[Bug rtl-optimization/109845] New: Addition overflow/carry flag unnecessarily put in a temporary register

[Bug rtl-optimization/109845] New: Addition overflow/carry flag unnecessarily put in a temporary register

[chfast at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109845

            Bug ID: 109845
           Summary: Addition overflow/carry flag unnecessarily put in a
                    temporary register
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chfast at gmail dot com
  Target Milestone: ---

When we have an addition and an overflow check and the overflow flag is
combined with some other condition the codegen may generate variant when the
overflow flag is temporary register.

    unsigned s = y + z;
    _Bool ov = s < y;

    if (x || ov) 
        return;

This produces

        add     esi, edx
        setc    al
        test    edi, edi
        jne     .L1
        test    eax, eax
        jne     .L1

while it could be

        add     esi, edx
        jc      .L6
        test    edi, edi
        jne     .L6


There are easy workaround to the C code which make the assembly optimal:

1. Change the order of checks 
    if (ov || x)

2. Split if into two
    if (x)
        return;
    if (ov) 
        return;

https://godbolt.org/z/rxsrnhPdc

[Bug rtl-optimization/109845] Addition overflow/carry flag unnecessarily put in a temporary register

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109845

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 55077
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55077&action=edit
full testcase

Next time please attach the full compilable testcase or paste it inline rather
than just including a godbolt link.

[Bug middle-end/109845] Addition overflow/carry flag unnecessarily put in a temporary register

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109845

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-05-13
             Status|UNCONFIRMED                 |NEW
           Severity|normal                      |enhancement
          Component|rtl-optimization            |middle-end
           Keywords|                            |missed-optimization
     Ever confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So the difference between good1 and bad1 at the gimple level is the order of
operands of the bit_ior:
good1:

  ov_8 = _13 != 0;
  _9 = x_2(D) != 0;
  _10 = ov_8 | _9;
  if (_10 != 0)

bad1:
  ov_7 = _13 != 0;
  _1 = x_8(D) != 0;
  _2 = _1 | ov_7;
  if (_2 != 0)

[Bug middle-end/109845] Addition overflow/carry flag unnecessarily put in a temporary register

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109845

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Yup, bad luck for later RTL optimization.  It might help to perform jump
expansions decision to go for jump or straight-line code earlier on GIMPLE (and
stick to that).