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

From: [email-masked] (Jakub Narębski)
Date: Sat, 06 Jun 2020 01:23:25 +0200

Abhishek Kumar <[email-masked]> writes:

> The struct member generation refers to "generation number" (or more

Again, a minor thing: 'generation'.

> 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.

It looks like the description is from earlier version of the commit,
before it was split -- because this commit does not remove 'generation'
member from the 'struct commit', actually.

This commit is about creating helper functions.

>
> Signed-off-by: Abhishek Kumar <[email-masked]>
> ---
>  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);

All right, we need this for the following helper functions to work.

We might want to encapsulate all commit-graph data together, in a single
struct (e.g. as 'struct commit_graph_data' instead of uint32_t here).

On the other hand other data is stored on slab often as separate
scalar data (contains_cache, commit_seen, indegree_slab,
author_date_slab, commit_base, commit_pos), but not always; sometimes it
is a struct (bloom_filter_slab, buffer_slab, commit_rev_name), sometimes
it is an array (commit_depth, ref_bitmap, commit_weight), and sometimes
it is an array/list of structs or pointer to struct (commit_names,
commit_name_slab, saved_parents, blame_suspects, commit_todo_item).

> +
> +uint32_t generation(const struct commit *c)
> +{
> +	uint32_t *gen = generation_slab_peek(&generation_slab, c);
> +
> +	return gen ? *gen : GENERATION_NUMBER_INFINITY;
> +}

All right, this is a synthetic getter using the fact that commits
outside the commit-graph should get GENERATION_NUMBER_INFINITY (because
[effective] commit-graph is closed under reachability, is full DAG).

Should we have something like that for 'graph_pos' and
COMMIT_NOT_FROM_GRAPH?

> +
> +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;
> +}

All right. I wonder if putting 'generation' and 'graph_pos' on the slab
together, gathered in 'struct commit_graph_data' would make this helper
more complex...

> +
>  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;

Why this change?

Best,
-- 
Jakub Narębski