name-rev: put struct rev_name into commit slab

The commit slab commit_rev_name contains a pointer to a struct rev_name,
and the actual struct is allocated separatly.  Avoid that allocation and
pointer indirection by storing the full struct in the commit slab.  Use
the tip_name member pointer to determine if the returned struct is
initialized.

Performance in the Linux repository measured with hyperfine before:

Benchmark #1: ./git -C ../linux/ name-rev --all
  Time (mean ± σ):     953.5 ms ±   6.3 ms    [User: 901.2 ms, System: 52.1 ms]
  Range (min … max):   945.2 ms … 968.5 ms    10 runs

... and with this patch:

Benchmark #1: ./git -C ../linux/ name-rev --all
  Time (mean ± σ):     851.0 ms ±   3.1 ms    [User: 807.4 ms, System: 43.6 ms]
  Range (min … max):   846.7 ms … 857.0 ms    10 runs

Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

builtin/name-rev.c

@@ -24,19 +24,24 @@ struct rev_name {
 	int from_tag;
 };
 
-define_commit_slab(commit_rev_name, struct rev_name *);
+define_commit_slab(commit_rev_name, struct rev_name);
 
 static timestamp_t cutoff = TIME_MAX;
 static struct commit_rev_name rev_names;
 
 /* How many generations are maximally preferred over _one_ merge traversal? */
 #define MERGE_TRAVERSAL_WEIGHT 65535
 
+static int is_valid_rev_name(const struct rev_name *name)
+{
+	return name && name->tip_name;
+}
+
 static struct rev_name *get_commit_rev_name(const struct commit *commit)
 {
-	struct rev_name **slot = commit_rev_name_peek(&rev_names, commit);
+	struct rev_name *name = commit_rev_name_peek(&rev_names, commit);
 
-	return slot ? *slot : NULL;
+	return is_valid_rev_name(name) ? name : NULL;
 }
 
 static int is_better_name(struct rev_name *name,
@@ -81,15 +86,12 @@ static struct rev_name *create_or_update_name(struct commit *commit,
 					      int generation, int distance,
 					      int from_tag)
 {
-	struct rev_name **slot = commit_rev_name_at(&rev_names, commit);
-	struct rev_name *name = *slot;
+	struct rev_name *name = commit_rev_name_at(&rev_names, commit);
 
-	if (name && !is_better_name(name, taggerdate, distance, from_tag))
+	if (is_valid_rev_name(name) &&
+	    !is_better_name(name, taggerdate, distance, from_tag))
 		return NULL;
 
-	if (!name)
-		name = *slot = xmalloc(sizeof(*name));
-
 	name->tip_name = tip_name;
 	name->taggerdate = taggerdate;
 	name->generation = generation;