21.5 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.