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| author | René Scharfe <l.s.r@web.de> | 2020-06-24 15:05:38 +0200 |
|---|---|---|
| committer | Junio C Hamano <gitster@pobox.com> | 2020-06-24 18:09:44 +0200 |
| commit | 23c4319f0d1bd68a15368e97a4612768333ff486 (patch) | |
| tree | 27964280b803234451472ad993626487f88dbe72 /object.h | |
| parent | The fourth batch (diff) | |
| download | git-23c4319f0d1bd68a15368e97a4612768333ff486.tar.xz git-23c4319f0d1bd68a15368e97a4612768333ff486.zip | |
revision: reallocate TOPO_WALK object flags
The bit fields in struct object have an unfortunate layout. Here's what
pahole reports on x86_64 GNU/Linux:
struct object {
unsigned int parsed:1; /* 0: 0 4 */
unsigned int type:3; /* 0: 1 4 */
/* XXX 28 bits hole, try to pack */
/* Force alignment to the next boundary: */
unsigned int :0;
unsigned int flags:29; /* 4: 0 4 */
/* XXX 3 bits hole, try to pack */
struct object_id oid; /* 8 32 */
/* size: 40, cachelines: 1, members: 4 */
/* sum members: 32 */
/* sum bitfield members: 33 bits, bit holes: 2, sum bit holes: 31 bits */
/* last cacheline: 40 bytes */
};
Notice the 1+3+29=33 bits in bit fields and 28+3=31 bits in holes.
There are holes inside the flags bit field as well -- while some object
flags are used for more than one purpose, 22, 23 and 24 are still free.
Use 23 and 24 instead of 27 and 28 for TOPO_WALK_EXPLORED and
TOPO_WALK_INDEGREE. This allows us to reduce FLAG_BITS by one so that
all bitfields combined fit into a single 32-bit slot:
struct object {
unsigned int parsed:1; /* 0: 0 4 */
unsigned int type:3; /* 0: 1 4 */
unsigned int flags:28; /* 0: 4 4 */
struct object_id oid; /* 4 32 */
/* size: 36, cachelines: 1, members: 4 */
/* last cacheline: 36 bytes */
};
With this tight packing the size of struct object is reduced by 10%.
Other architectures probably benefit as well.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'object.h')
| -rw-r--r-- | object.h | 4 |
1 files changed, 2 insertions, 2 deletions
@@ -59,7 +59,7 @@ struct object_array { /* * object flag allocation: - * revision.h: 0---------10 15 25----28 + * revision.h: 0---------10 15 23------26 * fetch-pack.c: 01 * negotiator/default.c: 2--5 * walker.c: 0-2 @@ -79,7 +79,7 @@ struct object_array { * builtin/show-branch.c: 0-------------------------------------------26 * builtin/unpack-objects.c: 2021 */ -#define FLAG_BITS 29 +#define FLAG_BITS 28 /* * The object type is stored in 3 bits. |