1 ------------------------------------------------------------------------------
3 -- GNAT COMPILER COMPONENTS --
9 -- Copyright (C) 1992-2020, Free Software Foundation, Inc. --
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. --
18 -- You should have received a copy of the GNU General Public License along --
19 -- with this program; see file COPYING3. If not see --
20 -- <http://www.gnu.org/licenses/>. --
22 -- GNAT was originally developed by the GNAT team at New York University. --
23 -- Extensive contributions were provided by Ada Core Technologies Inc. --
25 ------------------------------------------------------------------------------
27 -- Attribute handling is isolated in a separate package to ease the addition
28 -- of implementation defined attributes. Logically this processing belongs
29 -- in chapter 4. See Sem_Ch4 for a description of the relation of the
30 -- Analyze and Resolve routines for expression components.
32 -- This spec also documents all GNAT implementation defined pragmas
34 with Exp_Tss; use Exp_Tss;
35 with Namet; use Namet;
36 with Snames; use Snames;
37 with Types; use Types;
41 -----------------------------------------
42 -- Implementation Dependent Attributes --
43 -----------------------------------------
45 -- This section describes the implementation dependent attributes provided
46 -- in GNAT, as well as constructing an array of flags indicating which
47 -- attributes these are.
49 Attribute_Impl_Def : constant Attribute_Class_Array :=
50 Attribute_Class_Array'(
56 Attribute_Abort_Signal => True,
57 -- Standard'Abort_Signal (Standard is the only allowed prefix) provides
58 -- the entity for the special exception used to signal task abort or
59 -- asynchronous transfer of control. Normally this attribute should only
60 -- be used in the tasking runtime (it is highly peculiar, and completely
61 -- outside the normal semantics of Ada, for a user program to intercept
62 -- the abort exception).
68 Attribute_Address_Size => True,
69 -- Standard'Address_Size (Standard is the only allowed prefix) is
70 -- a static constant giving the number of bits in an Address. It
71 -- is used primarily for constructing the definition of Memory_Size
72 -- in package Standard, but may be freely used in user programs.
73 -- This is a static attribute.
79 Attribute_Asm_Input => True,
80 -- Used only in conjunction with the Asm subprograms in package
81 -- Machine_Code to construct machine instructions. See documentation
82 -- in package Machine_Code in file s-maccod.ads.
88 Attribute_Asm_Output => True,
89 -- Used only in conjunction with the Asm subprograms in package
90 -- Machine_Code to construct machine instructions. See documentation
91 -- in package Machine_Code in file s-maccod.ads.
97 Attribute_Bit => True,
98 -- Obj'Bit, where Obj is any object, yields the bit offset within the
99 -- storage unit (byte) that contains the first bit of storage allocated
100 -- for the object. The attribute value is of type Universal_Integer,
101 -- and is always a non-negative number not exceeding the value of
102 -- System.Storage_Unit.
104 -- For an object that is a variable or a constant allocated in a
105 -- register, the value is zero. (The use of this attribute does not
106 -- force the allocation of a variable to memory).
108 -- For an object that is a formal parameter, this attribute applies to
109 -- either the matching actual parameter or to a copy of the matching
112 -- For an access object the value is zero. Note that Obj.all'Bit is
113 -- subject to an Access_Check for the designated object. Similarly
114 -- for a record component X.C'Bit is subject to a discriminant check
115 -- and X(I).Bit and X(I1..I2)'Bit are subject to index checks.
117 -- This attribute is designed to be compatible with the DEC Ada
118 -- definition and implementation of the Bit attribute.
124 Attribute_Code_Address => True,
125 -- The reference subp'Code_Address, where subp is a subprogram entity,
126 -- gives the address of the first generated instruction for the sub-
127 -- program. This is often, but not always the same as the 'Address
128 -- value, which is the address to be used in a call. The differences
129 -- occur in the case of a nested procedure (where Address yields the
130 -- address of the trampoline code used to load the static link), and on
131 -- some systems which use procedure descriptors (in which case Address
132 -- yields the address of the descriptor).
134 -----------------------
135 -- Default_Bit_Order --
136 -----------------------
138 Attribute_Default_Bit_Order => True,
139 -- Standard'Default_Bit_Order (Standard is the only permissible prefix)
140 -- provides the value System.Default_Bit_Order as a Pos value (0 for
141 -- High_Order_First, 1 for Low_Order_First). This is used to construct
142 -- the definition of Default_Bit_Order in package System. This is a
145 ----------------------------------
146 -- Default_Scalar_Storage_Order --
147 ----------------------------------
149 Attribute_Default_Scalar_Storage_Order => True,
150 -- Standard'Default_Scalar_Storage_Order (Standard is the
151 -- only permissible prefix) provides the current value of the
152 -- default scalar storage order (as specified using pragma
153 -- Default_Scalar_Storage_Order, or equal to Default_Bit_Order if
154 -- unspecified) as a System.Bit_Order value. This is a static attribute.
