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-------------------------------------------------------------------------------
--- --
--- GNAT COMPILER COMPONENTS --
--- --
--- E X P A N D E R --
--- --
--- S p e c --
--- --
--- Copyright (C) 1992-2008, 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 3, 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 COPYING3. If not, go to --
--- http://www.gnu.org/licenses for a complete copy of the license. --
--- --
--- 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 Analyzed 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 spec expression (In_Spec_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.
- --
- -- Note: this routine has no effect in ASIS_Mode. In ASIS_Mode, all
- -- expansion activity is always off, since we want the original semantic
- -- tree for ASIS purposes without any expansion. This is achieved by
- -- setting Expander_Active False in ASIS_Mode. In situations such as
- -- the call to Instantiate_Bodies in Frontend, Expander_Mode_Save_And_Set
- -- may be called to temporarily turn the expander on, but this will have
- -- no effect in ASIS mode.
-
- 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. Disabled for ASIS_Mode (see above).
-
-end Expander;