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jlechner |
------------------------------------------------------------------------------
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-- --
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-- GNAT COMPILER COMPONENTS --
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-- --
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-- E X P A N D E R --
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-- --
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-- S p e c --
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-- --
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-- Copyright (C) 1992-2005, Free Software Foundation, Inc. --
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-- --
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-- GNAT is free software; you can redistribute it and/or modify it under --
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-- terms of the GNU General Public License as published by the Free Soft- --
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-- ware Foundation; either version 2, or (at your option) any later ver- --
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-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
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-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
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-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
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-- for more details. You should have received a copy of the GNU General --
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-- Public License distributed with GNAT; see file COPYING. If not, write --
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-- to the Free Software Foundation, 51 Franklin Street, Fifth Floor, --
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-- Boston, MA 02110-1301, USA. --
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-- --
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-- GNAT was originally developed by the GNAT team at New York University. --
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-- Extensive contributions were provided by Ada Core Technologies Inc. --
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-- --
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------------------------------------------------------------------------------
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-- This procedure performs any required expansion for the specified node.
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-- The argument is the node that is a candidate for possible expansion.
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-- If no expansion is required, then Expand returns without doing anything.
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-- If the node does need expansion, then the subtree is replaced by the
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-- tree corresponding to the required rewriting. This tree is a syntactic
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-- tree, except that all Entity fields must be correctly set on all
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-- direct names, since the expander presumably knows what it wants, and in
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-- any case it doesn't work to have the semantic analyzer perform visibility
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-- analysis on these trees (they may have references to non-visible runtime
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-- routines etc.) There are a few exceptions to this rule in special cases,
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-- but they must be documented clearly.
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-- Expand is called in two different situations:
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-- Nodes that are not subexpressions (Nkind not in N_Subexpr)
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-- In this case, Expand is called from the body of Sem, immediately
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-- after completing semantic analysis by calling the corresponding
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-- Analyze_N_xxx procedure. If expansion occurs, the given node must
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-- be replaced with another node that is also not a subexpression.
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-- This seems naturally to be the case, since it is hard to imagine any
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-- situation in which it would make sense to replace a non-expression
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-- subtree with an expression. Once the substitution is completed, the
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-- Expand routine must call Analyze on the resulting node to do any
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-- required semantic analysis. Note that references to children copied
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-- from the old tree won't be reanalyzed, since their Analyzed flag
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-- is set.
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-- Nodes that are subexpressions (Nkind in N_Subexpr)
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-- In this case, Expand is called from Sem_Res.Resolve after completing
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-- the resolution of the subexpression (this means that the expander sees
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-- the fully typed subtree). If expansion occurs, the given node must be
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-- replaced by a node that is also a subexpression. Again it is hard
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-- to see how this restriction could possibly be violated. Once the
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-- substitution is completed, the Expand routine must first call Analyze
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-- on the resulting node to do any required semantic analysis, and then
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-- call Resolve on the node to set the type (typically the type will be
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-- the same as the original type of the input node, but this is not
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-- always the case).
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-- In both these cases, Replace or Rewrite must be used to achieve the
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-- of the node, since the Expander routine is only passed the Node_Id
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-- of the node to be expanded, and the resulting expanded Node_Id must
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-- be the same (the parameter to Expand is mode in, not mode in-out).
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-- For nodes other than subexpressions, it is not necessary to preserve the
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-- original tree in the Expand routines, unlike the case for modifications
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-- to the tree made in the semantic analyzer. This is because anyone who is
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-- interested in working with the original tree (like ASIS) is required to
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-- compile in semantics checks only mode. Thus Replace may be freely used
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-- in such instances.
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-- For subexpressions, preservation of the original tree is required because
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-- of the need for conformance checking of default expressions, which occurs
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-- on expanded trees. This means that Replace should not ever be used on
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-- on subexpression nodes. Instead use Rewrite.
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-- Note: the front end avoids calls to any of the expand routines if code
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-- is not being generated. This is done for three reasons:
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-- 1. Make sure tree does not get mucked up by the expander if no
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-- code is being generated, and is thus usable by ASIS etc.
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-- 2. Save time, since expansion is not needed if a compilation is
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-- being done only to check the semantics, or if code generation
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-- has been canceled due to previously detected errors.
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-- 3. Allow the expand routines to assume that the tree is error free.
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-- This results from the fact that code generation mode is always
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-- cancelled when any error occurs.
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-- If we ever decide to implement a feature allowing object modules to be
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-- generated even if errors have been detected, then point 3 will no longer
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-- hold, and the expand routines will have to be modified to operate properly
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-- in the presence of errors (for many reasons this is not currently true).
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-- Note: a consequence of this approach is that error messages must never
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-- be generated in the expander, since this would mean that such error
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-- messages are not generated when the expander is not being called.
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-- Expansion is the last stage of analyzing a node, so Expand sets the
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-- Analyzed flag of the node being analyzed as its last action. This is
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-- done even if expansion is off (in this case, the only effect of the
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-- call to Expand is to set the Analyzed flag to True).
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with Types; use Types;
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package Expander is
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-- The flag Opt.Expander_Active controls whether expansion is active
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-- (True) or deactivated (False). When expansion is deactivated all
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-- calls to expander routines have no effect. To temporarily disable
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-- expansion, always call the routines defined below, do NOT change
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-- Expander_Active directly.
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--
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-- You should not use this flag to test if you are currently processing
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-- a generic spec or body. Use the flag Inside_A_Generic instead (see
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-- the spec of package Sem).
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--
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-- There is no good reason for permanently changing the value of this flag
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-- except after detecting a syntactic or semantic error. In this event
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-- this flag is set to False to disable all subsequent expansion activity.
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--
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-- In general this flag should be used as a read only value. The only
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-- exceptions where it makes sense to temporarily change its value are:
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--
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-- (a) when starting/completing the processing of a generic definition
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-- or declaration (see routines Start_Generic_Processing and
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-- End_Generic_Processing in Sem_Ch12)
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--
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-- (b) when starting/completing the pre-analysis of an expression
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-- (see the spec of package Sem for more info on pre-analysis.)
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--
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-- Note that when processing a default expression (In_Default_Expression
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-- is True) or performing semantic analysis of a generic spec or body
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-- (Inside_A_Generic) or when performing pre-analysis (Full_Analysis is
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-- False) the Expander_Active flag is False.
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procedure Expand (N : Node_Id);
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-- Expand node N, as described above
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procedure Expander_Mode_Save_And_Set (Status : Boolean);
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-- Saves the current setting of the Expander_Active flag on an internal
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-- stack and then sets the flag to the given value.
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--
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-- Note: this routine has no effect in ASIS_Mode. In ASIS_Mode, all
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-- expansion activity is always off, since we want the original semantic
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-- tree for ASIS purposes without any expansion. This is achieved by
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-- setting Expander_Active False in ASIS_Mode. In situations such as
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-- the call to Instantiate_Bodies in Frontend, Expander_Mode_Save_And_Set
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-- may be called to temporarily turn the expander on, but this will have
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-- no effect in ASIS mode.
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procedure Expander_Mode_Restore;
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-- Restores the setting of the Expander_Active flag using the top entry
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-- pushed onto the stack by Expander_Mode_Save_And_Reset, popping the
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-- stack, except that if any errors have been detected, then the state
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-- of the flag is left set to False. Disabled for ASIS_Mode (see above).
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end Expander;
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