Implement ClobberMemory() and fix DoNotOptimize on MSVC. (#352)
I recently learned Windows provides a function called _ReadWriteBarrier
which is literally ClobberMemory under a different name. This patch
uses it to implement ClobberMemory under MSVC.
diff --git a/README.md b/README.md
index 456b0a6..4446637 100644
--- a/README.md
+++ b/README.md
@@ -365,7 +365,7 @@
}
```
-Note that `ClobberMemory()` is only available for GNU based compilers.
+Note that `ClobberMemory()` is only available for GNU or MSVC based compilers.
### Set time unit manually
If a benchmark runs a few milliseconds it may be hard to visually compare the
diff --git a/include/benchmark/benchmark_api.h b/include/benchmark/benchmark_api.h
index f72a64a..8cde61b 100644
--- a/include/benchmark/benchmark_api.h
+++ b/include/benchmark/benchmark_api.h
@@ -165,6 +165,10 @@
#include <utility>
#endif
+#if defined(_MSC_VER)
+#include <intrin.h> // for _ReadWriteBarrier
+#endif
+
namespace benchmark {
class BenchmarkReporter;
@@ -215,11 +219,16 @@
} // end namespace internal
+
+#if !defined(__GNUC__) || defined(__pnacl__) || defined(EMSCRIPTN)
+# define BENCHMARK_HAS_NO_INLINE_ASSEMBLY
+#endif
+
// The DoNotOptimize(...) function can be used to prevent a value or
// expression from being optimized away by the compiler. This function is
// intended to add little to no overhead.
// See: https://youtu.be/nXaxk27zwlk?t=2441
-#if defined(__GNUC__) && !defined(__pnacl__) && !defined(EMSCRIPTEN)
+#ifndef BENCHMARK_HAS_NO_INLINE_ASSEMBLY
template <class Tp>
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
asm volatile("" : : "g"(value) : "memory");
@@ -229,12 +238,22 @@
inline BENCHMARK_ALWAYS_INLINE void ClobberMemory() {
asm volatile("" : : : "memory");
}
+#elif defined(_MSC_VER)
+template <class Tp>
+inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
+ internal::UseCharPointer(&reinterpret_cast<char const volatile&>(value));
+ _ReadWriteBarrier();
+}
+
+inline BENCHMARK_ALWAYS_INLINE void ClobberMemory() {
+ _ReadWriteBarrier();
+}
#else
template <class Tp>
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
internal::UseCharPointer(&reinterpret_cast<char const volatile&>(value));
}
-// FIXME Add ClobberMemory() for non-gnu compilers
+// FIXME Add ClobberMemory() for non-gnu and non-msvc compilers
#endif
