Do something sensible for timestamp wrapping.

Before we had this result in our frame timings (with the after/before
counters printed):

0x0000000fffa65090 0x0000000ffea7f9a0
frame: 99 16.6684 16.4952 0.0195919 0
0x0000000000a540a0 0x0000000fffa65090
frame: 100 1.84467e+13 1.84467e+13 1.7174e-14 0
0x0000000001a2fd30 0x0000000000a540a0
frame: 101 16.6289 16.4461 0.0216595 0

And now we get:

0x0000000fff3ab690 0x0000000ffe3c4f10
frame: 2387 16.6726 16.609 0.0173506 0
0x000000000038b420 0x0000000fff3ab690
frame: 2388 16.6455 16.5858 0.0169653 0
0x000000000137b4c0 0x000000000038b420
frame: 2389 16.7118 16.6509 0.0168314 0

diff --git a/metrics.c b/metrics.c
index 36ef90bf1c86b661204745b83aced491e675ed84..c52bf2ee6c3557dd6b8b41dda820ac24bced4e5b 100644 (file)
--- a/metrics.c
+++ b/metrics.c
@@ -113,6 +113,28 @@ struct metrics
        struct timespec cpu_time_ts;
 };
 
+/* Return (a - b) while accounting for the fact that between a and b,
+ * the counter may have wrapped around (with a maximum number of bits
+ * in the counter of GL_QUERY_COUNTER_BITS). */
+static uint64_t
+subtract_timestamp (uint64_t a, uint64_t b)
+{
+       GLint timestamp_bits;
+
+       glGetQueryiv (GL_TIMESTAMP, GL_QUERY_COUNTER_BITS, &timestamp_bits);
+
+       /* Note: "<<" isn't defined for the number of bits in the
+        * type, and the compiler will punish you.
+        */
+       if (timestamp_bits == 64)
+               return a - b;
+
+       if (b > a)
+               return (1ull << timestamp_bits) + a - b;
+       else
+               return a - b;
+}
+
 metrics_t *
 metrics_create (metrics_info_t *info)
 {
@@ -836,9 +858,11 @@ metrics_end_frame_pre_swap (metrics_t *metrics)
                printf ("# frame: Frame_Number Frame_Time_milliseconds Frame_latency_milliseconds CPU_load GPU_load\n");
        } else {
                /* Subtract previous frame's times to get frame times. */
-               frame_time_ns = swap_end_timestamp - metrics->previous_swap_end_timestamp;
+               frame_time_ns = subtract_timestamp (swap_end_timestamp,
+                                                   metrics->previous_swap_end_timestamp);
 
-               latency_ns = swap_end_timestamp - metrics->swap_begin_timestamp;
+               latency_ns = subtract_timestamp (swap_end_timestamp,
+                                                metrics->swap_begin_timestamp);
 
                cpu_time = TIMESPEC_DIFF (metrics->previous_cpu_time_ts, metrics->cpu_time_ts);