C ++ 2011：基于范围的循环展开？ [英] C++ 2011 : range-based loop unrolling?

问题描述

我不知道C ++编译器是否会展开基于范围的循环，就像他们目前为正常循环所做的那样，以最大限度地提高性能，或者在某些情况下基于范围的循环会比正常循环慢？

$ b $基于范围的循环等效于

解决方案

>

  {
 auto&& __range =（/ expression /）; 
 for（auto __begin = begin（__ range），
 __end = end（__ range）; 
 __begin！= __end; 
 ++ __ begin）{
 / = * __ begin; 
 / statement / 
} 
} 
  

迭代次数，它可以解决循环依赖性或循环是独立的，然后编译器可以自由滚动。

一般来说，循环展开只会改善较小循环的性能。所以，IMO，不管基于范围的循环是否展开无关紧要。你当然可以使用 -O3 和 -funroll-loops 和相关选项进行基准化，看看两者之间是否有任何区别。

I wonder if C++ compilers will unroll range-based loop the same way they currently do for "normal" loops to maximize performance or in some case range-based loops will be slower than normal loops ?

Thank you very much.

解决方案

for range-based loop is equivalent to:

{
  auto && __range = ( /expression/ );
  for (auto __begin = begin(__range),
            __end   = end(__range);
       __begin != __end;
       ++__begin) {
    /declaration/ = *__begin;
    /statement/
  }
}

If compiler knows the number of iterations and it can solve the loop dependencies or loops are independent, then compiler is free to un-roll.

In general, loop unrolling is going to improve performance only for smaller loops. So, IMO, it does not matter whether range-based loops are unrolled or not. You can certainly benchmark with -O3 and -funroll-loops and relevant options to see if there's indeed any difference between two.

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