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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;