move WC_BARRIER() definition from wolfssl/wolfcrypt/types.h to wolfssl/wolfcrypt/wc_port.h, and strengthen it.

wolfssl/wolfcrypt/types.h

@@ -489,12 +489,6 @@ enum {
     static WC_INLINE WARN_UNUSED_RESULT int WC_WUR_INT(int x) { return x; }
 #endif
 
-/* XFENCE() is a no-op on some targets.  WC_BARRIER() uses C89 intrinsics as an
- * additional portable barrier.
- */
-#define WC_BARRIER() do { volatile byte _xfence = 0; (void)_xfence; XFENCE(); \
-        } while(0)
-
 #ifdef WORD64_AVAILABLE
     #define WC_MAX_UINT_OF(x)                                        \
         ((x)((((word64)1 << ((sizeof(x) * (word64)CHAR_BIT) -        \

wolfssl/wolfcrypt/wc_port.h

@@ -1808,6 +1808,18 @@ WOLFSSL_ABI WOLFSSL_API int wolfCrypt_Cleanup(void);
     #define XFENCE() WC_DO_NOTHING
 #endif
 
+#ifdef WC_BARRIER
+    /* use user-supplied WC_BARRIER() definition. */
+#elif defined(__GNUC__) && !defined(WOLFSSL_NO_ASM)
+    #define WC_BARRIER() __asm__ __volatile__("" ::: "memory")
+#else
+    /* XFENCE() is a no-op on some targets.  The fallback construct uses C89
+     * intrinsics as an additional (but weak) portable barrier.
+     */
+    #define WC_BARRIER() do { volatile byte _xfence = 0; (void)_xfence; XFENCE(); \
+        } while(0)
+#endif
+
 
     /* AFTER user_settings.h is loaded,
     ** determine if POSIX multi-threaded: HAVE_PTHREAD  */