gcc/ada/freeze.ads

   1 ------------------------------------------------------------------------------
   2 --                                                                          --
   3 --                         GNAT COMPILER COMPONENTS                         --
   4 --                                                                          --
   5 --                               F R E E Z E                                --
   6 --                                                                          --
   7 --                                 S p e c                                  --
   8 --                                                                          --
   9 --                            $Revision: 1.1 $
  10 --                                                                          --
  11 --          Copyright (C) 1992-2000, Free Software Foundation, Inc.         --
  12 --                                                                          --
  13 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
  14 -- terms of the  GNU General Public License as published  by the Free Soft- --
  15 -- ware  Foundation;  either version 2,  or (at your option) any later ver- --
  16 -- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
  17 -- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
  18 -- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License --
  19 -- for  more details.  You should have  received  a copy of the GNU General --
  20 -- Public License  distributed with GNAT;  see file COPYING.  If not, write --
  21 -- to  the Free Software Foundation,  59 Temple Place - Suite 330,  Boston, --
  22 -- MA 02111-1307, USA.                                                      --
  23 --                                                                          --
  24 -- GNAT was originally developed  by the GNAT team at  New York University. --
  25 -- It is now maintained by Ada Core Technologies Inc (http://www.gnat.com). --
  26 --                                                                          --
  27 ------------------------------------------------------------------------------
  28
  29 with Types; use Types;
  30
  31 package Freeze is
  32
  33    --------------------------
  34    -- Handling of Freezing --
  35    --------------------------
  36
  37    --  In the formal Ada semantics, freezing of entities occurs at a well
  38    --  defined point, described in (RM 13.14). The model in GNAT of freezing
  39    --  is that a Freeze_Entity node is generated at the point where an entity
  40    --  is frozen, and the entity contains a pointer (Freeze_Node) to this
  41    --  generated freeze node.
  42
  43    --  The freeze node is processed in the expander to generate associated
  44    --  data and subprograms (e.g. an initialization procedure) which must
  45    --  be delayed until the type is frozen and its representation can be
  46    --  fully determined. Subsequently the freeze node is used by Gigi to
  47    --  determine the point at which it should elaborate the corresponding
  48    --  entity (this elaboration also requires the representation of the
  49    --  entity to be fully determinable). The freeze node is also used to
  50    --  provide additional diagnostic information (pinpointing the freeze
  51    --  point), when order of freezing errors are detected.
  52
  53    --  If we were fully faithful to the Ada model, we would generate freeze
  54    --  nodes for all entities, but that is a bit heavy so we optimize (that
  55    --  is the nice word) or cut corners (which is a bit more honest). For
  56    --  many entities, we do not need to delay the freeze and instead can
  57    --  freeze them at the point of declaration. The conditions for this
  58    --  early freezing being permissible are as follows:
  59
  60    --    There is no associated expander activity that needs to be delayed
  61
  62    --    Gigi can fully elaborate the entity at the point of occurrence (or,
  63    --    equivalently, no real elaboration is required for the entity).
  64
  65    --  In order for these conditions to be met (especially the second), it
  66    --  must be the case that all representation characteristics of the entity
  67    --  can be determined at declaration time.
  68
  69    --  The following indicates how freezing is handled for all entity kinds:
  70
  71    --    Types
  72
  73    --      All declared types have freeze nodes, as well as anonymous base
  74    --      types created for type declarations where the defining identifier
  75    --      is a first subtype of the anonymous type.
  76
  77    --    Subtypes
  78
  79    --      All first subtypes have freeze nodes. Other subtypes need freeze
  80    --      nodes if the corresponding base type has not yet been frozen. If
  81    --      the base type has been frozen, then there is no need for a freeze
  82    --      node, since no rep clauses can appear for the subtype in any case.
  83
  84    --    Implicit types and subtypes
  85
  86    --      As noted above, implicit base types always have freeze nodes. Other
  87    --      implicit types and subtypes typically do not require freeze nodes,
  88    --      because there is no possibility of delaying any information about
  89    --      their representation.
  90
  91    --    Subprograms
  92    --
  93    --      Are frozen at the point of declaration unless one or more of the
  94    --      formal types or return type themselves have delayed freezing and
  95    --      are not yet frozen. This includes the case of a formal access type
  96    --      where the designated type is not frozen. Note that we are talking
  97    --      about subprogram specs here (subprogram body entities have no
  98    --      relevance), and in any case, subprogram bodies freeze everything.
  99
 100    --    Objects with dynamic address clauses
 101    --
 102    --      These have a delayed freeze. Gigi will generate code to evaluate
 103    --      the initialization expression if present and store it in a temp.
 104    --      The actual object is created at the point of the freeze, and if
 105    --      necessary initialized by copying the value of this temporary.
 106
 107    --    Formal Parameters
 108    --
 109    --      Are frozen when the associated subprogram is frozen, so there is
 110    --      never any need for them to have delayed freezing.
 111
 112    --    Other Objects
 113    --
 114    --      Are always frozen at the point of declaration
 115
 116    --    All Other Entities
 117
 118    --      Are always frozen at the point of declaration
 119
 120    --  The flag Has_Delayed_Freeze is used for to indicate that delayed
 121    --  freezing is required. Usually the associated freeze node is allocated
 122    --  at the freezing point. One special exception occurs with anonymous
 123    --  base types, where the freeze node is preallocated at the point of
 124    --  declaration, so that the First_Subtype_Link field can be set.
