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1 706 jeremybenn
------------------------------------------------------------------------------
2
--                                                                          --
3
--                         GNAT COMPILER COMPONENTS                         --
4
--                                                                          --
5
--                               C H E C K S                                --
6
--                                                                          --
7
--                                 S p e c                                  --
8
--                                                                          --
9
--          Copyright (C) 1992-2011, 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
--  Package containing routines used to deal with runtime checks. These
27
--  routines are used both by the semantics and by the expander. In some
28
--  cases, checks are enabled simply by setting flags for gigi, and in
29
--  other cases the code for the check is expanded.
30
 
31
--  The approach used for range and length checks, in regards to suppressed
32
--  checks, is to attempt to detect at compilation time that a constraint
33
--  error will occur. If this is detected a warning or error is issued and the
34
--  offending expression or statement replaced with a constraint error node.
35
--  This always occurs whether checks are suppressed or not. Dynamic range
36
--  checks are, of course, not inserted if checks are suppressed.
37
 
38
with Namet;  use Namet;
39
with Table;
40
with Types;  use Types;
41
with Uintp;  use Uintp;
42
 
43
package Checks is
44
 
45
   procedure Initialize;
46
   --  Called for each new main source program, to initialize internal
47
   --  variables used in the package body of the Checks unit.
48
 
49
   function Access_Checks_Suppressed          (E : Entity_Id) return Boolean;
50
   function Accessibility_Checks_Suppressed   (E : Entity_Id) return Boolean;
51
   function Alignment_Checks_Suppressed       (E : Entity_Id) return Boolean;
52
   function Atomic_Synchronization_Disabled   (E : Entity_Id) return Boolean;
53
   function Discriminant_Checks_Suppressed    (E : Entity_Id) return Boolean;
54
   function Division_Checks_Suppressed        (E : Entity_Id) return Boolean;
55
   function Elaboration_Checks_Suppressed     (E : Entity_Id) return Boolean;
56
   function Index_Checks_Suppressed           (E : Entity_Id) return Boolean;
57
   function Length_Checks_Suppressed          (E : Entity_Id) return Boolean;
58
   function Overflow_Checks_Suppressed        (E : Entity_Id) return Boolean;
59
   function Range_Checks_Suppressed           (E : Entity_Id) return Boolean;
60
   function Storage_Checks_Suppressed         (E : Entity_Id) return Boolean;
61
   function Tag_Checks_Suppressed             (E : Entity_Id) return Boolean;
62
   function Validity_Checks_Suppressed        (E : Entity_Id) return Boolean;
63
   --  These functions check to see if the named check is suppressed, either
64
   --  by an active scope suppress setting, or because the check has been
65
   --  specifically suppressed for the given entity. If no entity is relevant
66
   --  for the current check, then Empty is used as an argument. Note: the
67
   --  reason we insist on specifying Empty is to force the caller to think
68
   --  about whether there is any relevant entity that should be checked.
69
 
70
   -------------------------------------------
71
   -- Procedures to Activate Checking Flags --
72
   -------------------------------------------
73
 
74
   procedure Activate_Division_Check (N : Node_Id);
75
   pragma Inline (Activate_Division_Check);
76
   --  Sets Do_Division_Check flag in node N, and handles possible local raise.
77
   --  Always call this routine rather than calling Set_Do_Division_Check to
78
   --  set an explicit value of True, to ensure handling the local raise case.
79
 
80
   procedure Activate_Overflow_Check (N : Node_Id);
81
   pragma Inline (Activate_Overflow_Check);
82
   --  Sets Do_Overflow_Check flag in node N, and handles possible local raise.
83
   --  Always call this routine rather than calling Set_Do_Overflow_Check to
84
   --  set an explicit value of True, to ensure handling the local raise case.
85
 
86
   procedure Activate_Range_Check (N : Node_Id);
87
   pragma Inline (Activate_Range_Check);
88
   --  Sets Do_Range_Check flag in node N, and handles possible local raise
89
   --  Always call this routine rather than calling Set_Do_Range_Check to
90
   --  set an explicit value of True, to ensure handling the local raise case.
91
 
92
   --------------------------------
93
   -- Procedures to Apply Checks --
94
   --------------------------------
95
 
96
   --  General note on following checks. These checks are always active if
97
   --  Expander_Active and not Inside_A_Generic. They are inactive and have
98
   --  no effect Inside_A_Generic. In the case where not Expander_Active
99
   --  and not Inside_A_Generic, most of them are inactive, but some of them
100
   --  operate anyway since they may generate useful compile time warnings.
101
 
