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From: "chfast at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/109845] New: Addition overflow/carry flag unnecessarily put in a temporary register
Date: Sat, 13 May 2023 18:32:00 +0000	[thread overview]
Message-ID: <bug-109845-4@http.gcc.gnu.org/bugzilla/> (raw)

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

             reply	other threads:[~2023-05-13 18:32 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-13 18:32 chfast at gmail dot com [this message]
2023-05-13 18:39 ` [Bug rtl-optimization/109845] " pinskia at gcc dot gnu.org
2023-05-13 18:45 ` [Bug middle-end/109845] " pinskia at gcc dot gnu.org
2023-05-15  6:52 ` rguenth at gcc dot gnu.org

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