[PATCH 1/1] commit-graph.c: avoid unnecessary tag dereference when merging

[Taylor Blau <me@ttaylorr.com>]

When performing a 'git commit-graph write' with '--split', the
commit-graph machinery calls 'merge_commit_graph()' after deciding on a
split strategy to optionally clean up any existing commit-graph
layers that were made obsolete by the split strategy [1].

At this time, 'merge_commit_graph()' checks each commit that it writes
into the merged graph to make sure that it still exists in the object
store.

To do this, it uses 'lookup_commit_reference_gently()', which accepts
either a commit object, or a tag that refers to a commit. However, since
all 'oid' arguments passed to this function are from within the
commit-graphs being merged, we never pass a commit reference, and so any
time we spend in 'deref_tag()' is wasted.

Improve the situation by using 'repo_has_object_file' to check if the
object still exists, and '{lookup,repo_parse}_commit()' to turn it into
a bona-fide 'struct commit *'.

In my testing environment, this improves the time to "merge" a split
commit-graph containing all reachable commits in the kernel by
re-writing the same commit-graph (effectively measuring the time it
takes to check that all of those commits still exist) from:

  Attempt 1: 9.614
  Attempt 2: 10.984
  Attempt 3: 10.39
  Attempt 4: 9.14
  Attempt 5: 9.439

  real	0m9.140s
  user	0m8.207s
  sys	0m0.602s

to:

  Attempt 1: 9.12
  Attempt 2: 8.904
  Attempt 3: 9.361
  Attempt 4: 9.288
  Attempt 5: 9.677

  real	0m8.904s
  user	0m8.208s
  sys	0m0.596s

yielding a modest ~2.6% improvement in the best timings from each run,
and ~7.4% improvement on average.

[1]: This can happen if, for example, the new commit-graph exceeds the
     maximum allowed factor on the number of commits.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 commit-graph.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index f013a84e29..c7cfadc786 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1599,14 +1599,18 @@ static void merge_commit_graph(struct write_commit_graph_context *ctx,
 
 	for (i = 0; i < g->num_commits; i++) {
 		struct object_id oid;
-		struct commit *result;
+		struct commit *result = NULL;
 
 		display_progress(ctx->progress, i + 1);
 
 		load_oid_from_graph(g, i + offset, &oid);
 
 		/* only add commits if they still exist in the repo */
-		result = lookup_commit_reference_gently(ctx->r, &oid, 1);
+		if (repo_has_object_file(ctx->r, &oid)) {
+			result = lookup_commit(ctx->r, &oid);
+			if (repo_parse_commit(ctx->r, result))
+				result = NULL;
+		}
 
 		if (result) {
 			ctx->commits.list[ctx->commits.nr] = result;
-- 
2.26.0.rc2.311.g8e52d2684b