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>

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.

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