GCC's -ffast-math behavior

[xunxun]

于 2012/2/9 17:47, Andrew Haley 写道:
> On 02/09/2012 09:35 AM, xunxun wrote:
>> 于 2012/2/9 17:21, Andrew Haley 写道:
>>> On 02/09/2012 08:33 AM, xunxun wrote:
>>>>           When using -ffast-math, gcc don't generate the math function
>>>> symbol: U _exp
>>> No, it doesn't.  Instead gcc uses the F2XM1 instruction.  Why would
>>> you want to call a library when gcc has an instruction to do the
>>> job?
>>>
>> Because other math lib works faster than gcc itself (even with
>> fastmath), and I want to use fastmath to make other caculation faster, too.
> Hmm, I think that'll be difficult.  We tend to assume that when a
> processor has built-in instructions to do something, that's the
> fastest way to do it.  It's usually true, and I am wondering what
> tricks Intel uses.  Granted, the floating-point transcendental
> instructions aren't super-fast, and perhaps Intel doesn't optimize
> them any more.
>
> Andrew.
>
Thank you for the explanation.

I think I can separate all the math functions from other code, put the 
math functions within one lib, and don't use fastmath to build the lib. :)

-- 
Best Regards,
xunxun