Adding Google benchmarking library.

third_party/google_benchmark/test/donotoptimize_test.cc

+#include "benchmark/benchmark.h"
+
+#include <cstdint>
+
+namespace {
+#if defined(__GNUC__)
+std::uint64_t double_up(const std::uint64_t x) __attribute__((const));
+#endif
+std::uint64_t double_up(const std::uint64_t x) { return x * 2; }
+}
+
+int main(int, char*[]) {
+  // this test verifies compilation of DoNotOptimize() for some types
+
+  char buffer8[8];
+  benchmark::DoNotOptimize(buffer8);
+
+  char buffer20[20];
+  benchmark::DoNotOptimize(buffer20);
+
+  char buffer1024[1024];
+  benchmark::DoNotOptimize(buffer1024);
+  benchmark::DoNotOptimize(&buffer1024[0]);
+
+  int x = 123;
+  benchmark::DoNotOptimize(x);
+  benchmark::DoNotOptimize(&x);
+  benchmark::DoNotOptimize(x += 42);
+
+  benchmark::DoNotOptimize(double_up(x));
+
+  return 0;
+}