Fix for bz#13658 - using fsincos on x86-64

[Andreas Jaeger]

On 02/23/2012 10:29 PM, Joseph S. Myers wrote:
> On Sun, 12 Feb 2012, Carlos O'Donell wrote:
>
>> On Thu, Feb 9, 2012 at 4:49 PM, Andreas Jaeger<aj@suse.com>  wrote:
>>> We should not use the inaccurate fsincos instruction on x86-64 (see bugzilla
>>> and the discussion on the gcc mailing list). Let's just remove the double
>>> version and use the IBM libm sincos implementation.
>>>
>>> Unfortunately the same fix does not work for the long double version - since
>>> there's no sincos implementation for ldbl-96 yet ;-(. Jakub, you wrote the
>>> ldb-128 versions - do you have any suggestions?
>
> What's actually missing is k_sinl.c / k_cosl.c.  Apart from the bits
> dealing with extracting integer values from the bits of a long double, you
> could probably just reuse the ldbl-128 versions (the numbers in the tables
> may have more decimal digits specified than needed, and approximations may
> be computed with more terms than needed, but they should still work once
> you fix up the integer bits).

Sorry, I won't have time in the near future to do this myself.

>>> 2012-02-09  Andreas Jaeger<aj@suse.de>
>>>
>>>         [BZ#13658]
>>>         * sysdeps/x86_64/fpu/s_sincos.S: Remove, let's not use the
>>>         inaccurate fsincos instruction.
>>
>> Is there any reason we don't do long-form argument reduction (as
>> normal sin and cos do) followed by fsincos?
>
> The long-form reduction actually produces a sum of two values rather than
> just a single reduced value, for greater accuracy (although not correctly
> rounded results).  I don't know how accurate fsincos is for reduced values
> (and if it agrees with fsin and fcos on those values, which was the issue
> in the gcc list discussion).
>
>> Does this effect i386 which also uses fsincos?
>
> I'd guess so - but that would also be a separate bug, which could be fixed
> separately as long as it's entered in Bugzilla.
>
> However, the patch should have a testcase added to the testsuite (that
> fails with a huge number of ULPs before the patch, passes with at most a
> few ULPs after it).  There don't appear to be any tests at all for large
> arguments to sin, cos, sincos or tan at present.  And if such a test then
> fails for i386, or for x86_64 long double, either it will be necessary to
> add entries with huge numbers of ULPs to libm-test-ulps files (at present
> 763 is the largest number in any file, and only i386 and x86_64 have any
> numbers 26 or more), or it will be necessary to fix more cases as part of
> the patch.

I can add a couple of testcases. How do we want to continue? Should we 
use the patch to fix the double case - and then later handle long 
double, or do you want everything fixed at the same time?

Andreas
-- 
  Andreas Jaeger aj@{suse.com,opensuse.org} Twitter/Identica: jaegerandi
   SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
    GF: Jeff Hawn,Jennifer Guild,Felix Imendörffer,HRB16746 (AG Nürnberg)
     GPG fingerprint = 93A3 365E CE47 B889 DF7F  FED1 389A 563C C272 A126