Subject: [PATCH 0/7] Rejecting useless merge bases

From: Junio C Hamano <[email-masked]>
Date: Tue, 18 Oct 2016 21:23:38 -0700

This is a continuation of

    http://public-inbox.org/git/[email-masked]

In a workflow where topic branches are first merged to the 'next'
integration branch to be tested before getting merged down to the
'master' integration branch to be consumed by the end users, merging
the 'master' branch back to the 'next' happens after topics graduate
to 'master' and release notes entries are written for them.

Git finds many merge bases between 'master' and 'next' while
creating this merge.  In addition to the tip of 'master' back when
we made such a merge back from 'master' to 'next' was made the last
time, which is the most reasonable merge base to explain the
histories of both branches, all the tips of topic branches that
graduated recently are merge bases.  Because these tips of topic
branches were already in 'next', the tip of 'next' reaches them, and
because they just graduated to 'master', the tip of 'master' reaches
them, too.  And these topics are independent most of the time (that
is the point of employing the topic-branch workflow), so they cannot
be reduced.

The merge-recursive machinery is very inefficient to compute this
merge, because it needs to create pointless virtual merge-base
commits across these many merge bases.  Conceptually, the point
where the histories of 'master' and 'next' diverged was the tip of
'master' back when we created such a merge back from 'master' to
'next' the last time, and in practice that is the only merge base
that matters.

The series allows us to ignore these tips of topics, which are
uninteresting merge bases, when running "git merge".  The example
merge with 12 merge bases:

    git checkout 4868def05e && git merge 659889482a

in our history takes about 1.22-1.33 seconds without the series,
while running the latter "git merge" with the "--fp-base-only"
option takes about 0.54-0.59 seconds.

Junio C Hamano (7):
  commit: simplify fastpath of merge-base
  sha1_name: remove ONELINE_SEEN bit
  merge-base: stop moving commits around in remove_redundant()
  merge-base: expose get_merge_bases_many_0() a bit more
  merge-base: mark bases that are on first-parent chain
  merge-base: limit the output to bases that are on first-parent chain
  merge: allow to use only the fp-only merge bases

 Documentation/git-merge-base.txt |  8 +++-
 Documentation/merge-options.txt  |  9 +++++
 builtin/merge-base.c             | 10 +++--
 builtin/merge.c                  | 15 ++++++--
 commit.c                         | 81 ++++++++++++++++++++++++----------------
 commit.h                         |  7 +++-
 object.h                         |  3 +-
 sha1_name.c                      | 10 ++---
 8 files changed, 94 insertions(+), 49 deletions(-)

-- 
2.10.1-631-gb2c64dcf30