160 Attribute_Deref => True,
161 -- typ'Deref (expr) is valid only if expr is of type System'Address.
162 -- The result is an object of type typ that is obtained by treating the
163 -- address as an access-to-typ value that points to the result. It is
164 -- basically equivalent to (atyp!expr).all where atyp is an access type
171 Attribute_Elab_Body => True,
172 -- This attribute can only be applied to a program unit name. It
173 -- returns the entity for the corresponding elaboration procedure for
174 -- elaborating the body of the referenced unit. This is used in the main
175 -- generated elaboration procedure by the binder, and is not normally
176 -- used in any other context, but there may be specialized situations in
177 -- which it is useful to be able to call this elaboration procedure from
178 -- Ada code, e.g. if it is necessary to do selective reelaboration to
185 Attribute_Elab_Subp_Body => True,
186 -- This attribute can only be applied to a library level subprogram
187 -- name and is only relevant in CodePeer mode. It returns the entity
188 -- for the corresponding elaboration procedure for elaborating the body
189 -- of the referenced subprogram unit. This is used in the main generated
190 -- elaboration procedure by the binder in CodePeer mode only.
196 Attribute_Elab_Spec => True,
197 -- This attribute can only be applied to a program unit name. It
198 -- returns the entity for the corresponding elaboration procedure for
199 -- elaborating the spec of the referenced unit. This is used in the main
200 -- generated elaboration procedure by the binder, and is not normally
201 -- used in any other context, but there may be specialized situations in
202 -- which it is useful to be able to call this elaboration procedure from
203 -- Ada code, e.g. if it is necessary to do selective reelaboration to
210 Attribute_Elaborated => True,
211 -- Lunit'Elaborated, where Lunit is a library unit, yields a boolean
212 -- value indicating whether or not the body of the designated library
213 -- unit has been elaborated yet.
215 -----------------------
216 -- Finalization_Size --
217 -----------------------
219 Attribute_Finalization_Size => True,
220 -- For every object or non-class-wide-type, Finalization_Size returns
221 -- the size of the hidden header used for finalization purposes as if
222 -- the object or type was allocated on the heap. The size of the header
223 -- does take into account any extra padding due to alignment issues.
229 Attribute_Fixed_Value => True,
230 -- For every fixed-point type S, S'Fixed_Value denotes a function
231 -- with the following specification:
233 -- function S'Fixed_Value (Arg : universal_integer) return S;
235 -- The value returned is the fixed-point value V such that
239 -- The effect is thus equivalent to first converting the argument to
240 -- the integer type used to represent S, and then doing an unchecked
241 -- conversion to the fixed-point type. This attribute is primarily
242 -- intended for use in implementation of the input-output functions
243 -- for fixed-point values.
245 -----------------------
246 -- Has_Discriminants --
247 -----------------------
249 Attribute_Has_Discriminants => True,
250 -- Gtyp'Has_Discriminants, where Gtyp is a generic formal type, yields
251 -- a Boolean value indicating whether or not the actual instantiation
252 -- type has discriminants.
258 Attribute_Img => True,
259 -- The 'Img function is defined for any prefix, P, that denotes an
260 -- object of scalar type T. P'Img is equivalent to T'Image (P). This
261 -- is convenient for debugging. For example:
263 -- Put_Line ("X = " & X'Img);
265 -- has the same meaning as the more verbose:
267 -- Put_Line ("X = " & Temperature_Type'Image (X));
269 -- where Temperature_Type is the subtype of the object X.
275 Attribute_Integer_Value => True,
276 -- For every integer type S, S'Integer_Value denotes a function
277 -- with the following specification:
279 -- function S'Integer_Value (Arg : universal_fixed) return S;
281 -- The value returned is the integer value V, such that
283 -- Arg = V * fixed-type'Small
285 -- The effect is thus equivalent to first doing an unchecked convert
286 -- from the fixed-point type to its corresponding implementation type,
287 -- and then converting the result to the target integer type. This
288 -- attribute is primarily intended for use in implementation of the
289 -- standard input-output functions for fixed-point values.
