[Bug middle-end/102566] [i386] GCC should emit LOCK BTS for simple bit-test-and-set operations with std::atomic

[thiago at kde dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102566

--- Comment #15 from Thiago Macieira <thiago at kde dot org> ---
Works now for the failing case. Additionally:

bool tbit(std::atomic<long> &i)
{
  return i.fetch_and(~CONSTANT, std::memory_order_relaxed) & (CONSTANT);
}

Will properly produce LOCK BTR (CONSTANT=2):

        lock btrq       $1, (%rdi)
        setc    %al
        ret

CONSTANT=(1L<<62):

        lock btrq       $62, (%rdi)
        setc    %al
        ret

But not for CONSTANT=1 or CONSTANT=(1L<<63):
        movq    (%rdi), %rax
.L2:
        movq    %rax, %rcx
        movq    %rax, %rdx
        andq    $-2, %rcx
        lock cmpxchgq   %rcx, (%rdi)
        jne     .L2
        movl    %edx, %eax
        andl    $1, %eax
        ret

Same applies to 1<<31 for atomic<int>.