xdiff: optimize patience diff's LCS search

The find_longest_common_sequence() function in patience diff is
inefficient as it calls binary_search() for every unique line it
encounters when deciding where to put it in the sequence. From
instrumentation (using xctrace) on popular repositories, binary_search()
takes up 50-60% of the run time within patience_diff() when performing a
diff.

To optimize this, add a boundary condition check before binary_search()
is called to see if the encountered unique line is located after the
entire currently tracked longest subsequence. If so, skip the
unnecessary binary search and simply append the entry to the end of
sequence. Given that most files compared in a diff are usually quite
similar to each other, this condition is very common, and should be hit
much more frequently than the binary search.

Below are some end-to-end performance results by timing `git log
--shortstat --oneline -500 --patience` on different repositories with
the old and new code. Generally speaking this seems to give at least
8-10% speed up. The "binary search hit %" column describes how often the
algorithm enters the binary search path instead of the new faster path.
Even in the WebKit case we can see that it's quite rare (1.46%).

| Repo     | Speed difference | binary search hit % |
|----------|------------------|---------------------|
| vim      | 1.27x            | 0.01%               |
| pytorch  | 1.16x            | 0.02%               |
| cpython  | 1.14x            | 0.06%               |
| ripgrep  | 1.14x            | 0.03%               |
| git      | 1.13x            | 0.12%               |
| vscode   | 1.09x            | 0.10%               |
| WebKit   | 1.08x            | 1.46%               |

The benchmarks were done using hyperfine, on an Apple M1 Max laptop,
with git compiled with `-O3 -flto`.

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/xdiff/xpatience.c b/xdiff/xpatience.c
index 77dc411d1937abbaf0e7c98f5d5bc68f4553ae64..d4094e6acf88104383798c38932e21d059f4c157 100644 (file)
--- a/xdiff/xpatience.c
+++ b/xdiff/xpatience.c
@@ -211,7 +211,10 @@ static int find_longest_common_sequence(struct hashmap *map, struct entry **res)
        for (entry = map->first; entry; entry = entry->next) {
                if (!entry->line2 || entry->line2 == NON_UNIQUE)
                        continue;
-               i = binary_search(sequence, longest, entry);
+               if (longest == 0 || entry->line2 > sequence[longest - 1]->line2)
+                       i = longest - 1;
+               else
+                       i = binary_search(sequence, longest, entry);
                entry->previous = i < 0 ? NULL : sequence[i];
                ++i;
                if (i <= anchor_i)