关于loop unwinding

今天看到一个有趣的东西，叫loop unwinding，不理解，查了下，发现答案如下。loop unwinding是为了减少循环语句的判断次数来达到提升速度的效果。这样的方法并不是通过修改算法达到程序加速的效果，很巧妙的方法，但又取巧的成分。

    Loop unwinding, also known as loop unrolling, is a loop transformation technique that attempts to optimize a program's execution speed at the expense of its binary size (space-time tradeoff). The transformation can be undertaken manually by the programmer or by an optimizing compiler.

    The goal of loop unwinding is to increase a program's speed by reducing (or eliminating) instructions that control the loop, such as pointer arithmetic and "end of loop" tests on each iteration;reducing branch penalties; as well as "hiding latencies, in particular, the delay in reading data from memory".

    Loops can be re-written instead as a repeated sequence of similar independent statements eliminating this overhead.

    gcc 优化指令：-funroll-loops

 

A simple manual example in C Language

（表格复制有问题，就不给了）

from:http://en.wikipedia.org/wiki/Loop_unwinding

转载于:https://my.oschina.net/u/782560/blog/305753