Create a field trial to test if we can detect good QPC implementations.

base/cpu.cc

Index: base/cpu.cc
diff --git a/base/cpu.cc b/base/cpu.cc
index d6976bab33b316aed50e3a133edf1a7e8a060a97..0f07ef674d4ae81eb63902982393b5dc7ef58490 100644
--- a/base/cpu.cc
+++ b/base/cpu.cc
@@ -32,6 +32,7 @@ CPU::CPU()
     has_ssse3_(false),
     has_sse41_(false),
     has_sse42_(false),
+    has_nonstoptsc_(false),
     cpu_vendor_("unknown") {
   Initialize();
 }
@@ -123,6 +124,7 @@ void CPU::Initialize() {
   // Get the brand string of the cpu.
   __cpuid(cpu_info, 0x80000000);
   const int parameter_end = 0x80000004;
+  int max_parameter = cpu_info[0];
 
   if (cpu_info[0] >= parameter_end) {
     char* cpu_string_ptr = cpu_string;
@@ -135,6 +137,12 @@ void CPU::Initialize() {
     }
     cpu_brand_.assign(cpu_string, cpu_string_ptr - cpu_string);
   }
+
+  const int parameter_containing_nonstoptsc = 0x80000007;
+  if (max_parameter >= parameter_containing_nonstoptsc) {
+    __cpuid(cpu_info, parameter_containing_nonstoptsc);
+    has_nonstoptsc_ = (cpu_info[3] & (1 << 8)) != 0;

[jar (doing other things), 2013/04/04 00:26:13]

nit: unless there is a great reason to write this so compactly, I'd greatly
prefer something like:

has_non_stop_tsc

Even "tsc" bugs me a bit, but I'm not sure about "time_stamp_counter"

hence:
has_non_stop_time_stamp_counter

I don't mind using a special name when there is some crazy intel defined name to
match (example: sse41), but for places where we're not forced to use a name,
perchance we can push toward our style.

[James Simonsen, 2013/04/04 02:26:51]

Done.

+  }
 #endif
 }