gcc/ada/expander.ads

   1 ------------------------------------------------------------------------------
   2 --                                                                          --
   3 --                         GNAT COMPILER COMPONENTS                         --
   4 --                                                                          --
   5 --                             E X P A N D E R                              --
   6 --                                                                          --
   7 --                                 S p e c                                  --
   8 --                                                                          --
   9 --          Copyright (C) 1992-2005, 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 2,  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 COPYING.  If not, write --
  19 -- to  the  Free Software Foundation,  51  Franklin  Street,  Fifth  Floor, --
  20 -- Boston, MA 02110-1301, USA.                                              --
  21 --                                                                          --
  22 -- GNAT was originally developed  by the GNAT team at  New York University. --
  23 -- Extensive contributions were provided by Ada Core Technologies Inc.      --
  24 --                                                                          --
  25 ------------------------------------------------------------------------------
  26
  27 --  This procedure performs any required expansion for the specified node.
  28 --  The argument is the node that is a candidate for possible expansion.
  29 --  If no expansion is required, then Expand returns without doing anything.
  30
  31 --  If the node does need expansion, then the subtree is replaced by the
  32 --  tree corresponding to the required rewriting. This tree is a syntactic
  33 --  tree, except that all Entity fields must be correctly set on all
  34 --  direct names, since the expander presumably knows what it wants, and in
  35 --  any case it doesn't work to have the semantic analyzer perform visibility
  36 --  analysis on these trees (they may have references to non-visible runtime
  37 --  routines etc.) There are a few exceptions to this rule in special cases,
  38 --  but they must be documented clearly.
  39
  40 --  Expand is called in two different situations:
  41
  42 --    Nodes that are not subexpressions (Nkind not in N_Subexpr)
  43
  44 --      In this case, Expand is called from the body of Sem, immediately
  45 --      after completing semantic analysis by calling the corresponding
  46 --      Analyze_N_xxx procedure. If expansion occurs, the given node must
  47 --      be replaced with another node that is also not a subexpression.
  48 --      This seems naturally to be the case, since it is hard to imagine any
  49 --      situation in which it would make sense to replace a non-expression
  50 --      subtree with an expression. Once the substitution is completed, the
  51 --      Expand routine must call Analyze on the resulting node to do any
  52 --      required semantic analysis. Note that references to children copied
  53 --      from the old tree won't be reanalyzed, since their Analyzed flag
  54 --      is set.
  55
  56 --    Nodes that are subexpressions (Nkind in N_Subexpr)
  57
  58 --      In this case, Expand is called from Sem_Res.Resolve after completing
  59 --      the resolution of the subexpression (this means that the expander sees
  60 --      the fully typed subtree). If expansion occurs, the given node must be
  61 --      replaced by a node that is also a subexpression. Again it is hard
  62 --      to see how this restriction could possibly be violated. Once the
  63 --      substitution is completed, the Expand routine must first call Analyze
  64 --      on the resulting node to do any required semantic analysis, and then
  65 --      call Resolve on the node to set the type (typically the type will be
  66 --      the same as the original type of the input node, but this is not
  67 --      always the case).
  68
  69 --  In both these cases, Replace or Rewrite must be used to achieve the
  70 --  of the node, since the Expander routine is only passed the Node_Id
  71 --  of the node to be expanded, and the resulting expanded Node_Id must
  72 --  be the same (the parameter to Expand is mode in, not mode in-out).
  73
  74 --  For nodes other than subexpressions, it is not necessary to preserve the
  75 --  original tree in the Expand routines, unlike the case for modifications
  76 --  to the tree made in the semantic analyzer. This is because anyone who is
  77 --  interested in working with the original tree (like ASIS) is required to
  78 --  compile in semantics checks only mode. Thus Replace may be freely used
  79 --  in such instances.
  80
  81 --  For subexpressions, preservation of the original tree is required because
  82 --  of the need for conformance checking of default expressions, which occurs
  83 --  on expanded trees. This means that Replace should not ever be used on
  84 --  on subexpression nodes. Instead use Rewrite.
