sha1_file.c: avoid gcc signed overflow warnings

With the recent gcc, we get:

sha1_file.c: In check_packed_git_:
sha1_file.c:527: warning: assuming signed overflow does not
occur when assuming that (X + c) < X is always false
sha1_file.c:527: warning: assuming signed overflow does not
occur when assuming that (X + c) < X is always false

for a piece of code that tries to make sure that off_t is large
enough to hold more than 2^32 offset.  The test tried to make
sure these do not wrap-around:

    /* make sure we can deal with large pack offsets */
    off_t x = 0x7fffffffUL, y = 0xffffffffUL;
    if (x > (x + 1) || y > (y + 1)) {

but gcc assumes it can do whatever optimization it wants for a
signed overflow (undefined behaviour) and warns about this
construct.

Follow Linus's suggestion to check sizeof(off_t) instead to work
around the problem.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/sha1_file.c b/sha1_file.c
index 9978a58da68bbf6f3482545d9f290fbfa3f3fe34..95b5a403d8d7e8c1657fc8d4c1a8efd15c6a4ca3 100644 (file)
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -521,13 +521,15 @@ static int check_packed_git_idx(const char *path,  struct packed_git *p)
                        munmap(idx_map, idx_size);
                        return error("wrong index v2 file size in %s", path);
                }
-               if (idx_size != min_size) {
-                       /* make sure we can deal with large pack offsets */
-                       off_t x = 0x7fffffffUL, y = 0xffffffffUL;
-                       if (x > (x + 1) || y > (y + 1)) {
-                               munmap(idx_map, idx_size);
-                               return error("pack too large for current definition of off_t in %s", path);
-                       }
+               if (idx_size != min_size &&
+                   /*
+                    * make sure we can deal with large pack offsets.
+                    * 31-bit signed offset won't be enough, neither
+                    * 32-bit unsigned one will be.
+                    */
+                   (sizeof(off_t) <= 4)) {
+                       munmap(idx_map, idx_size);
+                       return error("pack too large for current definition of off_t in %s", path);
                }
        }