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, 7 insertions, 1 deletions

diff --git a/src/string/memmove.c b/src/string/memmove.c
index f225bb30..5dc9cdb9 100644
--- a/src/string/memmove.c
+++ b/src/string/memmove.c
@@ -1,8 +1,10 @@
 #include <string.h>
 #include <stdint.h>
 
-#define WT size_t
+#ifdef __GNUC__
+typedef __attribute__((__may_alias__)) size_t WT;
 #define WS (sizeof(WT))
+#endif
 
 void *memmove(void *dest, const void *src, size_t n)
 {
@@ -13,6 +15,7 @@ void *memmove(void *dest, const void *src, size_t n)
 	if ((uintptr_t)s-(uintptr_t)d-n <= -2*n) return memcpy(d, s, n);
 
 	if (d<s) {
+#ifdef __GNUC__
 		if ((uintptr_t)s % WS == (uintptr_t)d % WS) {
 			while ((uintptr_t)d % WS) {
 				if (!n--) return dest;
@@ -20,8 +23,10 @@ void *memmove(void *dest, const void *src, size_t n)
 			}
 			for (; n>=WS; n-=WS, d+=WS, s+=WS) *(WT *)d = *(WT *)s;
 		}
+#endif
 		for (; n; n--) *d++ = *s++;
 	} else {
+#ifdef __GNUC__
 		if ((uintptr_t)s % WS == (uintptr_t)d % WS) {
 			while ((uintptr_t)(d+n) % WS) {
 				if (!n--) return dest;
@@ -29,6 +34,7 @@ void *memmove(void *dest, const void *src, size_t n)
 			}
 			while (n>=WS) n-=WS, *(WT *)(d+n) = *(WT *)(s+n);
 		}
+#endif
 		while (n) n--, d[n] = s[n];
 	}