Re: [RFH] git cherry vs. git rev-list --cherry, or: Why does "..." suck?

Michael J Gruber [off-list ref] writes:

Adding some recent insight:

Michael J Gruber venit, vidit, dixit 22.03.2011 13:07:

quoted

Performance
===========

I don't get this:

git cherry A B: 0.4s
git rev-list --cherry A...B: 1.7s
(more details below)

I can get the latter down to 0.95s and this

quoted

merge-base A B: 0.95s
merge-base --all A B: 0.95s
rev-parse A...B: 0.95s

to 0.16s each. The downside is that merge-base may give a few
unneccessary candidates (commits which are ancestors of other commits it
returns), but this does not change the results for rev-list, of course.

I get this dramatic speedup by removing the check for duplicates from
get_merge_bases_many() in commit.c. After a first merge_bases_many()
run, returning N commits, that check calls merge_bases_many() again for
each pair (N choose 2) to check whether one is contained in the other.
Quite a bottleneck. Removing it works great. But can we live with a few
additional merge bases?

When we run merge-base as the top-level command (this includes
reduce_heads() that is used by "git merge"), we have to cull unnecessary
phantom bases that can be reached by other bases, so you are not allowed
to make such a change unconditionally.

Passing down a parameter from a caller that is prepared to handle phantom
merge bases correctly is probably the right approach.  Existing callers
can make "safer" calls for now; you can later examine them and turn them
into "faster" calls if they operate correctly given a result that contain
phantom bases.