Re: [PATCH v3 3/4] revision: avoid loading object headers multiple times

> [[PGP Signed Part:Undecided]]
> When loading references, we try to optimize loading of commits by using
> the commit graph. To do so, we first need to determine whether the
> object actually is a commit or not, which is why we always execute
> `oid_object_info()` first. Like this, we'll unpack the object header of
> each object first.
>
> This pattern can be quite inefficient in case many references point to
> the same commit: if the object didn't end up in the cached objects, then
> we'll repeatedly unpack the same object header, even if we've already
> seen the object before.
>
> Optimize this pattern by using `lookup_unknown_object()` first in order
> to determine whether we've seen the object before. If so, then we don't
> need to re-parse the header but can directly use its object information
> and thus gain a modest performance improvement. Executed in a real-world
> repository with around 2.2 million references:
>
>     Benchmark #1: HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
>       Time (mean ± σ):      4.771 s ±  0.238 s    [User: 4.440 s, System: 0.330 s]
>       Range (min … max):    4.539 s …  5.219 s    10 runs
>
>     Benchmark #2: HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
>       Time (mean ± σ):      4.454 s ±  0.037 s    [User: 4.122 s, System: 0.332 s]
>       Range (min … max):    4.375 s …  4.496 s    10 runs
>
>     Summary
>       'HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev' ran
>         1.07 ± 0.05 times faster than 'HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev'
>
> The downside is that `lookup_unknown_object()` is forced to always
> allocate an object such that it's big enough to host all object types'
> structs and thus we may waste memory here. This tradeoff is probably
> worth it though considering the following struct sizes:
>
>     - commit: 72 bytes
>     - tree: 56 bytes
>     - blob: 40 bytes
>     - tag: 64 bytes
>
> Assuming that in almost all repositories, most references will point to
> either a tag or a commit, we'd have a modest increase in memory
> consumption of about 12.5% here.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  revision.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/revision.c b/revision.c
> index f06a5d63a3..671b6d6513 100644
> --- a/revision.c
> +++ b/revision.c
> @@ -359,14 +359,22 @@ static struct object *get_reference(struct rev_info *revs, const char *name,
>  				    const struct object_id *oid,
>  				    unsigned int flags)
>  {
> -	struct object *object;
> +	struct object *object = lookup_unknown_object(revs->repo, oid);
> +
> +	if (object->type == OBJ_NONE) {
> +		int type = oid_object_info(revs->repo, oid, NULL);
> +		if (type < 0 || !object_as_type(object, type, 1)) {