gcc/ada/sem_attr.ads

   1 ------------------------------------------------------------------------------
   2 --                                                                          --
   3 --                         GNAT COMPILER COMPONENTS                         --
   4 --                                                                          --
   5 --                             S E M _ A T T R                              --
   6 --                                                                          --
   7 --                                 S p e c                                  --
   8 --                                                                          --
   9 --                                                                          --
  10 --          Copyright (C) 1992-2002, Free Software Foundation, Inc.         --
  11 --                                                                          --
  12 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
  13 -- terms of the  GNU General Public License as published  by the Free Soft- --
  14 -- ware  Foundation;  either version 2,  or (at your option) any later ver- --
  15 -- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
  16 -- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
  17 -- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License --
  18 -- for  more details.  You should have  received  a copy of the GNU General --
  19 -- Public License  distributed with GNAT;  see file COPYING.  If not, write --
  20 -- to  the Free Software Foundation,  59 Temple Place - Suite 330,  Boston, --
  21 -- MA 02111-1307, USA.                                                      --
  22 --                                                                          --
  23 -- GNAT was originally developed  by the GNAT team at  New York University. --
  24 -- Extensive contributions were provided by Ada Core Technologies Inc.      --
  25 --                                                                          --
  26 ------------------------------------------------------------------------------
  27
  28 --  Attribute handling is isolated in a separate package to ease the addition
  29 --  of implementation defined attributes. Logically this processing belongs
  30 --  in chapter 4. See Sem_Ch4 for a description of the relation of the
  31 --  Analyze and Resolve routines for expression components.
  32
  33 --  This spec also documents all GNAT implementation defined pragmas
  34
  35 with Snames; use Snames;
  36 with Types;  use Types;
  37
  38 package Sem_Attr is
  39
  40    type Attribute_Class_Array is array (Attribute_Id) of Boolean;
  41    --  Type used to build attribute classification flag arrays
  42
  43    -----------------------------------------
  44    -- Implementation Dependent Attributes --
  45    -----------------------------------------
  46
  47    --  This section describes the implementation dependent attributes
  48    --  provided in GNAT, as well as constructing an array of flags
  49    --  indicating which attributes these are.
  50
  51    Attribute_Impl_Def : Attribute_Class_Array := Attribute_Class_Array'(
  52
  53       ------------------
  54       -- Abort_Signal --
  55       ------------------
  56
  57       Attribute_Abort_Signal => True,
  58       --
  59       --  Standard'Abort_Signal (Standard is the only allowed prefix)
  60       --  provides the entity for the special exception used to signal
  61       --  task abort or asynchronous transfer of control. Normally this
  62       --  attribute should only be used in the tasking runtime (it is
  63       --  highly peculiar, and completely outside the normal semantics
  64       --  of Ada, for a user program to intercept the abort exception).
  65
  66       ------------------
  67       -- Address_Size --
  68       ------------------
  69
  70       Attribute_Address_Size => True,
  71       --
  72       --  Standard'Address_Size (Standard is the only allowed prefix) is
  73       --  a static constant giving the number of bits in an Address. It
  74       --  is used primarily for constructing the definition of Memory_Size
  75       --  in package Standard, but may be freely used in user programs.
  76       --  This is a static attribute.
  77
  78       ---------------
  79       -- Asm_Input --
  80       ---------------
  81
  82       Attribute_Asm_Input => True,
  83       --
  84       --  Used only in conjunction with the Asm and Asm_Volatile subprograms
  85       --  in package Machine_Code to construct machine instructions. See
  86       --  documentation in package Machine_Code in file s-maccod.ads.
  87
  88       ----------------
  89       -- Asm_Output --
  90       ----------------
  91
  92       Attribute_Asm_Output => True,
  93       --
  94       --  Used only in conjunction with the Asm and Asm_Volatile subprograms
  95       --  in package Machine_Code to construct machine instructions. See
  96       --  documentation in package Machine_Code in file s-maccod.ads.
  97
  98       ---------------
  99       -- AST_Entry --
 100       ---------------
 101
 102       Attribute_AST_Entry => True,
 103       --
 104       --  E'Ast_Entry, where E is a task entry, yields a value of the
 105       --  predefined type System.DEC.AST_Handler, that enables the given
 106       --  entry to be called when an AST occurs. If the name to which the
 107       --  attribute applies has not been specified with the pragma AST_Entry,
 108       --  the attribute returns the value No_Ast_Handler, and no AST occurs.
