Support GCC and clang versions that lack <stdatomic.h>.

Add a code path to use the GCC builtins for atomics, which are supported
in older compiler versions, rather than the new <stdatomic.h>.

wasmtime-wasi/sandboxed-system-primitives/src/refcount.h

@@ -13,14 +13,20 @@
 #define REFCOUNT_H
 
 #include <assert.h>
+#if !defined(__GNUC__)
 #include <stdatomic.h>
+#endif
 #include <stdbool.h>
 
 #include "locking.h"
 
 // Simple reference counter.
 struct LOCKABLE refcount {
+#if defined(__GNUC__)
+  unsigned count;
+#else
   atomic_uint count;
+#endif
 };
 
 #define PRODUCES(...) LOCKS_SHARED(__VA_ARGS__) NO_LOCK_ANALYSIS
@@ -28,18 +34,30 @@ struct LOCKABLE refcount {
 
 // Initialize the reference counter.
 static void refcount_init(struct refcount *r, unsigned int count) PRODUCES(*r) {
+#if defined(__GNUC__)
+  __atomic_store_n(&r->count, count, __ATOMIC_SEQ_CST);
+#else
   atomic_init(&r->count, count);
+#endif
 }
 
 // Increment the reference counter.
 static inline void refcount_acquire(struct refcount *r) PRODUCES(*r) {
+#if defined(__GNUC__)
+  __atomic_fetch_add(&r->count, 1, __ATOMIC_ACQUIRE);
+#else
   atomic_fetch_add_explicit(&r->count, 1, memory_order_acquire);
+#endif
 }
 
 // Decrement the reference counter, returning whether the reference
 // dropped to zero.
 static inline bool refcount_release(struct refcount *r) CONSUMES(*r) {
+#if defined(__GNUC__)
+  int old = __atomic_fetch_sub(&r->count, 1, __ATOMIC_RELEASE);
+#else
   int old = atomic_fetch_sub_explicit(&r->count, 1, memory_order_release);
+#endif
   assert(old != 0 && "Reference count becoming negative");
   return old == 1;
 }