291 Attribute_Invalid_Value => True,
292 -- For every scalar type, S'Invalid_Value designates an undefined value
293 -- of the type. If possible this value is an invalid value, and in fact
294 -- is identical to the value that would be set if Initialize_Scalars
295 -- mode were in effect (including the behavior of its value on
296 -- environment variables or binder switches). The intended use is to
297 -- set a value where initialization is required (e.g. as a result of the
298 -- coding standards in use), but logically no initialization is needed,
299 -- and the value should never be accessed.
301 Attribute_Loop_Entry => True,
302 -- For every object of a non-limited type, S'Loop_Entry [(Loop_Name)]
303 -- denotes the constant value of prefix S at the point of entry into the
304 -- related loop. The type of the attribute is the type of the prefix.
310 Attribute_Machine_Size => True,
311 -- This attribute is identical to the Object_Size attribute. It is
312 -- provided for compatibility with the DEC attribute of this name.
314 -----------------------
315 -- Maximum_Alignment --
316 -----------------------
318 Attribute_Maximum_Alignment => True,
319 -- Standard'Maximum_Alignment (Standard is the only permissible prefix)
320 -- provides the maximum useful alignment value for the target. This is a
321 -- static value that can be used to specify the alignment for an object,
322 -- guaranteeing that it is properly aligned in all cases. The time this
323 -- is useful is when an external object is imported and its alignment
324 -- requirements are unknown. This is a static attribute.
330 Attribute_Mechanism_Code => True,
331 -- function'Mechanism_Code yields an integer code for the mechanism
332 -- used for the result of function, and subprogram'Mechanism_Code (n)
333 -- yields the mechanism used for formal parameter number n (a static
334 -- integer value, 1 = first parameter). The code returned is:
336 -- 1 = by copy (value)
338 -- 3 = by descriptor (default descriptor type)
339 -- 4 = by descriptor (UBS unaligned bit string)
340 -- 5 = by descriptor (UBSB aligned bit string with arbitrary bounds)
341 -- 6 = by descriptor (UBA unaligned bit array)
342 -- 7 = by descriptor (S string, also scalar access type parameter)
343 -- 8 = by descriptor (SB string with arbitrary bounds)
344 -- 9 = by descriptor (A contiguous array)
345 -- 10 = by descriptor (NCA non-contiguous array)
351 Attribute_Null_Parameter => True,
352 -- A reference T'Null_Parameter denotes an (imaginary) object of type
353 -- or subtype T allocated at (machine) address zero. The attribute is
354 -- allowed only as the default expression of a formal parameter, or
355 -- as an actual expression of a subprogram call. In either case, the
356 -- subprogram must be imported.
358 -- The identity of the object is represented by the address zero in
359 -- the argument list, independent of the passing mechanism (explicit
362 -- The reason that this capability is needed is that for a record or
363 -- other composite object passed by reference, there is no other way
364 -- of specifying that a zero address should be passed.
370 Attribute_Object_Size => True,
371 -- Type'Object_Size is the same as Type'Size for all types except
372 -- fixed-point types and discrete types. For fixed-point types and
373 -- discrete types, this attribute gives the size used for default
374 -- allocation of objects and components of the size. See section in
375 -- Einfo ("Handling of Type'Size values") for further details.
377 -------------------------
378 -- Passed_By_Reference --
379 -------------------------
381 Attribute_Passed_By_Reference => True,
382 -- T'Passed_By_Reference for any subtype T returns a boolean value that
383 -- is true if the type is normally passed by reference and false if the
384 -- type is normally passed by copy in calls. For scalar types, the
385 -- result is always False and is static. For non-scalar types, the
386 -- result is non-static (since it is computed by Gigi).
392 Attribute_Range_Length => True,
393 -- T'Range_Length for any discrete type T yields the number of values
394 -- represented by the subtype (zero for a null range). The result is
395 -- static for static subtypes. Note that Range_Length applied to the
396 -- index subtype of a one dimensional array always gives the same result
397 -- as Range applied to the array itself. The result is of type universal
404 Attribute_Reduce => True,
411 Attribute_Ref => True,
412 -- System.Address'Ref (Address is the only permissible prefix) is
413 -- equivalent to System'To_Address, provided for compatibility with
420 Attribute_Storage_Unit => True,
421 -- Standard'Storage_Unit (Standard is the only permissible prefix)
422 -- provides the value System.Storage_Unit, and is intended primarily
423 -- for constructing this definition in package System (see note above
424 -- in Default_Bit_Order description). The is a static attribute.
