145 lines
8.0 KiB
Ada
145 lines
8.0 KiB
Ada
------------------------------------------------------------------------------
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-- --
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-- GNAT COMPILER COMPONENTS --
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-- --
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-- S E M _ M E C H --
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-- --
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-- S p e c --
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-- --
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-- Copyright (C) 1996-2020, Free Software Foundation, Inc. --
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-- --
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-- GNAT is free software; you can redistribute it and/or modify it under --
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-- terms of the GNU General Public License as published by the Free Soft- --
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-- ware Foundation; either version 3, or (at your option) any later ver- --
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-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
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-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
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-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
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-- for more details. You should have received a copy of the GNU General --
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-- Public License distributed with GNAT; see file COPYING3. If not, go to --
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-- http://www.gnu.org/licenses for a complete copy of the license. --
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-- --
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-- GNAT was originally developed by the GNAT team at New York University. --
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-- Extensive contributions were provided by Ada Core Technologies Inc. --
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-- --
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------------------------------------------------------------------------------
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-- This package contains the routine used to establish calling mechanisms
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-- The reason we separate this off into its own package is that it is
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-- entirely possible that it may need some target specific specialization.
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with Types; use Types;
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package Sem_Mech is
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-------------------------------------------------
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-- Definitions for Parameter Mechanism Control --
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-------------------------------------------------
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-- For parameters passed to subprograms, and for function return values,
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-- a passing mechanism is defined. The entity attribute Mechanism returns
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-- an indication of the mechanism, and Set_Mechanism can be used to set
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-- the mechanism. At the program level, there are three ways to explicitly
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-- set the mechanism:
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-- An Import_xxx or Export_xxx pragma (where xxx is Function, Procedure,
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-- or Valued_Procedure) can explicitly set the mechanism for either a
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-- parameter or a function return value. A mechanism explicitly set by
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-- such a pragma overrides the effect of C_Pass_By_Copy described below.
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-- If convention C_Pass_By_Copy is set for a record, and the record type
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-- is used as the formal type of a subprogram with a foreign convention,
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-- then the mechanism is set to By_Copy.
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-- If a pragma C_Pass_By_Copy applies, and a record type has Convention
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-- C, and the record type is used as the formal type of a subprogram
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-- with a foreign convention, then the mechanism is set to use By_Copy
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-- if the size of the record is sufficiently small (as determined by
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-- the value of the parameter to pragma C_Pass_By_Copy).
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-- The subtype Mechanism_Type (declared in Types) is used to describe
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-- the mechanism to be used. The following special values of this type
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-- specify the mechanism, as follows.
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Default_Mechanism : constant Mechanism_Type := 0;
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-- The default setting indicates that the backend will choose the proper
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-- default mechanism. This depends on the convention of the subprogram
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-- involved, and is generally target dependent. In the compiler, the
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-- backend chooses the mechanism in this case in accordance with any
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-- requirements imposed by the ABI. Note that Default is never used for
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-- record types on foreign convention subprograms, since By_Reference
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-- is forced for such types unless one of the above described approaches
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-- is used to explicitly force By_Copy.
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By_Copy : constant Mechanism_Type := -1;
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-- Passing by copy is forced. The exact meaning of By_Copy (e.g. whether
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-- at a low level the value is passed in registers, or the value is copied
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-- and a pointer is passed), is determined by the backend in accordance
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-- with requirements imposed by the ABI. Note that in the extended import
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-- and export pragma mechanisms, this is called Value, rather than Copy.
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By_Reference : constant Mechanism_Type := -2;
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-- Passing by reference is forced. This is always equivalent to passing
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-- a simple pointer in the case of subprograms with a foreign convention.
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-- For unconstrained arrays passed to foreign convention subprograms, the
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-- address of the first element of the array is passed. For convention
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-- Ada, the result is logically to pass a reference, but the precise
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-- mechanism (e.g. to pass bounds of unconstrained types and other needed
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-- special information) is determined by the backend in accordance with
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-- requirements imposed by the ABI as interpreted for Ada.
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pragma Assert (Mechanism_Type'First = -2);
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-- Check definition in types is right!
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-- All the above special values are non-positive. Positive values for
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-- Mechanism_Type values have a special meaning. They are used only in
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-- the case of records, as a result of the use of the C_Pass_By_Copy
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-- pragma, and the meaning is that if the size of the record is known
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-- at compile time and does not exceed the mechanism type value, then
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-- By_Copy passing is forced, otherwise By_Reference is forced.
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----------------------
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-- Global Variables --
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----------------------
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Default_C_Record_Mechanism : Mechanism_Type := By_Reference;
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-- This value is the default mechanism used for C convention records
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-- in foreign-convention subprograms if no mechanism is otherwise
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-- specified. This value is modified appropriately by the occurrence
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-- of a C_Pass_By_Copy configuration pragma.
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-----------------
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-- Subprograms --
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-----------------
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procedure Set_Mechanisms (E : Entity_Id);
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-- E is a subprogram or subprogram type that has been frozen, so the
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-- convention of the subprogram and all its formal types and result
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-- type in the case of a function are established. The function of
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-- this call is to set mechanism values for formals and for the
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-- function return if they have not already been explicitly set by
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-- a use of an extended Import or Export pragma. The idea is to set
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-- mechanism values wherever the semantics is dictated by either
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-- requirements or implementation advice in the RM, and to leave
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-- the mechanism set to Default if there is no requirement, so that
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-- the back-end is free to choose the most efficient method.
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procedure Set_Mechanism_Value (Ent : Entity_Id; Mech_Name : Node_Id);
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-- Mech is a parameter passing mechanism (see Import_Function syntax
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-- for MECHANISM_NAME). This routine checks that the mechanism argument
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-- has the right form, and if not issues an error message. If the
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-- argument has the right form then the Mechanism field of Ent is
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-- set appropriately. It also performs some error checks. Note that
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-- the mechanism name has not been analyzed (and cannot indeed be
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-- analyzed, since it is semantic nonsense), so we get it in the
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-- exact form created by the parser.
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procedure Set_Mechanism_With_Checks
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(Ent : Entity_Id;
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Mech : Mechanism_Type;
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Enod : Node_Id);
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-- Sets the mechanism of Ent to the given Mech value, after first checking
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-- that the request makes sense. If it does not make sense, a warning is
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-- posted on node Enod, and the Mechanism of Ent is unchanged.
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end Sem_Mech;
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