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From: "hjl.tools at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/102566] [i386] GCC should emit LOCK BTS for simple
 bit-test-and-set operations with std::atomic
Date: Sun, 03 Oct 2021 13:06:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102566

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
This works:

[hjl@gnu-cfl-2 pr102566]$ cat y.c=20
#include <stdatomic.h>

_Atomic int v;

unsigned int
foo ()
{
  return atomic_fetch_or_explicit (&v, 1, memory_order_relaxed) & 1;
}
[hjl@gnu-cfl-2 pr102566]$ make y.s
/export/build/gnu/tools-build/gcc-gitlab-debug/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/tools-build/gcc-gitlab-debug/build-x86_64-linux/gcc/ -O=
2 -S
y.c
[hjl@gnu-cfl-2 pr102566]$ cat y.s
        .file   "y.c"
        .text
        .p2align 4
        .globl  foo
        .type   foo, @function
foo:
.LFB0:
        .cfi_startproc
        xorl    %eax, %eax
        lock btsl       $0, v(%rip)
        setc    %al
        ret
        .cfi_endproc
.LFE0:
        .size   foo, .-foo
        .globl  v
        .bss
        .align 4
        .type   v, @object
        .size   v, 4
v:
        .zero   4
        .ident  "GCC: (GNU) 12.0.0 20211003 (experimental)"
        .section        .note.GNU-stack,"",@progbits
[hjl@gnu-cfl-2 pr102566]$=