[PATCH v3 2/4] commit-graph: introduce commit_graph_data_slab

[Abhishek Kumar <abhishekkumar8222@gmail.com>]

The struct commit is used in many contexts. However, members
`generation` and `graph_pos` are only used for commit-graph related
operations and otherwise waste memory.

This wastage would have been more pronounced as we transition to
generation number v2, which uses 64-bit generation number instead of
current 32-bits.

As they are often accessed together, let's introduce struct
commit_graph_data and move them to a commit_graph_data slab.

While the overall test suite runs just as fast as master,
(series: 27m10s, master: 27m34s), certain commands like
`git merge-base --is-ancestor` are slowed by nearly 40% as
discovered by SDEZER Gabor [1].

Derrick Stolee believes the slow down is attributable to the
underlying algorithm rather slowness of commit-slab access [2] and we will
follow-up on that in a later series.

[1]: https://lore.kernel.org/git/20200607195347.GA8232@szeder.dev/
[2]: https://lore.kernel.org/git/13db757a-9412-7f1e-805c-8a028c4ab2b1@gmail.com/

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
---
 commit-graph.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 commit-graph.h | 10 ++++++++++
 2 files changed, 59 insertions(+)

diff --git a/commit-graph.c b/commit-graph.c
index 2ff042fbf4..91120ba3d3 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -87,6 +87,55 @@ static int commit_pos_cmp(const void *va, const void *vb)
 	       commit_pos_at(&commit_pos, b);
 }
 
+define_commit_slab(commit_graph_data_slab, struct commit_graph_data);
+static struct commit_graph_data_slab commit_graph_data_slab =
+	COMMIT_SLAB_INIT(1, commit_graph_data_slab);
+
+uint32_t commit_graph_position(const struct commit *c)
+{
+	struct commit_graph_data *data =
+		commit_graph_data_slab_peek(&commit_graph_data_slab, c);
+
+	return data ? data->graph_pos : COMMIT_NOT_FROM_GRAPH;
+}
+
+uint32_t commit_graph_generation(const struct commit *c)
+{
+	struct commit_graph_data *data =
+		commit_graph_data_slab_peek(&commit_graph_data_slab, c);
+
+	if (!data)
+		return GENERATION_NUMBER_INFINITY;
+	else if (data->graph_pos == COMMIT_NOT_FROM_GRAPH)
+		return GENERATION_NUMBER_INFINITY;
+
+	return data->generation;
+}
+
+static struct commit_graph_data *commit_graph_data_at(const struct commit *c)
+{
+	uint32_t i = commit_graph_data_slab.slab_count, j;
+	uint32_t slab_size = commit_graph_data_slab.slab_size;
+	struct commit_graph_data *data =
+		commit_graph_data_slab_at(&commit_graph_data_slab, c);
+
+	/*
+	 * commit-slab initializes elements with zero, overwrite this with
+	 * COMMIT_NOT_FROM_GRAPH for graph_pos.
+	 *
+	 * We avoid initializing generation with checking if graph position
+	 * is not COMMIT_NOT_FROM_GRAPH.
+	 */
+	for (; i < commit_graph_data_slab.slab_count; i++) {
+		for (j = 0; j < slab_size; j++) {
+			commit_graph_data_slab[i][j].graph_pos =
+				COMMIT_NOT_FROM_GRAPH;
+		}
+	}
+
+	return data;
+}
+
 static int commit_gen_cmp(const void *va, const void *vb)
 {
 	const struct commit *a = *(const struct commit **)va;
diff --git a/commit-graph.h b/commit-graph.h
index 3ba0da1e5f..cc76757007 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -135,4 +135,14 @@ void free_commit_graph(struct commit_graph *);
  */
 void disable_commit_graph(struct repository *r);
 
+struct commit_graph_data {
+	uint32_t graph_pos;
+	uint32_t generation;
+};
+
+/* 
+ * Commits should be parsed before accessing generation, graph positions.
+ */
+uint32_t commit_graph_generation(const struct commit *);
+uint32_t commit_graph_position(const struct commit *);
 #endif
-- 
2.27.0