commit-graph: fix regression when computing Bloom filters

Before computing Bloom filters, the commit-graph machinery uses
commit_gen_cmp to sort commits by generation order for improved diff
performance. 3d11275 (commit-graph: examine commits by generation
number, 2020-03-30) claims that this sort can reduce the time spent to
compute Bloom filters by nearly half.

But since c49c82a (commit: move members graph_pos, generation to a
slab, 2020-06-17), this optimization is broken, since asking for a
'commit_graph_generation()' directly returns GENERATION_NUMBER_INFINITY
while writing.

Not all hope is lost, though: 'commit_gen_cmp()' falls back to
comparing commits by their date when they have equal generation number,
and so since c49c82a is purely a date comparison function. This
heuristic is good enough that we don't seem to loose appreciable
performance while computing Bloom filters.

Applying this patch (compared with v2.30.0) speeds up computing Bloom
filters by factors ranging from 0.40% to 5.19% on various repositories [1].

So, avoid the useless 'commit_graph_generation()' while writing by
instead accessing the slab directly. This returns the newly-computed
generation numbers, and allows us to avoid the heuristic by directly
comparing generation numbers.

[1]: https://lore.kernel.org/git/[email protected]/

Signed-off-by: Abhishek Kumar <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Reviewed-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: abhishekkumar2718

commit-graph.c

@@ -139,13 +139,17 @@ static struct commit_graph_data *commit_graph_data_at(const struct commit *c)
 	return data;
 }
 
+/*
+ * Should be used only while writing commit-graph as it compares
+ * generation value of commits by directly accessing commit-slab.
+ */
 static int commit_gen_cmp(const void *va, const void *vb)
 {
 	const struct commit *a = *(const struct commit **)va;
 	const struct commit *b = *(const struct commit **)vb;
 
-	uint32_t generation_a = commit_graph_generation(a);
-	uint32_t generation_b = commit_graph_generation(b);
+	uint32_t generation_a = commit_graph_data_at(a)->generation;
+	uint32_t generation_b = commit_graph_data_at(b)->generation;
 	/* lower generation commits first */
 	if (generation_a < generation_b)
 		return -1;