@leroysheep / BookAlchemy / commits / 1760c37258

pack-bitmap: check subtree bits before recursing

In the previous commit, we adjusted the callers of `fill_bitmap_tree()` to pass in the bit position of the tree they wish to fill. This commit makes use of that information at the call site to avoid setting up a stack frame for fill_bitmap_tree() entirely whenever a tree's bit position is already set. Since this is such a hot path, the avoided cost of setting up and tearing down stack frames for each noop'd call to `fill_bitmap_tree()` is significant: +--------------+-------------+-------------+-------------------+ | | HEAD^ | HEAD | Delta | +--------------+-------------+-------------+-------------------+ | elapsed | 582.4 s | 562.8 s | -19.6 s (-3.4%) | | cycles | 2,713.3 B | 2,621.3 B | -92.0 B (-3.4%) | | instructions | 2,415.5 B | 2,348.9 B | -66.6 B (-2.8%) | | CPI | 1.123 | 1.116 | -0.007 (-0.7%) | +--------------+-------------+-------------+-------------------+ In the same repository as in the previous commit, our timings dropped from ~582.4 seconds down to ~562.77 seconds. While the cycles-per-instruction ratio is basically unchanged, we execute significantly fewer instructions, and correspondingly fewer cycles. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Taylor Blau committed May 27, 2026 at 15:55 UTC 1760c372589af09ff0b986c57bfe0b9101275674

1 file changed +17 -6

pack-bitmap-write.c

+17 -6

index 2d5ff8fd40..7261039702 100644 --- a/pack-bitmap-write.c +++ b/pack-bitmap-write.c @@ -463,12 +463,6 @@ static int fill_bitmap_tree(struct bitmap_writer *writer, struct tree_desc desc; struct name_entry entry; - /* - * If our bit is already set, then there is nothing to do. Both this - * tree and all of its children will be set. - */ - if (bitmap_get(bitmap, pos)) - return 0; bitmap_set(bitmap, pos); if (repo_parse_tree(writer->repo, tree) < 0) @@ -482,6 +476,15 @@ static int fill_bitmap_tree(struct bitmap_writer *writer, pos = find_object_pos(writer, &entry.oid, &found); if (!found) return -1; + if (bitmap_get(bitmap, pos)) { + /* + * If our bit is already set, then there + * is nothing to do. Both this tree and + * all of its children will be set. + */ + break; + } + if (fill_bitmap_tree(writer, bitmap, lookup_tree(writer->repo, &entry.oid), pos) < 0) @@ -582,6 +585,14 @@ static int fill_bitmap_commit(struct bitmap_writer *writer, pos = find_object_pos(writer, &t->object.oid, &found); if (!found) return -1; + if (bitmap_get(ent->bitmap, pos)) { + /* + * If our bit is already set, then there is + * nothing to do. Both this tree and all of its + * children will be set. + */ + continue; + } if (fill_bitmap_tree(writer, ent->bitmap, t, pos) < 0) return -1;