Path: utzoo!dptcdc!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!rice!sun-spots-request
From: self@bayes.arc.nasa.gov (Matthew Self)
Newsgroups: comp.sys.sun
Subject: Re: libm for 68881 and Sun fpa is incredibly slow
Keywords: Software
Message-ID: <8903222320.AA01458@bayes.arc.nasa.gov>
Date: 18 Apr 89 20:19:04 GMT
Sender: usenet@rice.edu
Organization: Sun-Spots
Lines: 57
Approved: Sun-Spots@rice.edu
Original-Date: Wed, 22 Mar 89 15:20:34 PST
X-Sun-Spots-Digest: Volume 7, Issue 230, message 7 of 16

John Schultz compiled the following timings for Sun's math libraries using
GCC and CC with various options:

> My results, running on Sun 3/60, Sun OS 3.5, GNU CC 1.32 built using
> default switches were
> 
> gcc -lm                  ===   4.6 real         4.2 user         0.0 sys
> gcc -m68881  -lm         ===   4.4 real         4.2 user         0.0 sys
> gcc -O -m68881 -lm       ===   4.4 real         4.1 user         0.0 sys  
> gcc -O -g -m68881 -lm    ===   5.4 real         4.2 user         0.1 sys
> cc -lm                   === 159.4 real       146.7 user         0.6 sys
> cc -O -lm                === 155.4 real       146.2 user         0.4 sys
> cc -f68881 -lm           ===   6.6 real         4.6 user         0.1 sys 
> cc  /usr/lib/f68881.il   ===   9.9 real         6.7 user         0.1 sys
> cc -O /usr/lib/f68881.il ===   6.5 real         6.4 user         0.0 sys 
> **********************************************************************/
> #include <math.h>
> 
> main()
> {
>   register int i;
>   register double x, y;
>   for(i = 0, x = 0; i < 100000; i++, x += 2*M_PI/100000.0)
>     y = cos(x);
> }

I have written an inline math library for GCC which is more than twice as
fast as any of these options for this test program.  In fact, it permits
GCC to determine that the program does nothing at all, so it optimizes it
away entirely!  I modified the test program slightly to make the return
value depend on the computations in the loop so this won't happen.  Even
with the extra addition I introduced, the program now executes in only
2.5s, more than twice as fast as before.  Here is the new test program:

#include <math.h>  /* my inline ANSI math library */

#define M_PI 3.1415792  /* this isn't defined in ANSI C's math.h */

main()
{
  int i;		/* GCC doesn't need register declarations */
  double x, y = 0;
  for(i = 0, x = 0; i < 100000; i++, x += 2*M_PI/100000.0)
    y += cos(x);
  if (y == 0)
    return 0;
  else
    return 1;
}

Availability of this inline math library will be announced soon on the
info-gcc mailing list.  Mail to info-gcc-request@prep.ai.mit.edu to
subscribe.

			Matthew Self
		  NASA Ames Research Center
		   self@bayes.arc.nasa.gov