[PATCH] commit: remove padding from struct commit

["Kristofer Karlsson via GitGitGadget" <gitgitgadget@gmail.com>]

From: Kristofer Karlsson <krka@spotify.com>

On 64-bit platforms, "struct commit" is 80 bytes, 8 of which are
padding:
 - 4 bytes between .object and .date
 - 4 bytes of tail padding after .index

Reduce memory usage by moving .index into the hole after .object.
On 64-bit platforms this shrinks the struct to 72 bytes and
eliminates the padding, resulting in 10% less memory for every commit
allocated, with no change in behaviour.

On platforms where 8-byte members only need 4-byte alignment (e.g.
i386) both layouts are already free of padding, so the size stays the
same there.

Measuring peak RSS of "git rev-list --all" over an internal monorepo
with ~4.5 million commits (three runs each):

             peak RSS     time
  before     ~849 MiB   12.8-13.7s
  after      ~817 MiB   12.8-13.0s

The ~32 MiB saved is close to the expected 8 bytes * 4.5M commits
(~34 MiB); running time is unchanged within noise.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
---
    commit: remove padding from struct commit
    
    I stumbled on this while rabbit-holing on an unrelated change to the
    commit walk, by looking at the layout with gdb's "ptype /o".
    
    Digging into the history showed that the size and padding of "struct
    commit" have changed several times, sometimes undoing earlier
    optimizations.
    
    I also prototyped a regression test that compares the size of "struct
    commit" and "struct object" against the sum of their members' sizes,
    plus any expected padding. I left it out of this patch because I am not
    sure whether:
    
     1. such a test is worth the cost of updating it whenever members are
        added or removed, or
    
     2. a unit test is the right place for it.
    
    I am happy to send it as a follow-up if there is interest.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2309%2Fspkrka%2Fcommit-index-hole-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2309/spkrka/commit-index-hole-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2309

 commit.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/commit.h b/commit.h
index 1061ed791b..66a5d245f2 100644
--- a/commit.h
+++ b/commit.h
@@ -26,6 +26,7 @@ struct commit_list {
  */
 struct commit {
 	struct object object;
+	unsigned int index;
 	timestamp_t date;
 	struct commit_list *parents;
 
@@ -35,7 +36,6 @@ struct commit {
 	 * or get_commit_tree_oid().
 	 */
 	struct tree *maybe_tree;
-	unsigned int index;
 };
 
 extern int save_commit_buffer;

base-commit: 1891de46f79dc444a47db1c87bd0c8c76b5463f2
-- 
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