diff_filespec: reorder is_binary field

The middle of the diff_filespec struct contains a mixture of
ints, shorts, and bit-fields, followed by a pointer. On an
x86-64 system with an LP64 or LLP64 data model (i.e., most
of them), the integers and flags end up being padded out by
41 bits to put the pointer at an 8-byte boundary.

After the pointer, we have the "int is_binary" field, which
is only 32 bits. We end up wasting another 32 bits to pad
the struct size up to a multiple of 64 bits.

We can move the is_binary field before the pointer, which
lets the compiler store it where we used to have padding.
This shrinks the top padding to only 9 bits (from the
bit-fields), and eliminates the bottom padding entirely,
dropping the struct size from 88 to 80 bytes.

On a 32-bit system, there is no benefit, but nor should
there be any harm (we only need 4-byte alignment there, so
we were already using only 9 bits of padding).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/diffcore.h b/diffcore.h
index 22993e1effc8a557f9b169abbe15fbed26e0fbb4..d911bf0a42eeef68023459d6b03a8d1cafa74683 100644 (file)
--- a/diffcore.h
+++ b/diffcore.h
@@ -45,9 +45,9 @@ struct diff_filespec {
 #define DIRTY_SUBMODULE_MODIFIED  2
        unsigned is_stdin : 1;
        unsigned has_more_entries : 1; /* only appear in combined diff */
-       struct userdiff_driver *driver;
        /* data should be considered "binary"; -1 means "don't know yet" */
        int is_binary;
+       struct userdiff_driver *driver;
 };
 
 extern struct diff_filespec *alloc_filespec(const char *);