[PATCH 25/34] commit-graph: check embedded modified path Bloom filters with a mask

["SZEDER Gábor" <szeder.dev@gmail.com>]

Since the previous patch we check the presence of all leading
directories of a pathspec in modified path Bloom filters to
significantly lower the false positive rate.  This means that we are
checking a lot of bit positions in the Bloom filters.  However, as
shown earlier in this series, a significant portion of commits have
embedded modified path Bloom filters, all with the same size of 63
bits.

Use a 64 bit mask to check all relevant bits in embedded modified path
Bloom filters in one step.

I don't have benchmark results at hand, but as far as I can recall
some old results this reduces the time spent loading and checking
modified path Bloom filters by up to 10%.  Checking Bloom filters is
of course only a small part of the whole runtime of a pathspec-limited
revision walk, so the overall improvement is only about 1-2%.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 commit-graph.c | 17 +++++++++++++++--
 pathspec.c     |  1 +
 pathspec.h     |  1 +
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 1fd1b4f8dd..56b8f33d10 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -918,10 +918,16 @@ enum bloom_result check_modified_path_bloom_filter(struct repository *r,
 	for (i = 0; i < pathspec->nr; i++) {
 		struct pathspec_item *pi = &pathspec->items[i];
 
-		if (bloom_filter_check_bits(&bf,
+		if (bf.nr_bits == GRAPH_MODIFIED_PATH_BLOOM_FILTER_EMBEDDED_NR_BITS) {
+			/* Gross!  And potential alignment issues?! */
+			if ((*((const uint64_t*)bf.bits) & pi->modified_path_bloom_mask) == pi->modified_path_bloom_mask)
+				return BLOOM_POSSIBLY_YES;
+		} else {
+			if (bloom_filter_check_bits(&bf,
 					pi->modified_path_bloom_hashes,
 					pi->modified_path_bloom_hashes_nr))
-			return BLOOM_POSSIBLY_YES;
+				return BLOOM_POSSIBLY_YES;
+		}
 	}
 
 	return BLOOM_DEFINITELY_NOT;
@@ -981,6 +987,7 @@ void init_pathspec_bloom_fields(struct repository *r,
 		size_t len = pi->len;
 		int path_component_nr = 0, j;
 		uint32_t *hashes;
+		struct bloom_filter embedded_bf;
 
 		/*
 		 * Pathspec parsing has normalized away any consecutive
@@ -1012,6 +1019,12 @@ void init_pathspec_bloom_fields(struct repository *r,
 					graph->num_modified_path_bloom_hashes,
 					hashes);
 		}
+
+		embedded_bf.nr_bits = GRAPH_MODIFIED_PATH_BLOOM_FILTER_EMBEDDED_NR_BITS;
+		embedded_bf.bits = (uint8_t*) &pi->modified_path_bloom_mask;
+		bloom_filter_set_bits(&embedded_bf,
+				      pi->modified_path_bloom_hashes,
+				      pi->modified_path_bloom_hashes_nr);
 	}
 
 	pathspec->can_use_modified_path_bloom_filters = 1;
diff --git a/pathspec.c b/pathspec.c
index 01e7ae6944..29cadce013 100644
--- a/pathspec.c
+++ b/pathspec.c
@@ -408,6 +408,7 @@ static void init_pathspec_item(struct pathspec_item *item, unsigned flags,
 	item->attr_match_nr = 0;
 	item->modified_path_bloom_hashes_nr = 0;
 	item->modified_path_bloom_hashes = NULL;
+	item->modified_path_bloom_mask = 0;
 
 	/* PATHSPEC_LITERAL_PATH ignores magic */
 	if (flags & PATHSPEC_LITERAL_PATH) {
diff --git a/pathspec.h b/pathspec.h
index 0c661e1b03..c9975aea7a 100644
--- a/pathspec.h
+++ b/pathspec.h
@@ -64,6 +64,7 @@ struct pathspec {
 		 */
 		uint32_t modified_path_bloom_hashes_nr;
 		uint32_t *modified_path_bloom_hashes;
+		uint64_t modified_path_bloom_mask;
 	} *items;
 };
 
-- 
2.27.0.rc1.431.g5c813f95dc