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------------------------------------------------------------------------------
-- --
-- GNAT COMPILER COMPONENTS --
-- --
-- E X P A N D E R --
-- --
-- S p e c --
-- --
-- --
-- Copyright (C) 1992-2000 Free Software Foundation, Inc. --
-- --
-- GNAT is free software; you can redistribute it and/or modify it under --
-- terms of the GNU General Public License as published by the Free Soft- --
-- ware Foundation; either version 2, or (at your option) any later ver- --
-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
-- for more details. You should have received a copy of the GNU General --
-- Public License distributed with GNAT; see file COPYING. If not, write --
-- to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, --
-- MA 02111-1307, USA. --
-- --
-- GNAT was originally developed by the GNAT team at New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc. --
-- --
------------------------------------------------------------------------------
-- This procedure performs any required expansion for the specified node.
-- The argument is the node that is a candidate for possible expansion.
-- If no expansion is required, then Expand returns without doing anything.
-- If the node does need expansion, then the subtree is replaced by the
-- tree corresponding to the required rewriting. This tree is a syntactic
-- tree, except that all Entity fields must be correctly set on all
-- direct names, since the expander presumably knows what it wants, and in
-- any case it doesn't work to have the semantic analyzer perform visibility
-- analysis on these trees (they may have references to non-visible runtime
-- routines etc.) There are a few exceptions to this rule in special cases,
-- but they must be documented clearly.
-- Expand is called in two different situations:
-- Nodes that are not subexpressions (Nkind not in N_Subexpr)
-- In this case, Expand is called from the body of Sem, immediately
-- after completing semantic analysis by calling the corresponding
-- Analyze_N_xxx procedure. If expansion occurs, the given node must
-- be replaced with another node that is also not a subexpression.
-- This seems naturally to be the case, since it is hard to imagine any
-- situation in which it would make sense to replace a non-expression
-- subtree with an expression. Once the substitution is completed, the
-- Expand routine must call Analyze on the resulting node to do any
-- required semantic analysis. Note that references to children copied
-- from the old tree won't be reanalyzed, since their Analyze flag is set.
-- Nodes that are subexpressions (Nkind in N_Subexpr)
-- In this case, Expand is called from Sem_Res.Resolve after completing
-- the resolution of the subexpression (this means that the expander sees
-- the fully typed subtree). If expansion occurs, the given node must be
-- replaced by a node that is also a subexpression. Again it is hard
-- to see how this restriction could possibly be violated. Once the
-- substitution is completed, the Expand routine must first call Analyze
-- on the resulting node to do any required semantic analysis, and then
-- call Resolve on the node to set the type (typically the type will be
-- the same as the original type of the input node, but this is not
-- always the case).
-- In both these cases, Replace or Rewrite must be used to achieve the
-- of the node, since the Expander routine is only passed the Node_Id
-- of the node to be expanded, and the resulting expanded Node_Id must
-- be the same (the parameter to Expand is mode in, not mode in-out).
-- For nodes other than subexpressions, it is not necessary to preserve the
-- original tree in the Expand routines, unlike the case for modifications
-- to the tree made in the semantic analyzer. This is because anyone who is
-- interested in working with the original tree (like ASIS) is required to
-- compile in semantics checks only mode. Thus Replace may be freely used
-- in such instances.
-- For subexpressions, preservation of the original tree is required because
-- of the need for conformance checking of default expressions, which occurs
-- on expanded trees. This means that Replace should not ever be used on
-- on subexpression nodes. Instead use Rewrite.
-- Note: the front end avoids calls to any of the expand routines if code
-- is not being generated. This is done for three reasons:
-- 1. Make sure tree does not get mucked up by the expander if no
-- code is being generated, and is thus usable by ASIS etc.
-- 2. Save time, since expansion is not needed if a compilation is
-- being done only to check the semantics, or if code generation
-- has been canceled due to previously detected errors.
-- 3. Allow the expand routines to assume that the tree is error free.
-- This results from the fact that code generation mode is always
-- cancelled when any error occurs.
-- If we ever decide to implement a feature allowing object modules to be
-- generated even if errors have been detected, then point 3 will no longer
-- hold, and the expand routines will have to be modified to operate properly
-- in the presence of errors (for many reasons this is not currently true).
-- Note: a consequence of this approach is that error messages must never
-- be generated in the expander, since this would mean that such error
-- messages are not generated when the expander is not being called.
-- Expansion is the last stage of analyzing a node, so Expand sets the
-- Analyzed flag of the node being analyzed as its last action. This is
-- done even if expansion is off (in this case, the only effect of the
-- call to Expand is to set the Analyzed flag to True).
with Types; use Types;
package Expander is
-- The flag Opt.Expander_Active controls whether expansion is active
-- (True) or deactivated (False). When expansion is deactivated all
-- calls to expander routines have no effect. To temporarily disable
-- expansion, always call the routines defined below, do NOT change
-- Expander_Active directly.
--
-- You should not use this flag to test if you are currently processing
-- a generic spec or body. Use the flag Inside_A_Generic instead (see
-- the spec of package Sem).
--
-- There is no good reason for permanently changing the value of this flag
-- except after detecting a syntactic or semantic error. In this event
-- this flag is set to False to disable all subsequent expansion activity.
--
-- In general this flag should be used as a read only value. The only
-- exceptions where it makes sense to temporarily change its value are:
--
-- (a) when starting/completing the processing of a generic definition
-- or declaration (see routines Start_Generic_Processing and
-- End_Generic_Processing in Sem_Ch12)
--
-- (b) when starting/completing the pre-analysis of an expression
-- (see the spec of package Sem for more info on pre-analysis.)
--
-- Note that when processing a default expression (In_Default_Expression
-- is True) or performing semantic analysis of a generic spec or body
-- (Inside_A_Generic) or when performing pre-analysis (Full_Analysis is
-- False) the Expander_Active flag is False.
procedure Expand (N : Node_Id);
-- Expand node N, as described above
procedure Expander_Mode_Save_And_Set (Status : Boolean);
-- Saves the current setting of the Expander_Active flag on an internal
-- stack and then sets the flag to the given value.
procedure Expander_Mode_Restore;
-- Restores the setting of the Expander_Active flag using the top entry
-- pushed onto the stack by Expander_Mode_Save_And_Reset, popping the
-- stack, except that if any errors have been detected, then the state
-- of the flag is left set to False.
end Expander;
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