 109       --  If the entry is for a task that is not callable (T'Callable False),
 110       --  the exception program error is raised. If an AST occurs for an
 111       --  entry of a task that is terminated, the program is erroneous.
 112       --
 113       --  The attribute AST_Entry is supported only in OpenVMS versions
 114       --  of GNAT. It will be rejected as illegal in other GNAT versions.
 115
 116       ---------
 117       -- Bit --
 118       ---------
 119
 120       Attribute_Bit => True,
 121       --
 122       --  Obj'Bit, where Obj is any object, yields the bit offset within
 123       --  the storage unit (byte) that contains the first bit of storage
 124       --  allocated for the object. The value of this attribute is of the
 125       --  type Universal_Integer, and is always a non-negative number not
 126       --  exceeding the value of System.Storage_Unit.
 127       --
 128       --  For an object that is a variable or a constant allocated in a
 129       --  register, the value is zero. (The use of this attribute does not
 130       --  force the allocation of a variable to memory).
 131       --
 132       --  For an object that is a formal parameter, this attribute applies
 133       --  to either the matching actual parameter or to a copy of the
 134       --  matching actual parameter.
 135       --
 136       --  For an access object the value is zero. Note that Obj.all'Bit is
 137       --  subject to an Access_Check for the designated object. Similarly
 138       --  for a record component X.C'Bit is subject to a discriminant check
 139       --  and X(I).Bit and X(I1..I2)'Bit are subject to index checks.
 140       --
 141       --  This attribute is designed to be compatible with the DEC Ada
 142       --  definition and implementation of the Bit attribute.
 143
 144       ------------------
 145       -- Code_Address --
 146       ------------------
 147
 148       Attribute_Code_Address => True,
 149       --
 150       --  subp'Code_Address, where subp is a subprogram entity, gives the
 151       --  address of the first generated instruction for a subprogram. This
 152       --  is often, but not always the same as the 'Address value, which is
 153       --  the address to be used in a call. The differences occur in the case
 154       --  of a nested procedure (where Address yields the address of the
 155       --  trampoline code used to load the static link), and on some systems
 156       --  which use procedure descriptors (in which case Address yields the
 157       --  address of the descriptor).
 158
 159       -----------------------
 160       -- Default_Bit_Order --
 161       -----------------------
 162
 163       Attribute_Default_Bit_Order => True,
 164       --
 165       --  Standard'Default_Bit_Order (Standard is the only permissible prefix),
 166       --  provides the value System.Default_Bit_Order as a Pos value (0 for
 167       --  High_Order_First, 1 for Low_Order_First). This is used to construct
 168       --  the definition of Default_Bit_Order in package System. This is a
 169       --  static attribute.
 170
 171       ---------------
 172       -- Elab_Body --
 173       ---------------
 174
 175       Attribute_Elab_Body => True,
 176       --
 177       --  This attribute can only be applied to a program unit name. It
 178       --  returns the entity for the corresponding elaboration procedure
 179       --  for elaborating the body of the referenced unit. This is used
 180       --  in the main generated elaboration procedure by the binder, and
 181       --  is not normally used in any other context, but there may be
 182       --  specialized situations in which it is useful to be able to
 183       --  call this elaboration procedure from Ada code, e.g. if it
 184       --  is necessary to do selective reelaboration to fix some error.
 185
 186       ---------------
 187       -- Elab_Spec --
 188       ---------------
 189
 190       Attribute_Elab_Spec => True,
 191       --
 192       --  This attribute can only be applied to a program unit name. It
 193       --  returns the entity for the corresponding elaboration procedure
 194       --  for elaborating the spec of the referenced unit. This is used
 195       --  in the main generated elaboration procedure by the binder, and
 196       --  is not normally used in any other context, but there may be
 197       --  specialized situations in which it is useful to be able to
 198       --  call this elaboration procedure from Ada code, e.g. if it
 199       --  is necessary to do selective reelaboration to fix some error.
 200
 201       ----------------
 202       -- Elaborated --
 203       ----------------
 204
 205       Attribute_Elaborated => True,
 206       --
 207       --  Lunit'Elaborated, where Lunit is a library unit, yields a boolean
 208       --  value indicating whether or not the body of the designated library
 209       --  unit has been elaborated yet.
