Re: [GSOC Patch v2 1/4] commit-graph: introduce commit_graph_data_slab

[Taylor Blau <me@ttaylorr.com>]

On Mon, Jun 08, 2020 at 01:02:34AM +0530, Abhishek Kumar wrote:
> 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.
>
> As they are often accessed together, let's introduce struct
> commit_graph_data and move them to a commit_graph_data slab.
>
> 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 e3420ddcbf..7d887a6a2c 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;
> +	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 the cost of initializing `generation` as generation
> +	 * number would be GENERATION_NUMBER_INFINITY if graph position
> +	 * is COMMIT_NOT_FROM_GRAPH.
> +	 */
> +	for (; i < commit_graph_data_slab.slab_count; i++) {
> +		for (j = 0; j < slab_size; j++) {
> +			commit_graph_data_slab.slab[i][j].graph_pos = COMMIT_NOT_FROM_GRAPH;
> +		}
> +	}
> +
> +	return data;
> +}
> +

It looks like this function ('commit_graph_data_at') is unused in this
patch. No big deal, especially since I can see that you are using it in
the second patch, but I think that you should remove it from this patch
and instead introduce it there.

In fact, this causes a build error (with 'make DEVELOPER=1') because of
'-Wunused-function'. It's good practice to "git rebase -x 'make
DEVELOPER=1' origin/master" before sending.

>  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..9d22f98f44 100644
> --- a/commit-graph.h
> +++ b/commit-graph.h
> @@ -137,4 +137,14 @@ 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

Thanks,
Taylor