pack-bitmap.c: support 'tree:0' filtering

In the previous patch, we made it easy to define other filters that
exclude all objects of a certain type. Use that in order to implement
bitmap-level filtering for the '--filter=tree:<n>' filter when 'n' is
equal to 0.

The general case is not helped by bitmaps, since for values of 'n > 0',
the object filtering machinery requires a full-blown tree traversal in
order to determine the depth of a given tree. Caching this is
non-obvious, too, since the same tree object can have a different depth
depending on the context (e.g., a tree was moved up in the directory
hierarchy between two commits).

But, the 'n = 0' case can be helped, and this patch does so. Running
p5310.11 in this tree and on master with the kernel, we can see that
this case is helped substantially:

  Test                                  master              this tree
  --------------------------------------------------------------------------------
  5310.11: rev-list count with tree:0   10.68(10.39+0.27)   0.06(0.04+0.01) -99.4%

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

pack-bitmap.c

@@ -749,7 +749,7 @@ static void filter_bitmap_exclude_type(struct bitmap_index *bitmap_git,
 	eword_t mask;
 	uint32_t i;
 
-	if (type != OBJ_BLOB)
+	if (type != OBJ_BLOB && type != OBJ_TREE)
 		BUG("filter_bitmap_exclude_type: unsupported type '%d'", type);
 
 	/*
@@ -867,6 +867,20 @@ static void filter_bitmap_blob_limit(struct bitmap_index *bitmap_git,
 	bitmap_free(tips);
 }
 
+static void filter_bitmap_tree_depth(struct bitmap_index *bitmap_git,
+				     struct object_list *tip_objects,
+				     struct bitmap *to_filter,
+				     unsigned long limit)
+{
+	if (limit)
+		BUG("filter_bitmap_tree_depth given non-zero limit");
+
+	filter_bitmap_exclude_type(bitmap_git, tip_objects, to_filter,
+				   OBJ_TREE);
+	filter_bitmap_exclude_type(bitmap_git, tip_objects, to_filter,
+				   OBJ_BLOB);
+}
+
 static int filter_bitmap(struct bitmap_index *bitmap_git,
 			 struct object_list *tip_objects,
 			 struct bitmap *to_filter,
@@ -890,6 +904,15 @@ static int filter_bitmap(struct bitmap_index *bitmap_git,
 		return 0;
 	}
 
+	if (filter->choice == LOFC_TREE_DEPTH &&
+	    filter->tree_exclude_depth == 0) {
+		if (bitmap_git)
+			filter_bitmap_tree_depth(bitmap_git, tip_objects,
+						 to_filter,
+						 filter->tree_exclude_depth);
+		return 0;
+	}
+
 	/* filter choice not handled */
 	return -1;
 }

t/perf/p5310-pack-bitmaps.sh

@@ -53,6 +53,11 @@ test_perf 'rev-list count with blob:limit=1k' '
 		--filter=blob:limit=1k >/dev/null
 '
 
+test_perf 'rev-list count with tree:0' '
+	git rev-list --use-bitmap-index --count --objects --all \
+		--filter=tree:0 >/dev/null
+'
+
 test_perf 'simulated partial clone' '
 	git pack-objects --stdout --all --filter=blob:none </dev/null >/dev/null
 '

t/t6113-rev-list-bitmap-filters.sh

@@ -53,4 +53,25 @@ test_expect_success 'blob:limit filter with specified blob' '
 	test_bitmap_traversal expect actual
 '
 
+test_expect_success 'tree:0 filter' '
+	git rev-list --objects --filter=tree:0 HEAD >expect &&
+	git rev-list --use-bitmap-index \
+		     --objects --filter=tree:0 HEAD >actual &&
+	test_bitmap_traversal expect actual
+'
+
+test_expect_success 'tree:0 filter with specified blob, tree' '
+	git rev-list --objects --filter=tree:0 HEAD HEAD:two.t >expect &&
+	git rev-list --use-bitmap-index \
+		     --objects --filter=tree:0 HEAD HEAD:two.t >actual &&
+	test_bitmap_traversal expect actual
+'
+
+test_expect_success 'tree:1 filter' '
+	git rev-list --objects --filter=tree:1 HEAD >expect &&
+	git rev-list --use-bitmap-index \
+		     --objects --filter=tree:1 HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_done