diff --git a/diffcore-rename.c b/diffcore-rename.c
index 7cc2459261..dfbe65c917 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 cf8d4cb861..de01e64bec 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 de96fe4f63..ed21dc8724 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -763,15 +763,48 @@ 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);
@@ -2360,7 +2393,9 @@ 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)
@@ -2384,7 +2419,6 @@ static void resolve_diffpair_statuses(struct diff_queue_struct *q)
 	}
 }
 
-MAYBE_UNUSED
 static void prune_cached_from_relevant(struct rename_info *renames,
 				       unsigned side)
 {
@@ -2404,7 +2438,6 @@ 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)
@@ -2507,6 +2540,7 @@ 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
@@ -2535,7 +2569,8 @@ 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);
 
@@ -2643,6 +2678,8 @@ 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);
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 &&