[Bug middle-end/115802] Non-atomic load of static variable moved out of loop despite atomic fences

[securesneakers at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115802

--- Comment #5 from Ivan Bodrov <securesneakers at gmail dot com> ---
Without the mutex threads would race for the first access to non-atomic
variable.

Seq-Cst ordering is only used to simplify the example, it can be relaxed to
acquire/release/relaxed for different operations. I didn't want people to spend
too much time thinking about it.

I guess even shorter demonstration would be using inlint asm with memory
clobbering:

    while (!val)
       __asm__ volatile ("":::"memory");

The above forces GCC to re-load "val" on every iteration and the code compiles
to:

    .L8:
        mov     edx, DWORD PTR val[rip]
        test    edx, edx
        je      .L8

But if the same fence is within the function, it won't have any effect:

     __attribute__((__noinline__)) void fence(void) { __asm__ volatile
("":::"memory"); }
    ...
    while (!val)
        fence();

Compiles to:

    .L8:
        call    func
        jmp     .L8