How to add intrinsic functions to gfortran?

[Steve Kargl <sgk at troutmask dot apl dot washington dot edu>]

Is there a document that describes how to add an
intrinsic function to gfortran?

I've implemented G77's irand(), srand(), and rand()
procedures.  srand() is an intrinsic subroutine and
as far as I can tell it is seen, works, and is used by
gfortran.  On the other hand, irand() and rand() are
not working and I suspect some name mangling is need.

I'll restrict the discussion to irand() where the Fortran
prototypes is

   integer function irand(integer flag)

In my C code, I have tried the following declarations

GFC_INTEGER_4 prefix(irand) (GFC_INTEGER_4 flag)
GFC_INTEGER_4      __irand  (GFC_INTEGER_4 flag)
GFC_INTEGER_4       _irand  (GFC_INTEGER_4 flag)
GFC_INTEGER_4        irand  (GFC_INTEGER_4 flag)

In all cases, gfortran compiles 

   program cc
   integer i
   i = irand(0)
   end program cc

to the following tree

troutmask:sgk[286] more cc.f90.t03.original
   MAIN__ ()
   {
     int4 i;

     i = 0;
   }

In intrinsic.c, I have 

  add_sym_1 ("irand", 0, 1, BT_INTEGER, di,
	     gfc_check_irand, gfc_simplify_irand, NULL,
	     i, BT_INTEGER, di, 0);

  make_generic ("irand", GFC_ISYM_IRAND);

where GFC_ISYM_IRAND is added to "enum gfc_generic_isym_id" in
gfortran.h.   gfc_simplify_irand is required or gfortran yields
an ICE.  It looks like

   gfc_expr *
   gfc_simplify_irand (gfc_expr * e)
   {
     gfc_expr *result;
   
     if (e->expr_type != EXPR_CONSTANT) return NULL;
   
     result = gfc_constant_result (BT_INTEGER, gfc_default_integer_kind(),
       &e->where);

     mpz_set (result->value.integer, e->value.integer);
   
     return range_check (result, "IRAND");
   } 

I've tried various version of a gfc_resolve_irand function to
set the function name to each of the above 4 prototype function
names without much luck.

Any hints would be appreciated.

-- 
Steve