Subject: Re: [PATCH 0/7] Optimization batch 11: avoid repeatedly detecting same renames

From: Junio C Hamano <[email-masked]>
Date: Thu, 25 Mar 2021 11:59:55 -0700

Elijah Newren <[email-masked]> writes:

>> And each step in this "rebase" is a 3-way merge of commits A, B and
>> C onto HEAD, using the parent of the commit being cherrk-picked as a
>> virtual common ancestor.  Which means
>
> You generated nearly the same description and diagram I used in the
> commit message (the one in 3/7) describing this.  :-)
>
>>  - To transplant A (i.e. the first step), we'd compare the diff of
>>    A^..O (i.e. what our side did, including the renames done at X)
>>    and diff of A^..A (i.e. what the first commit did in the range),
>>    and the former does quite a lot of rename detection.
>>
>>  - After transplanting B (i.e. the second step), then we'd compare
>>    the diff of A^..A' (where A' is A cherry-picked on O, i.e. the
>
> Close, but for transplanting B we do the diff of B^..A', not A^...A'.
> (And in this diagram, B^ is A.)  That's critical below...

Yes, I upfront said "pretend that the parent of the commit being
picked is the common ancestor and run 3-way merge", but then got
confused by the ancestry graph myself, forgetting that the reason
why A^ is used in the first "pick" is *not* because the it is the
fork point of our history and the side branch, but it is because it
is A's parent.

And if the renames in B^..A' and A^..A' are different that must have
come only from the difference between A..B (which is B^..B), but
that comparison is what we do when cherry-picking B on top of A',
so it is easy to take into account to reuse the renames precisely
without "assuming they are the same".

Thanks.