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pack-bitmap: check subtree bits before recursing

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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>
This commit is contained in:
Taylor Blau
2026-05-27 15:55:53 -04:00
committed by Junio C Hamano
parent e3959cc78c
commit 1760c37258
1 changed files with 17 additions and 6 deletions
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23
pack-bitmap-write.c
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@@ -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;