string: Use memcpy() within memmove() when we can

A common use of memmove() can be handled by memcpy(). Also memcpy()
includes an optimization for large sizes: it copies a word at a time. So
we can get a speed-up by calling memcpy() to handle our move in this case.

Update memmove() to call also memcpy() if the source don't overlap
the destination (src + count <= dest).

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>

lib/string.c

@@ -567,7 +567,19 @@ void * memmove(void * dest,const void *src,size_t count)
 {
 	char *tmp, *s;
 
-	if (dest <= src) {
+	if (dest <= src || (src + count) <= dest) {
+	/*
+	 * Use the fast memcpy implementation (ARCH optimized or lib/string.c) when it is possible:
+	 * - when dest is before src (assuming that memcpy is doing forward-copying)
+	 * - when destination don't overlap the source buffer (src + count <= dest)
+	 *
+	 * WARNING: the first optimisation cause an issue, when __HAVE_ARCH_MEMCPY is defined,
+	 *          __HAVE_ARCH_MEMMOVE is not defined and if the memcpy ARCH-specific
+	 *          implementation is not doing a forward-copying.
+	 *
+	 * No issue today because memcpy is doing a forward-copying in lib/string.c and for ARM32
+	 * architecture; no other arches use __HAVE_ARCH_MEMCPY without __HAVE_ARCH_MEMMOVE.
+	 */
 		memcpy(dest, src, count);
 	} else {
 		tmp = (char *) dest + count;