On Mon, Aug 02 2021, Patrick Steinhardt wrote:

[[PGP Signed Part:Undecided]]
When queueing up references for the revision walk, `handle_one_ref()`
will resolve the reference's object ID and then queue the ID as pending
object via `add_pending_oid()`. But `add_pending_oid()` will again try
to resolve the object ID to an object, effectively duplicating the work
its caller already did before.

Fix the issue by instead calling `add_pending_object()`, which takes the
already-resolved object as input. In a repository with lots of refs,
this translates in a nearly 10% speedup:

    Benchmark #1: HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      5.015 s ±  0.038 s    [User: 4.698 s, System: 0.316 s]
      Range (min … max):    4.970 s …  5.089 s    10 runs

    Benchmark #2: HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      4.606 s ±  0.029 s    [User: 4.260 s, System: 0.345 s]
      Range (min … max):    4.565 s …  4.657 s    10 runs

    Summary
      'HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev' ran
        1.09 ± 0.01 times faster than 'HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev'

It might be worth calling out explicitly that it's not just
"effectively", but that add_pending_oid() is just a thin wrapper for
get_reference() followed by add_pending_object(), so we're guaranteed to
get the exact same result here as before, just without the duplicate
work.

I.e. we're not going down some other lookup path that uses object
lookups with different flags or whatever as a result of this change.