  85
  86 --  Note: the front end avoids calls to any of the expand routines if code
  87 --  is not being generated. This is done for three reasons:
  88
  89 --    1.  Make sure tree does not get mucked up by the expander if no
  90 --        code is being generated, and is thus usable by ASIS etc.
  91
  92 --    2.  Save time, since expansion is not needed if a compilation is
  93 --        being done only to check the semantics, or if code generation
  94 --        has been canceled due to previously detected errors.
  95
  96 --    3.  Allow the expand routines to assume that the tree is error free.
  97 --        This results from the fact that code generation mode is always
  98 --        cancelled when any error occurs.
  99
 100 --  If we ever decide to implement a feature allowing object modules to be
 101 --  generated even if errors have been detected, then point 3 will no longer
 102 --  hold, and the expand routines will have to be modified to operate properly
 103 --  in the presence of errors (for many reasons this is not currently true).
 104
 105 --  Note: a consequence of this approach is that error messages must never
 106 --  be generated in the expander, since this would mean that such error
 107 --  messages are not generated when the expander is not being called.
 108
 109 --  Expansion is the last stage of analyzing a node, so Expand sets the
 110 --  Analyzed flag of the node being analyzed as its last action. This is
 111 --  done even if expansion is off (in this case, the only effect of the
 112 --  call to Expand is to set the Analyzed flag to True).
 113
 114 with Types; use Types;
 115
 116 package Expander is
 117
 118    --  The flag Opt.Expander_Active controls whether expansion is active
 119    --  (True) or deactivated (False). When expansion is deactivated all
 120    --  calls to expander routines have no effect. To temporarily disable
 121    --  expansion, always call the routines defined below, do NOT change
 122    --  Expander_Active directly.
 123    --
 124    --  You should not use this flag to test if you are currently processing
 125    --  a generic spec or body. Use the flag Inside_A_Generic instead (see
 126    --  the spec of package Sem).
 127    --
 128    --  There is no good reason for permanently changing the value of this flag
 129    --  except after detecting a syntactic or semantic error. In this event
 130    --  this flag is set to False to disable all subsequent expansion activity.
 131    --
 132    --  In general this flag should be used as a read only value. The only
 133    --  exceptions where it makes sense to temporarily change its value are:
 134    --
 135    --    (a) when starting/completing the processing of a generic definition
 136    --        or declaration (see routines Start_Generic_Processing and
 137    --        End_Generic_Processing in Sem_Ch12)
 138    --
 139    --    (b) when starting/completing the pre-analysis of an expression
 140    --        (see the spec of package Sem for more info on pre-analysis.)
 141    --
 142    --  Note that when processing a default expression (In_Default_Expression
 143    --  is True) or performing semantic analysis of a generic spec or body
 144    --  (Inside_A_Generic) or when performing pre-analysis (Full_Analysis is
 145    --  False) the Expander_Active flag is False.
 146
 147    procedure Expand (N : Node_Id);
 148    --  Expand node N, as described above
 149
 150    procedure Expander_Mode_Save_And_Set (Status : Boolean);
 151    --  Saves the current setting of the Expander_Active flag on an internal
 152    --  stack and then sets the flag to the given value.
 153    --
 154    --  Note: this routine has no effect in ASIS_Mode. In ASIS_Mode, all
 155    --  expansion activity is always off, since we want the original semantic
 156    --  tree for ASIS purposes without any expansion. This is achieved by
 157    --  setting Expander_Active False in ASIS_Mode. In situations such as
 158    --  the call to Instantiate_Bodies in Frontend, Expander_Mode_Save_And_Set
 159    --  may be called to temporarily turn the expander on, but this will have
 160    --  no effect in ASIS mode.
 161
 162    procedure Expander_Mode_Restore;
 163    --  Restores the setting of the Expander_Active flag using the top entry
 164    --  pushed onto the stack by Expander_Mode_Save_And_Reset, popping the
 165    --  stack, except that if any errors have been detected, then the state
 166    --  of the flag is left set to False. Disabled for ASIS_Mode (see above).
 167
 168 end Expander;