Cast 64 bit off_t to 32 bit size_t

Some systems have sizeof(off_t) == 8 while sizeof(size_t) == 4.
This implies that we are able to access and work on files whose
maximum length is around 2^63-1 bytes, but we can only malloc or
mmap somewhat less than 2^32-1 bytes of memory.

On such a system an implicit conversion of off_t to size_t can cause
the size_t to wrap, resulting in unexpected and exciting behavior.
Right now we are working around all gcc warnings generated by the
-Wshorten-64-to-32 option by passing the off_t through xsize_t().

In the future we should make xsize_t on such problematic platforms
detect the wrapping and die if such a file is accessed.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diffcore-order.c

@@ -14,6 +14,7 @@ static void prepare_order(const char *orderfile)
 	void *map;
 	char *cp, *endp;
 	struct stat st;
+	size_t sz;
 
 	if (order)
 		return;
@@ -25,11 +26,12 @@ static void prepare_order(const char *orderfile)
 		close(fd);
 		return;
 	}
-	map = mmap(NULL, st.st_size, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
+	sz = xsize_t(st.st_size);
+	map = mmap(NULL, sz, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
 	close(fd);
 	if (map == MAP_FAILED)
 		return;
-	endp = (char *) map + st.st_size;
+	endp = (char *) map + sz;
 	for (pass = 0; pass < 2; pass++) {
 		cnt = 0;
 		cp = map;