[PATCH 7/7] merge-ort, diffcore-rename: employ cached renames when possible

From: Elijah Newren via GitGitGadget <gitgitgadget@gmail.com>
Date: Mar 24, 2021, 21:32 UTC
Message-ID: <d1016c342d690b25da6f438bbcdd200917442531.1616621554.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.859.git.1616621553.gitgitgadget@gmail.com>

From: Elijah Newren <newren@gmail.com>

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     5.624 s ±  0.077 s
    mega-renames:     11.435 s ±  0.158 s    10.213 s ±  0.032 s
    just-one-mega:   494.2  ms ±  6.1  ms   497.6  ms ±  5.3  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      5.814 s ±  0.094 s
    mega-renames:  1799.937 s ±  0.493 s    303.225 s ±  1.330 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.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 diffcore-rename.c | 22 ++++++++++++++++++----
 diffcore.h        |  3 ++-
 merge-ort.c       | 48 ++++++++++++++++++++++++++++++++++++++++++-----
 3 files changed, 63 insertions(+), 10 deletions(-)

diff --git a/diffcore-rename.c b/diffcore-rename.c
index 7cc24592617e..dfbe65c917e9 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -568,7 +568,8 @@ static void update_dir_rename_counts(struct dir_rename_info *info,
 static void initialize_dir_rename_info(struct dir_rename_info *info,
 				       struct strintmap *relevant_sources,
 				       struct strintmap *dirs_removed,
-				       struct strmap *dir_rename_count)
+				       struct strmap *dir_rename_count,
+				       struct strmap *cached_pairs)
 {
 	struct hashmap_iter iter;
 	struct strmap_entry *entry;
@@ -633,6 +634,17 @@ static void initialize_dir_rename_info(struct dir_rename_info *info,
 					 rename_dst[i].p->two->path);
 	}
 
+	/* Add cached_pairs to counts */
+	strmap_for_each_entry(cached_pairs, &iter, entry) {
+		const char *old_name = entry->key;
+		const char *new_name = entry->value;
+		if (!new_name)
+			/* known delete; ignore it */
+			continue;
+
+		update_dir_rename_counts(info, dirs_removed, old_name, new_name);
+	}
+
 	/*
 	 * Now we collapse
 	 *    dir_rename_count: old_directory -> {new_directory -> count}
@@ -1247,7 +1259,8 @@ static void handle_early_known_dir_renames(struct dir_rename_info *info,
 void diffcore_rename_extended(struct diff_options *options,
 			      struct strintmap *relevant_sources,
 			      struct strintmap *dirs_removed,
-			      struct strmap *dir_rename_count)
+			      struct strmap *dir_rename_count,
+			      struct strmap *cached_pairs)
 {
 	int detect_rename = options->detect_rename;
 	int minimum_score = options->rename_score;
@@ -1363,7 +1376,8 @@ void diffcore_rename_extended(struct diff_options *options,
 		/* Preparation for basename-driven matching. */
 		trace2_region_enter("diff", "dir rename setup", options->repo);
 		initialize_dir_rename_info(&info, relevant_sources,
-					   dirs_removed, dir_rename_count);
+					   dirs_removed, dir_rename_count,
+					   cached_pairs);
 		trace2_region_leave("diff", "dir rename setup", options->repo);
 
 		/* Utilize file basenames to quickly find renames. */
@@ -1561,5 +1575,5 @@ void diffcore_rename_extended(struct diff_options *options,
 
 void diffcore_rename(struct diff_options *options)
 {
-	diffcore_rename_extended(options, NULL, NULL, NULL);
+	diffcore_rename_extended(options, NULL, NULL, NULL, NULL);
 }
diff --git a/diffcore.h b/diffcore.h
index cf8d4cb8617d..de01e64becaf 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -181,7 +181,8 @@ void diffcore_rename(struct diff_options *);
 void diffcore_rename_extended(struct diff_options *options,
 			      struct strintmap *relevant_sources,
 			      struct strintmap *dirs_removed,
-			      struct strmap *dir_rename_count);
+			      struct strmap *dir_rename_count,
+			      struct strmap *cached_pairs);
 void diffcore_merge_broken(void);
 void diffcore_pickaxe(struct diff_options *);
 void diffcore_order(const char *orderfile);
diff --git a/merge-ort.c b/merge-ort.c
index 8d0353ffbca2..719222aa4364 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -752,15 +752,48 @@ static void add_pair(struct merge_options *opt,
 	struct rename_info *renames = &opt->priv->renames;
 	int names_idx = is_add ? side : 0;
 
-	if (!is_add) {
+	if (is_add) {
+		if (strset_contains(&renames->cached_target_names[side],
+				    pathname))
+			return;
+	} else {
 		unsigned content_relevant = (match_mask == 0);
 		unsigned location_relevant = (dir_rename_mask == 0x07);
 
