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 --                                                                          --
  10 --          Copyright (C) 1992-2001, Free Software Foundation, Inc.         --
  11 --                                                                          --
  12 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
  13 -- terms of the  GNU General Public License as published  by the Free Soft- --
  14 -- ware  Foundation;  either version 2,  or (at your option) any later ver- --
  15 -- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
  16 -- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
  17 -- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License --
  18 -- for  more details.  You should have  received  a copy of the GNU General --
  19 -- Public License  distributed with GNAT;  see file COPYING.  If not, write --
  20 -- to  the Free Software Foundation,  59 Temple Place - Suite 330,  Boston, --
  21 -- MA 02111-1307, USA.                                                      --
  22 --                                                                          --
  23 -- GNAT was originally developed  by the GNAT team at  New York University. --
  24 -- Extensive contributions were provided by Ada Core Technologies Inc.      --
  25 --                                                                          --
  26 ------------------------------------------------------------------------------
  27
  28 with Types; use Types;
  29
  30 package Freeze is
  31
  32    --------------------------
  33    -- Handling of Freezing --
  34    --------------------------
  35
  36    --  In the formal Ada semantics, freezing of entities occurs at a well
  37    --  defined point, described in (RM 13.14). The model in GNAT of freezing
  38    --  is that a Freeze_Entity node is generated at the point where an entity
  39    --  is frozen, and the entity contains a pointer (Freeze_Node) to this
  40    --  generated freeze node.
  41
  42    --  The freeze node is processed in the expander to generate associated
  43    --  data and subprograms (e.g. an initialization procedure) which must
  44    --  be delayed until the type is frozen and its representation can be
  45    --  fully determined. Subsequently the freeze node is used by Gigi to
  46    --  determine the point at which it should elaborate the corresponding
  47    --  entity (this elaboration also requires the representation of the
  48    --  entity to be fully determinable). The freeze node is also used to
  49    --  provide additional diagnostic information (pinpointing the freeze
  50    --  point), when order of freezing errors are detected.
  51
  52    --  If we were fully faithful to the Ada model, we would generate freeze
  53    --  nodes for all entities, but that is a bit heavy so we optimize (that
  54    --  is the nice word) or cut corners (which is a bit more honest). For
  55    --  many entities, we do not need to delay the freeze and instead can
  56    --  freeze them at the point of declaration. The conditions for this
  57    --  early freezing being permissible are as follows:
  58
  59    --    There is no associated expander activity that needs to be delayed
  60
  61    --    Gigi can fully elaborate the entity at the point of occurrence (or,
  62    --    equivalently, no real elaboration is required for the entity).
  63
  64    --  In order for these conditions to be met (especially the second), it
  65    --  must be the case that all representation characteristics of the entity
  66    --  can be determined at declaration time.
  67
  68    --  The following indicates how freezing is handled for all entity kinds:
  69
  70    --    Types
  71
  72    --      All declared types have freeze nodes, as well as anonymous base
  73    --      types created for type declarations where the defining identifier
  74    --      is a first subtype of the anonymous type.
  75
  76    --    Subtypes
  77
  78    --      All first subtypes have freeze nodes. Other subtypes need freeze
  79    --      nodes if the corresponding base type has not yet been frozen. If
  80    --      the base type has been frozen, then there is no need for a freeze
  81    --      node, since no rep clauses can appear for the subtype in any case.
  82
  83    --    Implicit types and subtypes
  84
  85    --      As noted above, implicit base types always have freeze nodes. Other
  86    --      implicit types and subtypes typically do not require freeze nodes,
  87    --      because there is no possibility of delaying any information about
  88    --      their representation.
  89
  90    --    Subprograms
  91    --
  92    --      Are frozen at the point of declaration unless one or more of the
  93    --      formal types or return type themselves have delayed freezing and
  94    --      are not yet frozen. This includes the case of a formal access type
  95    --      where the designated type is not frozen. Note that we are talking
  96    --      about subprogram specs here (subprogram body entities have no
  97    --      relevance), and in any case, subprogram bodies freeze everything.
  98
  99    --    Objects with dynamic address clauses
 100    --
 101    --      These have a delayed freeze. Gigi will generate code to evaluate
 102    --      the initialization expression if present and store it in a temp.
 103    --      The actual object is created at the point of the freeze, and if
 104    --      necessary initialized by copying the value of this temporary.
 105
 106    --    Formal Parameters
 107    --
 108    --      Are frozen when the associated subprogram is frozen, so there is
 109    --      never any need for them to have delayed freezing.
 110
 111    --    Other Objects
 112    --
 113    --      Are always frozen at the point of declaration
 114
 115    --    All Other Entities
 116
 117    --      Are always frozen at the point of declaration
 118
 119    --  The flag Has_Delayed_Freeze is used for to indicate that delayed
 120    --  freezing is required. Usually the associated freeze node is allocated
 121    --  at the freezing point. One special exception occurs with anonymous
 122    --  base types, where the freeze node is preallocated at the point of
 123    --  declaration, so that the First_Subtype_Link field can be set.
