Re: [PATCH] commit: remove padding from struct commit

[Derrick Stolee <stolee@gmail.com>]

On 11/12/2026 11:14 AM, Kristofer Karlsson via GitGitGadget wrote:
> 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

Good find.

> 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.

Thanks for measuring the memory load improvement. And of course this
won't have any impact on 32-bit machines as the struct will use
4-byte alignment by default.

>  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;
>  };
This certainly looks like it should be obviously correct. I
believe that the only order-critical part is the object
struct needing to be first so we can "cast" a commit to an
object.

Thanks,
-Stolee