fix aliasing-based undefined behavior in string functions

use the GNU C may_alias attribute if available, and fallback to naive
byte-by-byte loops if __GNUC__ is not defined.

this patch has been written to minimize changes so that history
remains reviewable; it does not attempt to bring the affected code
into a more consistent or elegant form.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/string/memchr.c b/src/string/memchr.c
index 4daff7bb..65f0d789 100644
--- a/src/string/memchr.c
+++ b/src/string/memchr.c
@@ -12,12 +12,16 @@ void *memchr(const void *src, int c, size_t n)
 {
 	const unsigned char *s = src;
 	c = (unsigned char)c;
+#ifdef __GNUC__
 	for (; ((uintptr_t)s & ALIGN) && n && *s != c; s++, n--);
 	if (n && *s != c) {
-		const size_t *w;
+		typedef size_t __attribute__((__may_alias__)) word;
+		const word *w;
 		size_t k = ONES * c;
 		for (w = (const void *)s; n>=SS && !HASZERO(*w^k); w++, n-=SS);
-		for (s = (const void *)w; n && *s != c; s++, n--);
+		s = (const void *)w;
 	}
+#endif
+	for (; n && *s != c; s++, n--);
 	return n ? (void *)s : 0;
 }