 124
 125    -----------------
 126    -- Subprograms --
 127    -----------------
 128
 129    function Build_Renamed_Body
 130      (Decl  : Node_Id;
 131       New_S : Entity_Id)
 132    return Node_Id;
 133    --  Rewrite renaming declaration as a subprogram body, whose single
 134    --  statement is a call to the renamed entity. New_S is the entity that
 135    --  appears in the renaming declaration. If this is a Renaming_As_Body,
 136    --  then Decl is the original subprogram declaration that is completed
 137    --  by the renaming, otherwise it is the renaming declaration itself.
 138    --  The caller inserts the body where required. If this call comes
 139    --  from a freezing action, the resulting body is analyzed at once.
 140
 141    procedure Check_Compile_Time_Size (T : Entity_Id);
 142    --  Check to see whether the size of the type T is known at compile time.
 143    --  There are three possible cases:
 144    --
 145    --    Size is not known at compile time. In this case, the call has no
 146    --    effect. Note that the processing is conservative here, in the sense
 147    --    that this routine may decide that the size is not known even if in
 148    --    fact Gigi decides it is known, but the opposite situation can never
 149    --    occur.
 150    --
 151    --    Size is known at compile time, but the actual value of the size is
 152    --    not known to the front end or is definitely 32 or more. In this case
 153    --    Size_Known_At_Compile_Time is set, but the Esize field is left set
 154    --    to zero (to be set by Gigi).
 155    --
 156    --    Size is known at compile time, and the actual value of the size is
 157    --    known to the front end and is less than 32. In this case, the flag
 158    --    Size_Known_At_Compile_Time is set, and in addition Esize is set to
 159    --    the required size, allowing for possible front end packing of an
 160    --    array using this type as a component type.
 161    --
 162    --  Note: the flag Size_Known_At_Compile_Time is used to determine if the
 163    --  secondary stack must be used to return a value of the type, and also
 164    --  to determine whether a component clause is allowed for a component
 165    --  of the given type.
 166    --
 167    --  Note: this is public because of one dubious use in Sem_Res???
 168    --
 169    --  Note: Check_Compile_Time_Size does not test the case of the size being
 170    --  known because a size clause is specifically given. That is because we
 171    --  do not allow a size clause if the size would not otherwise be known at
 172    --  compile time in any case.
 173
 174    function Freeze_Entity (E : Entity_Id; Loc : Source_Ptr) return List_Id;
 175    --  Freeze an entity, and return Freeze nodes, to be inserted at the
 176    --  point of call. Loc is a source location which corresponds to the
 177    --  freeze point. This is used in placing warning messages in the
 178    --  situation where it appears that a type has been frozen too early,
 179    --  e.g. when a primitive operation is declared after the freezing
 180    --  point of its tagged type. Returns No_List if no freeze nodes needed.
 181
 182    procedure Freeze_All (From : Entity_Id; After : in out Node_Id);
 183    --  Before a non-instance body, or at the end of a declarative part
 184    --  freeze all entities therein that are not yet frozen. Calls itself
 185    --  recursively to catch types in inner packages that were not frozen
 186    --  at the inner level because they were not yet completely defined.
 187    --  This routine also analyzes and freezes default parameter expressions
 188    --  in subprogram specifications (this has to be delayed until all the
 189    --  types are frozen). The resulting freeze nodes are inserted just
 190    --  after node After (which is a list node) and analyzed. On return,
 191    --  'After' is updated to point to the last node inserted (or is returned
 192    --  unchanged if no nodes were inserted). 'From' is the last entity frozen
 193    --  in the scope. It is used to prevent a quadratic traversal over already
 194    --  frozen entities.
 195
 196    procedure Freeze_Before (N : Node_Id; T : Entity_Id);
 197    --  Freeze T then Insert the generated Freeze nodes before the node N.
 198
 199    procedure Freeze_Expression (N : Node_Id);
 200    --  Freezes the required entities when the Expression N causes freezing.
 201    --  The node N here is either a subexpression node (a "real" expression)
 202    --  or a subtype mark, or a subtype indication. The latter two cases are
 203    --  not really expressions, but they can appear within expressions and
 204    --  so need to be similarly treated. Freeze_Expression takes care of
 205    --  determining the proper insertion point for generated freeze actions.
 206
 207    procedure Freeze_Fixed_Point_Type (Typ : Entity_Id);
 208    --  Freeze fixed point type. For fixed-point types, we have to defer
 209    --  setting the size and bounds till the freeze point, since they are
 210    --  potentially affected by the presence of size and small clauses.
 211
 212    procedure Freeze_Itype (T : Entity_Id; N : Node_Id);
 213    --  This routine is called when an Itype is created and must be frozen
 214    --  immediately at the point of creation (for the sake of the expansion
 215    --  activities in Exp_Ch3 (for example, the creation of packed array
 216    --  types). We can't just let Freeze_Expression do this job since it
 217    --  goes out of its way to make sure that the freeze node occurs at a
 218    --  point outside the current construct, e.g. outside the expression or
 219    --  outside the initialization procedure. That's normally right, but
 220    --  not in this case, since if we create an Itype in an expression it
 221    --  may be the case that it is not always elaborated (for example it
 222    --  may result from the right operand of a short circuit). In this case
 223    --  we want the freeze node to be inserted at the same point as the Itype.
 224    --  The node N provides both the location for the freezing and also the
 225    --  insertion point for the resulting freeze nodes.
 226
 227 end Freeze;