string: Add memory barrier to memset_s

The memset_s spec talks about how it's different from memset and has
stronger guarantees. I can't see how that actually requires the writes
if the memory is released afterwards (either through a call to free or
a function return), but it seems like the spec might encourage people
to think that it does. Adding a memory barrier ensures the compiler
will not elide the actual memory operations even when built with LTO.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

newlib/libc/string/memset_s.c

@@ -64,11 +64,15 @@ memset_s(void *s, rsize_t smax, int c, rsize_t n)
 
     // Normal return path
     (void)memset(s, c, n);
+    /* Compiler barrier.  */
+    __asm__ __volatile__ ("" ::"r"(s): "memory");
     return 0;
 
 handle_error:
     if (s != NULL) {
         (void)memset(s, c, smax);
+        /* Compiler barrier.  */
+        __asm__ __volatile__ ("" ::"r"(s): "memory");
     }
 
     if (__cur_handler != NULL) {