Rename PBD::QPC::get_timer_valid to check_timer_valid and perform timer test

I'm not sure if this test is going to be effective as I don't have hardware to
test on at the moment. As noted in the documentation, Windows XP should be the
only OS where QPC uses a timer source that is non-monotonic(multi-core with
non-syncronized TSC).

3 files changed, 27 insertions, 4 deletions

diff --git a/libs/pbd/pbd/windows_timer_utils.h b/libs/pbd/pbd/windows_timer_utils.h
index 72d3cc5431..4ebeacd6eb 100644
--- a/libs/pbd/pbd/windows_timer_utils.h
+++ b/libs/pbd/pbd/windows_timer_utils.h
@@ -61,8 +61,11 @@ namespace QPC {
 
 /**
  * @return true if QueryPerformanceCounter is usable as a timer source
+ * This should always return true for systems > XP as those versions of windows
+ * have there own tests to check timer validity and will select an appropriate
+ * timer source.
  */
-bool get_timer_valid ();
+bool check_timer_valid ();
 
 /**
  * @return the value of the performance counter converted to microseconds
diff --git a/libs/pbd/test/windows_timer_utils_test.cc b/libs/pbd/test/windows_timer_utils_test.cc
index fe5e1a24df..1566f37d96 100644
--- a/libs/pbd/test/windows_timer_utils_test.cc
+++ b/libs/pbd/test/windows_timer_utils_test.cc
@@ -13,7 +13,8 @@ CPPUNIT_TEST_SUITE_REGISTRATION (WindowsTimerUtilsTest);
 void
 WindowsTimerUtilsTest::testQPC ()
 {
-	CPPUNIT_ASSERT (PBD::QPC::get_timer_valid());
+	// performs basically the same test
+	CPPUNIT_ASSERT (PBD::QPC::check_timer_valid());
 
 	int64_t last_timer_val = PBD::QPC::get_microseconds ();
 	CPPUNIT_ASSERT (last_timer_val >= 0);
diff --git a/libs/pbd/windows_timer_utils.cc b/libs/pbd/windows_timer_utils.cc
index d95a69e0c7..ee7cd962c5 100644
--- a/libs/pbd/windows_timer_utils.cc
+++ b/libs/pbd/windows_timer_utils.cc
@@ -136,16 +136,35 @@ qpc_frequency_cached ()
 	return frequency;
 }
 
+bool
+test_qpc_validity ()
+{
+	int64_t last_timer_val = PBD::QPC::get_microseconds ();
+	if (last_timer_val < 0) return false;
+
+	for (int i = 0; i < 100000; ++i) {
+		int64_t timer_val = PBD::QPC::get_microseconds ();
+		if (timer_val < 0) return false;
+		// try and test for non-syncronized TSC(AMD K8/etc)
+		if (timer_val < last_timer_val) return false;
+		last_timer_val = timer_val;
+	}
+	return true;
+}
+
 } // anon namespace
 
 namespace QPC {
 
 bool
-get_timer_valid ()
+check_timer_valid ()
 {
 	// setup caching the timer frequency
 	qpc_frequency_cached ();
-	return qpc_frequency_success ();
+	if (!qpc_frequency_success ()) {
+		return false;
+	}
+	return test_qpc_validity ();
 }
 
 int64_t