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------------------------------------------------------------------------------

--                                                                          --

--                         GNAT COMPILER COMPONENTS                         --

--                                                                          --

--                             S E M _ M E C H                              --

--                                                                          --

--                                 S p e c                                  --

--                                                                          --

--          Copyright (C) 1996-2008, Free Software Foundation, Inc.         --

--                                                                          --

-- GNAT is free software;  you can  redistribute it  and/or modify it under --

-- terms of the  GNU General Public License as published  by the Free Soft- --

-- ware  Foundation;  either version 3,  or (at your option) any later ver- --

-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --

-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --

-- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License --

-- for  more details.  You should have  received  a copy of the GNU General --

-- Public License  distributed with GNAT; see file COPYING3.  If not, go to --

-- http://www.gnu.org/licenses for a complete copy of the license.          --

--                                                                          --

-- GNAT was originally developed  by the GNAT team at  New York University. --

-- Extensive contributions were provided by Ada Core Technologies Inc.      --

--                                                                          --

------------------------------------------------------------------------------

--  This package contains the routine used to establish calling mechanisms

--  The reason we separate this off into its own package is that it is

--  entirely possible that it may need some target specific specialization.

with Types; use Types;

package Sem_Mech is

   -------------------------------------------------

   -- Definitions for Parameter Mechanism Control --

   -------------------------------------------------

   --  For parameters passed to subprograms, and for function return values,

   --  as passing mechanism is defined. The entity attribute Mechanism returns

   --  an indication of the mechanism, and Set_Mechanism can be used to set

   --  the mechanism. At the program level, there are three ways to explicitly

   --  set the mechanism:

   --    An Import_xxx or Export_xxx pragma (where xxx is Function, Procedure,

   --    or Valued_Procedure) can explicitly set the mechanism for either a

   --    parameter or a function return value. A mechanism explicitly set by

   --    such a pragma overrides the effect of C_Pass_By_Copy described below.

   --    If convention C_Pass_By_Copy is set for a record, and the record type

   --    is used as the formal type of a subprogram with a foreign convention,

   --    then the mechanism is set to By_Copy.

   --    If a pragma C_Pass_By_Copy applies, and a record type has Convention

   --    C, and the record type is used as the formal type of a subprogram

   --    with a foreign convention, then the mechanism is set to use By_Copy

   --    if the size of the record is sufficiently small (as determined by

   --    the value of the parameter to pragma C_Pass_By_Copy).

   --  The subtype Mechanism_Type (declared in Types) is used to describe

   --  the mechanism to be used. The following special values of this type

   --  specify the mechanism, as follows.

   Default_Mechanism : constant Mechanism_Type := 0;

   --  The default setting indicates that the backend will choose the proper

   --  default mechanism. This depends on the convention of the subprogram

   --  involved, and is generally target dependent. In the compiler, the

   --  backend chooses the mechanism in this case in accordance with any

   --  requirements imposed by the ABI. Note that Default is never used for

   --  record types on foreign convention subprograms, since By_Reference

   --  is forced for such types unless one of the above described approaches

   --  is used to explicitly force By_Copy.

   By_Copy : constant Mechanism_Type := -1;

   --  Passing by copy is forced. The exact meaning of By_Copy (e.g. whether

   --  at a low level the value is passed in registers, or the value is copied

   --  and a pointer is passed), is determined by the backend in accordance

   --  with requirements imposed by the ABI. Note that in the extended import

   --  and export pragma mechanisms, this is called Value, rather than Copy.

   By_Reference : constant Mechanism_Type := -2;

   --  Passing by reference is forced. This is always equivalent to passing

   --  a simple pointer in the case of subprograms with a foreign convention.

   --  For unconstrained arrays passed to foreign convention subprograms, the

   --  address of the first element of the array is passed. For convention

   --  Ada, the result is logically to pass a reference, but the precise

   --  mechanism (e.g. to pass bounds of unconstrained types and other needed

   --  special information) is determined by the backend in accordance with

   --  requirements imposed by the ABI as interpreted for Ada.

