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@xxxxxx>
---
object.h | 4 ++--
revision.h | 7 ++++---
2 files changed, 6 insertions(+), 5 deletions(-)
diff --git a/object.h b/object.h
index a496d2e4e1..b207f550b2 100644
--- a/object.h
+++ b/object.h
@@ -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.
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
--
2.27.0
