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.
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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