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Implement ClobberMemory() and fix DoNotOptimize on MSVC. (#352)

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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.
This commit is contained in:
Eric 2017-03-10 18:47:39 -07:00 committed by GitHub
parent 8ae6448cc7
commit f682f7e9a4
2 changed files with 22 additions and 3 deletions
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2
README.md
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@ -365,7 +365,7 @@ static void BM_vector_push_back(benchmark::State& state) {
}
```
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

23
include/benchmark/benchmark_api.h
View File

@ -165,6 +165,10 @@ BENCHMARK(BM_test)->Unit(benchmark::kMillisecond);
#include <utility>
#endif
#if defined(_MSC_VER)
#include <intrin.h> // for _ReadWriteBarrier
#endif
namespace benchmark {
class BenchmarkReporter;
@ -215,11 +219,16 @@ BENCHMARK_UNUSED static int stream_init_anchor = InitializeStreams();
} // 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 DoNotOptimize(Tp const& value) {
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