102
   procedure Apply_Access_Check (N : Node_Id);
103
   --  Determines whether an expression node requires a runtime access
104
   --  check and if so inserts the appropriate run-time check.
105
 
106
   procedure Apply_Accessibility_Check
107
     (N           : Node_Id;
108
      Typ         : Entity_Id;
109
      Insert_Node : Node_Id);
110
   --  Given a name N denoting an access parameter, emits a run-time
111
   --  accessibility check (if necessary), checking that the level of
112
   --  the object denoted by the access parameter is not deeper than the
113
   --  level of the type Typ. Program_Error is raised if the check fails.
114
   --  Insert_Node indicates the node where the check should be inserted.
115
 
116
   procedure Apply_Address_Clause_Check (E : Entity_Id; N : Node_Id);
117
   --  E is the entity for an object which has an address clause. If checks
118
   --  are enabled, then this procedure generates a check that the specified
119
   --  address has an alignment consistent with the alignment of the object,
120
   --  raising PE if this is not the case. The resulting check (if one is
121
   --  generated) is inserted before node N. check is also made for the case of
122
   --  a clear overlay situation that the size of the overlaying object is not
123
   --  larger than the overlaid object.
124
 
125
   procedure Apply_Arithmetic_Overflow_Check (N : Node_Id);
126
   --  Given a binary arithmetic operator (+ - *) expand a software integer
127
   --  overflow check using range checks on a larger checking type or a call
128
   --  to an appropriate runtime routine. This is used for all three operators
129
   --  for the signed integer case, and for +/- in the fixed-point case. The
130
   --  check is expanded only if Software_Overflow_Checking is enabled and
131
   --  Do_Overflow_Check is set on node N. Note that divide is handled
132
   --  separately using Apply_Arithmetic_Divide_Overflow_Check.
133
 
134
   procedure Apply_Constraint_Check
135
     (N          : Node_Id;
136
      Typ        : Entity_Id;
137
      No_Sliding : Boolean := False);
138
   --  Top-level procedure, calls all the others depending on the class of
139
   --  Typ. Checks that expression N satisfies the constraint of type Typ.
140
   --  No_Sliding is only relevant for constrained array types, if set to
141
   --  True, it checks that indexes are in range.
142
 
143
   procedure Apply_Discriminant_Check
144
     (N   : Node_Id;
145
      Typ : Entity_Id;
146
      Lhs : Node_Id := Empty);
147
   --  Given an expression N of a discriminated type, or of an access type
148
   --  whose designated type is a discriminanted type, generates a check to
149
   --  ensure that the expression can be converted to the subtype given as
150
   --  the second parameter. Lhs is empty except in the case of assignments,
151
   --  where the target object may be needed to determine the subtype to
152
   --  check against (such as the cases of unconstrained formal parameters
153
   --  and unconstrained aliased objects). For the case of unconstrained
154
   --  formals, the check is performed only if the corresponding actual is
155
   --  constrained, i.e., whether Lhs'Constrained is True.
156
 
157
   procedure Apply_Predicate_Check (N : Node_Id; Typ : Entity_Id);
158
   --  N is an expression to which a predicate check may need to be applied
159
   --  for Typ, if Typ has a predicate function. The check is applied only
160
   --  if the type of N does not match Typ.
161
 
162
   function Build_Discriminant_Checks
163
     (N     : Node_Id;
164
      T_Typ : Entity_Id)
165
      return  Node_Id;
166
   --  Subsidiary routine for Apply_Discriminant_Check. Builds the expression
167
   --  that compares discriminants of the expression with discriminants of the
168
   --  type. Also used directly for membership tests (see Exp_Ch4.Expand_N_In).
169
 
170
   procedure Apply_Divide_Check (N : Node_Id);
171
   --  The node kind is N_Op_Divide, N_Op_Mod, or N_Op_Rem. An appropriate
172
   --  check is generated to ensure that the right operand is non-zero. In
173
   --  the divide case, we also check that we do not have the annoying case
174
   --  of the largest negative number divided by minus one.
175
 
176
   procedure Apply_Type_Conversion_Checks (N : Node_Id);
177
   --  N is an N_Type_Conversion node. A type conversion actually involves
178
   --  two sorts of checks. The first check is the checks that ensures that
179
   --  the operand in the type conversion fits onto the base type of the
180
   --  subtype it is being converted to (see RM 4.6 (28)-(50)). The second
181
   --  check is there to ensure that once the operand has been converted to
182
   --  a value of the target type, this converted value meets the
183
   --  constraints imposed by the target subtype (see RM 4.6 (51)).
184
 