 210
 211       --------------
 212       -- Enum_Rep --
 213       --------------
 214
 215       Attribute_Enum_Rep => True,
 216       --
 217       --  For every enumeration subtype S, S'Enum_Rep denotes a function
 218       --  with the following specification:
 219       --
 220       --    function S'Enum_Rep (Arg : S'Base) return universal_integer;
 221       --
 222       --  The function returns the representation value for the given
 223       --  enumeration value. This will be equal to the 'Pos value in the
 224       --  absence of an enumeration representation clause. This is a static
 225       --  attribute (i.e. the result is static if the argument is static).
 226
 227       -----------------
 228       -- Fixed_Value --
 229       -----------------
 230
 231       Attribute_Fixed_Value => True,
 232       --
 233       --  For every fixed-point type S, S'Fixed_Value denotes a function
 234       --  with the following specification:
 235       --
 236       --    function S'Fixed_Value (Arg : universal_integer) return S;
 237       --
 238       --  The value returned is the fixed-point value V such that
 239       --
 240       --    V = Arg * S'Small
 241       --
 242       --  The effect is thus equivalent to first converting the argument
 243       --  to the integer type used to represent S, and then doing an
 244       --  unchecked conversion to the fixed-point type. This attribute is
 245       --  primarily intended for use in implementation of the input-output
 246       --  functions for fixed-point values.
 247
 248       -----------------------
 249       -- Has_Discriminants --
 250       -----------------------
 251
 252       Attribute_Has_Discriminants => True,
 253       --
 254       --  Gtyp'Has_Discriminants, where Gtyp is a generic formal type, yields
 255       --  a Boolean value indicating whether or not the actual instantiation
 256       --  type has discriminants.
 257
 258       ---------
 259       -- Img --
 260       ---------
 261
 262       Attribute_Img => True,
 263       --
 264       --  The 'Img function is defined for any prefix, P, that denotes an
 265       --  object of scalar type T. P'Img is equivalent to T'Image (P). This
 266       --  is convenient for debugging. For example:
 267       --
 268       --     Put_Line ("X = " & X'Img);
 269       --
 270       --  has the same meaning as the more verbose:
 271       --
 272       --     Put_Line ("X = " & Temperature_Type'Image (X));
 273       --
 274       --  where Temperature_Type is the subtype of the object X.
 275
 276       -------------------
 277       -- Integer_Value --
 278       -------------------
 279
 280       Attribute_Integer_Value => True,
 281       --
 282       --  For every integer type S, S'Integer_Value denotes a function
 283       --  with the following specification:
 284       --
 285       --    function S'Integer_Value (Arg : universal_fixed) return S;
 286       --
 287       --  The value returned is the integer value V, such that
 288       --
 289       --    Arg = V * fixed-type'Small
 290       --
 291       --  The effect is thus equivalent to first doing an unchecked convert
 292       --  from the fixed-point type to its corresponding implementation type,
 293       --  and then converting the result to the target integer type. This
 294       --  attribute is primarily intended for use in implementation of the
 295       --  standard input-output functions for fixed-point values.
 296
 297       ------------------
 298       -- Machine_Size --
 299       ------------------
 300
 301       Attribute_Machine_Size => True,
 302       --
 303       --  This attribute is identical to the Object_Size attribute. It is
 304       --  provided for compatibility with the DEC attribute of this name.
 305
 306       -----------------------
 307       -- Maximum_Alignment --
 308       -----------------------
 309
 310       Attribute_Maximum_Alignment => True,
 311       --
 312       --  Standard'Maximum_Alignment (Standard is the only permissible prefix)
 313       --  provides the maximum useful alignment value for the target. This
 314       --  is a static value that can be used to specify the alignment for an
 315       --  object, guaranteeing that it is properly aligned in all cases. The
 316       --  time this is useful is when an external object is imported and its
 317       --  alignment requirements are unknown. This is a static attribute.
