使用volatile来防止基准测试代码中的编译器优化？ [英] Using volatile to prevent compiler optimization in benchmarking code?

问题描述

我正在创建一个小程序来测量 boost :: shared_ptr 和 boost :: intrusive_ptr 。为了防止编译器优化副本，我将变量声明为volatile。循环如下所示：

I am creating a little program measure the performance difference between containers of types boost::shared_ptr and boost::intrusive_ptr. In order to prevent the compiler from optimizing away the copy I declare the variable as volatile. The loop looks like this:

// TestCopy measures the time required to create n copies of the given container.
// Returns time in milliseconds.
template<class Container>
time_t TestCopy(const Container & inContainer, std::size_t n) {
    Poco::Stopwatch stopwatch;
    stopwatch.start();
    for (std::size_t idx = 0; idx < n; ++idx)
    {
        volatile Container copy = inContainer; // Volatile!
    }

    // convert microseconds to milliseconds
    return static_cast<time_t>(0.5 + (double(stopwatch.elapsed()) / 1000.0));
}

其余代码可在此处找到： main.cpp 。

The rest of the code can be found here: main.cpp.

• 在这里使用volatile会阻止编译器优化副本吗？


• 是否存在任何可能使结果无效的陷阱？ / li>
  

• Will using volatile here prevent the compiler from optimizing away the copy?
• Are there any pitfalls that may invalidate the results?

响应@Neil Butterworth。即使在使用副本时，在我看来，编译器仍然可以轻松避免该副本：

In response to @Neil Butterworth. Even when using the copy it still seems to me that the compiler could easily avoid the copy:

for (std::size_t idx = 0; idx < n; ++idx)
{
    // gcc won't remove this copy?
    Container copy = inContainer;
    gNumCopies += copy.size();        
}

推荐答案

C ++ 03标准表示对易失性数据进行读写是可以观察到的行为（C ++ 2003，1.9 [intro.execution] / 6）。我相信，这保证了无法优化分配给易失数据的工作。另一种可观察到的行为是对I / O函数的调用。
在这方面，C ++ 11标准更加明确：在1.9 / 8中，它明确指出

The C++03 standard says that reads and writes to volatile data is observable behavior (C++ 2003, 1.9 [intro.execution] / 6). I believe this guarantees that assignment to volatile data cannot be optimized away. Another kind of observable behavior is calls to I/O functions. The C++11 standard is even more unambiguous in this regard: in 1.9/8 it explicitly says that

符合标准的实现的要求如下：

—严格根据抽象机的规则评估对易失对象的访问。

The least requirements on a conforming implementation are:
— Access to volatile objects are evaluated strictly according to the rules of the abstract machine.

如果编译器可以证明代码没有产生可观察到的行为，那么它可以优化代码。在您的更新中（不使用volatile），复制构造函数和其他函数调用&重载的运算符可能会避免任何I / O调用和对易失性数据的访问，并且编译器可能会很好地理解它。但是，如果 gNumCopies 是稍后在具有可观察到的行为（例如打印）的表达式中使用的全局变量，则不会删除此代码。

If a compiler can prove that a code does not produce an observable behavior then it can optimize the code away. In your update (where volatile is not used), copy constructor and other function calls & overloaded operators might avoid any I/O calls and access to volatile data, and the compiler might well understand it. However if gNumCopies is a global variable that later used in an expression with observable behavior (e.g. printed), then this code will not be removed.

这篇关于使用volatile来防止基准测试代码中的编译器优化？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IT屋！