[Bug target/86693] inefficient atomic_fetch_xor

[nruslan_devel at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86693

--- Comment #2 from Ruslan Nikolaev <nruslan_devel at yahoo dot com> ---
Also may be (partially) related the following cases:

1.

#include <stdatomic.h>
#include <stdbool.h>

void func2();

void func(_Atomic(unsigned long) * obj, void * obj2)
{
        if (atomic_fetch_sub(obj, 1) == 1 && obj2)
                func2();
}

generates 'xadd' when 'sub' suffices:
func:
.LFB0:
        .cfi_startproc
        movq    $-1, %rax
        lock xaddq      %rax, (%rdi)
        testq   %rsi, %rsi
        je      .L1
        cmpq    $1, %rax
        je      .L10
.L1:
        rep ret
        .p2align 4,,10
        .p2align 3
.L10:
        xorl    %eax, %eax
        jmp     func2@PLT


2.

#include <stdatomic.h>

int func(_Atomic(unsigned long) * obj, unsigned long a)
{
        return atomic_fetch_add(obj, a) == -a;
}

generates 'xadd' when 'add' suffices:
func:
.LFB0:
        .cfi_startproc
        movq    %rsi, %rax
        lock xaddq      %rax, (%rdi)
        addq    %rsi, %rax
        sete    %al
        movzbl  %al, %eax
        ret