[Bug c++/103966] std::atomic relaxed load, inc, store sub-optimal codegen

[witold.baryluk+gcc at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103966

--- Comment #2 from Witold Baryluk <witold.baryluk+gcc at gmail dot com> ---
Similarly, dec, add, sub, are affected, as well mul.

Example:

#include <atomic>
#include <cstdint>

uint64_t x;
void add_a() {
    x += 5;
}

std::atomic<uint64_t> y;

void add_b_non_atomic() {
    y.store(y.load(std::memory_order_relaxed) + 5, std::memory_order_relaxed);
}



Producing:

add_a():
        add     QWORD PTR x[rip], 5
        ret
add_b_non_atomic():
        mov     rax, QWORD PTR y[rip]
        add     rax, 5
        mov     QWORD PTR y[rip], rax
        ret
y:
        .zero   8
x:
        .zero   8