add functions that prevent optimization

include/benchmark/benchmark_api.h

@@ -174,8 +174,23 @@ struct EnableIfString<T, typename Voider<typename T::basic_string>::type> {
     typedef int type;
 };
 
+void UseCharPointer(char const volatile*);
+
 } // end namespace internal
 
+#if defined(__GNUC__)
+template <class Tp>
+inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp& value) {
+    asm volatile("" : "+r" (value));
+}
+#else
+template <class Tp>
+inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp& value) {
+    internal::UseCharPointer(&reinterpret_cast<char const volatile&>(value));
+}
+#endif
+
+
 // State is passed to a running Benchmark and contains state for the
 // benchmark to use.
 class State {

src/benchmark.cc

@@ -84,6 +84,8 @@ namespace benchmark {
 
 namespace internal {
 
+void UseCharPointer(char const volatile*) {}
+
 // NOTE: This is a dummy "mutex" type used to denote the actual mutex
 // returned by GetBenchmarkLock(). This is only used to placate the thread
 // safety warnings by giving the return of GetBenchmarkLock() a name.