[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

llvm-svn: 300525

3 files changed, 23 insertions, 6 deletions

diff --git a/compiler-rt/lib/xray/xray_fdr_logging.cc b/compiler-rt/lib/xray/xray_fdr_logging.cc
index c5b63b0a564..e538b477a3d 100644
--- a/compiler-rt/lib/xray/xray_fdr_logging.cc
+++ b/compiler-rt/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
diff --git a/compiler-rt/lib/xray/xray_inmemory_log.cc b/compiler-rt/lib/xray/xray_inmemory_log.cc
index cdaa6d1b5c8..83aecfaf770 100644
--- a/compiler-rt/lib/xray/xray_inmemory_log.cc
+++ b/compiler-rt/lib/xray/xray_inmemory_log.cc
@@ -79,15 +79,19 @@ static int __xray_OpenLogFile() XRAY_NEVER_INSTRUMENT {
   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.
diff --git a/compiler-rt/lib/xray/xray_x86_64.cc b/compiler-rt/lib/xray/xray_x86_64.cc
index 8c2a4e313e3..2e9a8d270c3 100644
--- a/compiler-rt/lib/xray/xray_x86_64.cc
+++ b/compiler-rt/lib/xray/xray_x86_64.cc
@@ -214,6 +214,12 @@ bool probeRequiredCPUFeatures() XRAY_NEVER_INSTRUMENT {
     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;
 }