185
   procedure Apply_Universal_Integer_Attribute_Checks (N : Node_Id);
186
   --  The argument N is an attribute reference node intended for processing
187
   --  by gigi. The attribute is one that returns a universal integer, but
188
   --  the attribute reference node is currently typed with the expected
189
   --  result type. This routine deals with range and overflow checks needed
190
   --  to make sure that the universal result is in range.
191
 
192
   procedure Determine_Range
193
     (N            : Node_Id;
194
      OK           : out Boolean;
195
      Lo           : out Uint;
196
      Hi           : out Uint;
197
      Assume_Valid : Boolean := False);
198
   --  N is a node for a subexpression. If N is of a discrete type with no
199
   --  error indications, and no other peculiarities (e.g. missing type
200
   --  fields), then OK is True on return, and Lo and Hi are set to a
201
   --  conservative estimate of the possible range of values of N. Thus if OK
202
   --  is True on return, the value of the subexpression N is known to like in
203
   --  the range Lo .. Hi (inclusive). If the expression is not of a discrete
204
   --  type, or some kind of error condition is detected, then OK is False on
205
   --  exit, and Lo/Hi are set to No_Uint. Thus the significance of OK being
206
   --  False on return is that no useful information is available on the range
207
   --  of the expression. Assume_Valid determines whether the processing is
208
   --  allowed to assume that values are in range of their subtypes. If it is
209
   --  set to True, then this assumption is valid, if False, then processing
210
   --  is done using base types to allow invalid values.
211
 
212
   procedure Install_Null_Excluding_Check (N : Node_Id);
213
   --  Determines whether an access node requires a runtime access check and
214
   --  if so inserts the appropriate run-time check.
215
 
216
   -------------------------------------------------------
217
   -- Control and Optimization of Range/Overflow Checks --
218
   -------------------------------------------------------
219
 
220
   --  Range checks are controlled by the Do_Range_Check flag. The front end
221
   --  is responsible for setting this flag in relevant nodes. Originally
222
   --  the back end generated all corresponding range checks. But later on
223
   --  we decided to generate many range checks in the front end. We are now
224
   --  in the transitional phase where some of these checks are still done
225
   --  by the back end, but many are done by the front end. It is possible
226
   --  that in the future we might move all the checks to the front end. The
227
   --  main remaining back end checks are for subscript checking.
228
 
229
   --  Overflow checks are similarly controlled by the Do_Overflow_Check flag.
230
   --  The difference here is that if back end overflow checks are inactive
231
   --  (Backend_Overflow_Checks_On_Target set False), then the actual overflow
232
   --  checks are generated by the front end, but if back end overflow checks
233
   --  are active (Backend_Overflow_Checks_On_Target set True), then the back
234
   --  end does generate the checks.
235
 
236
   --  The following two routines are used to set these flags, they allow
237
   --  for the possibility of eliminating checks. Checks can be eliminated
238
   --  if an identical check has already been performed.
239
 
240
   procedure Enable_Overflow_Check (N : Node_Id);
241
   --  First this routine determines if an overflow check is needed by doing
242
   --  an appropriate range check. If a check is not needed, then the call
243
   --  has no effect. If a check is needed then this routine sets the flag
244
   --  Do_Overflow_Check in node N to True, unless it can be determined that
245
   --  the check is not needed. The only condition under which this is the
246
   --  case is if there was an identical check earlier on.
247
 
248
   procedure Enable_Range_Check (N : Node_Id);
249
   --  Set Do_Range_Check flag in node N True, unless it can be determined
250
   --  that the check is not needed. The only condition under which this is
251
   --  the case is if there was an identical check earlier on. This routine
252
   --  is not responsible for doing range analysis to determine whether or
253
   --  not such a check is needed -- the caller is expected to do this. The
254
   --  one other case in which the request to set the flag is ignored is
255
   --  when Kill_Range_Check is set in an N_Unchecked_Conversion node.
256
 
257
   --  The following routines are used to keep track of processing sequences
258
   --  of statements (e.g. the THEN statements of an IF statement). A check
259
   --  that appears within such a sequence can eliminate an identical check
260
   --  within this sequence of statements. However, after the end of the
261
   --  sequence of statements, such a check is no longer of interest, since
262
   --  it may not have been executed.
263
 
