Re: [PATCH] revision: reallocate TOPO_WALK object flags
From: Derrick Stolee <hidden>
Date: 2020-06-24 13:51:06
On 6/24/2020 9:05 AM, René Scharfe wrote:
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.This is certainly unfortunate.
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.Excellent improvement!
quoted hunk ↗ jump to hunk
- * 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
The only collision is here in builtin/show-branch.c, and when I added the TOPO_WALK_* bits, I didn't understand that these bits were sufficiently independent from the topo-walk logic that we could add collisions here. I think I realized this behavior when doing the commit-reach.c refactor, but I never revisited these flag values.
quoted hunk ↗ jump to hunk
* builtin/unpack-objects.c: 2021 */ -#define FLAG_BITS 29 +#define FLAG_BITS 28 /* * The object type is stored in 3 bits.diff --git a/revision.h b/revision.h index 93491b79d4..f412ae85eb 100644 --- a/revision.h +++ b/revision.h@@ -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
Thankfully, the changes are very simple! Thanks, -Stolee