   By_Descriptor      : constant Mechanism_Type := -3;

   By_Descriptor_UBS  : constant Mechanism_Type := -4;

   By_Descriptor_UBSB : constant Mechanism_Type := -5;

   By_Descriptor_UBA  : constant Mechanism_Type := -6;

   By_Descriptor_S    : constant Mechanism_Type := -7;

   By_Descriptor_SB   : constant Mechanism_Type := -8;

   By_Descriptor_A    : constant Mechanism_Type := -9;

   By_Descriptor_NCA  : constant Mechanism_Type := -10;

   By_Short_Descriptor      : constant Mechanism_Type := -11;

   By_Short_Descriptor_UBS  : constant Mechanism_Type := -12;

   By_Short_Descriptor_UBSB : constant Mechanism_Type := -13;

   By_Short_Descriptor_UBA  : constant Mechanism_Type := -14;

   By_Short_Descriptor_S    : constant Mechanism_Type := -15;

   By_Short_Descriptor_SB   : constant Mechanism_Type := -16;

   By_Short_Descriptor_A    : constant Mechanism_Type := -17;

   By_Short_Descriptor_NCA  : constant Mechanism_Type := -18;

   --  These values are used only in OpenVMS ports of GNAT. Pass by descriptor

   --  is forced, as described in the OpenVMS ABI. The suffix indicates the

   --  descriptor type:

   --

   --     UBS    unaligned bit string

   --     UBSB   aligned bit string with arbitrary bounds

   --     UBA    unaligned bit array

   --     S      string, also a scalar or access type parameter

   --     SB     string with arbitrary bounds

   --     A      contiguous array

   --     NCA    non-contiguous array

   --

   --  Note: the form with no suffix is used if the Import/Export pragma

   --  uses the simple form of the mechanism name where no descriptor

   --  type is supplied. In this case the back end assigns a descriptor

   --  type based on the Ada type in accordance with the OpenVMS ABI.

   subtype Descriptor_Codes is Mechanism_Type

     range By_Short_Descriptor_NCA .. By_Descriptor;

   --  Subtype including all descriptor mechanisms

   --  All the above special values are non-positive. Positive values for

   --  Mechanism_Type values have a special meaning. They are used only in

   --  the case of records, as a result of the use of the C_Pass_By_Copy

   --  pragma, and the meaning is that if the size of the record is known

   --  at compile time and does not exceed the mechanism type value, then

   --  By_Copy passing is forced, otherwise By_Reference is forced.

   ----------------------

   -- Global Variables --

   ----------------------

   Default_C_Record_Mechanism : Mechanism_Type := By_Reference;

   --  This value is the default mechanism used for C convention records

   --  in foreign-convention subprograms if no mechanism is otherwise

   --  specified. This value is modified appropriately by the occurrence

   --  of a C_Pass_By_Copy configuration pragma.

   -----------------

   -- Subprograms --

   -----------------

   procedure Set_Mechanisms (E : Entity_Id);

   --  E is a subprogram or subprogram type that has been frozen, so the

   --  convention of the subprogram and all its formal types and result

   --  type in the case of a function are established. The function of

   --  this call is to set mechanism values for formals and for the

   --  function return if they have not already been explicitly set by

   --  a use of an extended Import or Export pragma. The idea is to set

   --  mechanism values wherever the semantics is dictated by either

   --  requirements or implementation advice in the RM, and to leave

   --  the mechanism set to Default if there is no requirement, so that

   --  the back-end is free to choose the most efficient method.

   procedure Set_Mechanism_Value (Ent : Entity_Id; Mech_Name : Node_Id);

   --  Mech is a parameter passing mechanism (see Import_Function syntax

   --  for MECHANISM_NAME). This routine checks that the mechanism argument

   --  has the right form, and if not issues an error message. If the

   --  argument has the right form then the Mechanism field of Ent is

   --  set appropriately. It also performs some error checks. Note that

   --  the mechanism name has not been analyzed (and cannot indeed be

   --  analyzed, since it is semantic nonsense), so we get it in the

   --  exact form created by the parser.

   procedure Set_Mechanism_With_Checks

     (Ent  : Entity_Id;

      Mech : Mechanism_Type;

      Enod : Node_Id);

   --  Sets the mechanism of Ent to the given Mech value, after first checking

   --  that the request makes sense. If it does not make sense, a warning is

   --  posted on node Enod, and the Mechanism of Ent is unchanged.

end Sem_Mech;