Commits on Aug 5, 2021

1. Change default merge backend from recursive to ort …

   There are a few reasons to switch the default:
     * Correctness
     * Extensibility
     * Performance
   
   I'll provide some summaries about each.
   
   === Correctness ===
   
   The original impetus for a new merge backend was to fix issues that were
   difficult to fix within recursive's design.  The success with this goal
   is perhaps most easily demonstrated by running the following:
   
     $ git grep -2 KNOWN_FAILURE t/ | grep -A 4 GIT_TEST_MERGE_ALGORITHM
     $ git grep test_expect_merge_algorithm.failure.success t/
     $ git grep test_expect_merge_algorithm.success.failure t/
   
   In order, these greps show:
   
     * Seven sets of submodule tests (10 total tests) that fail with
       recursive but succeed with ort
     * 22 other tests that fail with recursive, but succeed with ort
     * 0 tests that pass with recursive, but fail with ort
   
   === Extensibility ===
   
   Being able to perform merges without touching the working tree or index
   makes it possible to create new features that were difficult with the
   old backend:
   
     * Merging, cherry-picking, rebasing, reverting in bare repositories...
       or just on branches that aren't checked out.
   
     * `git diff AUTO_MERGE` -- ability to see what changes the user has
       made to resolve conflicts so far (see commit 5291828 ("merge-ort:
       write $GIT_DIR/AUTO_MERGE whenever we hit a conflict", 2021-03-20)
   
     * A --remerge-diff option for log/show, used to show diffs for merges
       that display the difference between what an automatic merge would
       have created and what was recorded in the merge.  (This option will
       often result in an empty diff because many merges are clean, but for
       the non-clean ones it will show how conflicts were fixed including
       the removal of conflict markers, and also show additional changes
       made outside of conflict regions to e.g. fix semantic conflicts.)
   
     * A --remerge-diff-only option for log/show, similar to --remerge-diff
       but also showing how cherry-picks or reverts differed from what an
       automatic cherry-pick or revert would provide.
   
   The last three have been implemented already (though only one has been
   submitted upstream so far; the others were waiting for performance work
   to complete), and I still plan to implement the first one.
   
   === Performance ===
   
   I'll quote from the summary of my final optimization for merge-ort
   (while fixing the testcase name from 'no-renames' to 'few-renames'):
   
                                  Timings
   
                                             Infinite
                    merge-       merge-     Parallelism
                   recursive    recursive    of rename    merge-ort
                    v2.30.0      current     detection     current
                   ----------   ---------   -----------   ---------
   few-renames:      18.912 s    18.030 s     11.699 s     198.3 ms
   mega-renames:   5964.031 s   361.281 s    203.886 s     661.8 ms
   just-one-mega:   149.583 s    11.009 s      7.553 s     264.6 ms
   
                              Speedup factors
   
                                             Infinite
                    merge-       merge-     Parallelism
                   recursive    recursive    of rename
                    v2.30.0      current     detection    merge-ort
                   ----------   ---------   -----------   ---------
   few-renames:        1           1.05         1.6           95
   mega-renames:       1          16.5         29           9012
   just-one-mega:      1          13.6         20            565
   
   And, for partial clone users:
   
                Factor reduction in number of objects needed
   
                                             Infinite
                    merge-       merge-     Parallelism
                   recursive    recursive    of rename
                    v2.30.0      current     detection    merge-ort
                   ----------   ---------   -----------   ---------
   mega-renames:       1            1            1          181.3
   
   Signed-off-by: Elijah Newren <newren@gmail.com>
   Signed-off-by: Junio C Hamano <gitster@pobox.com>

   

   newren authored and gitster committed Aug 5, 2021
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2. Update docs for change of default merge backend …

   Make it clear that `ort` is the default merge strategy now rather than
   `recursive`, including moving `ort` to the front of the list of merge
   strategies.
   
   Signed-off-by: Elijah Newren <newren@gmail.com>
   Signed-off-by: Junio C Hamano <gitster@pobox.com>

   

   newren authored and gitster committed Aug 5, 2021
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