264
   procedure Conditional_Statements_Begin;
265
   --  This call marks the start of processing of a sequence of statements.
266
   --  Every call to this procedure must be followed by a matching call to
267
   --  Conditional_Statements_End.
268
 
269
   procedure Conditional_Statements_End;
270
   --  This call removes from consideration all saved checks since the
271
   --  corresponding call to Conditional_Statements_Begin. These two
272
   --  procedures operate in a stack like manner.
273
 
274
   --  The mechanism for optimizing checks works by remembering checks
275
   --  that have already been made, but certain conditions, for example
276
   --  an assignment to a variable involved in a check, may mean that the
277
   --  remembered check is no longer valid, in the sense that if the same
278
   --  expression appears again, another check is required because the
279
   --  value may have changed.
280
 
281
   --  The following routines are used to note conditions which may render
282
   --  some or all of the stored and remembered checks to be invalidated.
283
 
284
   procedure Kill_Checks (V : Entity_Id);
285
   --  This procedure records an assignment or other condition that causes
286
   --  the value of the variable to be changed, invalidating any stored
287
   --  checks that reference the value. Note that all such checks must
288
   --  be discarded, even if they are not in the current statement range.
289
 
290
   procedure Kill_All_Checks;
291
   --  This procedure kills all remembered checks
292
 
293
   -----------------------------
294
   -- Length and Range Checks --
295
   -----------------------------
296
 
297
   --  In the following procedures, there are three arguments which have
298
   --  a common meaning as follows:
299
 
300
   --    Expr        The expression to be checked. If a check is required,
301
   --                the appropriate flag will be placed on this node. Whether
302
   --                this node is further examined depends on the setting of
303
   --                the parameter Source_Typ, as described below.
304
 
305
   --    ??? Apply_Length_Check and Apply_Range_Check do not have an Expr
306
   --        formal
307
 
308
   --    ??? Apply_Length_Check and Apply_Range_Check have a Ck_Node formal
309
   --        which is undocumented, is it the same as Expr?
310
 
311
   --    Target_Typ  The target type on which the check is to be based. For
312
   --                example, if we have a scalar range check, then the check
313
   --                is that we are in range of this type.
314
 
315
   --    Source_Typ  Normally Empty, but can be set to a type, in which case
316
   --                this type is used for the check, see below.
317
 
318
   --  The checks operate in one of two modes:
319
 
320
   --    If Source_Typ is Empty, then the node Expr is examined, at the very
321
   --    least to get the source subtype. In addition for some of the checks,
322
   --    the actual form of the node may be examined. For example, a node of
323
   --    type Integer whose actual form is an Integer conversion from a type
324
   --    with range 0 .. 3 can be determined to have a value in range 0 .. 3.
325
 
326
   --    If Source_Typ is given, then nothing can be assumed about the Expr,
327
   --    and indeed its contents are not examined. In this case the check is
328
   --    based on the assumption that Expr can be an arbitrary value of the
329
   --    given Source_Typ.
330
 
331
   --  Currently, the only case in which a Source_Typ is explicitly supplied
332
   --  is for the case of Out and In_Out parameters, where, for the conversion
333
   --  on return (the Out direction), the types must be reversed. This is
334
   --  handled by the caller.
335
 
336
   procedure Apply_Length_Check
337
     (Ck_Node    : Node_Id;
338
      Target_Typ : Entity_Id;
339
      Source_Typ : Entity_Id := Empty);
340
   --  This procedure builds a sequence of declarations to do a length check
341
   --  that checks if the lengths of the two arrays Target_Typ and source type
342
   --  are the same. The resulting actions are inserted at Node using a call
343
   --  to Insert_Actions.
344
   --
345
   --  For access types, the Directly_Designated_Type is retrieved and
346
   --  processing continues as enumerated above, with a guard against null
347
   --  values.
348
   --
349
   --  Note: calls to Apply_Length_Check currently never supply an explicit
350
   --  Source_Typ parameter, but Apply_Length_Check takes this parameter and
351
   --  processes it as described above for consistency with the other routines
352
   --  in this section.
353
 
