File : 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) 1992-2013, 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 3, 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 COPYING3. If not, go to --
19 -- http://www.gnu.org/licenses for a complete copy of the license. --
20 -- --
21 -- GNAT was originally developed by the GNAT team at New York University. --
22 -- Extensive contributions were provided by Ada Core Technologies Inc. --
23 -- --
24 ------------------------------------------------------------------------------
25
26 -- This procedure performs any required expansion for the specified node.
27 -- The argument is the node that is a candidate for possible expansion.
28 -- If no expansion is required, then Expand returns without doing anything.
29
30 -- If the node does need expansion, then the subtree is replaced by the
31 -- tree corresponding to the required rewriting. This tree is a syntactic
32 -- tree, except that all Entity fields must be correctly set on all
33 -- direct names, since the expander presumably knows what it wants, and in
34 -- any case it doesn't work to have the semantic analyzer perform visibility
35 -- analysis on these trees (they may have references to non-visible runtime
36 -- routines etc.) There are a few exceptions to this rule in special cases,
37 -- but they must be documented clearly.
38
39 -- Expand is called in two different situations:
40
41 -- Nodes that are not subexpressions (Nkind not in N_Subexpr)
42
43 -- In this case, Expand is called from the body of Sem, immediately
44 -- after completing semantic analysis by calling the corresponding
45 -- Analyze_N_xxx procedure. If expansion occurs, the given node must
46 -- be replaced with another node that is also not a subexpression.
47 -- This seems naturally to be the case, since it is hard to imagine any
48 -- situation in which it would make sense to replace a non-expression
49 -- subtree with an expression. Once the substitution is completed, the
50 -- Expand routine must call Analyze on the resulting node to do any
51 -- required semantic analysis. Note that references to children copied
52 -- from the old tree won't be reanalyzed, since their Analyzed flag
53 -- is set.
54
55 -- Nodes that are subexpressions (Nkind in N_Subexpr)
56
57 -- In this case, Expand is called from Sem_Res.Resolve after completing
58 -- the resolution of the subexpression (this means that the expander sees
59 -- the fully typed subtree). If expansion occurs, the given node must be
60 -- replaced by a node that is also a subexpression. Again it is hard
61 -- to see how this restriction could possibly be violated. Once the
62 -- substitution is completed, the Expand routine must first call Analyze
63 -- on the resulting node to do any required semantic analysis, and then
64 -- call Resolve on the node to set the type (typically the type will be
65 -- the same as the original type of the input node, but this is not
66 -- always the case).
67
68 -- In both these cases, Replace or Rewrite must be used to achieve the
69 -- of the node, since the Expander routine is only passed the Node_Id
70 -- of the node to be expanded, and the resulting expanded Node_Id must
71 -- be the same (the parameter to Expand is mode in, not mode in-out).
72
73 -- For nodes other than subexpressions, it is not necessary to preserve the
74 -- original tree in the Expand routines, unlike the case for modifications
75 -- to the tree made in the semantic analyzer. This is because anyone who is
76 -- interested in working with the original tree (like ASIS) is required to
77 -- compile in semantics checks only mode. Thus Replace may be freely used
78 -- in such instances.
79
80 -- For subexpressions, preservation of the original tree is required because
81 -- of the need for conformance checking of default expressions, which occurs
82 -- on expanded trees. This means that Replace should not ever be used on
83 -- on subexpression nodes. Instead use Rewrite.
84
85 -- Note: the front end avoids calls to any of the expand routines if code
86 -- is not being generated. This is done for three reasons:
87
88 -- 1. Make sure tree does not get mucked up by the expander if no
89 -- code is being generated, and is thus usable by ASIS etc.
90
91 -- 2. Save time, since expansion is not needed if a compilation is
92 -- being done only to check the semantics, or if code generation
93 -- has been canceled due to previously detected errors.
94
95 -- 3. Allow the expand routines to assume that the tree is error free.
96 -- This results from the fact that code generation mode is always
97 -- cancelled when any error occurs.
98
99 -- If we ever decide to implement a feature allowing object modules to be
100 -- generated even if errors have been detected, then point 3 will no longer
101 -- hold, and the expand routines will have to be modified to operate properly
102 -- in the presence of errors (for many reasons this is not currently true).
103
104 -- Note: a consequence of this approach is that error messages must never
105 -- be generated in the expander, since this would mean that such error
106 -- messages are not generated when the expander is not being called.
107
108 -- Expansion is the last stage of analyzing a node, so Expand sets the
109 -- Analyzed flag of the node being analyzed as its last action. This is
110 -- done even if expansion is off (in this case, the only effect of the
111 -- call to Expand is to set the Analyzed flag to True).
112
113 with Types; use Types;
114
115 package Expander is
116
117 -- The flag Opt.Expander_Active controls whether expansion is active
118 -- (True) or deactivated (False). When expansion is deactivated all
119 -- calls to expander routines have no effect. To temporarily disable
120 -- expansion, always call the routines defined below, do NOT change
121 -- Expander_Active directly.
122 --
123 -- You should not use this flag to test if you are currently processing
124 -- a generic spec or body. Use the flag Inside_A_Generic instead (see
125 -- the spec of package Sem).
126 --
127 -- There is no good reason for permanently changing the value of this flag
128 -- except after detecting a syntactic or semantic error. In this event
129 -- this flag is set to False to disable all subsequent expansion activity.
130 --
131 -- In general this flag should be used as a read only value. The only
132 -- exceptions where it makes sense to temporarily change its value are:
133 --
134 -- (a) when starting/completing the processing of a generic definition
135 -- or declaration (see routines Start_Generic_Processing and
136 -- End_Generic_Processing in Sem_Ch12)
137 --
138 -- (b) when starting/completing the pre-analysis of an expression
139 -- (see the spec of package Sem for more info on pre-analysis.)
140 --
141 -- Note that when processing a spec expression (In_Spec_Expression
142 -- is True) or performing semantic analysis of a generic spec or body
143 -- (Inside_A_Generic) or when performing pre-analysis (Full_Analysis is
144 -- False) the Expander_Active flag is False.
145
146 procedure Expand (N : Node_Id);
147 -- Expand node N, as described above
148
149 procedure Expander_Mode_Save_And_Set (Status : Boolean);
150 -- Saves the current setting of the Expander_Active flag on an internal
151 -- stack and then sets the flag to the given value.
152 --
153 -- Note: this routine has no effect in ASIS and GNATprove modes. In ASIS
154 -- mode, all expansion activity is always off, since we want the original
155 -- semantic tree for ASIS purposes without any expansion. In GNATprove
156 -- mode, a very light expansion is performed on specific nodes. Both are
157 -- achieved by setting Expander_Active False in ASIS and GNATprove modes.
158 -- In situations such as the call to Instantiate_Bodies in Frontend,
159 -- Expander_Mode_Save_And_Set may be called to temporarily turn the
160 -- expander on, but this will have no effect in ASIS and GNATprove modes.
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 of
166 -- the flag is left set to False. Disabled for ASIS and GNATprove modes
167 -- (see above).
168
169 end Expander;