Porting codes using mppl
			 Lynda Lo Destro, LLNL

Mppl has several features that permit the robust and easy porting of old
codes, particularly those developed on Cray computers and that have been
little changed since, to a wide variety of platforms often with no changes
at all (exceptions noted below) to the old Fortran source.

The first of these is the mppl execute line option -rN, where N=4, 8, or 16.
On machines with word-size 32, mppl translates "real" as
   "real", "doubleprecision", or "doubleprecision," respectively,
and on machines with word-size 64, as
   "real", "real", or "doubleprecision."
Similarly with complex and implicit real(...).
Real constants such as 1. or 1.e0 are converted to 1.d0 in the appropriate
circumstances. (Constructs such as real*8 or 1.d0 are left as they are.)
In addition, Mppl has a special macro named Prolog, which is invoked near
the top of every function and subroutine, and which can be defined, for
example, as
       implicit none
or
       implicit real(a-h)
       implicit real(o-z)

A second important feature for precision control is mppl's macro facility.
This can be used to select single- vs. double-precision versions of
library routines. See tst/input8 for an example showing how to do this
in a fairly general way.

Thus fortran codes written with real (as opposed to real*8) declarations
can be reliably ported to 32-bit machines with 8-byte arithmetic without
relying on the not-always-available -r8 or similar compiler option.
Furthermore, such codes retain the capability, selectable at compile time,
of running with 16-byte precision on 64-bit machines.

Qualifications:
   -- integers are never converted to reals by mppl.
   -- there is no provision for limiting the source-code line-length to
      n columns, so the reserved use of columns 73:80 for comments employed
      by some codes will fail. This option may be added in the future.
      At present, one can apply a simple unix script to such codes to
      insert a # or ! (comment characters recognized by mppl) in column 73
      prior to running mppl.

Final note: these remarks have been addressed to the porting of old
codes in particular. Mppl, however, also provides most Fortran 90 syntax
for real and integer declarations, literal constants, and intrinsic
functions---the more modern approach to portability.  In circumstances
where you need to use library functions with one precision in a code at
a higher or lower precision, Mppl has the tools that allow this to be
expressed both precisely and portably:

      real(Size8) function foobar
      external libfunc
      real(Size4) libfunc
      real(Size4) a
      a = 1.0_Size4
      foobar = real(libfunc(a), Size8)
      return
      end