core: bitop: add atomic compare-exchange

core/include/blue/core/bitop.h

@@ -7,4 +7,15 @@ BLUE_API int b_popcountl(long v);
 BLUE_API int b_ctzl(long v);
 BLUE_API int b_clzl(long v);
 
-#endif
+#if defined(__GNUC__) || defined(__clang__)
+#define b_cmpxchg(v, expected_val, new_val)                                    \
+	__atomic_compare_exchange_n(                                           \
+		v, expected_val, new_val, 0, __ATOMIC_RELAXED, __ATOMIC_RELAXED)
+#elif defined(_MSC_VER)
+/* TODO add MSVC support */
+#error MSVC intrinsics not yet supported
+#else
+#error Unsupported compiler
+#endif
+
+#endif