opensourcemirror
/
git
mirror of https://github.com/git/git.git
1
0
Fork 0
Code Issues Releases Wiki Activity

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>
This commit is contained in:
René Scharfe 2020-06-24 15:05:38 +02:00 committed by Junio C Hamano
parent c9c318d6bf
commit 23c4319f0d
2 changed files with 6 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
object.h
View File

@ -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.

7
revision.h
View File

@ -37,6 +37,10 @@
/* WARNING: This is also used as REACHABLE in commit-graph.c. */
#define PULL_MERGE (1u<<15)
#define TOPO_WALK_EXPLORED (1u<<23)
#define TOPO_WALK_INDEGREE (1u<<24)
/*
* Indicates object was reached by traversal. i.e. not given by user on
* command-line or stdin.
@ -48,9 +52,6 @@
#define TRACK_LINEAR (1u<<26)
#define ALL_REV_FLAGS (((1u<<11)-1) | NOT_USER_GIVEN | TRACK_LINEAR | PULL_MERGE)
#define TOPO_WALK_EXPLORED (1u<<27)
#define TOPO_WALK_INDEGREE (1u<<28)
#define DECORATE_SHORT_REFS 1
#define DECORATE_FULL_REFS 2