Re: [PATCH v2] target/i386: reimplement fpatan using floatx80 operations

[Joseph Myers]

On Tue, 23 Jun 2020, Paolo Bonzini wrote:

> On 23/06/20 02:01, Joseph Myers wrote:
> > The x87 fpatan emulation is currently based around conversion to
> > double.  This is inherently unsuitable for a good emulation of any
> > floatx80 operation.  Reimplement using the soft-float operations, as
> > for other such instructions.
> > 
> > Signed-off-by: Joseph Myers <joseph@codesourcery.com>
> 
> Queued, thanks.
> 
> Just one question: do recent processors still use the same CORDIC
> approximations as the 8087, and if so would it be better or simpler to
> do that instead of using a good implementation such as this one?

I don't know what approximations the processors use, but they're 
definitely different for at least some instructions between Intel and AMD 
processors (as shown by glibc test ulps baselines created on one processor 
sometimes needing increasing to work on other processors; avoiding test 
problems means the emulation needs to be at least as accurate as 
hardware).  (Whereas the AVX-512 approximation instructions have reference 
implementations for their exact semantics.)

-- 
Joseph S. Myers
joseph@codesourcery.com