revision.c: use Bloom filters to speed up path based revision walks

Revision walk will now use Bloom filters for commits to speed up
revision walks for a particular path (for computing history for
that path), if they are present in the commit-graph file.

We load the Bloom filters during the prepare_revision_walk step,
currently only when dealing with a single pathspec. Extending
it to work with multiple pathspecs can be explored and built on
top of this series in the future.

While comparing trees in rev_compare_trees(), if the Bloom filter
says that the file is not different between the two trees, we don't
need to compute the expensive diff. This is where we get our
performance gains. The other response of the Bloom filter is '`:maybe',
in which case we fall back to the full diff calculation to determine
if the path was changed in the commit.

We do not try to use Bloom filters when the '--walk-reflogs' option
is specified. The '--walk-reflogs' option does not walk the commit
ancestry chain like the rest of the options. Incorporating the
performance gains when walking reflog entries would add more
complexity, and can be explored in a later series.

Performance Gains:
We tested the performance of `git log -- <path>` on the git repo, the linux
and some internal large repos, with a variety of paths of varying depths.

On the git and linux repos:
- we observed a 2x to 5x speed up.

On a large internal repo with files seated 6-10 levels deep in the tree:
- we observed 10x to 20x speed ups, with some paths going up to 28 times
  faster.

Helped-by: Derrick Stolee <dstolee@microsoft.com
Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 118 insertions, 2 deletions

diff --git a/bloom.c b/bloom.c
index 0f714dd76a..c5b461d1cf 100644
--- a/bloom.c
+++ b/bloom.c
@@ -253,3 +253,23 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
 
 	return filter;
 }
+
+int bloom_filter_contains(const struct bloom_filter *filter,
+			  const struct bloom_key *key,
+			  const struct bloom_filter_settings *settings)
+{
+	int i;
+	uint64_t mod = filter->len * BITS_PER_WORD;
+
+	if (!mod)
+		return -1;
+
+	for (i = 0; i < settings->num_hashes; i++) {
+		uint64_t hash_mod = key->hashes[i] % mod;
+		uint64_t block_pos = hash_mod / BITS_PER_WORD;
+		if (!(filter->data[block_pos] & get_bitmask(hash_mod)))
+			return 0;
+	}
+
+	return 1;
+}
\ No newline at end of file
diff --git a/bloom.h b/bloom.h
index 760d712237..b935186425 100644
--- a/bloom.h
+++ b/bloom.h
@@ -83,4 +83,8 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
 				      struct commit *c,
 				      int compute_if_not_present);
 
+int bloom_filter_contains(const struct bloom_filter *filter,
+			  const struct bloom_key *key,
+			  const struct bloom_filter_settings *settings);
+
 #endif
\ No newline at end of file
diff --git a/revision.c b/revision.c
index 8136929e23..d3fcb7c6ff 100644
--- a/revision.c
+++ b/revision.c
@@ -29,6 +29,7 @@
 #include "prio-queue.h"
 #include "hashmap.h"
 #include "utf8.h"
+#include "bloom.h"
 
 volatile show_early_output_fn_t show_early_output;
 
@@ -624,11 +625,80 @@ static void file_change(struct diff_options *options,
 	options->flags.has_changes = 1;
 }
 