430 Attribute_Stub_Type => True,
431 -- The GNAT implementation of remote access-to-classwide types is
432 -- organised as described in AARM E.4(20.t): a value of an RACW type
433 -- (designating a remote object) is represented as a normal access
434 -- value, pointing to a "stub" object which in turn contains the
435 -- necessary information to contact the designated remote object. A
436 -- call on any dispatching operation of such a stub object does the
437 -- remote call, if necessary, using the information in the stub object
438 -- to locate the target partition, etc.
440 -- For a prefix T that denotes a remote access-to-classwide type,
441 -- T'Stub_Type denotes the type of the corresponding stub objects.
443 -- By construction, the layout of T'Stub_Type is identical to that of
444 -- System.Partition_Interface.RACW_Stub_Type (see implementation notes
445 -- in body of Exp_Dist).
451 Attribute_Target_Name => True,
452 -- Standard'Target_Name yields the string identifying the target for the
453 -- compilation, taken from Sdefault.Target_Name.
459 Attribute_To_Address => True,
460 -- System'To_Address (System is the only permissible prefix) is a
461 -- function that takes any integer value, and converts it into an
462 -- address value. The semantics is to first convert the integer value to
463 -- type Integer_Address according to normal conversion rules, and then
464 -- to convert this to an address using the same semantics as the
465 -- System.Storage_Elements.To_Address function. The important difference
466 -- is that this is a static attribute so it can be used in
467 -- initializations in preelaborate packages.
473 Attribute_Type_Class => True,
474 -- T'Type_Class for any type or subtype T yields the value of the type
475 -- class for the full type of T. If T is a generic formal type, then the
476 -- value is the value for the corresponding actual subtype. The value of
477 -- this attribute is of type System.Aux_DEC.Type_Class, which has the
478 -- following definition:
480 -- type Type_Class is
481 -- (Type_Class_Enumeration,
482 -- Type_Class_Integer,
483 -- Type_Class_Fixed_Point,
484 -- Type_Class_Floating_Point,
486 -- Type_Class_Record,
487 -- Type_Class_Access,
489 -- Type_Class_Address);
491 -- Protected types yield the value Type_Class_Task, which thus applies
492 -- to all concurrent types. This attribute is designed to be compatible
493 -- with the DEC Ada attribute of the same name.
495 -- Note: if pragma Extend_System is used to merge the definitions of
496 -- Aux_DEC into System, then the type Type_Class can be referenced
497 -- as an entity within System, as can its enumeration literals.
499 ------------------------------
500 -- Universal_Literal_String --
501 ------------------------------
503 Attribute_Universal_Literal_String => True,
504 -- The prefix of 'Universal_Literal_String must be a named number.
505 -- The static result is the string consisting of the characters of
506 -- the number as defined in the original source. This allows the
507 -- user program to access the actual text of named numbers without
508 -- intermediate conversions and without the need to enclose the
509 -- strings in quotes (which would preclude their use as numbers).
511 -------------------------
512 -- Unrestricted_Access --
513 -------------------------
515 Attribute_Unrestricted_Access => True,
516 -- The Unrestricted_Access attribute is similar to Access except that
517 -- all accessibility and aliased view checks are omitted. This is very
518 -- much a user-beware attribute. Basically its status is very similar
519 -- to Address, for which it is a desirable replacement where the value
520 -- desired is an access type. In other words, its effect is identical
521 -- to first taking 'Address and then doing an unchecked conversion to
522 -- a desired access type. Note that in GNAT, but not necessarily in
523 -- other implementations, the use of static chains for inner level
524 -- subprograms means that Unrestricted_Access applied to a subprogram
525 -- yields a value that can be called as long as the subprogram is in
526 -- scope (normal Ada 95 accessibility rules restrict this usage).
532 Attribute_VADS_Size => True,
533 -- Typ'VADS_Size yields the Size value typically yielded by some Ada 83
534 -- compilers. The differences between VADS_Size and Size is that for
535 -- scalar types for which no Size has been specified, VADS_Size yields
536 -- the Object_Size rather than the Value_Size. For example, while
537 -- Natural'Size is typically 31, the value of Natural'VADS_Size is 32.
538 -- For all other types, Size and VADS_Size yield the same value.
544 Attribute_Valid_Scalars => True,
545 -- Obj'Valid_Scalars can be applied to any object. The result depends
546 -- on the type of the object:
548 -- For a scalar type, the result is the same as obj'Valid
550 -- For an array object, the result is True if the result of applying
551 -- Valid_Scalars to every component is True. For an empty array the
554 -- For a record object, the result is True if the result of applying
555 -- Valid_Scalars to every component is True. For class-wide types,
556 -- only the components of the base type are checked. For variant
557 -- records, only the components actually present are checked. The
558 -- discriminants, if any, are also checked. If there are no components
559 -- or discriminants, the result is True.