+		/*
+		 * If pathname is found in cached_irrelevant[side] due to
+		 * previous pick but for this commit content is relevant,
+		 * then we need to remove it from cached_irrelevant.
+		 */
+		if (content_relevant)
+			/* strset_remove is no-op if strset doesn't have key */
+			strset_remove(&renames->cached_irrelevant[side],
+				      pathname);
+
+		/*
+		 * We do not need to re-detect renames for paths that we already
+		 * know the pairing, i.e. for cached_pairs (or
+		 * cached_irrelevant).  However, handle_deferred_entries() needs
+		 * to loop over the union of keys from relevant_sources[side] and
+		 * cached_pairs[side], so for simplicity we set relevant_sources
+		 * for all the cached_pairs too and then strip them back out in
+		 * prune_cached_from_relevant() at the beginning of
+		 * detect_regular_renames().
+		 */
 		if (content_relevant || location_relevant) {
 			/* content_relevant trumps location_relevant */
 			strintmap_set(&renames->relevant_sources[side], pathname,
 				      content_relevant ? RELEVANT_CONTENT : RELEVANT_LOCATION);
 		}
+
+		/*
+		 * Avoid creating pair if we've already cached rename results.
+		 * Note that we do this after setting relevant_sources[side]
+		 * as noted in the comment above.
+		 */
+		if (strmap_contains(&renames->cached_pairs[side], pathname) ||
+		    strset_contains(&renames->cached_irrelevant[side], pathname))
+			return;
 	}
 
 	one = alloc_filespec(pathname);
@@ -2336,7 +2369,9 @@ static inline int possible_side_renames(struct rename_info *renames,
 static inline int possible_renames(struct rename_info *renames)
 {
 	return possible_side_renames(renames, 1) ||
-	       possible_side_renames(renames, 2);
+	       possible_side_renames(renames, 2) ||
+	       !strmap_empty(&renames->cached_pairs[1]) ||
+	       !strmap_empty(&renames->cached_pairs[2]);
 }
 
 static void resolve_diffpair_statuses(struct diff_queue_struct *q)
@@ -2360,7 +2395,6 @@ static void resolve_diffpair_statuses(struct diff_queue_struct *q)
 	}
 }
 
-MAYBE_UNUSED
 static void prune_cached_from_relevant(struct rename_info *renames,
 				       unsigned side)
 {
@@ -2380,7 +2414,6 @@ static void prune_cached_from_relevant(struct rename_info *renames,
 	}
 }
 
-MAYBE_UNUSED
 static void use_cached_pairs(struct merge_options *opt,
 			     struct strmap *cached_pairs,
 			     struct diff_queue_struct *pairs)
@@ -2463,6 +2496,7 @@ static void detect_regular_renames(struct merge_options *opt,
 	struct diff_options diff_opts;
 	struct rename_info *renames = &opt->priv->renames;
 
+	prune_cached_from_relevant(renames, side_index);
 	if (!possible_side_renames(renames, side_index)) {
 		/*
 		 * No rename detection needed for this side, but we still need
@@ -2473,6 +2507,7 @@ static void detect_regular_renames(struct merge_options *opt,
 		return;
 	}
 
+	partial_clear_dir_rename_count(&renames->dir_rename_count[side_index]);
 	repo_diff_setup(opt->repo, &diff_opts);
 	diff_opts.flags.recursive = 1;
 	diff_opts.flags.rename_empty = 0;
@@ -2490,7 +2525,8 @@ static void detect_regular_renames(struct merge_options *opt,
 	diffcore_rename_extended(&diff_opts,
 				 &renames->relevant_sources[side_index],
 				 &renames->dirs_removed[side_index],
-				 &renames->dir_rename_count[side_index]);
+				 &renames->dir_rename_count[side_index],
+				 &renames->cached_pairs[side_index]);
 	trace2_region_leave("diff", "diffcore_rename", opt->repo);
 	resolve_diffpair_statuses(&diff_queued_diff);
 
@@ -2597,6 +2633,8 @@ static int detect_and_process_renames(struct merge_options *opt,
 	trace2_region_enter("merge", "regular renames", opt->repo);
 	detect_regular_renames(opt, MERGE_SIDE1);
 	detect_regular_renames(opt, MERGE_SIDE2);
+	use_cached_pairs(opt, &renames->cached_pairs[1], &renames->pairs[1]);
+	use_cached_pairs(opt, &renames->cached_pairs[2], &renames->pairs[2]);
 	trace2_region_leave("merge", "regular renames", opt->repo);
 
 	trace2_region_enter("merge", "directory renames", opt->repo);

-- 
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