File : sem_mech.ads
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) 1996-2014, 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
30 with Types; use Types;
31
32 package 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
144 end Sem_Mech;