From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 17 Jun 2020 20:46:56 GMT
Subject: Re: [PATCH 0/2] Accelerate "git merge-base --is-ancestor"
Message-ID: <xmqqzh91s9mn.fsf@gitster.c.googlers.com>
In-Reply-To: <pull.664.git.1592414670.gitgitgadget@gmail.com>

"Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:

> It was recently [1] reported (and not-so-recently [2] reported) that "git
> merge-base --is-ancestor" can be pretty slow. In fact, it is regularly
> slower than "git branch --contains" or "git tag --contains", which are
> answering a "harder" query.
>
> [1] https://lore.kernel.org/git/20200607195347.GA8232@szeder.dev/
>
> [2] https://lore.kernel.org/git/87608bawoa.fsf@evledraar.gmail.com/
>
> The root cause is that the in_merge_base() implementation is skipping the
> fast can_all_from_reach() implementation and using paint_down_to_common()
> instead. Note that these are equivalent: a commit A is in the set of
> merge-bases between A and B if and only if B can reach A.

OK, so in short, this codepath was not taking advantage of the
generation numbers and that was the difference?