gcc的快速数学实际上做了什么？ [英] What does gcc's ffast-math actually do?

问题描述

我知道gcc的 - ffast-math 标志可以大大提高浮点操作的速度，并超出IEEE标准，但我似乎无法找到关于当它开始时真的发生了什么。任何人都可以请解释一些细节，也许给一个明确的例子，如果国旗是打开还是关闭，会发生什么样的变化？

我曾尝试通过S.O.对于类似的问题，但找不到解释ffast数学运算的任何东西。

例如：

pre $ code > x = x * x * x * x * x * x * x * x;

至

  x * = x; 
 x * = x; 
 x * = x;由于浮点运算不是关联的，所以操作的排序和因式分解会影响结果，这是由于以下原因造成的：
  

四舍五入。因此，这种优化不是在严格的FP行为下完成的。

编辑：我没有真正检查GCC是否确实做了这个特定的优化。但是这个想法是一样的。

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?

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

解决方案

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

An example is this:

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

to

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

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.

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

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