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From: "josephcsible at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/105875] New: Toggling an atomic_bool is inefficient
Date: Tue, 07 Jun 2022 15:10:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105875

            Bug ID: 105875
           Summary: Toggling an atomic_bool is inefficient
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: josephcsible at gmail dot com
  Target Milestone: ---
            Target: x86_64-pc-linux-gnu

Consider this C code:

#include <stdatomic.h>

atomic_bool b;
atomic_char c;
_Bool b2;

void f1(void) {
    b ^=3D 1;
}

void f2(void) {
    c ^=3D 1;
}

void f3(void) {
    b2 ^=3D 1;
}

At -O3, those functions compile into this:

f1:
        movzbl  b(%rip), %eax
.L5:
        movb    %al, -1(%rsp)
        xorl    $1, %eax
        movl    %eax, %edx
        movzbl  -1(%rsp), %eax
        lock cmpxchgb   %dl, b(%rip)
        jne     .L5
        ret
f2:
        lock xorb       $1, c(%rip)
        ret
f3:
        xorb    $1, b2(%rip)
        ret

The code generated for f1 is inefficient. It should have just done a "lock =
xorb
      $1, b(%rip)".=