Comment on possible data-dependent branch in `!res'.

common/lib/libc/string/consttime_memequal.c

@@ -1,4 +1,4 @@
-/* $NetBSD: consttime_memequal.c,v 1.3 2013/08/28 17:47:07 riastradh Exp $ */
+/* $NetBSD: consttime_memequal.c,v 1.4 2013/08/28 19:31:14 riastradh Exp $ */
 
 #if !defined(_KERNEL) && !defined(_STANDALONE)
 #include "namespace.h"
@@ -18,5 +18,15 @@ consttime_memequal(const void *b1, const void *b2, size_t len)
 
 	while (len --)
 		res |= *c1++ ^ *c2++;
+
+	/*
+	 * If the compiler for your favourite architecture generates a
+	 * conditional branch for `!res', it will be a data-dependent
+	 * branch, in which case this should be replaced by
+	 *
+	 *	return (1 - (1 & ((res - 1) >> 8)));
+	 *
+	 * or rewritten in assembly.
+	 */
 	return !res;
 }