Split platform specific time measurement away

We now have a `stopwatch` to measure time between two moments.

src/platform_bsdlike.c

@@ -3,8 +3,64 @@
 #include <err.h>
 #include <stdarg.h>
 
+#ifdef __MACH__
+#include <mach/clock.h>
+#include <mach/mach.h>
+#endif
+
+#ifdef __NetBSD__
+#include <sys/resource.h>
+#endif
+
 void h_platform_errx(int err, const char* format, ...) {
   va_list ap;
   va_start(ap, format);
   verrx(err, format, ap);
 }
+
+// TODO: replace this with a posix timer-based benchmark. (cf. timerfd_create, timer_create, setitimer)
+
+static void gettime(struct timespec *ts) {
+  if (ts == NULL)
+    return;
+#ifdef __MACH__ // OS X does not have clock_gettime, use clock_get_time
+  /*
+   * This returns real time, not CPU time. See http://stackoverflow.com/a/6725161
+   * Possible solution: http://stackoverflow.com/a/11659289
+   */
+  clock_serv_t cclock;
+  mach_timespec_t mts;
+  host_get_clock_service(mach_host_self(), CALENDAR_CLOCK, &cclock);
+  clock_get_time(cclock, &mts);
+  mach_port_deallocate(mach_task_self(), cclock);
+  ts->tv_sec = mts.tv_sec;
+  ts->tv_nsec = mts.tv_nsec;
+#elif defined(__NetBSD__)
+  // NetBSD doesn't have CLOCK_THREAD_CPUTIME_ID. We'll use getrusage instead
+  struct rusage rusage;
+  getrusage(RUSAGE_SELF, &rusage);
+  ts->tv_nsec = (rusage.ru_utime.tv_usec + rusage.ru_stime.tv_usec) * 1000;
+  // not going to overflow; can be at most 2e9-2
+  ts->tv_sec = rusage.ru_utime.tv_sec + rusage.ru_utime.tv_sec;
+  if (ts->tv_nsec >= 1000000000) {
+    ts->tv_nsec -=   1000000000; // subtract a second
+    ts->tv_sec += 1; // add it back.
+  }
+  assert (ts->tv_nsec <= 1000000000);
+#else
+  clock_gettime(CLOCK_THREAD_CPUTIME_ID, ts);
+#endif
+}
+
+void h_platform_stopwatch_reset(struct HStopWatch* stopwatch) {
+  gettime(&stopwatch->start);
+}
+
+int64_t h_platform_stopwatch_ns(struct HStopWatch* stopwatch) {
+  struct timespec ts_now;
+  gettime(&ts_now);
+
+  // time_diff is in ns
+  return (ts_now.tv_sec - stopwatch->start.tv_sec) * 1000000000
+	  + (ts_now.tv_nsec - stopwatch->start.tv_nsec);
+}