pack-bitmap: avoid traversal of objects referenced by uninteresting tag

When preparing the bitmap walk, we first establish the set of of have
and want objects by iterating over the set of pending objects: if an
object is marked as uninteresting, it's declared as an object we already
have, otherwise as an object we want. These two sets are then used to
compute which transitively referenced objects we need to obtain.

One special case here are tag objects: when a tag is requested, we
resolve it to its first not-tag object and add both resolved objects as
well as the tag itself into either the have or want set. Given that the
uninteresting-property always propagates to referenced objects, it is
clear that if the tag is uninteresting, so are its children and vice
versa. But we fail to propagate the flag, which effectively means that
referenced objects will always be interesting except for the case where
they have already been marked as uninteresting explicitly.

This mislabeling does not impact correctness: we now have it in our
"wants" set, and given that we later do an `AND NOT` of the bitmaps of
"wants" and "haves" sets it is clear that the result must be the same.
But we now start to needlessly traverse the tag's referenced objects in
case it is uninteresting, even though we know that each referenced
object will be uninteresting anyway. In the worst case, this can lead to
a complete graph walk just to establish that we do not care for any
object.

Fix the issue by propagating the `UNINTERESTING` flag to pointees of tag
objects and add a benchmark with negative revisions to p5310. This shows
some nice performance benefits, tested with linux.git:

Test                                                          HEAD~                  HEAD
---------------------------------------------------------------------------------------------------------------
5310.3: repack to disk                                        193.18(181.46+16.42)   194.61(183.41+15.83) +0.7%
5310.4: simulated clone                                       25.93(24.88+1.05)      25.81(24.73+1.08) -0.5%
5310.5: simulated fetch                                       2.64(5.30+0.69)        2.59(5.16+0.65) -1.9%
5310.6: pack to file (bitmap)                                 58.75(57.56+6.30)      58.29(57.61+5.73) -0.8%
5310.7: rev-list (commits)                                    1.45(1.18+0.26)        1.46(1.22+0.24) +0.7%
5310.8: rev-list (objects)                                    15.35(14.22+1.13)      15.30(14.23+1.07) -0.3%
5310.9: rev-list with tag negated via --not --all (objects)   22.49(20.93+1.56)      0.11(0.09+0.01) -99.5%
5310.10: rev-list with negative tag (objects)                 0.61(0.44+0.16)        0.51(0.35+0.16) -16.4%
5310.11: rev-list count with blob:none                        12.15(11.19+0.96)      12.18(11.19+0.99) +0.2%
5310.12: rev-list count with blob:limit=1k                    17.77(15.71+2.06)      17.75(15.63+2.12) -0.1%
5310.13: rev-list count with tree:0                           1.69(1.31+0.38)        1.68(1.28+0.39) -0.6%
5310.14: simulated partial clone                              20.14(19.15+0.98)      19.98(18.93+1.05) -0.8%
5310.16: clone (partial bitmap)                               12.78(13.89+1.07)      12.72(13.99+1.01) -0.5%
5310.17: pack to file (partial bitmap)                        42.07(45.44+2.72)      41.44(44.66+2.80) -1.5%
5310.18: rev-list with tree filter (partial bitmap)           0.44(0.29+0.15)        0.46(0.32+0.14) +4.5%

While most benchmarks are probably in the range of noise, the newly
added 5310.9 and 5310.10 benchmarks consistenly perform better.

Signed-off-by: Patrick Steinhardt <ps@pks.im>.
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap.c

@@ -969,6 +969,7 @@ struct bitmap_index *prepare_bitmap_walk(struct rev_info *revs,
 				object_list_insert(object, &wants);
 
 			object = parse_object_or_die(get_tagged_oid(tag), NULL);
+			object->flags |= (tag->object.flags & UNINTERESTING);
 		}
 
 		if (object->flags & UNINTERESTING)

t/perf/p5310-pack-bitmaps.sh

@@ -15,6 +15,12 @@ test_expect_success 'setup bitmap config' '
 	git config pack.writebitmaps true
 '
 
+# we need to create the tag up front such that it is covered by the repack and
+# thus by generated bitmaps.
+test_expect_success 'create tags' '
+	git tag --message="tag pointing to HEAD" perf-tag HEAD
+'
+
 test_perf 'repack to disk' '
 	git repack -ad
 '
@@ -43,6 +49,14 @@ test_perf 'rev-list (objects)' '
 	git rev-list --all --use-bitmap-index --objects >/dev/null
 '
 
+test_perf 'rev-list with tag negated via --not --all (objects)' '
+	git rev-list perf-tag --not --all --use-bitmap-index --objects >/dev/null
+'
+
+test_perf 'rev-list with negative tag (objects)' '
+	git rev-list HEAD --not perf-tag --use-bitmap-index --objects >/dev/null
+'
+
 test_perf 'rev-list count with blob:none' '
 	git rev-list --use-bitmap-index --count --objects --all \
 		--filter=blob:none >/dev/null