RE: Sine and Cosine Accuracy

["Menezes, Evandro" <evandro.menezes@amd.com>]

Scott, 

> > This is not true.  Compare results on an x86 systems with 
> those on an
> > x86_64 or ppc.  As I said before, shortcuts were taken in x87 that 
> > sacrificed accuracy for the sake of speed initially and later of 
> > compatibility.
> 
> It *is* true for the case where the argument is in the range 
> [0, 2*PI), at least according to the tests I published 
> earlier in this thread. If you think there is something 
> erroneous in the test code, I sincerely would like to know.

Your code just tests every 3.6°, perhaps you won't trip at the problems...  

As I said, x87 can be off by hundreds of ulps, whereas the routines for x86_64 which ships with SUSE are accurate to less than 1ulp over their entire domain.

Besides, you're also comparing 80-bit calculations with 64-bit calculations, not only the accuracy of sin and cos.  Try using -ffloat-store along with -mfpmath=387 and see yet another set of results.  At the end of the day, which one do you trust?  I wouldn't trust my check balance to x87 microcode... ;-)

HTH


_______________________________________________________
Evandro Menezes            Software Strategy & Alliance
512-602-9940                                        AMD
evandro.menezes@amd.com                      Austin, TX