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From: alan@dmsmelb.mel.dms.CSIRO.AU (Alan Miller)
Newsgroups: comp.lang.fortran
Subject: Re: why does optimization take so long ?
Summary: Edinburgh f77 v. Sun f77
Message-ID: <1991May22.073127.4540@monu0.cc.monash.edu.au>
Date: 22 May 91 07:31:27 GMT
References: <1236@nikhefh.nikhef.nl>
Sender: news@monu0.cc.monash.edu.au (0000-news(0000))
Organization: CSIRO, Division of Mathematics & Statistics
Lines: 33

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.
: 
: Geert Jan van Oldenborgh
: gj@hep.physik.uni-muenchen.de


Geert sent me the piece of code, which was in the form of a Fortran function
without a main program.   I compiled it with the Sun f77 compiler (v 1.3.1)
and with version 2.6.3 of the Edinburgh f77 compiler, with and without the
optimization option -O.   The times taken were as follows:

   Compiler + options    Time
   f77 -c               16.1u + 1.8s
   f77 -O -c            Gave up after 37 mins. of CPU time !
   epcf77 -c             5.2u + 0.6s
   epcf77 -O -c        156.0u + 3.1s

The Edinburgh compiler certainly struggled with the 485-line function, while
the Sun compiler looks as if it was firmly stuck.

Alan Miller