354
   procedure Apply_Range_Check
355
     (Ck_Node    : Node_Id;
356
      Target_Typ : Entity_Id;
357
      Source_Typ : Entity_Id := Empty);
358
   --  For a Node of kind N_Range, constructs a range check action that tests
359
   --  first that the range is not null and then that the range is contained in
360
   --  the Target_Typ range.
361
   --
362
   --  For scalar types, constructs a range check action that first tests that
363
   --  the expression is contained in the Target_Typ range. The difference
364
   --  between this and Apply_Scalar_Range_Check is that the latter generates
365
   --  the actual checking code in gigi against the Etype of the expression.
366
   --
367
   --  For constrained array types, construct series of range check actions
368
   --  to check that each Expr range is properly contained in the range of
369
   --  Target_Typ.
370
   --
371
   --  For a type conversion to an unconstrained array type, constructs a range
372
   --  check action to check that the bounds of the source type are within the
373
   --  constraints imposed by the Target_Typ.
374
   --
375
   --  For access types, the Directly_Designated_Type is retrieved and
376
   --  processing continues as enumerated above, with a guard against null
377
   --  values.
378
   --
379
   --  The source type is used by type conversions to unconstrained array
380
   --  types to retrieve the corresponding bounds.
381
 
382
   procedure Apply_Static_Length_Check
383
     (Expr       : Node_Id;
384
      Target_Typ : Entity_Id;
385
      Source_Typ : Entity_Id := Empty);
386
   --  Tries to determine statically whether the two array types source type
387
   --  and Target_Typ have the same length. If it can be determined at compile
388
   --  time that they do not, then an N_Raise_Constraint_Error node replaces
389
   --  Expr, and a warning message is issued.
390
 
391
   procedure Apply_Scalar_Range_Check
392
     (Expr       : Node_Id;
393
      Target_Typ : Entity_Id;
394
      Source_Typ : Entity_Id := Empty;
395
      Fixed_Int  : Boolean   := False);
396
   --  For scalar types, determines whether an expression node should be
397
   --  flagged as needing a runtime range check. If the node requires such a
398
   --  check, the Do_Range_Check flag is turned on. The Fixed_Int flag if set
399
   --  causes any fixed-point values to be treated as though they were discrete
400
   --  values (i.e. the underlying integer value is used).
401
 
402
   type Check_Result is private;
403
   --  Type used to return result of Get_Range_Checks call, for later use in
404
   --  call to Insert_Range_Checks procedure.
405
 
406
   function Get_Range_Checks
407
     (Ck_Node    : Node_Id;
408
      Target_Typ : Entity_Id;
409
      Source_Typ : Entity_Id := Empty;
410
      Warn_Node  : Node_Id   := Empty) return Check_Result;
411
   --  Like Apply_Range_Check, except it does not modify anything. Instead
412
   --  it returns an encapsulated result of the check operations for later
413
   --  use in a call to Insert_Range_Checks. If Warn_Node is non-empty, its
414
   --  Sloc is used, in the static case, for the generated warning or error.
415
   --  Additionally, it is used rather than Expr (or Low/High_Bound of Expr)
416
   --  in constructing the check.
417
 
418
   procedure Append_Range_Checks
419
     (Checks       : Check_Result;
420
      Stmts        : List_Id;
421
      Suppress_Typ : Entity_Id;
422
      Static_Sloc  : Source_Ptr;
423
      Flag_Node    : Node_Id);
424
   --  Called to append range checks as returned by a call to Get_Range_Checks.
425
   --  Stmts is a list to which either the dynamic check is appended or the
426
   --  raise Constraint_Error statement is appended (for static checks).
427
   --  Static_Sloc is the Sloc at which the raise CE node points, Flag_Node is
428
   --  used as the node at which to set the Has_Dynamic_Check flag. Checks_On
429
   --  is a boolean value that says if range and index checking is on or not.
430
 
431
   procedure Insert_Range_Checks
432
     (Checks       : Check_Result;
433
      Node         : Node_Id;
434
      Suppress_Typ : Entity_Id;
435
      Static_Sloc  : Source_Ptr := No_Location;
436
      Flag_Node    : Node_Id    := Empty;
437
      Do_Before    : Boolean    := False);
438
   --  Called to insert range checks as returned by a call to Get_Range_Checks.
439
   --  Node is the node after which either the dynamic check is inserted or
440
   --  the raise Constraint_Error statement is inserted (for static checks).
441
   --  Suppress_Typ is the type to check to determine if checks are suppressed.
442
   --  Static_Sloc, if passed, is the Sloc at which the raise CE node points,
443
   --  otherwise Sloc (Node) is used. The Has_Dynamic_Check flag is normally
444
   --  set at Node. If Flag_Node is present, then this is used instead as the
445
   --  node at which to set the Has_Dynamic_Check flag. Normally the check is
446
   --  inserted after, if Do_Before is True, the check is inserted before
447
   --  Node.
448
 
