Bug#572746: libm: sinf/cosf performance is awful on amd64

[Vincent Lefevre <vincent@vinc17.net>]

On 2010-03-21 16:14:49 +0100, Aurelien Jarno wrote:
> On Wed, Mar 17, 2010 at 11:29:00AM +0100, Vincent Lefevre wrote:
> > It may be optimized, but completely buggy. For instance, on 1e22,
> > sincos returns 0.46261304076460174617 for the sine instead of
> > -0.85220084976718879499 (correctly rounded value). Even the sign
> > is incorrect!
> 
> Where did you get this result? In my tests both the x87 FPU and the
> current glibc code give the following result when using double variables
> and the sincos() function:
> 
> sin (1e22) =  0.46261304076460174617
> cos (1e22) = -0.88656030506363692201

Actually the sincos() function uses the x87 FPU (fsincos instruction),
so that's not surprising that you get the same result.

Try with GCC >= 4.4 (so that constant expressions are evaluated
with MPFR):

xvii:~> cat tst.c
#include <stdio.h>
#include <math.h>
int main (void)
{
  printf ("sin (1e22) = %.17g\n", sin(1e22));
  printf ("cos (1e22) = %.17g\n", cos(1e22));
  return 0;
}
xvii:~> gcc tst.c -o tst
xvii:~> ./tst
sin (1e22) = -0.85220084976718879
cos (1e22) = 0.52321478539513899

And if you don't trust MPFR, you can still try with gp (pari).

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <http://www.vinc17.net/>
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