tree 27964280b803234451472ad993626487f88dbe72
parent c9c318d6bf26bcecdca5b6f31683b9d5887a83ee
author René Scharfe <l.s.r@web.de> 1593003938 +0200
committer Junio C Hamano <gitster@pobox.com> 1593014984 -0700

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>