pack-bitmap-write: better reuse bitmaps

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
into a separate list of reusable commits that are eventually stored 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 |  88.478 |   53.218 |   2.157 |    2.224 |
  this patch |  86.681 |   16.164 |   2.157 |    2.222 |

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap-write.c

@@ -195,10 +195,13 @@ 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;
+	struct commit_list *reusable = NULL;
+	struct commit_list *r;
 	unsigned int i, num_maximal = 0;
 
 	memset(bb, 0, sizeof(*bb));
@@ -234,6 +237,31 @@ static void bitmap_builder_init(struct bitmap_builder *bb,
 
 		c_ent = bb_data_at(&bb->data, commit);
 
+		/*
+		 * If there is no commit_mask, there is no reason to iterate
+		 * over this commit; it is not selected (if it were, it would
+		 * not have a blank commit mask) and all its children have
+		 * existing bitmaps (see the comment starting with "This commit
+		 * has an existing bitmap" below), so it does not contribute
+		 * anything to the final bitmap file or its descendants.
+		 */
+		if (!c_ent->commit_mask)
+			continue;
+
+		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. That is, it is reusable as-is and there is no
+			 * need to continue walking beyond it.
+			 *
+			 * Mark it as such and add it to bb->commits separately
+			 * to avoid allocating a position in the commit mask.
+			 */
+			commit_list_insert(commit, &reusable);
+			goto next;
+		}
+
 		if (c_ent->maximal) {
 			num_maximal++;
 			ALLOC_GROW(bb->commits, bb->commits_nr + 1, bb->commits_alloc);
@@ -278,14 +306,22 @@ static void bitmap_builder_init(struct bitmap_builder *bb,
 			}
 		}
 
+next:
 		bitmap_free(c_ent->commit_mask);
 		c_ent->commit_mask = NULL;
 	}
 
+	for (r = reusable; r; r = r->next) {
+		ALLOC_GROW(bb->commits, bb->commits_nr + 1, bb->commits_alloc);
+		bb->commits[bb->commits_nr++] = r->item;
+	}
+
 	trace2_data_intmax("pack-bitmap-write", the_repository,
 			   "num_selected_commits", writer->selected_nr);
 	trace2_data_intmax("pack-bitmap-write", the_repository,
 			   "num_maximal_commits", num_maximal);
+
+	free_commit_list(reusable);
 }
 
 static void bitmap_builder_clear(struct bitmap_builder *bb)
@@ -420,7 +456,7 @@ 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);