gh-155628: Use relaxed adds for the GC allocation counter

The free-threaded build buffers per-thread allocation counts and
flushes them to gcstate->young.count in three places: when the local
threshold is reached, when a thread state is cleared, and in
gc.get_count(). The counter is a collection heuristic: its readers use
relaxed loads (gc_should_collect()) or a compare-exchange loop, it is
reset during a stop-the-world pause, and it publishes no other memory,
so the flushes need atomicity but no ordering.

Author: kddnewton

Modules/gcmodule.c

@@ -222,7 +222,7 @@ gc_get_count_impl(PyObject *module)
     struct _gc_thread_state *gc = &tstate->gc;
 
     // Flush the local allocation count to the global count
-    _Py_atomic_add_int(&gcstate->young.count, (int)gc->alloc_count);
+    _Py_atomic_add_int_relaxed(&gcstate->young.count, (int)gc->alloc_count);
     gc->alloc_count = 0;
 #endif
 

Python/gc_free_threading.c

@@ -2025,7 +2025,7 @@ record_allocation(PyThreadState *tstate)
     if (gc->alloc_count >= LOCAL_ALLOC_COUNT_THRESHOLD) {
         // TODO: Use Py_ssize_t for the generation count.
         GCState *gcstate = &tstate->interp->gc;
-        _Py_atomic_add_int(&gcstate->young.count, (int)gc->alloc_count);
+        _Py_atomic_add_int_relaxed(&gcstate->young.count, (int)gc->alloc_count);
         gc->alloc_count = 0;
 
         if (gc_should_collect(gcstate) &&

Python/pystate.c

@@ -1879,8 +1879,8 @@ PyThreadState_Clear(PyThreadState *tstate)
     // Flush the thread's local GC allocation count to the global count
     // before the thread state is cleared, otherwise the count is lost.
     _PyThreadStateImpl *tstate_impl = (_PyThreadStateImpl *)tstate;
-    _Py_atomic_add_int(&tstate->interp->gc.young.count,
-                       (int)tstate_impl->gc.alloc_count);
+    _Py_atomic_add_int_relaxed(&tstate->interp->gc.young.count,
+                               (int)tstate_impl->gc.alloc_count);
     tstate_impl->gc.alloc_count = 0;
 
     // Merge our thread-local refcounts into the type's own refcount and