 318
 319       --------------------
 320       -- Mechanism_Code --
 321       --------------------
 322
 323       Attribute_Mechanism_Code => True,
 324       --
 325       --  function'Mechanism_Code yeilds an integer code for the mechanism
 326       --  used for the result of function, and subprogram'Mechanism_Code (n)
 327       --  yields the mechanism used for formal parameter number n (a static
 328       --  integer value, 1 = first parameter). The code returned is:
 329       --
 330       --     1 = by copy (value)
 331       --     2 = by reference
 332       --     3 = by descriptor (default descriptor type)
 333       --     4 = by descriptor (UBS  unaligned bit string)
 334       --     5 = by descriptor (UBSB aligned bit string with arbitrary bounds)
 335       --     6 = by descriptor (UBA  unaligned bit array)
 336       --     7 = by descriptor (S    string, also scalar access type parameter)
 337       --     8 = by descriptor (SB   string with arbitrary bounds)
 338       --     9 = by descriptor (A    contiguous array)
 339       --    10 = by descriptor (NCA  non-contiguous array)
 340
 341       --------------------
 342       -- Null_Parameter --
 343       --------------------
 344
 345       Attribute_Null_Parameter => True,
 346       --
 347       --  A reference T'Null_Parameter denotes an (imaginary) object of
 348       --  type or subtype T allocated at (machine) address zero. The
 349       --  attribute is allowed only as the default expression of a formal
 350       --  parameter, or as an actual expression of a subporgram call. In
 351       --  either case, the subprogram must be imported.
 352       --
 353       --  The identity of the object is represented by the address zero
 354       --  in the argument list, independent of the passing mechanism
 355       --  (explicit or default).
 356       --
 357       --  The reason that this capability is needed is that for a record
 358       --  or other composite object passed by reference, there is no other
 359       --  way of specifying that a zero address should be passed.
 360
 361       -----------------
 362       -- Object_Size --
 363       -----------------
 364
 365       Attribute_Object_Size => True,
 366       --
 367       --  Type'Object_Size is the same as Type'Size for all types except
 368       --  fixed-point types and discrete types. For fixed-point types and
 369       --  discrete types, this attribute gives the size used for default
 370       --  allocation of objects and components of the size. See section
 371       --  in Einfo ("Handling of type'Size values") for further details.
 372
 373       -------------------------
 374       -- Passed_By_Reference --
 375       -------------------------
 376
 377       Attribute_Passed_By_Reference => True,
 378       --
 379       --  T'Passed_By_Reference for any subtype T returns a boolean value
 380       --  that is true if the type is normally passed by reference and
 381       --  false if the type is normally passed by copy in calls. For scalar
 382       --  types, the result is always False and is static. For non-scalar
 383       --  types, the result is non-static (since it is computed by Gigi).
 384
 385       ------------------
 386       -- Range_Length --
 387       ------------------
 388
 389       Attribute_Range_Length => True,
 390       --
 391       --  T'Range_Length for any discrete type T yields the number of
 392       --  values represented by the subtype (zero for a null range). The
 393       --  result is static for static subtypes. Note that Range_Length
 394       --  applied to the index subtype of a one dimensional array always
 395       --  gives the same result as Range applied to the array itself.
 396       --  The result is of type universal integer.
 397
 398       ------------------
 399       -- Storage_Unit --
 400       ------------------
 401
 402       Attribute_Storage_Unit => True,
 403       --
 404       --  Standard'Storage_Unit (Standard is the only permissible prefix)
 405       --  provides the value System.Storage_Unit, and is intended primarily
 406       --  for constructing this definition in package System (see note above
 407       --  in Default_Bit_Order description). The is a static attribute.
 408
 409       ----------------
 410       -- To_Address --
 411       ----------------
 412
 413       Attribute_To_Address => True,
 414       --
 415       --  System'To_Address (Address is the only permissible prefix)
 416       --  is a function that takes any integer value, and converts it into
 417       --  an address value. The semantics is to first convert the integer
 418       --  value to type Integer_Address according to normal conversion
 419       --  rules, and then to convert this to an address using the same
 420       --  semantics as the System.Storage_Elements.To_Address function.
 421       --  The important difference is that this is a static attribute
 422       --  so it can be used in initializations in preealborate packages.
 423
 424       ----------------
 425       -- Type_Class --
 426       ----------------
 427
 428       Attribute_Type_Class => True,
 429       --
 430       --  T'Type_Class for any type or subtype T yields the value of the
 431       --  type class for the full type of T. If T is a generic formal type,
 432       --  then the value is the value for the corresponding actual subtype.
