[Bug target/66867] Suboptimal code generation for C11 atomic_compare_exchange_strong_explicit()

[sebastian.huber@embedded-brains.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66867

--- Comment #1 from Sebastian Huber <sebastian.huber@embedded-brains.de> ---
This problem is also present on x86.  The depreated
__sync_bool_compare_and_swap() produces better code.

#include <stdatomic.h>

void f(atomic_uint *a)
{
  unsigned int e = 0;
  atomic_compare_exchange_strong_explicit(a, &e, 1, memory_order_relaxed,
memory_order_relaxed);
}

void g(unsigned int *a)
{
  __sync_bool_compare_and_swap(a, 0, 1);
}

        .file   "cas.c"
        .section        .text.unlikely,"ax",@progbits
.LCOLDB0:
        .text
.LHOTB0:
        .p2align 4,,15
        .globl  f
        .type   f, @function
f:
.LFB0:
        .cfi_startproc
        movl    $1, %edx
        xorl    %eax, %eax
        movl    $0, -4(%rsp) <- Superfluous
        lock cmpxchgl   %edx, (%rdi)
        ret
        .cfi_endproc
.LFE0:
        .size   f, .-f
        .section        .text.unlikely
.LCOLDE0:
        .text
.LHOTE0:
        .section        .text.unlikely
.LCOLDB1:
        .text
.LHOTB1:
        .p2align 4,,15
        .globl  g
        .type   g, @function
g:
.LFB1:
        .cfi_startproc
        movl    $1, %edx
        xorl    %eax, %eax
        lock cmpxchgl   %edx, (%rdi)
        ret
        .cfi_endproc
.LFE1:
        .size   g, .-g
        .section        .text.unlikely
.LCOLDE1:
        .text
.LHOTE1:
        .ident  "GCC: (GNU) 6.0.0 20150717 (experimental)"
        .section        .note.GNU-stack,"",@progbits