Subject: Re: [PATCH v2 05/15] rev-list: factor out bitmap-optimized routines

From: Taylor Blau <[email-masked]>
Date: Fri, 14 Feb 2020 16:35:51 -0800

On Fri, Feb 14, 2020 at 01:22:18PM -0500, Jeff King wrote:
> There are a few operations in rev-list that are optimized for bitmaps.
> Rather than having the code inline in cmd_rev_list(), let's move them
> into helpers. This not only makes the flow of the main function simpler,
> but it lets us replace the complex "can we do the optimization?"
> conditionals with a series of early returns from the functions. That
> also makes it easy to add comments explaining those conditions.

Makes sense.

> Signed-off-by: Jeff King <[email-masked]>
> ---
>  builtin/rev-list.c | 88 +++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 67 insertions(+), 21 deletions(-)
>
> diff --git a/builtin/rev-list.c b/builtin/rev-list.c
> index 4cb5a52dee..38c5ca5603 100644
> --- a/builtin/rev-list.c
> +++ b/builtin/rev-list.c
> @@ -364,6 +364,69 @@ static inline int parse_missing_action_value(const char *value)
>  	return 0;
>  }
>
> +static int try_bitmap_count(struct rev_info *revs)
> +{
> +	uint32_t commit_count;
> +	int max_count;
> +	struct bitmap_index *bitmap_git;
> +
> +	/* This function only handles counting, not general traversal. */
> +	if (!revs->count)
> +		return -1;
> +
> +	/*
> +	 * A bitmap result can't know left/right, etc, because we don't
> +	 * actually traverse.
> +	 */
> +	if (revs->left_right || revs->cherry_mark)
> +		return -1;
> +
> +	/*
> +	 * This must be saved before doing any walking, since the revision
> +	 * machinery will count it down to zero while traversing.
> +	 */
> +	max_count = revs->max_count;
> +
> +	bitmap_git = prepare_bitmap_walk(revs);
> +	if (!bitmap_git)
> +		return -1;
> +
> +	count_bitmap_commit_list(bitmap_git, &commit_count, NULL, NULL, NULL);
> +	if (max_count >= 0 && max_count < commit_count)
> +		commit_count = max_count;
> +
> +	printf("%d\n", commit_count);
> +	free_bitmap_index(bitmap_git);
> +	return 0;
> +}
> +
> +static int try_bitmap_traversal(struct rev_info *revs)
> +{
> +	struct bitmap_index *bitmap_git;
> +
> +	/*
> +	 * We can't use a bitmap result with a traversal limit, since the set
> +	 * of commits we'd get would be essentially random.
> +	 */
> +	if (revs->max_count >= 0)
> +		return -1;
> +
> +	/*
> +	 * Our bitmap result will return all objects, and we're not
> +	 * yet prepared to show only particular types.
> +	 */
> +	if (!revs->tag_objects || !revs->tree_objects || !revs->blob_objects)
> +		return -1;
> +
> +	bitmap_git = prepare_bitmap_walk(revs);
> +	if (!bitmap_git)
> +		return -1;
> +
> +	traverse_bitmap_commit_list(bitmap_git, &show_object_fast);
> +	free_bitmap_index(bitmap_git);
> +	return 0;
> +}
> +
>  int cmd_rev_list(int argc, const char **argv, const char *prefix)
>  {
>  	struct rev_info revs;
> @@ -534,27 +597,10 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
>  		progress = start_delayed_progress(show_progress, 0);
>
>  	if (use_bitmap_index) {
> -		if (revs.count && !revs.left_right && !revs.cherry_mark) {
> -			uint32_t commit_count;
> -			int max_count = revs.max_count;
> -			struct bitmap_index *bitmap_git;
> -			if ((bitmap_git = prepare_bitmap_walk(&revs))) {
> -				count_bitmap_commit_list(bitmap_git, &commit_count, NULL, NULL, NULL);
> -				if (max_count >= 0 && max_count < commit_count)
> -					commit_count = max_count;
> -				printf("%d\n", commit_count);
> -				free_bitmap_index(bitmap_git);
> -				return 0;
> -			}
> -		} else if (revs.max_count < 0 &&
> -			   revs.tag_objects && revs.tree_objects && revs.blob_objects) {
> -			struct bitmap_index *bitmap_git;
> -			if ((bitmap_git = prepare_bitmap_walk(&revs))) {
> -				traverse_bitmap_commit_list(bitmap_git, &show_object_fast);
> -				free_bitmap_index(bitmap_git);
> -				return 0;
> -			}
> -		}
> +		if (!try_bitmap_count(&revs))
> +			return 0;
> +		if (!try_bitmap_traversal(&revs))
> +			return 0;
>  	}
>
>  	if (prepare_revision_walk(&revs))
> --
> 2.25.0.796.gcc29325708

The refactoring here is straightforward, and looks correct to my
reading.

Thanks,
Taylor