0
                   |sign flag                   |(or greater or equal to
                   |                            |than 0) after a subtraction
                   |                            |does not use the flags
                   |                            |regster

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I am not sure this is always better ...

[Bug middle-end/114547] comparison than less than 0 (or greater or equal to than 0) after a subtraction does not use the flags regster

[goon.pri.low at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114547

--- Comment #2 from gooncreeper <goon.pri.low at gmail dot com> ---
(In reply to Andrew Pinski from comment #1)
> I am not sure this is always better ...

sets and setns are 1 uop, just like any other setcc
why wouldn't it be better?

[Bug middle-end/114547] comparison than less than 0 (or greater or equal to than 0) after a subtraction does not use the flags regster

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114547

Uroš Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |ubizjak at gmail dot com
   Last reconfirmed|                            |2024-04-02
             Status|UNCONFIRMED                 |NEW

--- Comment #3 from Uroš Bizjak <ubizjak at gmail dot com> ---
The similar testcase with comparison to zero:

int z(int *v, int n) {
        *v -= n;
        return *v == 0;
}

int nz(int *v, int n) {
        *v -= n;
        return *v != 0;
}

generates expected code:

z:
        xorl    %eax, %eax
        subl    %esi, (%rdi)
        sete    %al
        ret

nz:
        xorl    %eax, %eax
        subl    %esi, (%rdi)
        setne   %al
        ret

The middle end expands via standard sequence:

(insn 10 9 11 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg:SI 83 [ _2 ])
            (const_int 0 [0]))) "s.c":3:12 -1
     (nil))

(insn 11 10 12 (set (reg:QI 91)
        (eq:QI (reg:CCZ 17 flags)
            (const_int 0 [0]))) "s.c":3:12 -1
     (nil))

(insn 12 11 13 (set (reg:SI 90)
        (zero_extend:SI (reg:QI 91))) "s.c":3:12 -1
     (nil))

(insn 13 12 14 (set (reg:SI 85 [ <retval> ])
        (reg:SI 90)) "s.c":3:12 -1
     (nil))

OTOH, the sign compare is expanded via:

(insn 12 9 13 (parallel [
            (set (reg:SI 90)
                (lshiftrt:SI (reg:SI 83 [ _2 ])
                    (const_int 31 [0x1f])))
            (clobber (reg:CC 17 flags))
        ]) "pr114547.c":4:12 -1
     (nil))

(insn 13 12 14 (set (reg:SI 85 [ <retval> ])
        (reg:SI 90)) "pr114547.c":4:12 -1
     (nil))

(The above shift also interferes with RMW creation, resulting in unoptimal asm
sequence with two extra moves, and additional NEG insn in "ns" case.)

Middle-end expansion should avoid premature optimization in this case, at least
for targets that can merge sign comparison with the arith instruction.