Index: compiler-rt/trunk/lib/xray/xray_fdr_logging.cc
===================================================================
--- compiler-rt/trunk/lib/xray/xray_fdr_logging.cc
+++ compiler-rt/trunk/lib/xray/xray_fdr_logging.cc
@@ -118,11 +118,15 @@
     return Result;
   }
 
+  // Test for required CPU features and cache the cycle frequency
+  static bool TSCSupported = probeRequiredCPUFeatures();
+  static uint64_t CycleFrequency = TSCSupported ? getTSCFrequency()
+                                   : __xray::NanosecondsPerSecond;
+
   XRayFileHeader Header;
   Header.Version = 1;
   Header.Type = FileTypes::FDR_LOG;
-  Header.CycleFrequency = probeRequiredCPUFeatures()
-                          ? getTSCFrequency() : __xray::NanosecondsPerSecond;
+  Header.CycleFrequency = CycleFrequency;
   // FIXME: Actually check whether we have 'constant_tsc' and 'nonstop_tsc'
   // before setting the values in the header.
   Header.ConstantTSC = 1;
@@ -196,7 +200,10 @@
   unsigned char CPU;
   uint64_t TSC;
 
-  if(probeRequiredCPUFeatures()) {
+  // Test once for required CPU features
+  static bool TSCSupported = probeRequiredCPUFeatures();
+
+  if(TSCSupported) {
     TSC = __xray::readTSC(CPU);
   } else {
     // FIXME: This code needs refactoring as it appears in multiple locations
Index: compiler-rt/trunk/lib/xray/xray_inmemory_log.cc
===================================================================
--- compiler-rt/trunk/lib/xray/xray_inmemory_log.cc
+++ compiler-rt/trunk/lib/xray/xray_inmemory_log.cc
@@ -79,15 +79,19 @@
   int F = getLogFD();
   if (F == -1)
     return -1;
+
+  // Test for required CPU features and cache the cycle frequency
+  static bool TSCSupported = probeRequiredCPUFeatures();
+  static uint64_t CycleFrequency = TSCSupported ? getTSCFrequency()
+                                   : __xray::NanosecondsPerSecond;
+
   // Since we're here, we get to write the header. We set it up so that the
   // header will only be written once, at the start, and let the threads
   // logging do writes which just append.
   XRayFileHeader Header;
   Header.Version = 1;
   Header.Type = FileTypes::NAIVE_LOG;
-  Header.CycleFrequency = probeRequiredCPUFeatures()
-                              ? getTSCFrequency()
-                              : __xray::NanosecondsPerSecond;
+  Header.CycleFrequency = CycleFrequency;
 
   // FIXME: Actually check whether we have 'constant_tsc' and 'nonstop_tsc'
   // before setting the values in the header.
Index: compiler-rt/trunk/lib/xray/xray_x86_64.cc
===================================================================
--- compiler-rt/trunk/lib/xray/xray_x86_64.cc
+++ compiler-rt/trunk/lib/xray/xray_x86_64.cc
@@ -214,6 +214,12 @@
     Report("Missing rdtscp support.\n");
     return false;
   }
+  // Also check whether we can determine the CPU frequency, since if we cannot,
+  // we should use the emulated TSC instead.
+  if (!getTSCFrequency()) {
+    Report("Unable to determine CPU frequency.\n");
+    return false;
+  }
   return true;
 }