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

Junio C Hamano <gitster@pobox.com> writes:

I scanned all two-parent merges in git.git leading to 'next' and 'pu' and reproduced them with or without the new --fp-base-only option. I also did the same experiment in linux.git, but only for the latest 10,000 merges.

This exercise yielded some interesting numbers.

Among the total 10,995 merges, 10,487 merges had only one merge base and that merge base was on the first-parent chain, i.e. the result of reproduction is the same with or without "--fp-base-only".

There were 65 merges with multiple merge-bases but all of these merge bases were on the first-parent chain, i.e. again the result is the same with or without "--fp-base-only".

The remaining 443 merges had merge bases that are not on the first-parent chain. These merges, when recreated with "--fp-base-only", would use different/reduced set of merge bases to drive merge-recursive machinery and could produce different results.

Among these 443, "git merge" with or without the "--fp-base-only" option successfully auto-resolved and produced the same result as recorded in the real history for 214 of them. They stopped in conflicts but "git ls-files -s" output in their conflicted states were the same, i.e. they left the identical conflicts, for 221 of them.

The remaining 8 merges were auto-resolved the same way with or without the "--fp-base-only" option, but they were different from the merge in the real history (i.e. the real history had an evil merge there to adjust for non-textual conflicts).

The most important numbers was 0. There was no merges whose reproduction with and without "--fp-base-only" produced different results.

For linux.git the numbers are:

 - total merges looked at:                         10,000
 - single merge base:                               9,705
 - multi merge base all on the first parent chain:    123
 - fp-base-only eligible:                             172
 - cleanly resolved the same way w/ or w/o fp-only:   118
 - conflicted the same way w/ or w/o fp-only:          54
 - evil merges among fp-base-only eligible merges:      0
 - fp-base-only mismerges:                              0