GNU Info

(g77-295.info)Challenges Posed

---

Next: Transforming Statements Prev: Two-pass Design Up: Front End

---

Enter node , (file) or (file)node

Challenges Posed
================

   Consider the following Fortran code, which uses various extensions
(including some to Fortran 90):

     SUBROUTINE X(A)
     CHARACTER*(*) A
     COMPLEX CFUNC
     INTEGER*2 CLOCKS(200)
     INTEGER IFUNC
     
     CALL SYSTEM_CLOCK (CLOCKS (IFUNC (CFUNC ('('//A//')'))))

   The above poses the following challenges to any Fortran compiler
that uses run-time interfaces, and a run-time library, roughly similar
to those used by `g77':

   * Assuming the library routine that supports `SYSTEM_CLOCK' expects
     to set an `INTEGER*4' variable via its `COUNT' argument, the
     compiler must make available to it a temporary variable of that
     type.

   * Further, after the `SYSTEM_CLOCK' library routine returns, the
     compiler must ensure that the temporary variable it wrote is
     copied into the appropriate element of the `CLOCKS' array.  (This
     assumes the compiler doesn't just reject the code, which it should
     if it is compiling under some kind of a "strict" option.)

   * To determine the correct index into the `CLOCKS' array, (putting
     aside the fact that the index, in this particular case, need not
     be computed until after the `SYSTEM_CLOCK' library routine
     returns), the compiler must ensure that the `IFUNC' function is
     called.

     That requires evaluating its argument, which requires, for `g77'
     (assuming `-ff2c' is in force), reserving a temporary variable of
     type `COMPLEX' for use as a repository for the return value being
     computed by `CFUNC'.

   * Before invoking `CFUNC', is argument must be evaluated, which
     requires allocating, at run time, a temporary large enough to hold
     the result of the concatenation, as well as actually performing
     the concatenation.

   * The large temporary needed during invocation of `CFUNC' should,
     ideally, be deallocated (or, at least, left to the GBE to dispose
     of, as it sees fit) as soon as `CFUNC' returns, which means before
     `IFUNC' is called (as it might need a lot of dynamically allocated
     memory).

   `g77' currently doesn't support all of the above, but, so that it
might someday, it has evolved to handle at least some of the above
requirements.

   Meeting the above requirements is made more challenging by
conforming to the requirements of the GBEL/GBE combination.