merge-ort: add data structures for in-memory caching of rename detection

When there are many renames between the old base of a series of commits
and the new base for a series of commits, the sequence of merges
employed to transplant those commits (from a cherry-pick or rebase
operation) will repeatedly detect the exact same renames.  This is
wasted effort.

Add data structures which will be used to cache rename detection
results, along with the initialization and deallocation of these data
structures.  Future commits will populate these caches, detect the
appropriate circumstances when they can be used, and employ them to
avoid re-detecting the same renames repeatedly.

Signed-off-by: Elijah Newren <[email protected]>

Author: newren

merge-ort.c

@@ -138,6 +138,37 @@ struct rename_info {
 	int callback_data_nr, callback_data_alloc;
 	char *callback_data_traverse_path;
 
+	/*
+	 * cached_pairs: Caching of renames and deletions.
+	 *
+	 * These are mappings recording renames and deletions of individual
+	 * files (not directories).  They are thus a map from an old
+	 * filename to either NULL (for deletions) or a new filename (for
+	 * renames).
+	 */
+	struct strmap cached_pairs[3];
+
+	/*
+	 * cached_target_names: just the destinations from cached_pairs
+	 *
+	 * We sometimes want a fast lookup to determine if a given filename
+	 * is one of the destinations in cached_pairs.  cached_target_names
+	 * is thus duplicative information, but it provides a fast lookup.
+	 */
+	struct strset cached_target_names[3];
+
+	/*
+	 * cached_irrelevant: Caching of rename_sources that aren't relevant.
+	 *
+	 * cached_pairs records both renames and deletes.  Sometimes we
+	 * do not know if a path is a rename or a delete because we pass
+	 * RELEVANT_LOCATION to diffcore_rename_extended() and based on
+	 * various optimizations it returns without detecting whether that
+	 * path is actually a rename or a delete.  We need to cache such
+	 * paths too, but separately from cached_pairs.
+	 */
+	struct strset cached_irrelevant[3];
+
 	/*
 	 * needed_limit: value needed for inexact rename detection to run
 	 *
@@ -370,6 +401,8 @@ static void clear_or_reinit_internal_opts(struct merge_options_internal *opti,
 		reinitialize ? strmap_partial_clear : strmap_clear;
 	void (*strintmap_func)(struct strintmap *) =
 		reinitialize ? strintmap_partial_clear : strintmap_clear;
+	void (*strset_func)(struct strset *) =
+		reinitialize ? strset_partial_clear : strset_clear;
 
 	/*
 	 * We marked opti->paths with strdup_strings = 0, so that we
@@ -410,6 +443,9 @@ static void clear_or_reinit_internal_opts(struct merge_options_internal *opti,
 		strmap_func(&renames->dir_renames[i], 0);
 
 		strintmap_func(&renames->relevant_sources[i]);
+		strset_func(&renames->cached_target_names[i]);
+		strmap_func(&renames->cached_pairs[i], 1);
+		strset_func(&renames->cached_irrelevant[i]);
 	}
 
 	if (!reinitialize) {
@@ -3498,6 +3534,12 @@ static void merge_start(struct merge_options *opt, struct merge_result *result)
 					 NULL, 0);
 		strintmap_init_with_options(&renames->relevant_sources[i],
 					    0, NULL, 0);
+		strmap_init_with_options(&renames->cached_pairs[i],
+					 NULL, 1);
+		strset_init_with_options(&renames->cached_irrelevant[i],
+					 NULL, 1);
+		strset_init_with_options(&renames->cached_target_names[i],
+					 NULL, 0);
 	}
 
 	/*