1------------------------------------------------------------------------------ 2-- -- 3-- GNAT COMPILER COMPONENTS -- 4-- -- 5-- S E M _ M E C H -- 6-- -- 7-- S p e c -- 8-- -- 9-- Copyright (C) 1996-2019, Free Software Foundation, Inc. -- 10-- -- 11-- GNAT is free software; you can redistribute it and/or modify it under -- 12-- terms of the GNU General Public License as published by the Free Soft- -- 13-- ware Foundation; either version 3, or (at your option) any later ver- -- 14-- sion. GNAT is distributed in the hope that it will be useful, but WITH- -- 15-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY -- 16-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License -- 17-- for more details. You should have received a copy of the GNU General -- 18-- Public License distributed with GNAT; see file COPYING3. If not, go to -- 19-- http://www.gnu.org/licenses for a complete copy of the license. -- 20-- -- 21-- GNAT was originally developed by the GNAT team at New York University. -- 22-- Extensive contributions were provided by Ada Core Technologies Inc. -- 23-- -- 24------------------------------------------------------------------------------ 25 26-- This package contains the routine used to establish calling mechanisms 27-- The reason we separate this off into its own package is that it is 28-- entirely possible that it may need some target specific specialization. 29 30with Types; use Types; 31 32package Sem_Mech is 33 34 ------------------------------------------------- 35 -- Definitions for Parameter Mechanism Control -- 36 ------------------------------------------------- 37 38 -- For parameters passed to subprograms, and for function return values, 39 -- a passing mechanism is defined. The entity attribute Mechanism returns 40 -- an indication of the mechanism, and Set_Mechanism can be used to set 41 -- the mechanism. At the program level, there are three ways to explicitly 42 -- set the mechanism: 43 44 -- An Import_xxx or Export_xxx pragma (where xxx is Function, Procedure, 45 -- or Valued_Procedure) can explicitly set the mechanism for either a 46 -- parameter or a function return value. A mechanism explicitly set by 47 -- such a pragma overrides the effect of C_Pass_By_Copy described below. 48 49 -- If convention C_Pass_By_Copy is set for a record, and the record type 50 -- is used as the formal type of a subprogram with a foreign convention, 51 -- then the mechanism is set to By_Copy. 52 53 -- If a pragma C_Pass_By_Copy applies, and a record type has Convention 54 -- C, and the record type is used as the formal type of a subprogram 55 -- with a foreign convention, then the mechanism is set to use By_Copy 56 -- if the size of the record is sufficiently small (as determined by 57 -- the value of the parameter to pragma C_Pass_By_Copy). 58 59 -- The subtype Mechanism_Type (declared in Types) is used to describe 60 -- the mechanism to be used. The following special values of this type 61 -- specify the mechanism, as follows. 62 63 Default_Mechanism : constant Mechanism_Type := 0; 64 -- The default setting indicates that the backend will choose the proper 65 -- default mechanism. This depends on the convention of the subprogram 66 -- involved, and is generally target dependent. In the compiler, the 67 -- backend chooses the mechanism in this case in accordance with any 68 -- requirements imposed by the ABI. Note that Default is never used for 69 -- record types on foreign convention subprograms, since By_Reference 70 -- is forced for such types unless one of the above described approaches 71 -- is used to explicitly force By_Copy. 72 73 By_Copy : constant Mechanism_Type := -1; 74 -- Passing by copy is forced. The exact meaning of By_Copy (e.g. whether 75 -- at a low level the value is passed in registers, or the value is copied 76 -- and a pointer is passed), is determined by the backend in accordance 77 -- with requirements imposed by the ABI. Note that in the extended import 78 -- and export pragma mechanisms, this is called Value, rather than Copy. 79 80 By_Reference : constant Mechanism_Type := -2; 81 -- Passing by reference is forced. This is always equivalent to passing 82 -- a simple pointer in the case of subprograms with a foreign convention. 83 -- For unconstrained arrays passed to foreign convention subprograms, the 84 -- address of the first element of the array is passed. For convention 85 -- Ada, the result is logically to pass a reference, but the precise 86 -- mechanism (e.g. to pass bounds of unconstrained types and other needed 87 -- special information) is determined by the backend in accordance with 88 -- requirements imposed by the ABI as interpreted for Ada. 89 90 pragma Assert (Mechanism_Type'First = -2); 91 -- Check definition in types is right! 92 93 -- All the above special values are non-positive. Positive values for 94 -- Mechanism_Type values have a special meaning. They are used only in 95 -- the case of records, as a result of the use of the C_Pass_By_Copy 96 -- pragma, and the meaning is that if the size of the record is known 97 -- at compile time and does not exceed the mechanism type value, then 98 -- By_Copy passing is forced, otherwise By_Reference is forced. 99 100 ---------------------- 101 -- Global Variables -- 102 ---------------------- 103 104 Default_C_Record_Mechanism : Mechanism_Type := By_Reference; 105 -- This value is the default mechanism used for C convention records 106 -- in foreign-convention subprograms if no mechanism is otherwise 107 -- specified. This value is modified appropriately by the occurrence 108 -- of a C_Pass_By_Copy configuration pragma. 109 110 ----------------- 111 -- Subprograms -- 112 ----------------- 113 114 procedure Set_Mechanisms (E : Entity_Id); 115 -- E is a subprogram or subprogram type that has been frozen, so the 116 -- convention of the subprogram and all its formal types and result 117 -- type in the case of a function are established. The function of 118 -- this call is to set mechanism values for formals and for the 119 -- function return if they have not already been explicitly set by 120 -- a use of an extended Import or Export pragma. The idea is to set 121 -- mechanism values wherever the semantics is dictated by either 122 -- requirements or implementation advice in the RM, and to leave 123 -- the mechanism set to Default if there is no requirement, so that 124 -- the back-end is free to choose the most efficient method. 125 126 procedure Set_Mechanism_Value (Ent : Entity_Id; Mech_Name : Node_Id); 127 -- Mech is a parameter passing mechanism (see Import_Function syntax 128 -- for MECHANISM_NAME). This routine checks that the mechanism argument 129 -- has the right form, and if not issues an error message. If the 130 -- argument has the right form then the Mechanism field of Ent is 131 -- set appropriately. It also performs some error checks. Note that 132 -- the mechanism name has not been analyzed (and cannot indeed be 133 -- analyzed, since it is semantic nonsense), so we get it in the 134 -- exact form created by the parser. 135 136 procedure Set_Mechanism_With_Checks 137 (Ent : Entity_Id; 138 Mech : Mechanism_Type; 139 Enod : Node_Id); 140 -- Sets the mechanism of Ent to the given Mech value, after first checking 141 -- that the request makes sense. If it does not make sense, a warning is 142 -- posted on node Enod, and the Mechanism of Ent is unchanged. 143 144end Sem_Mech; 145