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1 ------------------------------------------------------------------------------
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2 -- --
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3 -- GNAT COMPILER COMPONENTS --
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4 -- --
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5 -- S E M _ M E C H --
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6 -- --
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7 -- S p e c --
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8 -- --
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9 -- Copyright (C) 1996-2014, Free Software Foundation, Inc. --
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10 -- --
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11 -- GNAT is free software; you can redistribute it and/or modify it under --
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12 -- terms of the GNU General Public License as published by the Free Soft- --
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13 -- ware Foundation; either version 3, or (at your option) any later ver- --
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14 -- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
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15 -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
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16 -- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
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17 -- for more details. You should have received a copy of the GNU General --
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18 -- Public License distributed with GNAT; see file COPYING3. If not, go to --
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19 -- http://www.gnu.org/licenses for a complete copy of the license. --
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20 -- --
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21 -- GNAT was originally developed by the GNAT team at New York University. --
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22 -- Extensive contributions were provided by Ada Core Technologies Inc. --
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23 -- --
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24 ------------------------------------------------------------------------------
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25
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26 -- This package contains the routine used to establish calling mechanisms
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27 -- The reason we separate this off into its own package is that it is
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28 -- entirely possible that it may need some target specific specialization.
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29
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30 with Types; use Types;
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31
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32 package Sem_Mech is
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33
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34 -------------------------------------------------
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35 -- Definitions for Parameter Mechanism Control --
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36 -------------------------------------------------
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37
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38 -- For parameters passed to subprograms, and for function return values,
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39 -- a passing mechanism is defined. The entity attribute Mechanism returns
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40 -- an indication of the mechanism, and Set_Mechanism can be used to set
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41 -- the mechanism. At the program level, there are three ways to explicitly
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42 -- set the mechanism:
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43
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44 -- An Import_xxx or Export_xxx pragma (where xxx is Function, Procedure,
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45 -- or Valued_Procedure) can explicitly set the mechanism for either a
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46 -- parameter or a function return value. A mechanism explicitly set by
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47 -- such a pragma overrides the effect of C_Pass_By_Copy described below.
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48
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49 -- If convention C_Pass_By_Copy is set for a record, and the record type
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50 -- is used as the formal type of a subprogram with a foreign convention,
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51 -- then the mechanism is set to By_Copy.
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52
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53 -- If a pragma C_Pass_By_Copy applies, and a record type has Convention
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54 -- C, and the record type is used as the formal type of a subprogram
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55 -- with a foreign convention, then the mechanism is set to use By_Copy
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56 -- if the size of the record is sufficiently small (as determined by
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57 -- the value of the parameter to pragma C_Pass_By_Copy).
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58
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59 -- The subtype Mechanism_Type (declared in Types) is used to describe
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60 -- the mechanism to be used. The following special values of this type
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61 -- specify the mechanism, as follows.
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62
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63 Default_Mechanism : constant Mechanism_Type := 0;
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64 -- The default setting indicates that the backend will choose the proper
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65 -- default mechanism. This depends on the convention of the subprogram
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66 -- involved, and is generally target dependent. In the compiler, the
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67 -- backend chooses the mechanism in this case in accordance with any
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68 -- requirements imposed by the ABI. Note that Default is never used for
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69 -- record types on foreign convention subprograms, since By_Reference
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70 -- is forced for such types unless one of the above described approaches
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71 -- is used to explicitly force By_Copy.
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72
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73 By_Copy : constant Mechanism_Type := -1;
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74 -- Passing by copy is forced. The exact meaning of By_Copy (e.g. whether
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75 -- at a low level the value is passed in registers, or the value is copied
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76 -- and a pointer is passed), is determined by the backend in accordance
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77 -- with requirements imposed by the ABI. Note that in the extended import
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78 -- and export pragma mechanisms, this is called Value, rather than Copy.
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79
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80 By_Reference : constant Mechanism_Type := -2;
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81 -- Passing by reference is forced. This is always equivalent to passing
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82 -- a simple pointer in the case of subprograms with a foreign convention.
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83 -- For unconstrained arrays passed to foreign convention subprograms, the
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84 -- address of the first element of the array is passed. For convention
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85 -- Ada, the result is logically to pass a reference, but the precise
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86 -- mechanism (e.g. to pass bounds of unconstrained types and other needed
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87 -- special information) is determined by the backend in accordance with
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88 -- requirements imposed by the ABI as interpreted for Ada.
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89
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90 pragma Assert (Mechanism_Type'First = -2);
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91 -- Check definition in types is right!
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92
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93 -- All the above special values are non-positive. Positive values for
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94 -- Mechanism_Type values have a special meaning. They are used only in
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95 -- the case of records, as a result of the use of the C_Pass_By_Copy
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96 -- pragma, and the meaning is that if the size of the record is known
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97 -- at compile time and does not exceed the mechanism type value, then
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98 -- By_Copy passing is forced, otherwise By_Reference is forced.
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99
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100 ----------------------
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101 -- Global Variables --
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102 ----------------------
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103
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104 Default_C_Record_Mechanism : Mechanism_Type := By_Reference;
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105 -- This value is the default mechanism used for C convention records
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106 -- in foreign-convention subprograms if no mechanism is otherwise
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107 -- specified. This value is modified appropriately by the occurrence
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108 -- of a C_Pass_By_Copy configuration pragma.
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109
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110 -----------------
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111 -- Subprograms --
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112 -----------------
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113
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114 procedure Set_Mechanisms (E : Entity_Id);
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115 -- E is a subprogram or subprogram type that has been frozen, so the
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116 -- convention of the subprogram and all its formal types and result
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117 -- type in the case of a function are established. The function of
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118 -- this call is to set mechanism values for formals and for the
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119 -- function return if they have not already been explicitly set by
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120 -- a use of an extended Import or Export pragma. The idea is to set
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121 -- mechanism values wherever the semantics is dictated by either
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122 -- requirements or implementation advice in the RM, and to leave
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123 -- the mechanism set to Default if there is no requirement, so that
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124 -- the back-end is free to choose the most efficient method.
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125
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126 procedure Set_Mechanism_Value (Ent : Entity_Id; Mech_Name : Node_Id);
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127 -- Mech is a parameter passing mechanism (see Import_Function syntax
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128 -- for MECHANISM_NAME). This routine checks that the mechanism argument
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129 -- has the right form, and if not issues an error message. If the
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130 -- argument has the right form then the Mechanism field of Ent is
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131 -- set appropriately. It also performs some error checks. Note that
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132 -- the mechanism name has not been analyzed (and cannot indeed be
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133 -- analyzed, since it is semantic nonsense), so we get it in the
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134 -- exact form created by the parser.
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135
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136 procedure Set_Mechanism_With_Checks
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137 (Ent : Entity_Id;
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138 Mech : Mechanism_Type;
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139 Enod : Node_Id);
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140 -- Sets the mechanism of Ent to the given Mech value, after first checking
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141 -- that the request makes sense. If it does not make sense, a warning is
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142 -- posted on node Enod, and the Mechanism of Ent is unchanged.
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143
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144 end Sem_Mech;
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