From 23c4319f0d1bd68a15368e97a4612768333ff486 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Ren=C3=A9=20Scharfe?= Date: Wed, 24 Jun 2020 15:05:38 +0200 Subject: revision: reallocate TOPO_WALK object flags MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Signed-off-by: Junio C Hamano --- object.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'object.h') diff --git a/object.h b/object.h index b22328b838..941dd33ca7 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. -- cgit v1.2.3