Subject: Re: [PATCH] hash: reduce size of algo member of object ID

From: Eric Wong <[email-masked]>
Date: Mon, 4 Oct 2021 07:31:42 +0000

René Scharfe <[email-masked]> wrote:
> cf0983213c (hash: add an algo member to struct object_id, 2021-04-26)
> introduced the algo member as an int.  This increased the size of struct
> object_id by 4 bytes (12.5%) and imposed a 4-byte alignment.  Currently
> we only need to stored the values 0, 1 and 2 in it.  Let's use an
> unsigned char instead to reduce the size overhead and lift the alignment
> requirement.

I like it.  Btw, would a bitfield enum be portable enough for us?

	enum git_hash_algo algo:8;  /* or algo:2 */

I've used those in other projects, but AFAIK they were for
gcc||clang-only.

> Not sure how to measure the performance impact of this change.  The perf
> tests mentioned by cf0983213c don't show much of a difference with
> GIT_PERF_REPEAT_COUNT=10 for me:

Thanks for that info.  I'm not familiar enough with most git
internals to know if it ought to make a difference as-is.

In my experience, changes like this open the door to further
struct-packing opportunities (possibly combined with conditional
use of __attribute__((packed)) ).