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

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 4, 2020, 17:35 UTC
Message-ID: <xmqqk10mpwdy.fsf@gitster.c.googlers.com>
In-Reply-To: <20200604072759.19142-2-abhishekkumar8222@gmail.com>

Abhishek Kumar <abhishekkumar8222@gmail.com> writes:

Show 42 quoted lines

> 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 <abhishekkumar8222@gmail.com>
> ---
>  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) {

Style: favor post-increment over pre-increment when there is no
difference, especially when updating the loop control in for() loop.

Show 22 quoted lines

> +		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
> +

Makes sense to move it from commit.h, I guess.

Show 23 quoted lines

>  #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;