merge-ort, diffcore-rename: employ cached renames when possible

When there are many renames between the old base of a series of commits
and the new base, the way sequencer.c, merge-recursive.c, and
diffcore-rename.c have traditionally split the work resulted in
redetecting the same renames with each and every commit being
transplanted.  To address this, the last several commits have been
creating a cache of rename detection results, determining when it was
safe to use such a cache in subsequent merge operations, adding helper
functions, and so on.  See the previous half dozen commit messages for
additional discussion of this optimization, particularly the message a
few commits ago entitled "add code to check for whether cached renames
can be reused".  This commit finally ties all of that work together,
modifying the merge algorithm to make use of these cached renames.

For the testcases mentioned in commit 557ac0350d ("merge-ort: begin
performance work; instrument with trace2_region_* calls", 2020-10-28),
this change improves the performance as follows:

                            Before                  After
    no-renames:        5.665 s ±  0.129 s     5.622 s ±  0.059 s
    mega-renames:     11.435 s ±  0.158 s    10.127 s ±  0.073 s
    just-one-mega:   494.2  ms ±  6.1  ms   500.3  ms ±  3.8  ms

That's a fairly small improvement, but mostly because the previous
optimizations were so effective for these particular testcases; this
optimization only kicks in when the others don't.  If we undid the
basename-guided rename detection and skip-irrelevant-renames
optimizations, then we'd see that this series by itself improved
performance as follows:

                   Before Basename Series   After Just This Series
    no-renames:      13.815 s ±  0.062 s      5.697 s ±  0.080 s
    mega-renames:  1799.937 s ±  0.493 s    205.709 s ±  0.457 s

Since this optimization kicks in to help accelerate cases where the
previous optimizations do not apply, this last comparison shows that
this cached-renames optimization has the potential to help signficantly
in cases that don't meet the requirements for the other optimizations to
be effective.

The changes made in this optimization also lay some important groundwork
for a future optimization around having collect_merge_info() avoid
recursing into subtrees in more cases.

However, for this optimization to be effective, merge_switch_to_result()
should only be called when the rebase or cherry-pick operation has
either completed or hit a case where the user needs to resolve a
conflict or edit the result.  If it is called after every commit, as
sequencer.c does, then the working tree and index are needlessly updated
with every commit and the cached metadata is tossed, defeating this
optimization.  Refactoring sequencer.c to only call
merge_switch_to_result() at the end of the operation is a bigger
undertaking, and the practical benefits of this optimization will not be
realized until that work is performed.  Since `test-tool fast-rebase`
only updates at the end of the operation, it was used to obtain the
timings above.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 28 insertions, 20 deletions

diff --git a/t/t6429-merge-sequence-rename-caching.sh b/t/t6429-merge-sequence-rename-caching.sh
index f47d8924ee..035edc40b1 100755
--- a/t/t6429-merge-sequence-rename-caching.sh
+++ b/t/t6429-merge-sequence-rename-caching.sh
@@ -101,10 +101,10 @@ test_expect_success 'caching renames does not preclude finding new ones' '
 # dramatic change in size of the file, but remembering the rename and
 # reusing it is reasonable too.
 #
-# Rename detection (diffcore_rename_extended()) will run twice here; it is
-# not needed on the topic side of history for either of the two commits
-# being merged, but it is needed on the upstream side of history for each
-# commit being picked.
+# We do test here that we expect rename detection to only be run once total
+# (the topic side of history doesn't need renames, and with caching we
+# should be able to only run rename detection on the upstream side one
+# time.)
 test_expect_success 'cherry-pick both a commit and its immediate revert' '
 	test_create_repo pick-commit-and-its-immediate-revert &&
 	(
@@ -140,11 +140,11 @@ test_expect_success 'cherry-pick both a commit and its immediate revert' '
 		GIT_TRACE2_PERF="$(pwd)/trace.output" &&
 		export GIT_TRACE2_PERF &&
 
-		test_might_fail test-tool fast-rebase --onto HEAD upstream~1 topic &&
+		test-tool fast-rebase --onto HEAD upstream~1 topic &&
 		#git cherry-pick upstream~1..topic &&
 
 		grep region_enter.*diffcore_rename trace.output >calls &&
-		test_line_count = 2 calls
+		test_line_count = 1 calls
 	)
 '
 