449
   -----------------------
450
   -- Expander Routines --
451
   -----------------------
452
 
453
   --  Some of the earlier processing for checks results in temporarily setting
454
   --  the Do_Range_Check flag rather than actually generating checks. Now we
455
   --  are moving the generation of such checks into the front end for reasons
456
   --  of efficiency and simplicity (there were difficulties in handling this
457
   --  in the back end when side effects were present in the expressions being
458
   --  checked).
459
 
460
   --  Probably we could eliminate the Do_Range_Check flag entirely and
461
   --  generate the checks earlier, but this is a delicate area and it
462
   --  seemed safer to implement the following routines, which are called
463
   --  late on in the expansion process. They check the Do_Range_Check flag
464
   --  and if it is set, generate the actual checks and reset the flag.
465
 
466
   procedure Generate_Range_Check
467
     (N           : Node_Id;
468
      Target_Type : Entity_Id;
469
      Reason      : RT_Exception_Code);
470
   --  This procedure is called to actually generate and insert a range check.
471
   --  A check is generated to ensure that the value of N lies within the range
472
   --  of the target type. Note that the base type of N may be different from
473
   --  the base type of the target type. This happens in the conversion case.
474
   --  The Reason parameter is the exception code to be used for the exception
475
   --  if raised.
476
   --
477
   --  Note on the relation of this routine to the Do_Range_Check flag. Mostly
478
   --  for historical reasons, we often set the Do_Range_Check flag and then
479
   --  later we call Generate_Range_Check if this flag is set. Most probably we
480
   --  could eliminate this intermediate setting of the flag (historically the
481
   --  back end dealt with range checks, using this flag to indicate if a check
482
   --  was required, then we moved checks into the front end).
483
 
484
   procedure Generate_Index_Checks (N : Node_Id);
485
   --  This procedure is called to generate index checks on the subscripts for
486
   --  the indexed component node N. Each subscript expression is examined, and
487
   --  if the Do_Range_Check flag is set, an appropriate index check is
488
   --  generated and the flag is reset.
489
 
490
   --  Similarly, we set the flag Do_Discriminant_Check in the semantic
491
   --  analysis to indicate that a discriminant check is required for selected
492
   --  component of a discriminated type. The following routine is called from
493
   --  the expander to actually generate the call.
494
 
495
   procedure Generate_Discriminant_Check (N : Node_Id);
496
   --  N is a selected component for which a discriminant check is required to
497
   --  make sure that the discriminants have appropriate values for the
498
   --  selection. This is done by calling the appropriate discriminant checking
499
   --  routine for the selector.
500
 
501
   -----------------------
502
   -- Validity Checking --
503
   -----------------------
504
 
505
   --  In (RM 13.9.1(9-11)) we have the following rules on invalid values
506
 
507
   --    If the representation of a scalar object does not represent value of
508
   --    the object's subtype (perhaps because the object was not initialized),
509
   --    the object is said to have an invalid representation. It is a bounded
510
   --    error to evaluate the value of such an object. If the error is
511
   --    detected, either Constraint_Error or Program_Error is raised.
512
   --    Otherwise, execution continues using the invalid representation. The
513
   --    rules of the language outside this subclause assume that all objects
514
   --    have valid representations. The semantics of operations on invalid
515
   --    representations are as follows:
516
   --
517
   --       10  If the representation of the object represents a value of the
518
   --           object's type, the value of the type is used.
519
   --
520
   --       11  If the representation of the object does not represent a value
521
   --           of the object's type, the semantics of operations on such
522
   --           representations is implementation-defined, but does not by
523
   --           itself lead to erroneous or unpredictable execution, or to
524
   --           other objects becoming abnormal.
525
 
526
   --  We quote the rules in full here since they are quite delicate. Most
527
   --  of the time, we can just compute away with wrong values, and get a
528
   --  possibly wrong result, which is well within the range of allowed
529
   --  implementation defined behavior. The two tricky cases are subscripted
530
   --  array assignments, where we don't want to do wild stores, and case
531
   --  statements where we don't want to do wild jumps.
532
 
533
   --  In GNAT, we control validity checking with a switch -gnatV that can take
534
   --  three parameters, n/d/f for None/Default/Full. These modes have the
535
   --  following meanings:
536
 
537
   --    None (no validity checking)
538
 
539
   --      In this mode, there is no specific checking for invalid values
540
   --      and the code generator assumes that all stored values are always
541
   --      within the bounds of the object subtype. The consequences are as
542
   --      follows:
543
 
