perf parse-events: Minor grouping tidy up

Add comments. Ensure leader->group_name is freed before overwriting
it.
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Tested-by: Atish Patra <atishp@rivosinc.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Beeman Strong <beeman@rivosinc.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240416061533.921723-16-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/parse-events.c

@@ -1711,6 +1711,7 @@ void parse_events__set_leader(char *name, struct list_head *list)
 
 	leader = list_first_entry(list, struct evsel, core.node);
 	__perf_evlist__set_leader(list, &leader->core);
+	zfree(&leader->group_name);
 	leader->group_name = name;
 }
 

tools/perf/util/parse-events.y

@@ -193,7 +193,10 @@ PE_NAME '{' events '}'
 {
 	struct list_head *list = $3;
 
-	/* Takes ownership of $1. */
+	/*
+	 * Set the first entry of list to be the leader. Set the group name on
+	 * the leader to $1 taking ownership.
+	 */
 	parse_events__set_leader($1, list);
 	$$ = list;
 }
@@ -202,6 +205,7 @@ PE_NAME '{' events '}'
 {
 	struct list_head *list = $2;
 
+	/* Set the first entry of list to be the leader clearing the group name. */
 	parse_events__set_leader(NULL, list);
 	$$ = list;
 }