[src/trunk]: src/common/lib/libc/string Comment on possible data-dependent br...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/bb1e1d375e9b
branches:  trunk
changeset: 789633:bb1e1d375e9b
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Wed Aug 28 19:31:14 2013 +0000

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

diffstat:

 common/lib/libc/string/consttime_memequal.c |  12 +++++++++++-
 1 files changed, 11 insertions(+), 1 deletions(-)

diffs (25 lines):

diff -r 9cdce396d948 -r bb1e1d375e9b common/lib/libc/string/consttime_memequal.c
--- a/common/lib/libc/string/consttime_memequal.c       Wed Aug 28 19:04:12 2013 +0000
+++ b/common/lib/libc/string/consttime_memequal.c       Wed Aug 28 19:31:14 2013 +0000
@@ -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 @@
 
        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;
 }