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-------------------------------------------------------------------------------
--- --
--- GNAT COMPILER COMPONENTS --
--- --
--- S E M _ M E C H --
--- --
--- S p e c --
--- --
--- $Revision: 1.1.16.2 $
--- --
--- Copyright (C) 1996-1997 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 2, 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 COPYING. If not, write --
--- to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, --
--- MA 02111-1307, USA. --
--- --
--- 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;
- -- 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_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 whereever 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;
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