“逃脱"和“口语"在MSVC中等效

问题描述

在钱德勒·卡鲁斯(Chandler Carruth)的CppCon 2015演讲中，他介绍了两个神奇的功能，它们可以击败优化器而不会造成任何额外的性能损失.

In Chandler Carruth's CppCon 2015 talk he introduces two magical functions for defeating the optimizer without any extra performance penalties.

作为参考，以下是函数(使用GNU风格的内联汇编):

For reference, here are the functions (using GNU-style inline assembly):

void escape(void* p)
{
    asm volatile("" : : "g"(p) : "memory");
}

void clobber()
{
    asm volatile("" : : : "memory");
}

它可在任何支持GNU风格内联汇编的编译器(GCC，Clang，Intel的编译器，可能还有其他编译器)上使用.但是，他提到它在MSVC中不起作用.

It works on any compiler which supports GNU-style inline assembly (GCC, Clang, Intel's compiler, possibly others). However, he mentions it doesn't work in MSVC.

检查 Google Benchmark的实现，似乎他们使用了重新解释的强制转换volatile const char&并将其传递给非gcc/clang编译器上其他翻译单元中隐藏的函数.

Examining Google Benchmark's implementation, it seems they use a reinterpret cast to a volatile const char& and passes it to a function hidden in a different translation unit on non-gcc/clang compilers.

template <class Tp>
inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp const& value) {
    internal::UseCharPointer(&reinterpret_cast<char const volatile&>(value));
}

// some other translation unit
void UseCharPointer(char const volatile*) {}

但是，我对此有两个担忧:

However, there are two concerns I have with this:

1. 我可能会发生函数调用
2. 智能"链接时优化器可能会发现UseCharPointer很小，将其内联，然后丢弃我想要保留的所有代码，或者可能允许智能"优化器执行其他重新排序不想.

MSVC中是否有与GNU样式的汇编函数相当的低级等效项?还是这是MSVC上最好的?

Is there any lower-level equivalent in MSVC to the GNU-style assembly functions? Or is this the best it gets on MSVC?

推荐答案

虽然我不知道MSVC的等效汇编技巧，但Facebook在其Folly基准测试库中使用了以下内容:

While I don't know of an equivalent assembly trick for MSVC, Facebook uses the following in their Folly benchmark library:

/**
 * Call doNotOptimizeAway(var) against variables that you use for
 * benchmarking but otherwise are useless. The compiler tends to do a
 * good job at eliminating unused variables, and this function fools
 * it into thinking var is in fact needed.
 */
#ifdef _MSC_VER

#pragma optimize("", off)

template <class T>
void doNotOptimizeAway(T&& datum) {
  datum = datum;
}

#pragma optimize("", on)

#elif defined(__clang__)

template <class T>
__attribute__((__optnone__)) void doNotOptimizeAway(T&& /* datum */) {}

#else

template <class T>
void doNotOptimizeAway(T&& datum) {
  asm volatile("" : "+r" (datum));
}

#endif

此处是指向GitHub上代码的链接.

这篇关于“逃脱"和“口语"在MSVC中等效的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！