Optimization batch 14: trivial directory resolution

This series depends textually on ort-perf-batch-12, but is semantically independent. (It is both semantically and textually independent of ort-perf-batch-13.)

Most of my previous series dramatically accelerated cases with lots of renames, while providing comparatively minor benefits for cases with few or no renames. This series is the opposite; it provides huge benefits when there are few or no renames, and comparatively smaller (though still quite decent) benefits for cases with many uncached renames.

Changes since v3:

* Add Stolee's Acked-by.

Changes since v2, addressing feedback from Stolee:

* Created a separate struct for three related variables to hint they are related
* Simplified a lengthy comment that was duplicated by the commit message
* Various other minor cleanups

Changes since v1:

* Minor tweak to the final patch to correct implicit assumption that rename detection running implies all renames were found (rename limits could have been exceeded and prevented finding renames)

=== Basic Optimization idea ===

unpack_trees has had a concept of trivial merges for individual files (see Documentation/technical/trivial-merge.txt). The same idea can be applied in merge-ort. It'd be really nice to extend that idea to trees as well, as it could provide a huge performance boost; sadly however, applying it in general would wreck both regular rename detection (the unmatched side can have new files that serve as potential destinations in rename detection) and directory rename detection (the unmatched side could have a new directory that was moved into it).

If we somehow knew rename detection wasn't needed, we could do trivial directory resolution. In the past, this wasn't possible. However...

With recent optimizations we have created a possibility to do trivial directory resolutions in some cases. These came from the addition of the "skipping irrelevant renames" optimizations (from ort-perf-batch-9 and ort-perf-batch-10), and in particular noting that we added an ability to entirely skip rename detection in commit f89b4f2bee ("merge-ort: skip rename detection entirely if possible", 2021-03-11) when there are no relevant sources. We can detect if there are no relevant sources without recursing into the directories in question.

As a cherry on top, the caching of renames (from ort-perf-batch-11) allows us to cover additional cases.

This series is all about adding all the special checks needed to safely perform trival directory resolutions.

=== Results ===

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

Before Series After Series

no-renames:        5.235 s ±  0.042 s   204.2  ms ±  3.0  ms
mega-renames:      9.419 s ±  0.107 s     1.076 s ±  0.015 s
just-one-mega:   480.1  ms ±  3.9  ms   364.1  ms ±  7.0  ms

As a reminder, before any merge-ort/diffcore-rename performance work, the performance results we started with (for merge-recursive as of git-2.30.0) were:

no-renames-am:      6.940 s ±  0.485 s
no-renames:        18.912 s ±  0.174 s
mega-renames:    5964.031 s ± 10.459 s
just-one-mega:    149.583 s ±  0.751 s

Elijah Newren (7):
merge-ort: resolve paths early when we have sufficient information
merge-ort: add some more explanations in collect_merge_info_callback()
merge-ort: add data structures for allowable trivial directory
resolves
merge-ort: add a handle_deferred_entries() helper function
merge-ort: defer recursing into directories when merge base is matched
merge-ort: avoid recursing into directories when we don't need to
merge-ort: restart merge with cached renames to reduce process entry
cost

 merge-ort.c                         | 399 +++++++++++++++++++++++++++-
 t/t6423-merge-rename-directories.sh |   2 +-
 2 files changed, 389 insertions(+), 12 deletions(-)

base-commit: 2eeee12b02e441ac05054a5a5ecbcea6964a1e6b
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-988%2Fnewren%2Fort-perf-batch-14-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-988/newren/ort-perf-batch-14-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/988

Range-diff vs v3:

1: 5dca982c0b0 ! 1: 7fdfeb159d0 merge-ort: resolve paths early when we have sufficient information
@@ Commit message
range of noise. However, this idea serves as the beginning of some
bigger optimizations coming in the following patches.

+ Acked-by: Derrick Stolee <[email protected]>

## merge-ort.c ##
2: 8aea3713902 ! 2: 7a0085f2da9 merge-ort: add some more explanations in collect_merge_info_callback()
@@ Commit message
better. While we're at it, add a few comments to denote what a few
boolean '0' or '1' values stand for.

+ Acked-by: Derrick Stolee <[email protected]>

## merge-ort.c ##
3: c2b45fef1d7 ! 3: d8165740316 merge-ort: add data structures for allowable trivial directory resolves
@@ Commit message
Add some data structures that we will use to do these deferrals, with
some lengthy comments explaining their purpose.

+ Acked-by: Derrick Stolee <[email protected]>

## merge-ort.c ##
4: 1cf4a47562a ! 4: ff568a612f5 merge-ort: add a handle_deferred_entries() helper function
@@ Commit message
but a subsequent commit will change that. Future commits will also make the function sometimes resolve directories instead of traversing inside.

+ Acked-by: Derrick Stolee <[email protected]>

## merge-ort.c ##
5: 79c51536829 ! 5: 5b01c118f10 merge-ort: defer recursing into directories when merge base is matched
@@ Commit message
subdirectories. The extra conditions are there for such deferred cases
and will be used more as we do more with those variables.

+ Acked-by: Derrick Stolee <[email protected]>

## merge-ort.c ##
6: 572cc5e94d2 ! 6: 7b211271815 merge-ort: avoid recursing into directories when we don't need to
@@ Commit message
mega-renames: 9.419 s ± 0.107 s 1.564 s ± 0.010 s
just-one-mega: 480.1 ms ± 3.9 ms 479.5 ms ± 3.9 ms

+ Acked-by: Derrick Stolee <[email protected]>

## merge-ort.c ##
7: a9cbc1d4f18 ! 7: c9ada8369e6 merge-ort: restart merge with cached renames to reduce process entry cost
@@ Commit message
mega-renames: 1.564 s ± 0.010 s 1.076 s ± 0.015 s
just-one-mega: 479.5 ms ± 3.9 ms 364.1 ms ± 7.0 ms

+ Acked-by: Derrick Stolee <[email protected]>

## merge-ort.c ##

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