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From: quinlan@physics.utoronto.ca (Gerald Quinlan)
Newsgroups: comp.lang.fortran
Subject: Using cpp for macro substitution.
Message-ID: <1990Jul6.162153.17644@helios.physics.utoronto.ca>
Date: 6 Jul 90 20:21:54 GMT
Distribution: na
Organization: University of Toronto Physics/Astronomy/CITA
Lines: 64

I have a question about using the C preprocessor to perform macro
substitutions in a Fortran program.  The problem is the following.  I have
a short subroutine which adds two numbers (a,b) in extended precision,
returning their sum (sum) and the error made in the addition (error).  This
routine gets called about 8 times in a heavily-used do loop in my program,
so I thought I could speed things up by eliminating the subroutine call and
doing the add in-line.  I tried to get the preprocessor cpp to do the
in-line substitution automatically for me, as shown below in the sample
program adtest.F.

--------------------------- adtest.F ----------------------------------
#define CALL_ADD(a,b,sum,error)\
        dum=a+b \
        if(abs(a).gt.abs(b)) then \
          error=a-dum \
          error=error+b \
        else \
          error=b-dum \
          error=error+a \
        endif \
        sum=dum 

      program adtest
      double precision dum,err,resid,rn,sum
      integer n
      sum=0.d0
      resid=0.d0
      do 10 n=1,500000
        rn=sqrt(dble(n))
        CALL_ADD(sum,rn,sum,err)
        resid=resid+err
   10 continue
      sum=sum+resid
      write(6,*) 'addt:',sum,resid
      stop
      end


When I run this through cpp (on a Sun) to get adtest.f I get the result shown
below.

--------------------------- adtest.f ----------------------------------
      program adtest
      double precision dum,err,resid,rn,sum
      integer n
      sum=0.d0
      resid=0.d0
      do 10 n=1,500000
        rn=sqrt(dble(n))
        dum=sum+rn if(abs(sum).gt.abs(rn)) then err=sum-dum err=err+rn else err=rn-dum err=err+sum endif sum=dum
        resid=resid+err
   10 continue
      sum=sum+resid
      write(6,*) 'addt:',sum,resid
      stop
      end


Note how cpp has combined all the statements of the macro into one long line.
I suppose this isn't a problem in C, where the statements end with ";" and
more than one statement per line is allowed.  But in Fortran this doesn't
work; the compiler won't accept adtest.f.  Does anyone know how to get
around this?  I've tried various tricks to get the macro inserted properly,
but can't seem to find anything that works.