[compiler-rt] r300525 - [XRay][compiler-rt] Use emulated TSC when CPU supports rdtscp, but cannot determine the CPU frequency

[Douglas Yung via llvm-commits]

Author: dyung
Date: Mon Apr 17 22:25:11 2017
New Revision: 300525

URL: http://llvm.org/viewvc/llvm-project?rev=300525&view=rev
Log:
[XRay][compiler-rt] Use emulated TSC when CPU supports rdtscp, but cannot determine the CPU frequency

A problem arises if a machine supports the rdtscp instruction, but the processor
frequency cannot be determined by the function getTSCFrequency(). In this case,
we want to use the emulated TSC instead. This patch implements that by adding a
call to getTSCFrequency() from probeRequiredCPUFeatures(), and the function only
returns true if both the processor supports rdtscp and the CPU frequency can be
determined.

This should fix PR32620.

Reviewers: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32067

Modified:
    compiler-rt/trunk/lib/xray/xray_fdr_logging.cc
    compiler-rt/trunk/lib/xray/xray_inmemory_log.cc
    compiler-rt/trunk/lib/xray/xray_x86_64.cc

Modified: compiler-rt/trunk/lib/xray/xray_fdr_logging.cc
URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/xray/xray_fdr_logging.cc?rev=300525&r1=300524&r2=300525&view=diff
==============================================================================
--- compiler-rt/trunk/lib/xray/xray_fdr_logging.cc (original)
+++ compiler-rt/trunk/lib/xray/xray_fdr_logging.cc Mon Apr 17 22:25:11 2017
@@ -118,11 +118,15 @@ XRayLogFlushStatus fdrLoggingFlush() XRA
     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 @@ void fdrLoggingHandleArg0(int32_t FuncId
   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

Modified: compiler-rt/trunk/lib/xray/xray_inmemory_log.cc
URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/xray/xray_inmemory_log.cc?rev=300525&r1=300524&r2=300525&view=diff
==============================================================================
--- compiler-rt/trunk/lib/xray/xray_inmemory_log.cc (original)
+++ compiler-rt/trunk/lib/xray/xray_inmemory_log.cc Mon Apr 17 22:25:11 2017
@@ -79,15 +79,19 @@ static int __xray_OpenLogFile() XRAY_NEV
   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.

Modified: compiler-rt/trunk/lib/xray/xray_x86_64.cc
URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/xray/xray_x86_64.cc?rev=300525&r1=300524&r2=300525&view=diff
==============================================================================
--- compiler-rt/trunk/lib/xray/xray_x86_64.cc (original)
+++ compiler-rt/trunk/lib/xray/xray_x86_64.cc Mon Apr 17 22:25:11 2017
@@ -214,6 +214,12 @@ bool probeRequiredCPUFeatures() XRAY_NEV
     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;
 }