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   6 <TITLE>The SUIF Version 1 Library - Call-By-Ref Parameters</TITLE>
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  17 <H2><A NAME="SEC90" HREF="suif1_toc.html#TOC90">Call-By-Reference Parameters</A></H2>
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  19 <A NAME="IDX649"></A>
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  23 Procedure parameters in Fortran are passed by reference.  SUIF uses
  24 call-by-value parameters and implements call-by-reference parameters by
  25 passing pointers to the actual arguments.  When the formal parameters
  26 are used, the pointers must be dereferenced.  However, one of the
  27 primary advantages of compiling Fortran programs is that there are no
  28 pointers in the source code.  If not for the call-by-reference parameter
  29 pointers, most compiler passes that only deal with Fortran can be
  30 simplified by not having to deal with pointers.  Thus the SUIF Fortran
  31 form automatically converts the code to make it appear that the
  32 parameters are passed by reference.  The parameters are changed to
  33 call-by-reference types instead of pointers and the pointer dereferences
  34 are temporarily removed.
  35
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  37 <P>
  38 <A NAME="IDX650"></A>
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  40 The <CODE>make_ref_params</CODE> function is used to convert call-by-reference
  41 parameters in a procedure to the Fortran form.  It must be called before
  42 converting the procedure body to expression trees.  This is done
  43 automatically by the <CODE>read_proc</CODE> method for a <CODE>proc_sym</CODE> if
  44 the <CODE>use_fortran_form</CODE> flag is set.  Before writing a procedure to
  45 an output file, the call-by-reference parameters must be converted back
  46 to pointers using the <CODE>undo_ref_params</CODE> function.  Since it is
  47 illegal to write out a procedure in the Fortran form, the
  48 <CODE>write_proc</CODE> method for the <CODE>proc_sym</CODE> always calls
  49 <CODE>undo_ref_params</CODE>; you do not need to call it directly.  Both
  50 <CODE>make_ref_params</CODE> and <CODE>undo_ref_params</CODE> are defined in the
  51 file <TT>`callbyref.cc'</TT>.
  52
  53 </P>
  54 <P>
  55 The Fortran front-end must identify call-by-reference formal parameters
  56 by putting <CODE>call_by_ref</CODE> annotations (see section <A HREF="suif1_76.html#SEC76">Call-By-Reference Annotations</A>)
  57 on the pointer types used by those parameters.  Before actually changing
  58 the type of a call-by-reference parameter and removing its dereferences,
  59 <CODE>make_ref_params</CODE> checks that the pointer parameter is neither
  60 addressed nor assigned within the procedure.  If this check fails, the
  61 pointer is not a valid call-by-reference parameter, so
  62 <CODE>make_ref_params</CODE> prints a warning message and does not convert
  63 that parameter.  Otherwise, it goes ahead and changes the type of the
  64 parameter from a pointer to a call-by-reference type (see section <A HREF="suif1_67.html#SEC67">Modifier Types</A>).  Any dereferences of that pointer within the procedure are
  65 removed, and other references to the pointer are changed to <CODE>ldc</CODE>
  66 (load constant) instructions that take the address of the parameter.
  67 The end result of all this is that it appears that the parameters are
  68 passed by reference.  Note that the call sites are not changed; the
  69 actual arguments for call-by-reference parameters are still passed as
  70 pointers.
  71
  72 </P>
  73 <P>
  74 The <CODE>undo_ref_params</CODE> function does as its name suggests and undoes
  75 the transformations applied by <CODE>make_ref_params</CODE>.  The only
  76 potential complication is that while in the Fortran form the user may
  77 have used the call-by-reference parameters in ways that cannot be
  78 expressed outside of the Fortran form.  Specifically, the
  79 call-by-reference parameters cannot be used as index variables of
  80 <CODE>for</CODE> loops (see section <A HREF="suif1_17.html#SEC17">For Nodes</A>), and they cannot be used as bounds
  81 in array types (see section <A HREF="suif1_63.html#SEC63">Array Types</A>).  Both of those uses require direct
  82 references to variable symbols, and there is no place to insert the
  83 pointer dereferences required outside of Fortran form.
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