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
