[PATCH v2 24/24] pack-bitmap-write: better reuse bitmaps

[Taylor Blau <me@ttaylorr.com>]

From: Derrick Stolee <dstolee@microsoft.com>

If the old bitmap file contains a bitmap for a given commit, then that
commit does not need help from intermediate commits in its history to
compute its final bitmap. Eject that commit from the walk and insert it
as a maximal commit in the list of commits for computing bitmaps.

This helps the repeat bitmap computation task, even if the selected
commits shift drastically. This helps when a previously-bitmapped commit
exists in the first-parent history of a newly-selected commit. Since we
stop the walk at these commits and we use a first-parent walk, it is
harder to walk "around" these bitmapped commits. It's not impossible,
but we can greatly reduce the computation time for many selected
commits.

             |   runtime (sec)    |   peak heap (GB)   |
             |                    |                    |
             |   from  |   with   |   from  |   with   |
             | scratch | existing | scratch | existing |
  -----------+---------+----------+---------+-----------
  last patch | 100.641 |   35.560 |   2.152 |    2.224 |
  this patch |  99.720 |   11.696 |   2.152 |    2.217 |

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 pack-bitmap-write.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
index b0493d971d..3ac90ae410 100644
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@@ -195,7 +195,8 @@ struct bitmap_builder {
 };
 
 static void bitmap_builder_init(struct bitmap_builder *bb,
-				struct bitmap_writer *writer)
+				struct bitmap_writer *writer,
+				struct bitmap_index *old_bitmap)
 {
 	struct rev_info revs;
 	struct commit *commit;
@@ -234,12 +235,26 @@ static void bitmap_builder_init(struct bitmap_builder *bb,
 
 		c_ent = bb_data_at(&bb->data, commit);
 
+		if (old_bitmap && bitmap_for_commit(old_bitmap, commit)) {
+			/*
+			 * This commit has an existing bitmap, so we can
+			 * get its bits immediately without an object
+			 * walk. There is no need to continue walking
+			 * beyond this commit.
+			 */
+			c_ent->maximal = 1;
+			p = NULL;
+		}
+
 		if (c_ent->maximal) {
 			num_maximal++;
 			ALLOC_GROW(bb->commits, bb->commits_nr + 1, bb->commits_alloc);
 			bb->commits[bb->commits_nr++] = commit;
 		}
 
+		if (!c_ent->commit_mask)
+			continue;
+
 		if (p) {
 			struct bb_commit *p_ent = bb_data_at(&bb->data, p->item);
 			int c_not_p, p_not_c;
@@ -422,7 +437,7 @@ void bitmap_writer_build(struct packing_data *to_pack)
 	else
 		mapping = NULL;
 
-	bitmap_builder_init(&bb, &writer);
+	bitmap_builder_init(&bb, &writer, old_bitmap);
 	for (i = bb.commits_nr; i > 0; i--) {
 		struct commit *commit = bb.commits[i-1];
 		struct bb_commit *ent = bb_data_at(&bb.data, commit);
-- 
2.29.2.312.gabc4d358d8