be/c: replace __atomic-builtins by c11 atomics

[michaellilltokiwa]

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[michaellilltokiwa]

@fridis @maxteufel I need some guidance here. I'm all but certain that C11-atomics are preferable to the builtins.
We could keep both implementations defaulting to C11?

Microsoft added support in their compiler for C11-atomics at end of 2022:
https://devblogs.microsoft.com/cppblog/c11-atomics-in-visual-studio-2022-version-17-5-preview-2/

[maxteufel]

After pondering a while, I think if GCC, Clang, and MSVC support C11-style atomics, then we should use those. Why shouldn't we use the standard over custom implementations?

[fridis]

I am very much for using the C11 atomics as well, We need to understand the effects of ++ operation.

[fridis]

I think this might not be atomic, if there is no C function for this, you will need to do something like

  do
    {
      o = last_id;
    }
  while (compare_and_swap(&last_id, o, o+1) != o;

But maybe the C11 standard says something different.

[michaellilltokiwa]

I believe this to be atomic:

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdatomic.h>

int main()
{
    static atomic_uint_least64_t test = 0;
    ++test;
}

main:                                   # @main
        push    rbp
        mov     rbp, rsp
        lock            inc     qword ptr [rip + main.test]
        xor     eax, eax
        pop     rbp
        ret

https://godbolt.org/z/GbYj78Mvb