Subject: Re: [PATCH v3 6/9] commit: use generation numbers for in_merge_bases()

From: Jakub Narebski <[email-masked]>
Date: Thu, 19 Apr 2018 00:15:06 +0200

Derrick Stolee <[email-masked]> writes:

> The containment algorithm for 'git branch --contains' is different
> from that for 'git tag --contains' in that it uses is_descendant_of()
> instead of contains_tag_algo(). The expensive portion of the branch
> algorithm is computing merge bases.
>
> When a commit-graph file exists with generation numbers computed,
> we can avoid this merge-base calculation when the target commit has
> a larger generation number than the target commits.

You have "target" twice in above paragraph; one of those should probably
be something else.

>
> Performance tests were run on a copy of the Linux repository where
> HEAD is contained in v4.13 but no earlier tag. Also, all tags were
> copied to branches and 'git branch --contains' was tested:
>
> Before: 60.0s
> After:   0.4s
> Rel %: -99.3%

Nice...

>
> Reported-by: Jeff King <[email-masked]>
> Signed-off-by: Derrick Stolee <[email-masked]>
> ---
>  commit.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)

...especially for so small changes.

>
> diff --git a/commit.c b/commit.c
> index a44899c733..bceb79c419 100644
> --- a/commit.c
> +++ b/commit.c
> @@ -1053,12 +1053,19 @@ int in_merge_bases_many(struct commit *commit, int nr_reference, struct commit *
>  {
>  	struct commit_list *bases;
>  	int ret = 0, i;
> +	uint32_t min_generation = GENERATION_NUMBER_INFINITY;
>  
>  	if (parse_commit(commit))
>  		return ret;
> -	for (i = 0; i < nr_reference; i++)
> +	for (i = 0; i < nr_reference; i++) {
>  		if (parse_commit(reference[i]))
>  			return ret;
> +		if (min_generation > reference[i]->generation)
> +			min_generation = reference[i]->generation;
> +	}
> +
> +	if (commit->generation > min_generation)
> +		return 0;

Why not use "return ret;" instead of "return 0;", like the rest of the
code [cryptically] does, that is:

  +	if (commit->generation > min_generation)
  +		return ret;

>  
>  	bases = paint_down_to_common(commit, nr_reference, reference);
>  	if (commit->object.flags & PARENT2)

Otherwise, it looks good to me.