561 -- For any other type that has discriminants, the result is True if
562 -- the result of applying Valid_Scalars to each discriminant is True.
564 -- For all other types, the result is always True
566 -- A warning is given for a trivially True result, when the attribute
567 -- is applied to an object that is not of scalar, array, or record
568 -- type, or in the composite case if no scalar subcomponents exist. For
569 -- a variant record, the warning is given only if none of the variants
570 -- have scalar subcomponents. In addition, the warning is suppressed
571 -- for private types, or generic formal types in an instance.
577 Attribute_Value_Size => True,
578 -- Type'Value_Size is the number of bits required to represent value of
579 -- the given subtype. It is the same as Type'Size, but, unlike Size, may
580 -- be set for non-first subtypes. See section in Einfo ("Handling of
581 -- type'Size values") for further details.
587 Attribute_Word_Size => True,
588 -- Standard'Word_Size (Standard is the only permissible prefix)
589 -- provides the value System.Word_Size, and is intended primarily
590 -- for constructing this definition in package System (see note above
591 -- in Default_Bit_Order description). This is a static attribute.
595 -- The following table lists all attributes that yield a result of a
598 Universal_Type_Attribute : constant array (Attribute_Id) of Boolean :=
599 (Attribute_Aft => True,
600 Attribute_Alignment => True,
601 Attribute_Component_Size => True,
602 Attribute_Count => True,
603 Attribute_Delta => True,
604 Attribute_Digits => True,
605 Attribute_Exponent => True,
606 Attribute_First_Bit => True,
607 Attribute_Fore => True,
608 Attribute_Last_Bit => True,
609 Attribute_Length => True,
610 Attribute_Machine_Emax => True,
611 Attribute_Machine_Emin => True,
612 Attribute_Machine_Mantissa => True,
613 Attribute_Machine_Radix => True,
614 Attribute_Max_Alignment_For_Allocation => True,
615 Attribute_Max_Size_In_Storage_Elements => True,
616 Attribute_Model_Emin => True,
617 Attribute_Model_Epsilon => True,
618 Attribute_Model_Mantissa => True,
619 Attribute_Model_Small => True,
620 Attribute_Modulus => True,
621 Attribute_Pos => True,
622 Attribute_Position => True,
623 Attribute_Safe_First => True,
624 Attribute_Safe_Last => True,
625 Attribute_Scale => True,
626 Attribute_Size => True,
627 Attribute_Small => True,
628 Attribute_Wide_Wide_Width => True,
629 Attribute_Wide_Width => True,
630 Attribute_Width => True,
637 procedure Analyze_Attribute (N : Node_Id);
638 -- Performs bottom up semantic analysis of an attribute. Note that the
639 -- parser has already checked that type returning attributes appear only
640 -- in appropriate contexts (i.e. in subtype marks, or as prefixes for
641 -- other attributes).
643 function Name_Implies_Lvalue_Prefix (Nam : Name_Id) return Boolean;
644 -- Determine whether the name of an attribute reference categorizes its
645 -- prefix as an lvalue. The following attributes fall under this bracket
646 -- by directly or indirectly modifying their prefixes.
652 -- Unrestricted_Access
654 procedure Resolve_Attribute (N : Node_Id; Typ : Entity_Id);
655 -- Performs type resolution of attribute. If the attribute yields a
656 -- universal value, mark its type as that of the context. On the other
657 -- hand, if the context itself is universal (as in T'Val (T'Pos (X)), mark
658 -- the type as being the largest type of that class that can be used at
659 -- run-time. This is correct since either the value gets folded (in which
660 -- case it doesn't matter what type of the class we give if, since the
661 -- folding uses universal arithmetic anyway) or it doesn't get folded (in
662 -- which case it is going to be dealt with at runtime, and the largest type
665 function Stream_Attribute_Available
668 Partial_View : Entity_Id := Empty) return Boolean;
669 -- For a limited type Typ, return True if and only if the given attribute
670 -- is available. For Ada 2005, availability is defined by 13.13.2(36/1).
671 -- For Ada 95, an attribute is considered to be available if it has been
672 -- specified using an attribute definition clause for the type, or for its
673 -- full view, or for an ancestor of either. Parameter Partial_View is used
674 -- only internally, when checking for an attribute definition clause that
675 -- is not visible (Ada 95 only).
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