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

[witold.baryluk+gcc at gmail dot com]

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

            Bug ID: 103966
           Summary: std::atomic relaxed load, inc, store sub-optimal
                    codegen
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: witold.baryluk+gcc at gmail dot com
  Target Milestone: ---

Both functions below, should compile to the same assembly on x86:

#include <atomic>
#include <cstdint>

uint64_t x;
void inc_a() {
    x++;
}

std::atomic<uint64_t> y;

void inc_b_non_atomic() {
    y.store(y.load(std::memory_order_relaxed) + 1, std::memory_order_relaxed);
}


and it does so in clang.

It does not in gcc 12 (and earlier).

https://godbolt.org/z/GcM67xz8T



This pattern is very popular in approximate statistical counters / metrics,
where the flow of information is unidirectional (i.e. from one thread that does
updates, to another thread that only reads the counters), and its performance
is critical in many codebases.