perf: Add yield and fix double comparison

- add a yield () function that's called before every test. It reduced the std
dev slightly for me
- fix double comparisons to not just compare the integer part

perf/cairo-perf-timer-posix.c

@@ -28,6 +28,9 @@
 #include <signal.h>
 #include <sys/time.h>
 #include <unistd.h>
+#ifdef _POSIX_PRIORITY_SCHEDULING
+#include <sched.h>
+#endif
 
 #include "cairo-perf.h"
 
@@ -81,3 +84,12 @@ set_alarm (double seconds) {
 
     setitimer (ITIMER_REAL, &tr, NULL);
 }
+
+/* yield */
+
+void
+yield (void) {
+#ifdef _POSIX_PRIORITY_SCHEDULING
+    sched_yield ();
+#endif
+}

perf/cairo-perf-timer-win32.c

@@ -74,3 +74,10 @@ set_alarm (double seconds) {
     if (!SetWaitableTimer (hTimer, &expTime, 0, alarm_handler, &cairo_perf_alarm_expired, FALSE))
         fprintf (stderr, "SetWaitableTimer failed!\n");
 }
+
+/* yield */
+
+void
+yield (void) {
+    SleepEx(0, TRUE);
+}

perf/cairo-perf-timer.h

@@ -62,4 +62,9 @@ alarm_handler (int signal);
 void
 set_alarm (double seconds);
 
+/* yield */
+
+void
+yield (void);
+
 #endif

perf/cairo-perf.c

@@ -102,7 +102,11 @@ compare_doubles (const void *_a, const void *_b)
     const double *a = _a;
     const double *b = _b;
 
-    return *a - *b;
+    if (*a > *b)
+	return 1;
+    if (*a < *b)
+	return -1;
+    return 0;
 }
 
 static void
@@ -162,8 +166,10 @@ main (int argc, char *argv[])
 							   size, size,
 							   &target->closure);
 		cr = cairo_create (surface);
-		for (k =0; k < cairo_perf_iterations; k++)
+		for (k =0; k < cairo_perf_iterations; k++) {
+		    yield ();
 		    rates[k] = perf->run (cr, size, size);
+		}
 		_compute_stats (rates, cairo_perf_iterations, &stats);
 		if (i==0 && j==0 && size == perf->min_size)
 		    printf ("backend-content\ttest-size\trate\tstd dev.\titerations\n");