[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


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@300525 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 10 insertions, 3 deletions

diff --git a/lib/xray/xray_fdr_logging.cc b/lib/xray/xray_fdr_logging.cc
index c5b63b0a5..e538b477a 100644
--- a/lib/xray/xray_fdr_logging.cc
+++ b/lib/xray/xray_fdr_logging.cc
@@ -118,11 +118,15 @@ XRayLogFlushStatus fdrLoggingFlush() XRAY_NEVER_INSTRUMENT {
     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