[Bug libc/15424] modf benchtest gives bogus results

["neleai at seznam dot cz" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15424

--- Comment #2 from Ondrej Bilka <neleai at seznam dot cz> 2013-05-10 09:16:24 UTC ---
On Fri, May 10, 2013 at 08:19:48AM +0000, siddhesh at redhat dot com wrote:
> http://sourceware.org/bugzilla/show_bug.cgi?id=15424
> 
> Siddhesh Poyarekar <siddhesh at redhat dot com> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>              Status|NEW                         |WAITING
>                  CC|                            |siddhesh at redhat dot com
>          AssignedTo|unassigned at sourceware    |siddhesh at redhat dot com
>                    |dot org                     |
> 
> --- Comment #1 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2013-05-10 08:19:48 UTC ---
> Does this happen with latest master?  Changing from fixed iterations to fixed
> time showed such spurious results with bench-modf once (likely because it was
> not rebuilt), which went away when I did a bench-clean and rebuilt it.
>
Now it shows following results. According to them atan is ten times
faster than modf which is clearly wrong.

atan(): ITERS:4.48268e+09: TOTAL:7.79352s, MAX:166.672ns, MIN:0.638ns,
5.7518e+08 iter/s
atan(768bits): ITERS:4.8324e+08: TOTAL:9.83724s, MAX:38.805ns, MIN:17.654ns,
4.91235e+07 iter/s
modf(): ITERS:6.8975e+08: TOTAL:9.53964s, MAX:193.448ns, MIN:4.486ns,
7.23035e+07 iter/s

A following benchmark takes  8.9s which is order of magnitude more than
what benchtest tells.

extern double modf (double, double *);
int main(){ int i;
  double x=0;
  double ret=0;
  double f;
  for(i=0;i<1000000000;i++){
     ret+=modf(x,&f);
    x+=0.1;
  }
  return ret;
}

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