perf diff: Make hist_entry_diff fields union

The period_ratio_delta, period_ratio and wdiff are never by used at the
same time.  Instead, Just one of them is accessed according to a
comparison method.  So make it union to reduce memory footprint.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1429416255-12070-3-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/sort.h

@@ -58,7 +58,7 @@ struct he_stat {
 
 struct hist_entry_diff {
 	bool	computed;
-
+	union {
 		/* PERF_HPP__DELTA */
 		double	period_ratio_delta;
 
@@ -67,6 +67,7 @@ struct hist_entry_diff {
 
 		/* HISTC_WEIGHTED_DIFF */
 		s64	wdiff;
+	};
 };
 
 /**