544
   --        For case statements, an out of range invalid value will cause
545
   --        Constraint_Error to be raised, or an arbitrary one of the case
546
   --        alternatives will be executed. Wild jumps cannot result even
547
   --        in this mode, since we always do a range check
548
 
549
   --        For subscripted array assignments, wild stores will result in
550
   --        the expected manner when addresses are calculated using values
551
   --        of subscripts that are out of range.
552
 
553
   --      It could perhaps be argued that this mode is still conformant with
554
   --      the letter of the RM, since implementation defined is a rather
555
   --      broad category, but certainly it is not in the spirit of the
556
   --      RM requirement, since wild stores certainly seem to be a case of
557
   --      erroneous behavior.
558
 
559
   --    Default (default standard RM-compatible validity checking)
560
 
561
   --      In this mode, which is the default, minimal validity checking is
562
   --      performed to ensure no erroneous behavior as follows:
563
 
564
   --        For case statements, an out of range invalid value will cause
565
   --        Constraint_Error to be raised.
566
 
567
   --        For subscripted array assignments, invalid out of range
568
   --        subscript values will cause Constraint_Error to be raised.
569
 
570
   --    Full (Full validity checking)
571
 
572
   --      In this mode, the protections guaranteed by the standard mode are
573
   --      in place, and the following additional checks are made:
574
 
575
   --        For every assignment, the right side is checked for validity
576
 
577
   --        For every call, IN and IN OUT parameters are checked for validity
578
 
579
   --        For every subscripted array reference, both for stores and loads,
580
   --        all subscripts are checked for validity.
581
 
582
   --      These checks are not required by the RM, but will in practice
583
   --      improve the detection of uninitialized variables, particularly
584
   --      if used in conjunction with pragma Normalize_Scalars.
585
 
586
   --  In the above description, we talk about performing validity checks,
587
   --  but we don't actually generate a check in a case where the compiler
588
   --  can be sure that the value is valid. Note that this assurance must
589
   --  be achieved without assuming that any uninitialized value lies within
590
   --  the range of its type. The following are cases in which values are
591
   --  known to be valid. The flag Is_Known_Valid is used to keep track of
592
   --  some of these cases.
593
 
594
   --    If all possible stored values are valid, then any uninitialized
595
   --    value must be valid.
596
 
597
   --    Literals, including enumeration literals, are clearly always valid
598
 
599
   --    Constants are always assumed valid, with a validity check being
600
   --    performed on the initializing value where necessary to ensure that
601
   --    this is the case.
602
 
603
   --    For variables, the status is set to known valid if there is an
604
   --    initializing expression. Again a check is made on the initializing
605
   --    value if necessary to ensure that this assumption is valid. The
606
   --    status can change as a result of local assignments to a variable.
607
   --    If a known valid value is unconditionally assigned, then we mark
608
   --    the left side as known valid. If a value is assigned that is not
609
   --    known to be valid, then we mark the left side as invalid. This
610
   --    kind of processing does NOT apply to non-local variables since we
611
   --    are not following the flow graph (more properly the flow of actual
612
   --    processing only corresponds to the flow graph for local assignments).
613
   --    For non-local variables, we preserve the current setting, i.e. a
614
   --    validity check is performed when assigning to a knonwn valid global.
615
 
616
   --  Note: no validity checking is required if range checks are suppressed
617
   --  regardless of the setting of the validity checking mode.
618
 
619
   --  The following procedures are used in handling validity checking
620
 
621
   procedure Apply_Subscript_Validity_Checks (Expr : Node_Id);
622
   --  Expr is the node for an indexed component. If validity checking and
623
   --  range checking are enabled, all subscripts for this indexed component
624
   --  are checked for validity.
625
 
626
   procedure Check_Valid_Lvalue_Subscripts (Expr : Node_Id);
627
   --  Expr is a lvalue, i.e. an expression representing the target of an
628
   --  assignment. This procedure checks for this expression involving an
629
   --  assignment to an array value. We have to be sure that all the subscripts
630
   --  in such a case are valid, since according to the rules in (RM
631
   --  13.9.1(9-11)) such assignments are not permitted to result in erroneous
632
   --  behavior in the case of invalid subscript values.
633
 
