loop unrolling in C++

juliosun

于 2014-02-28 21:46:54 发布

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本文探讨了编译器如何通过循环展开技术减少循环控制结构的开销，从而提高程序执行效率。通过对比未优化与优化后的循环结构，解释了优化前后指令执行频率的变化。

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In the high-level view of a language, you're not going to see the optimization. The speed enhancement comes from what the compiler does with what you have.

In the first case, it's something like:

LOCATION_FLAG;
DO_SOMETHING;
TEST FOR LOOP COMPLETION;//Jumps to LOCATION_FLAG if false

In the second it's something like:

LOCATION_FLAG;
DO_SOMETHING;
DO_SOMETHING;
DO_SOMETHING;
TEST FOR LOOP COMPLETION;//Jumps to LOCATION_FLAG if false

You can see in the latter case, the overhead of testing and jumping is only 1 instruction per 3. In the first it's 1 instruction per 1; so it happens a lot more often.