@@ -304,9 +304,11 @@ test_expect_success 'rename same file identically, then add file to old dir' '
 # Here we are just concerned that cached renames might prevent us from seeing
 # the rename conflict, and we want to ensure that we do get a conflict.
 #
-# While at it, also test that we do rename detection three times.  We have to
-# detect renames on the upstream side of history once for each merge, plus
-# Topic_2 has renames.
+# While at it, though, we do test that we only try to detect renames 2
+# times and not three.  (The first merge needs to detect renames on the
+# upstream side.  Traditionally, the second merge would need to detect
+# renames on both sides of history, but our caching of upstream renames
+# should avoid the need to re-detect upstream renames.)
 #
 test_expect_success 'cached dir rename does not prevent noticing later conflict' '
 	test_create_repo dir-rename-cache-not-occluding-later-conflict &&
@@ -357,7 +359,7 @@ test_expect_success 'cached dir rename does not prevent noticing later conflict'
 		grep CONFLICT..rename/rename output &&
 
 		grep region_enter.*diffcore_rename trace.output >calls &&
-		test_line_count = 3 calls
+		test_line_count = 2 calls
 	)
 '
 
@@ -412,10 +414,17 @@ test_setup_upstream_rename () {
 # commit to mess up its location either.  We want to make sure that
 # olddir/newfile doesn't exist in the result and that newdir/newfile does.
 #
-# We also expect rename detection to occur three times.  Although it is
-# typically needed two times per commit, there are no deleted files on the
-# topic side of history, so we only need to detect renames on the upstream
-# side for each of the 3 commits we need to pick.
+# We also test that we only do rename detection twice.  We never need
+# rename detection on the topic side of history, but we do need it twice on
+# the upstream side of history.  For the first topic commit, we only need
+# the
+#   relevant-rename -> renamed
+# rename, because olddir is unmodified by Topic_1.  For Topic_2, however,
+# the new file being added to olddir means files that were previously
+# irrelevant for rename detection are now relevant, forcing us to repeat
+# rename detection for the paths we don't already have cached.  Topic_3 also
+# tweaks olddir/newfile, but the renames in olddir/ will have been cached
+# from the second rename detection run.
 #
 test_expect_success 'dir rename unneeded, then add new file to old dir' '
 	test_setup_upstream_rename dir-rename-unneeded-until-new-file &&
@@ -450,7 +459,7 @@ test_expect_success 'dir rename unneeded, then add new file to old dir' '
 		#git cherry-pick upstream..topic &&
 
 		grep region_enter.*diffcore_rename trace.output >calls &&
-		test_line_count = 3 calls &&
+		test_line_count = 2 calls &&
 
 		git ls-files >tracked &&
 		test_line_count = 5 tracked &&
@@ -516,7 +525,7 @@ test_expect_success 'dir rename unneeded, then rename existing file into old dir
 		#git cherry-pick upstream..topic &&
 
 		grep region_enter.*diffcore_rename trace.output >calls &&
-		test_line_count = 4 calls &&
+		test_line_count = 3 calls &&
 
 		test_path_is_missing somefile &&
 		test_path_is_missing olddir/newfile &&
@@ -648,9 +657,8 @@ test_expect_success 'caching renames only on upstream side, part 1' '
 # for the wrong side of history.
 #
 #
-# This testcase should only need three calls to diffcore_rename_extended(),
-# because there are no renames on the topic side of history for picking
-# Topic_2.
+# This testcase should only need two calls to diffcore_rename_extended(),
+# both for the first merge, one for each side of history.
 #
 test_expect_success 'caching renames only on upstream side, part 2' '
 	test_setup_topic_rename cache-renames-only-upstream-rename-file &&
@@ -677,7 +685,7 @@ test_expect_success 'caching renames only on upstream side, part 2' '
 		#git cherry-pick upstream..topic &&
 
 		grep region_enter.*diffcore_rename trace.output >calls &&
-		test_line_count = 3 calls &&
+		test_line_count = 2 calls &&
 
 		git ls-files >tracked &&
 		test_line_count = 4 tracked &&