Re: weird optimization in sin+cos, x86 backend

[Richard Guenther <richard dot guenther at gmail dot com>]

On Thu, Feb 9, 2012 at 11:35 AM, Andrew Haley <aph@redhat.com> wrote:
> On 02/09/2012 10:20 AM, James Courtier-Dutton wrote:
>> From what I can see, on x86_64, the hardware fsin(x) is more accurate
>> than the hardware fsincos(x).
>> As you gradually increase the size of X from 0 to 10e22, fsincos(x)
>> diverges from the correct accurate value quicker than fsin(x) does.
>>
>> So, from this I would say that using fsincos instead of fsin is not a
>> good idea, at least on x86_64 platforms.
>
> That's true iff you're using the hardware builtins, which we're not on
> GNU/Linux unless you're using -ffast-math. ?If you're using
> -ffast-math, the fsincos optimization is appropriate anyway because
> you want fast. ?If you're not using -ffast-math it's still
> appropriate, because we're using an accurate libm.

The point of course is that glibc happily uses fsin/fsincos (which isn't even
fast compared to a decent implementation using SSE math).

Richard.

> Andrew.