CycleClock: Add support for Alpha architecture (#1753)

* Add support for Alpha architecture

As documented, the real cycle counter is unsafe to use here, because it
is a 32-bit integer which wraps every ~4s.  Use gettimeofday instead,
which has a limitation of a low-precision real-time-clock (~1ms), but no
wrapping.  Passes test suite.

Support parsing /proc/cpuinfo on Alpha

tabular_test: add a missing DoNotOptimize call

src/cycleclock.h

@@ -218,6 +218,15 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
   uint64_t pcycle;
   asm volatile("%0 = C15:14" : "=r"(pcycle));
   return static_cast<double>(pcycle);
+#elif defined(__alpha__)
+  // Alpha has a cycle counter, the PCC register, but it is an unsigned 32-bit
+  // integer and thus wraps every ~4s, making using it for tick counts
+  // unreliable beyond this time range.  The real-time clock is low-precision,
+  // roughtly ~1ms, but it is the only option that can reasonable count
+  // indefinitely.
+  struct timeval tv;
+  gettimeofday(&tv, nullptr);
+  return static_cast<int64_t>(tv.tv_sec) * 1000000 + tv.tv_usec;
 #else
   // The soft failover to a generic implementation is automatic only for ARM.
   // For other platforms the developer is expected to make an attempt to create

src/sysinfo.cc

@@ -513,7 +513,11 @@ int GetNumCPUs() {
     std::cerr << "failed to open /proc/cpuinfo\n";
     return -1;
   }
+#if defined(__alpha__)
+  const std::string Key = "cpus detected";
+#else
   const std::string Key = "processor";
+#endif
   std::string ln;
   while (std::getline(f, ln)) {
     if (ln.empty()) continue;

test/user_counters_tabular_test.cc

@@ -63,6 +63,9 @@ ADD_CASES(TC_CSVOut, {{"%csv_header,"
 
 void BM_Counters_Tabular(benchmark::State& state) {
   for (auto _ : state) {
+    // This test requires a non-zero CPU time to avoid divide-by-zero
+    auto iterations = state.iterations();
+    benchmark::DoNotOptimize(iterations);
   }
   namespace bm = benchmark;
   state.counters.insert({