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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/105875] Toggling an atomic_bool is inefficient
Date: Wed, 17 May 2023 23:32:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105875

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|tree-optimization           |c
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-05-17

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It is the front-end that is producing the worse code:
  TARGET_EXPR <D.2818, 1>;
  TARGET_EXPR <D.2819, (void) (D.2819 =3D
VIEW_CONVERT_EXPR<atomic_bool>(__atomic_load_1 ((const volatile void *) &b,
5)))>;
  <D.2821>:;
  TARGET_EXPR <D.2820, ((int) TARGET_EXPR <D.2819, (void) (D.2819 =3D
VIEW_CONVERT_EXPR<atomic_bool>(__atomic_load_1 ((const volatile void *) &b,
5)))> ^ D.2818) !=3D 0>;
  if (__atomic_compare_exchange_1 ((volatile void *) &b, (void *) &D.2819,
(int) VIEW_CONVERT_EXPR<unsigned char>(D.2820), 0, 5, 5))
    {
      goto <D.2822>;
    }
  goto <D.2821>;
  <D.2822>:;, D.2820;

vs:
  TARGET_EXPR <D.2827, (char) __atomic_xor_fetch_1 ((volatile void *) &c, (=
int)
(unsigned char) TARGET_EXPR <D.2826, 1>, 5)>;, D.2827;


So confirmed.

Using __atomic_xor_fetch_1 directly works.
That is:
  __atomic_xor_fetch_1 (&b, 1, 5);

Produces:
        lock xorb       $1, b(%rip)=