Re: [GSoC Patch 1/3] commit: introduce helpers for generation slab

The struct member generation refers to "generation number" (or more
broadly, a reachablity index value) used by commit-graph to reduce time
taken to walk commits. However, generation is not useful in other
contexts and bloats the struct.

Let's move it to a commit-slab and shrink the struct by four bytes.

Signed-off-by: Abhishek Kumar <redacted>
---
 commit-graph.c | 27 +++++++++++++++++++++++++++
 commit-graph.h |  5 +++++
 commit.h       |  3 ---
 3 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index e3420ddcbf..63f419048d 100644
--- a/commit-graph.c
+++ b/commit-graph.c

@@ -87,6 +87,33 @@ static int commit_pos_cmp(const void *va, const void *vb)
 	       commit_pos_at(&commit_pos, b);
 }
 
+define_commit_slab(generation_slab, uint32_t);
+static struct generation_slab generation_slab = COMMIT_SLAB_INIT(1, generation_slab);
+
+uint32_t generation(const struct commit *c)
+{
+	uint32_t *gen = generation_slab_peek(&generation_slab, c);
+
+	return gen ? *gen : GENERATION_NUMBER_INFINITY;
+}
+
+static void set_generation(const struct commit *c, const uint32_t generation)
+{
+	unsigned int i = generation_slab.slab_count;
+	uint32_t *gen = generation_slab_at(&generation_slab, c);
+
+	/*
+	 * commit-slab initializes with zero, overwrite this with
+	 * GENERATION_NUMBER_INFINITY
+	 */
+	for (; i < generation_slab.slab_count; ++i) {

+		memset(generation_slab.slab[i], GENERATION_NUMBER_INFINITY,
+		       generation_slab.slab_size * sizeof(uint32_t));
+	}
+
+	*gen = generation;
+}
+
 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 4212766a4f..653bd041ad 100644
--- a/commit-graph.h
+++ b/commit-graph.h

@@ -8,6 +8,10 @@
 #include "object-store.h"
 #include "oidset.h"
 
+#define GENERATION_NUMBER_INFINITY 0xFFFFFFFF
+#define GENERATION_NUMBER_MAX 0x3FFFFFFF
+#define GENERATION_NUMBER_ZERO 0
+

 #define GIT_TEST_COMMIT_GRAPH "GIT_TEST_COMMIT_GRAPH"
 #define GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD "GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD"
 #define GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS "GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS"

@@ -137,4 +141,5 @@ void free_commit_graph(struct commit_graph *);
  */
 void disable_commit_graph(struct repository *r);
 
+uint32_t generation(const struct commit *c);
 #endif

diff --git a/commit.h b/commit.h
index 1b2dea5d85..cc610400d5 100644
--- a/commit.h
+++ b/commit.h

@@ -11,9 +11,6 @@
 #include "commit-slab.h"
 
 #define COMMIT_NOT_FROM_GRAPH 0xFFFFFFFF
-#define GENERATION_NUMBER_INFINITY 0xFFFFFFFF
-#define GENERATION_NUMBER_MAX 0x3FFFFFFF
-#define GENERATION_NUMBER_ZERO 0
 
 struct commit_list {
 	struct commit *item;