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.
