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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;
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