多少次应循环中unwinded？ [英] How many times should a loop be unwinded?

问题描述

我学习循环展开，以避免因依赖摊位。我发现互联网和文学的例子很多，但我发现算法如何用于获得优化code ++工程（如果有一个这样的算法，当然）没有解释。特别是，我不知道如何来定多少次应循环中展开。能不能事先计算出来的？

I'm learning about loop unrolling to avoid stalls caused by dependencies. I found many examples on internet and in literature, but I found no explanation on how the algorithm used to obtain the optimized code works (in case there is one such algorithm, of course). In particular, I don't know how to determinate how many times should the loop be unrolled. Can it be calculated beforehand?

推荐答案

经验法则是你放松，因此：

The rule of thumb is that you unwind so that:

• 操作都在自然的边界值4,8,16,32 ..字节
完成

• 您不引入过多的寄存器pressure（即你不开始杂耍寄存器内存）


• 您不要开始一遍又一遍地重复着相同的指令序列

基本上你解开只要你可以把更多的资源来工作，你停止当你不再可以测量任何性能提升。

Basically you unroll as long as you can put more resources to work and you stop when you no longer can measure any performance gains.

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