 125
 126    -----------------
 127    -- Subprograms --
 128    -----------------
 129
 130    function Build_Renamed_Body
 131      (Decl  : Node_Id;
 132       New_S : Entity_Id)
 133    return Node_Id;
 134    --  Rewrite renaming declaration as a subprogram body, whose single
 135    --  statement is a call to the renamed entity. New_S is the entity that
 136    --  appears in the renaming declaration. If this is a Renaming_As_Body,
 137    --  then Decl is the original subprogram declaration that is completed
 138    --  by the renaming, otherwise it is the renaming declaration itself.
 139    --  The caller inserts the body where required. If this call comes
 140    --  from a freezing action, the resulting body is analyzed at once.
 141
 142    procedure Check_Compile_Time_Size (T : Entity_Id);
 143    --  Check to see whether the size of the type T is known at compile time.
 144    --  There are three possible cases:
 145    --
 146    --    Size is not known at compile time. In this case, the call has no
 147    --    effect. Note that the processing is conservative here, in the sense
 148    --    that this routine may decide that the size is not known even if in
 149    --    fact Gigi decides it is known, but the opposite situation can never
 150    --    occur.
 151    --
 152    --    Size is known at compile time, but the actual value of the size is
 153    --    not known to the front end or is definitely 32 or more. In this case
 154    --    Size_Known_At_Compile_Time is set, but the Esize field is left set
 155    --    to zero (to be set by Gigi).
 156    --
 157    --    Size is known at compile time, and the actual value of the size is
 158    --    known to the front end and is less than 32. In this case, the flag
 159    --    Size_Known_At_Compile_Time is set, and in addition Esize is set to
 160    --    the required size, allowing for possible front end packing of an
 161    --    array using this type as a component type.
 162    --
 163    --  Note: the flag Size_Known_At_Compile_Time is used to determine if the
 164    --  secondary stack must be used to return a value of the type, and also
 165    --  to determine whether a component clause is allowed for a component
 166    --  of the given type.
 167    --
 168    --  Note: this is public because of one dubious use in Sem_Res???
 169    --
 170    --  Note: Check_Compile_Time_Size does not test the case of the size being
 171    --  known because a size clause is specifically given. That is because we
 172    --  do not allow a size clause if the size would not otherwise be known at
 173    --  compile time in any case.
 174
 175    function Freeze_Entity (E : Entity_Id; Loc : Source_Ptr) return List_Id;
 176    --  Freeze an entity, and return Freeze nodes, to be inserted at the
 177    --  point of call. Loc is a source location which corresponds to the
 178    --  freeze point. This is used in placing warning messages in the
 179    --  situation where it appears that a type has been frozen too early,
 180    --  e.g. when a primitive operation is declared after the freezing
 181    --  point of its tagged type. Returns No_List if no freeze nodes needed.
 182
 183    procedure Freeze_All (From : Entity_Id; After : in out Node_Id);
 184    --  Before a non-instance body, or at the end of a declarative part
 185    --  freeze all entities therein that are not yet frozen. Calls itself
 186    --  recursively to catch types in inner packages that were not frozen
 187    --  at the inner level because they were not yet completely defined.
 188    --  This routine also analyzes and freezes default parameter expressions
 189    --  in subprogram specifications (this has to be delayed until all the
 190    --  types are frozen). The resulting freeze nodes are inserted just
 191    --  after node After (which is a list node) and analyzed. On return,
 192    --  'After' is updated to point to the last node inserted (or is returned
 193    --  unchanged if no nodes were inserted). 'From' is the last entity frozen
 194    --  in the scope. It is used to prevent a quadratic traversal over already
 195    --  frozen entities.
 196
 197    procedure Freeze_Before (N : Node_Id; T : Entity_Id);
 198    --  Freeze T then Insert the generated Freeze nodes before the node N.
 199
 200    procedure Freeze_Expression (N : Node_Id);
 201    --  Freezes the required entities when the Expression N causes freezing.
 202    --  The node N here is either a subexpression node (a "real" expression)
 203    --  or a subtype mark, or a subtype indication. The latter two cases are
 204    --  not really expressions, but they can appear within expressions and
 205    --  so need to be similarly treated. Freeze_Expression takes care of
 206    --  determining the proper insertion point for generated freeze actions.
 207
 208    procedure Freeze_Itype (T : Entity_Id; N : Node_Id);
 209    --  This routine is called when an Itype is created and must be frozen
 210    --  immediately at the point of creation (for the sake of the expansion
 211    --  activities in Exp_Ch3 (for example, the creation of packed array
 212    --  types). We can't just let Freeze_Expression do this job since it
 213    --  goes out of its way to make sure that the freeze node occurs at a
 214    --  point outside the current construct, e.g. outside the expression or
 215    --  outside the initialization procedure. That's normally right, but
 216    --  not in this case, since if we create an Itype in an expression it
 217    --  may be the case that it is not always elaborated (for example it
 218    --  may result from the right operand of a short circuit). In this case
 219    --  we want the freeze node to be inserted at the same point as the Itype.
 220    --  The node N provides both the location for the freezing and also the
 221    --  insertion point for the resulting freeze nodes.
 222
 223 end Freeze;