[PATCH 4/5] diff_filespec: reorder is_binary field
- From
Jeff King <peff@peff.net>
- Date
- Jan 17, 2014, 01:22 UTC
- Message-ID
- <20140117012255.GD7249@sigill.intra.peff.net>
- In-Reply-To
- <20140117011844.GA6870@sigill.intra.peff.net>
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> ---
diffcore.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/diffcore.h b/diffcore.h index 22993e1..d911bf0 100644 --- 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 *);
-- 1.8.5.2.500.g8060133