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.
