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From: ake@dayton.saic.com (Earle Ake)
Newsgroups: comp.lang.fortran
Subject: Re: why does optimization take so long ?
Message-ID: <1991May21.204444.1623@dayton.saic.com>
Date: 22 May 91 00:44:44 GMT
References: <1236@nikhefh.nikhef.nl>
Organization: Science Applications Intl. Corp., Dayton, Ohio
Lines: 45

In article <1236@nikhefh.nikhef.nl>, t19@nikhefh.nikhef.nl (Geert J v Oldenborgh) writes:
> I am curious whether any of you know why compiling certain routines with a -O 
> flag takes a semi-infinite amount of time on certain computers (Apollo DN1000, 
> Sun Fortran 1.3.1) (i.e. I killed compilation after a few hours CPU time for a 
> single routine).  The code in question is generated by a computer algebra 
> program, and looks like this
> 
>       qqgg= n1i*n2i*pbqi*ptqi * (  - 2*pnpe*pbpt**2*qk + 2*pnpe*
>      + qk*mb2*mt2 + 2*pnpb*pepb*pbpt*qk - 2*pnpb*pepb*qk*mt2 + 2*
>      + pnpb*pept*pbpt*qk + 4*pnpb*pept*pbpt**2 - pnpb*pept*qk*mb2 - 
>       ...400 similar lines...	
> 
> As a lot of the time is spent here I'd like to be able to optimize it.

	My guess is you are running out of physical memory during the
optimization and the thing is paging like crazy.  Why not try to optimize it
some yourself and give the compiler a break?  I see a few things to try.


      xxxx = pnpe*pbpt
      xxx0 = pnpb*pepb
      xxx1 = xxx0*pbpt
      xxx2 = xxx0*qk
      xxx3 = pnpb*pept
      xxx4 = xxx3*pbpt
      xxx5 = xxx3*qk

      qqgg= n1i*n2i*pbqi*ptqi * (  - 2*xxxx**2*qk + 2*pnpe*
     + qk*mb2*mt2 + 2*xxx1*qk - 2*xxx2*mt2 + 2*
     + xxx4*qk + 4*xxx4**2 - xxx5*mb2 - 

And so on.  Move those parts of the equation that are computed more than once
outside the LARGE equation.  What you give up is readibility unless you start
giving the intermediate variables names like pnpbpept = pnpb*pept or
pnpbpeptqk = pnpb*pept*qk.



Earle
_____________________________________________________________________________
             ____ ____    ___
Earle Ake   /___ /___/ / /     Science Applications International Corporation
           ____//   / / /__                 Dayton, Ohio
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