+static void prepare_to_use_bloom_filter(struct rev_info *revs)
+{
+	struct pathspec_item *pi;
+	char *path_alloc = NULL;
+	const char *path;
+	int last_index;
+	int len;
+
+	if (!revs->commits)
+	    return;
+
+	repo_parse_commit(revs->repo, revs->commits->item);
+
+	if (!revs->repo->objects->commit_graph)
+		return;
+
+	revs->bloom_filter_settings = revs->repo->objects->commit_graph->bloom_filter_settings;
+	if (!revs->bloom_filter_settings)
+		return;
+
+	pi = &revs->pruning.pathspec.items[0];
+	last_index = pi->len - 1;
+
+	/* remove single trailing slash from path, if needed */
+	if (pi->match[last_index] == '/') {
+	    path_alloc = xstrdup(pi->match);
+	    path_alloc[last_index] = '\0';
+	    path = path_alloc;
+	} else
+	    path = pi->match;
+
+	len = strlen(path);
+
+	revs->bloom_key = xmalloc(sizeof(struct bloom_key));
+	fill_bloom_key(path, len, revs->bloom_key, revs->bloom_filter_settings);
+
+	free(path_alloc);
+}
+
+static int check_maybe_different_in_bloom_filter(struct rev_info *revs,
+						 struct commit *commit)
+{
+	struct bloom_filter *filter;
+	int result;
+
+	if (!revs->repo->objects->commit_graph)
+		return -1;
+
+	if (commit->generation == GENERATION_NUMBER_INFINITY)
+		return -1;
+
+	filter = get_bloom_filter(revs->repo, commit, 0);
+
+	if (!filter) {
+		return -1;
+	}
+
+	if (!filter->len) {
+		return -1;
+	}
+
+	result = bloom_filter_contains(filter,
+				       revs->bloom_key,
+				       revs->bloom_filter_settings);
+
+	return result;
+}
+
 static int rev_compare_tree(struct rev_info *revs,
-			    struct commit *parent, struct commit *commit)
+			    struct commit *parent, struct commit *commit, int nth_parent)
 {
 	struct tree *t1 = get_commit_tree(parent);
 	struct tree *t2 = get_commit_tree(commit);
+	int bloom_ret = 1;
 
 	if (!t1)
 		return REV_TREE_NEW;
@@ -653,11 +723,19 @@ static int rev_compare_tree(struct rev_info *revs,
 			return REV_TREE_SAME;
 	}
 
+	if (revs->bloom_key && !nth_parent) {
+		bloom_ret = check_maybe_different_in_bloom_filter(revs, commit);
+
+		if (bloom_ret == 0)
+			return REV_TREE_SAME;
+	}
+
 	tree_difference = REV_TREE_SAME;
 	revs->pruning.flags.has_changes = 0;
 	if (diff_tree_oid(&t1->object.oid, &t2->object.oid, "",
 			   &revs->pruning) < 0)
 		return REV_TREE_DIFFERENT;
+
 	return tree_difference;
 }
 
@@ -855,7 +933,7 @@ static void try_to_simplify_commit(struct rev_info *revs, struct commit *commit)
 			die("cannot simplify commit %s (because of %s)",
 			    oid_to_hex(&commit->object.oid),
 			    oid_to_hex(&p->object.oid));
-		switch (rev_compare_tree(revs, p, commit)) {
+		switch (rev_compare_tree(revs, p, commit, nth_parent)) {
 		case REV_TREE_SAME:
 			if (!revs->simplify_history || !relevant_commit(p)) {
 				/* Even if a merge with an uninteresting
@@ -3362,6 +3440,8 @@ int prepare_revision_walk(struct rev_info *revs)
 				       FOR_EACH_OBJECT_PROMISOR_ONLY);
 	}
 
+	if (revs->pruning.pathspec.nr == 1 && !revs->reflog_info)
+		prepare_to_use_bloom_filter(revs);
 	if (revs->no_walk != REVISION_WALK_NO_WALK_UNSORTED)
 		commit_list_sort_by_date(&revs->commits);
 	if (revs->no_walk)
@@ -3379,6 +3459,7 @@ int prepare_revision_walk(struct rev_info *revs)
 		simplify_merges(revs);
 	if (revs->children.name)
 		set_children(revs);
+
 	return 0;
 }
 
diff --git a/revision.h b/revision.h
index 475f048fb6..7c026fe41f 100644
--- a/revision.h
+++ b/revision.h
@@ -56,6 +56,8 @@ struct repository;
 struct rev_info;
 struct string_list;
 struct saved_parents;
+struct bloom_key;
+struct bloom_filter_settings;
 define_shared_commit_slab(revision_sources, char *);
 
 struct rev_cmdline_info {
@@ -291,6 +293,15 @@ struct rev_info {
 	struct revision_sources *sources;
 
 	struct topo_walk_info *topo_walk_info;
+
+	/* Commit graph bloom filter fields */
+	/* The bloom filter key for the pathspec */
+	struct bloom_key *bloom_key;
+	/*
+	 * The bloom filter settings used to generate the key.
+	 * This is loaded from the commit-graph being used.
+	 */
+	struct bloom_filter_settings *bloom_filter_settings;
 };
 
 int ref_excluded(struct string_list *, const char *path);