gcc 的 ffast-math 实际上是做什么的? [英] What does gcc's ffast-math actually do?

问题描述

我了解 gcc 的 --ffast-math 标志可以大大提高浮点运算的速度，并且超出了 IEEE 标准，但我似乎无法找到有关真正发生的事情的信息在.任何人都可以解释一些细节，也许可以给出一个清晰的例子，说明如果标志打开或关闭，某些事情会发生什么变化?

I understand gcc's --ffast-math flag can greatly increase speed for float ops, and goes outside of IEEE standards, but I can't seem to find information on what is really happening when it's on. Can anyone please explain some of the details and maybe give a clear example of how something would change if the flag was on or off?

我确实尝试过挖掘 S.O.对于类似的问题，但找不到任何解释 ffast-math 工作原理的内容.

I did try digging through S.O. for similar questions but couldn't find anything explaining the workings of ffast-math.

推荐答案

正如您提到的，它允许进行不严格遵守 IEEE 合规性的优化.

As you mentioned, it allows optimizations that do not preserve strict IEEE compliance.

一个例子是:

x = x*x*x*x*x*x*x*x;

到

x *= x;
x *= x;
x *= x;

因为浮点运算是不结合的，所以运算的排序和因式分解会因为四舍五入而影响结果.因此，这种优化不是在严格的 FP 行为下完成的.

Because floating-point arithmetic is not associative, the ordering and factoring of the operations will affect results due to round-off. Therefore, this optimization is not done under strict FP behavior.

我还没有真正检查过 GCC 是否真的进行了这种特定的优化.但想法是一样的.

I haven't actually checked to see if GCC actually does this particular optimization. But the idea is the same.

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