From - Fri Sep 20 19:40:34 1996
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From: "Omar G. Stradella" <omar@boston.sgi.com>
Newsgroups: comp.sys.sgi.misc
Subject: Re: Performance differences between f90 and cc
Date: Wed, 18 Sep 1996 11:26:17 -0400
Organization: Silicon Graphics, Inc.
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Add -OPT:alias=restrict to the cc compile line. The problem is that
the C compiler has no way of telling that the memory regions pointed
to by x and y do not overlap. In Fortran, argument aliasing is
forbidden by the standard.

Omar.

Eberhard Pasch wrote:
> 
> We have an Indigo2 with a R8000 and Irix 6.2.
> 
> The FORTRAN compiler seems to be much better than the
> C compiler. Please take a look at the following example.
> The code out f90 is FOUR times faster than the code
> produced by the cc Compiler.
> I've tried many of the cc optimization options without
> success.
> 
> FORTRAN source
> 
>       subroutine daxpy(n,da,dx,incx,dy,incy)
> 
>       double precision dx(*),dy(*),da
>       integer i,incx,incy,n
> 
>       do 10 i = 1,n,1
>         dy(i) = dy(i) + da*dx(i)
>    10 continue
>       return
>       end
> 
> out of .s file: (f90 -S -c -O3)
>  #<swps>
>  #<swps> Pipelined loop line 6 steady state
>  #<swps>
>  #<swps>     4 unrollings before pipelining
>  #<swps>     6 cycles per 4 iterations
>  #<swps>     8 flops        ( 33% of peak) (madds count as 2)
>  #<swps>     4 flops        ( 33% of peak) (madds count as 1)
>  #<swps>     4 madds        ( 33% of peak)
>  #<swps>    12 mem refs     (100% of peak)
>  #<swps>     2 integer ops  ( 16% of peak)
>  #<swps>    18 instructions ( 75% of peak)
>  #<swps>     1 short trip threshold
>  #<swps>     7 ireg registers used.
>  #<swps>    11 fgr registers used.
>  #<swps>
> 
> This version uses 100% of the processor performance.
> 
> C source
> 
> void daxpyc(int n, double a, double *x, double *y)
> {
>   int i;
>   for (i=0; i<n; i++)
>     y[i]=y[i]+a*x[i];
> }
> 
> out of .s file: (cc -S -c -O3)
>  #<swps>
>  #<swps> Pipelined loop line 4 steady state
>  #<swps>
>  #<swps>     4 unrollings before pipelining
>  #<swps>    24 cycles per 4 iterations
>  #<swps>     8 flops        (  8% of peak) (madds count as 2)
>  #<swps>     4 flops        (  8% of peak) (madds count as 1)
>  #<swps>     4 madds        (  8% of peak)
>  #<swps>    12 mem refs     ( 25% of peak)
>  #<swps>     2 integer ops  (  4% of peak)
>  #<swps>    18 instructions ( 18% of peak)
>  #<swps>     1 short trip threshold
>  #<swps>     7 ireg registers used.
>  #<swps>     4 fgr registers used.
>  #<swps>
>  #<swps>     6 min cycles required for resources
>  #<swps>    24 cycles required for recurrence(s)
>  #<swps>    12 operations in largest recurrence
>  #<swps>
> 
> This version uses only 25% of the processor.
> 
> Has anyone else made this observation?
> What are the correct options for the cc Compiler?
> 
> Any help is appreciated.
> 
> Eberhard Pasch
> --
> ************************************
> * Eberhard Pasch                   *
> * EMail: pasch@na.uni-tuebingen.de *
> ************************************

-- 
+-----------------------------------------------------------------------+
 Omar G. Stradella, Ph.D.                    Supercomputing Applications 
 Silicon Graphics, Inc.                          Computational Chemistry
 One Cabot Road, Hudson, MA 01749, USA           N 42 22'41" W 71 33'45"
 E-mail: omar@boston.sgi.com Phone: +1-508-567-2258 FAX: +1-508-562-4755 
 http://www.sgi.com/ChemBio                  http://reality.sgi.com/omar 
+--------  Ph-nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn 
---------+