Derrick Stolee <dsto...@microsoft.com> writes:
> A commit A can reach a commit B only if the generation number of A
> is strictly larger than the generation number of B. This condition
> allows significantly short-circuiting commit-graph walks.
>
> Use generation number for '--contains' type queries.
>
> On a copy of the Linux repository where HEAD is containd in v4.13
Minor typo: containd -> contained.
> but no earlier tag, the command 'git tag --contains HEAD' had the
> following peformance improvement:
>
> Before: 0.81s
> After: 0.04s
> Rel %: -95%
Very nice. I guess that any performance changes for when commit-graph
feature is not available are negligible / not measurable.
Rel % = (before - after)/before * 100%, isn't it?.
Good.
>
> Helped-by: Jeff King <p...@peff.net>
> Signed-off-by: Derrick Stolee <dsto...@microsoft.com>
> ---
> ref-filter.c | 24 ++++++++++++++++++++----
> 1 file changed, 20 insertions(+), 4 deletions(-)
>
> diff --git a/ref-filter.c b/ref-filter.c
> index aff24d93be..fb35067fc9 100644
> --- a/ref-filter.c
> +++ b/ref-filter.c
> @@ -16,6 +16,7 @@
> #include "trailer.h"
> #include "wt-status.h"
> #include "commit-slab.h"
> +#include "commit-graph.h"
>
> static struct ref_msg {
> const char *gone;
> @@ -1587,7 +1588,8 @@ static int in_commit_list(const struct commit_list
> *want, struct commit *c)
> */
> static enum contains_result contains_test(struct commit *candidate,
> const struct commit_list *want,
> - struct contains_cache *cache)
> + struct contains_cache *cache,
> + uint32_t cutoff)
> {
> enum contains_result *cached = contains_cache_at(cache, candidate);
>
> @@ -1603,6 +1605,10 @@ static enum contains_result contains_test(struct
> commit *candidate,
>
> /* Otherwise, we don't know; prepare to recurse */
> parse_commit_or_die(candidate);
> +
> + if (candidate->generation < cutoff)
> + return CONTAINS_NO;
> +
We use here weaker negative-cut criteria, which has the advantage of
simply automatic handling of special values: _INFINITY, _MAX, _ZERO.
Stronger version:
if A != B and A ---> B, then gen(A) > gen(B)
if gen(A) <= gen(B) and A != B, then A -/-> B
Weaker version:
if gen(A) < gen(B), then A -/-> B
If commit-graph feature is not available, then all generation numbers
would be _INFINITY, and cutoff would also be _INFINITY - which means
this operation is practically no-op. One memory access (probably from
cache) and one comparison is very cheap.
All right.
> return CONTAINS_UNKNOWN;
> }
>
> @@ -1618,8 +1624,18 @@ static enum contains_result contains_tag_algo(struct
> commit *candidate,
> struct contains_cache *cache)
> {
> struct contains_stack contains_stack = { 0, 0, NULL };
> - enum contains_result result = contains_test(candidate, want, cache);
> + enum contains_result result;
> + uint32_t cutoff = GENERATION_NUMBER_INFINITY;
> + const struct commit_list *p;
> +
> + for (p = want; p; p = p->next) {
> + struct commit *c = p->item;
> + load_commit_graph_info(c);
> + if (c->generation < cutoff)
> + cutoff = c->generation;
> + }
For each in wants, load generation numbers if needed and find lowest
one. Anything lower cannot reach any of wants. All right.
If commit-graph feature is not available, this is practically no-op. It
is fast, as it only accesses memory - it does not access disk, nor do it
needs to do any decompression, un-deltafication or parsing.
All right.
>
> + result = contains_test(candidate, want, cache, cutoff);
> if (result != CONTAINS_UNKNOWN)
> return result;
>
> @@ -1637,7 +1653,7 @@ static enum contains_result contains_tag_algo(struct
> commit *candidate,
> * If we just popped the stack, parents->item has been marked,
> * therefore contains_test will return a meaningful yes/no.
> */
> - else switch (contains_test(parents->item, want, cache)) {
> + else switch (contains_test(parents->item, want, cache, cutoff))
> {
> case CONTAINS_YES:
> *contains_cache_at(cache, commit) = CONTAINS_YES;
> contains_stack.nr--;
> @@ -1651,7 +1667,7 @@ static enum contains_result contains_tag_algo(struct
> commit *candidate,
> }
> }
> free(contains_stack.contains_stack);
> - return contains_test(candidate, want, cache);
> + return contains_test(candidate, want, cache, cutoff);
Those two just update callsite to new signatore. All right.
> }
>
> static int commit_contains(struct ref_filter *filter, struct commit *commit,
