make overflow test on delta base offset work regardless of variable size

This patch introduces the MSB() macro to obtain the desired number of
most significant bits from a given variable independently of the variable
type.

It is then used to better implement the overflow test on the OBJ_OFS_DELTA
base offset variable with the property of always working correctly
regardless of the type/size of that variable.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-pack-objects.c

@@ -1014,7 +1014,7 @@ static void check_object(struct object_entry *entry)
 				ofs = c & 127;
 				while (c & 128) {
 					ofs += 1;
-					if (!ofs || ofs & ~(~0UL >> 7))
+					if (!ofs || MSB(ofs, 7))
 						die("delta base offset overflow in pack for %s",
 						    sha1_to_hex(entry->sha1));
 					c = buf[used_0++];

builtin-unpack-objects.c

@@ -209,7 +209,7 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 		base_offset = c & 127;
 		while (c & 128) {
 			base_offset += 1;
-			if (!base_offset || base_offset & ~(~0UL >> 7))
+			if (!base_offset || MSB(base_offset, 7))
 				die("offset value overflow for delta base object");
 			pack = fill(1);
 			c = *pack;

git-compat-util.h

@@ -13,6 +13,14 @@
 
 #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
 
+#ifdef __GNUC__
+#define TYPEOF(x) (__typeof__(x))
+#else
+#define TYPEOF(x)
+#endif
+
+#define MSB(x, bits) ((x) & TYPEOF(x)(~0ULL << (sizeof(x) * 8 - (bits))))
+
 #if !defined(__APPLE__) && !defined(__FreeBSD__)
 #define _XOPEN_SOURCE 600 /* glibc2 and AIX 5.3L need 500, OpenBSD needs 600 for S_ISLNK() */
 #define _XOPEN_SOURCE_EXTENDED 1 /* AIX 5.3L needs this */

index-pack.c

@@ -249,7 +249,7 @@ static void *unpack_raw_entry(struct object_entry *obj, union delta_base *delta_
 		base_offset = c & 127;
 		while (c & 128) {
 			base_offset += 1;
-			if (!base_offset || base_offset & ~(~0UL >> 7))
+			if (!base_offset || MSB(base_offset, 7))
 				bad_object(obj->offset, "offset value overflow for delta base object");
 			p = fill(1);
 			c = *p;

sha1_file.c

@@ -1150,7 +1150,7 @@ static off_t get_delta_base(struct packed_git *p,
 		base_offset = c & 127;
 		while (c & 128) {
 			base_offset += 1;
-			if (!base_offset || base_offset & ~(~0UL >> 7))
+			if (!base_offset || MSB(base_offset, 7))
 				die("offset value overflow for delta base object");
 			c = base_info[used++];
 			base_offset = (base_offset << 7) + (c & 127);