634
   procedure Ensure_Valid (Expr : Node_Id; Holes_OK : Boolean := False);
635
   --  Ensure that Expr represents a valid value of its type. If this type
636
   --  is not a scalar type, then the call has no effect, since validity
637
   --  is only an issue for scalar types. The effect of this call is to
638
   --  check if the value is known valid, if so, nothing needs to be done.
639
   --  If this is not known, then either Expr is set to be range checked,
640
   --  or specific checking code is inserted so that an exception is raised
641
   --  if the value is not valid.
642
   --
643
   --  The optional argument Holes_OK indicates whether it is necessary to
644
   --  worry about enumeration types with non-standard representations leading
645
   --  to "holes" in the range of possible representations. If Holes_OK is
646
   --  True, then such values are assumed valid (this is used when the caller
647
   --  will make a separate check for this case anyway). If Holes_OK is False,
648
   --  then this case is checked, and code is inserted to ensure that Expr is
649
   --  valid, raising Constraint_Error if the value is not valid.
650
 
651
   function Expr_Known_Valid (Expr : Node_Id) return Boolean;
652
   --  This function tests it the value of Expr is known to be valid in the
653
   --  sense of RM 13.9.1(9-11). In the case of GNAT, it is only discrete types
654
   --  which are a concern, since for non-discrete types we simply continue
655
   --  computation with invalid values, which does not lead to erroneous
656
   --  behavior. Thus Expr_Known_Valid always returns True if the type of Expr
657
   --  is non-discrete. For discrete types the value returned is True only if
658
   --  it can be determined that the value is Valid. Otherwise False is
659
   --  returned.
660
 
661
   procedure Insert_Valid_Check (Expr : Node_Id);
662
   --  Inserts code that will check for the value of Expr being valid, in
663
   --  the sense of the 'Valid attribute returning True. Constraint_Error
664
   --  will be raised if the value is not valid.
665
 
666
   procedure Null_Exclusion_Static_Checks (N : Node_Id);
667
   --  Ada 2005 (AI-231): Check bad usages of the null-exclusion issue
668
 
669
   procedure Remove_Checks (Expr : Node_Id);
670
   --  Remove all checks from Expr except those that are only executed
671
   --  conditionally (on the right side of And Then/Or Else. This call
672
   --  removes only embedded checks (Do_Range_Check, Do_Overflow_Check).
673
 
674
   procedure Validity_Check_Range (N : Node_Id);
675
   --  If N is an N_Range node, then Ensure_Valid is called on its bounds,
676
   --  if validity checking of operands is enabled.
677
 
678
   -----------------------------
679
   -- Handling of Check Names --
680
   -----------------------------
681
 
682
   --  The following table contains Name_Id's for recognized checks. The first
683
   --  entries (corresponding to the values of the subtype Predefined_Check_Id)
684
   --  contain the Name_Id values for the checks that are predefined, including
685
   --  All_Checks (see Types). Remaining entries are those that are introduced
686
   --  by pragma Check_Names.
687
 
688
   package Check_Names is new Table.Table (
689
     Table_Component_Type => Name_Id,
690
     Table_Index_Type     => Check_Id,
691
     Table_Low_Bound      => 1,
692
     Table_Initial        => 30,
693
     Table_Increment      => 200,
694
     Table_Name           => "Name_Check_Names");
695
 
696
   function Get_Check_Id (N : Name_Id) return Check_Id;
697
   --  Function to search above table for matching name. If found returns the
698
   --  corresponding Check_Id value in the range 1 .. Check_Name.Last. If not
699
   --  found returns No_Check_Id.
700
 
701
private
702
 
703
   type Check_Result is array (Positive range 1 .. 2) of Node_Id;
704
   --  There are two cases for the result returned by Range_Check:
705
   --
706
   --    For the static case the result is one or two nodes that should cause
707
   --    a Constraint_Error. Typically these will include Expr itself or the
708
   --    direct descendents of Expr, such as Low/High_Bound (Expr)). It is the
709
   --    responsibility of the caller to rewrite and substitute the nodes with
710
   --    N_Raise_Constraint_Error nodes.
711
   --
712
   --    For the non-static case a single N_Raise_Constraint_Error node with a
713
   --    non-empty Condition field is returned.
714
   --
715
   --  Unused entries in Check_Result, if any, are simply set to Empty For
716
   --  external clients, the required processing on this result is achieved
717
   --  using the Insert_Range_Checks routine.
718
 
719
   pragma Inline (Apply_Length_Check);
720
   pragma Inline (Apply_Range_Check);
721
   pragma Inline (Apply_Static_Length_Check);
722
end Checks;

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