[PATCH v2 08/10] ref-filter: use generation number for --contains

[Derrick Stolee <dstolee@microsoft.com>]

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
but no earlier tag, the command 'git tag --contains HEAD' had the
following peformance improvement:

Before: 0.81s
After:  0.04s
Rel %:  -95%

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 ref-filter.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/ref-filter.c b/ref-filter.c
index 45fc56216a..2f5e79b5de 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -1584,7 +1584,8 @@ 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);
 
@@ -1598,8 +1599,11 @@ static enum contains_result contains_test(struct commit *candidate,
 		return CONTAINS_YES;
 	}
 
-	/* Otherwise, we don't know; prepare to recurse */
 	parse_commit_or_die(candidate);
+
+	if (candidate->generation < cutoff)
+		return CONTAINS_NO;
+
 	return CONTAINS_UNKNOWN;
 }
 
@@ -1615,8 +1619,18 @@ 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;
+		parse_commit_or_die(c);
+		if (c->generation < cutoff)
+			cutoff = c->generation;
+	}
 
+	result = contains_test(candidate, want, cache, cutoff);
 	if (result != CONTAINS_UNKNOWN)
 		return result;
 
@@ -1634,7 +1648,7 @@ 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--;
@@ -1648,7 +1662,7 @@ 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);
 }
 
 static int commit_contains(struct ref_filter *filter, struct commit *commit,
-- 
2.17.0