libgo patch committed: Use fence instruction before rdtsc

[Ian Lance Taylor]

This libgo patch uses an appropriate fence instruction before the
rdtsc instruction.  This is required on some multicore implementations
in order to consistent cycle counts on different cores.  Bootstrapped
and ran Go testsuite on x86_64-pc-linux-gnu.  Committed to mainline.

Ian

Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE     (revision 258259)
+++ gcc/go/gofrontend/MERGE     (working copy)
@@ -1,4 +1,4 @@
-3287064c24cbf0c50776cdb87a720d29130b4363
+2a07cd31927ac943104f55d2b696e53e7cd073b3
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: libgo/runtime/runtime_c.c
===================================================================
--- libgo/runtime/runtime_c.c   (revision 257914)
+++ libgo/runtime/runtime_c.c   (working copy)
@@ -33,13 +33,47 @@ runtime_atoi(const byte *p, intgo len)
        return n;
 }
 
+#if defined(__i386__) || defined(__x86_64__) || defined (__s390__) || defined 
(__s390x__)
+
+// When cputicks is just asm instructions, skip the split stack
+// prologue for speed.
+
+int64 runtime_cputicks(void) __attribute__((no_split_stack));
+
+#endif
+
+// Whether the processor supports SSE2.
+#if defined (__i386__)
+static _Bool hasSSE2;
+
+// Force appropriate CPU level so that we can call the lfence/mfence
+// builtins.
+
+#pragma GCC push_options
+#pragma GCC target("sse2")
+
+#elif defined(__x86_64__)
+#define hasSSE2 true
+#endif
+
+#if defined(__i386__) || defined(__x86_64__)
+// Whether to use lfence, as opposed to mfence.
+// Set based on cpuid.
+static _Bool lfenceBeforeRdtsc;
+#endif // defined(__i386__) || defined(__x86_64__)
+
 int64
 runtime_cputicks(void)
 {
-#if defined(__386__) || defined(__x86_64__)
-  uint32 low, high;
-  asm("rdtsc" : "=a" (low), "=d" (high));
-  return (int64)(((uint64)high << 32) | (uint64)low);
+#if defined(__i386__) || defined(__x86_64__)
+  if (hasSSE2) {
+    if (lfenceBeforeRdtsc) {
+      __builtin_ia32_lfence();
+    } else {
+      __builtin_ia32_mfence();
+    }
+  }
+  return __builtin_ia32_rdtsc();
 #elif defined (__s390__) || defined (__s390x__)
   uint64 clock = 0;
   /* stckf may not write the return variable in case of a clock error, so make
@@ -56,6 +90,10 @@ runtime_cputicks(void)
 #endif
 }
 
+#if defined(__i386__)
+#pragma GCC pop_options
+#endif
+
 void
 runtime_signalstack(byte *p, uintptr n)
 {
@@ -146,8 +184,21 @@ runtime_cpuinit()
 #if defined(__i386__) || defined(__x86_64__)
        unsigned int eax, ebx, ecx, edx;
 
+       if (__get_cpuid(0, &eax, &ebx, &ecx, &edx)) {
+               if (eax != 0
+                   && ebx == 0x756E6547    // "Genu"
+                   && edx == 0x49656E69    // "ineI"
+                   && ecx == 0x6C65746E) { // "ntel"
+                       lfenceBeforeRdtsc = true;
+               }
+       }
        if (__get_cpuid(1, &eax, &ebx, &ecx, &edx)) {
                setCpuidECX(ecx);
+#if defined(__i386__)
+               if ((edx & bit_SSE2) != 0) {
+                       hasSSE2 = true;
+               }
+#endif
        }
 
 #if defined(HAVE_AS_X86_AES)