GCC's -ffast-math behavior

[Andrew Haley]

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.