On 6/4/2020 3:27 AM, Abhishek Kumar wrote:
> 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@xxxxxxxxx>
> ---
> 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;
> +}
This is good: if we don't have the value, then use INFINITY.
In the header file, perhaps include a warning comment that a
caller _must_ first parse the commit or else we have no guarantee
that the generation slab is populated. This matches the current
expectations before accessing the generation member.
> +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));
> + }
Here is an example where combining the graph_pos and generation
slabs into one would be helpful. The only reason the generation
would be INFINITY is if graph_pos is COMMIT_NOT_FROM_GRAPH. If
the two values are side-by-side, we could just check graph_pos
first and return INFINITY instead of paying this initialization
cost as the slab grows.
I would also like to avoid initializing the slab if there is
no commit-graph present. I wonder if we can populate the slab
while parsing the commit-graph and check here if the slab is
NULL before doing any other logic? (I'm not sure if this is
possible, but it would be nice.)
> 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
I appreciate that you are able to relocate these constants to
a more appropriate location.
Thanks,
-Stolee
