时间与“假设”规则相对应(Timing vs the “as-if” rule)

 关于“as-if”规则有一个很好的问题 ，但是我想知道在测量时间方面是否有任何重复。 
 
 考虑一下（从这里略微修改）： 
 
using std::chrono;
auto begin = steady_clock::now();

auto result = some_lengthy_calculation(some_params);

auto end= std::chrono::steady_clock::now();

std::cout << "Time diff = " << duration_cast<microseconds>(end - begin).count() <<std::endl;
std::cout << "Result = " << result;
 
 允许编译器应用任何导致相同result优化。 这里的要点是“as-if”规则不直接适用于测量时间。 当然，在应用优化时，测量的时间不应该是恒定的。 
 
 所以我的问题是：当根据“as-if”规则允许编译器将其重新排列为以下之一时，如何使用上述代码可靠地测量时间？ 
 
auto temp = some_lengthy_calculation(some_params); // clever "optimization", precompute some stuff

auto begin = steady_clock::now();
auto result = temp;               // yay, I can use it here to pretend to be faster
auto end = steady_clock::now();

std::cout << "Time diff = " << duration_cast<microseconds>(end - begin).count() <<std::endl;
std::cout << "Result = " << result;
 
 甚至“更优化”： 
 
std::cout << "Time diff = " << 42 <<std::endl;
std::cout << "Result = " << some_lengthy_calculation(some_params);
 
 我假设没有理智的编译器会这样做，但究竟是什么阻止了编译器进行这样的“优化”？ 
 
 TL; DR ... 
 
 
 
 可以观察优化和非优化代码之间的运行时差异 
 
 
 如果允许编译器优化影响测量时间，那么是什么阻止了编译器根本不创建任何代码？ 
 

There is a great question about the "as-if" rule in general, but I wonder if there are any excpetions when it comes to measuring time.
 
Consider this (taken from here slightly modified):
 
using std::chrono;
auto begin = steady_clock::now();

auto result = some_lengthy_calculation(some_params);

auto end= std::chrono::steady_clock::now();

std::cout << "Time diff = " << duration_cast<microseconds>(end - begin).count() <<std::endl;
std::cout << "Result = " << result;
 
The compiler is allowed to apply any optimization that results in the same result. The point here is that the "as-if" rule does not apply to the measured time directly. Of course the measured time is not something that should be constant when optimizations are applied. 
 
So my question is: How is it possible that I can reliably measure time with the above code when, according to the "as-if" rule, the compiler is allowed to rearrange it to one of the following?
 
auto temp = some_lengthy_calculation(some_params); // clever "optimization", precompute some stuff

auto begin = steady_clock::now();
auto result = temp;               // yay, I can use it here to pretend to be faster
auto end = steady_clock::now();

std::cout << "Time diff = " << duration_cast<microseconds>(end - begin).count() <<std::endl;
std::cout << "Result = " << result;
 
or even "more optimized": 
 
std::cout << "Time diff = " << 42 <<std::endl;
std::cout << "Result = " << some_lengthy_calculation(some_params);
 
I assume no sane compiler would do that, but what exactly prevents a compiler from doing such "optimization"?
 
TL;DR...
 
 
 
One can observe the runtime difference between optimized and non-optimized code
 
 
If compiler optimizations are allowed to effect measured times, what prevents the compiler from not creating any code for the timing at all?
 

原文：https://stackoverflow.com/questions/46572817