 433       --  The value of this attribute is of type System.Aux_DEC.Type_Class,
 434       --  which has the following definition:
 435       --
 436       --    type Type_Class is
 437       --      (Type_Class_Enumeration,
 438       --       Type_Class_Integer,
 439       --       Type_Class_Fixed_Point,
 440       --       Type_Class_Floating_Point,
 441       --       Type_Class_Array,
 442       --       Type_Class_Record,
 443       --       Type_Class_Access,
 444       --       Type_Class_Task,
 445       --       Type_Class_Address);
 446       --
 447       --  Protected types yield the value Type_Class_Task, which thus
 448       --  applies to all concurrent types. This attribute is designed to
 449       --  be compatible with the DEC Ada attribute of the same name.
 450       --
 451       --  Note: if pragma Extend_System is used to merge the definitions of
 452       --  Aux_DEC into System, then the type Type_Class can be referenced
 453       --  as an entity within System, as can its enumeration literals.
 454
 455       -----------------
 456       -- UET_Address --
 457       -----------------
 458
 459       Attribute_UET_Address => True,
 460       --
 461       --  Unit'UET_Address, where Unit is a program unit, yields the address
 462       --  of the unit exception table for the specified unit. This is only
 463       --  used in the internal implementation of exception handling. See the
 464       --  implementation of unit Ada.Exceptions for details on its use.
 465
 466       ------------------------------
 467       -- Universal_Literal_String --
 468       ------------------------------
 469
 470       Attribute_Universal_Literal_String => True,
 471       --
 472       --  The prefix of 'Universal_Literal_String must be a named number.
 473       --  The static result is the string consisting of the characters of
 474       --  the number as defined in the original source. This allows the
 475       --  user program to access the actual text of named numbers without
 476       --  intermediate conversions and without the need to enclose the
 477       --  strings in quotes (which would preclude their use as numbers).
 478       --  This is used internally for the construction of values of the
 479       --  floating-point attributes from the file ttypef.ads, but may
 480       --  also be used by user programs.
 481
 482       -------------------------
 483       -- Unrestricted_Access --
 484       -------------------------
 485
 486       Attribute_Unrestricted_Access => True,
 487       --
 488       --  The Unrestricted_Access attribute is similar to Access except that
 489       --  all accessibility and aliased view checks are omitted. This is very
 490       --  much a user-beware attribute. Basically its status is very similar
 491       --  to Address, for which it is a desirable replacement where the value
 492       --  desired is an access type. In other words, its effect is identical
 493       --  to first taking 'Address and then doing an unchecked conversion to
 494       --  a desired access type. Note that in GNAT, but not necessarily in
 495       --  other implementations, the use of static chains for inner level
 496       --  subprograms means that Unrestricted_Access applied to a subprogram
 497       --  yields a value that can be called as long as the subprogram is in
 498       --  scope (normal Ada 95 accessibility rules restrict this usage).
 499
 500       ---------------
 501       -- VADS_Size --
 502       ---------------
 503
 504       Attribute_VADS_Size => True,
 505       --
 506       --  Typ'VADS_Size yields the Size value typically yielded by some
 507       --  Ada 83 compilers. The differences between VADS_Size and Size
 508       --  is that for scalar types for which no Size has been specified,
 509       --  VADS_Size yields the Object_Size rather than the Value_Size.
 510       --  For example, while Natural'Size is typically 31, the value of
 511       --  Natural'VADS_Size is 32. For all other types, Size and VADS_Size
 512       --  yield the same value.
 513
 514       ----------------
 515       -- Value_Size --
 516       ----------------
 517
 518       Attribute_Value_Size => True,
 519       --
 520       --  Type'Value_Size is the number of bits required to represent a
 521       --  value of the given subtype. It is the same as Type'Size, but,
 522       --  unlike Size, may be set for non-first subtypes. See section
 523       --  in Einfo ("Handling of type'Size values") for further details.
 524
 525       ---------------
 526       -- Word_Size --
 527       ---------------
 528
 529       Attribute_Word_Size => True,
 530       --
 531       --  Standard'Word_Size (Standard is the only permissible prefix)
 532       --  provides the value System.Word_Size, and is intended primarily
 533       --  for constructing this definition in package System (see note above
 534       --  in Default_Bit_Order description). This is a static attribute.
 535
 536       others => False);
 537
 538    -----------------
 539    -- Subprograms --
 540    -----------------
 541
 542    procedure Analyze_Attribute (N : Node_Id);
 543    --  Performs bottom up semantic analysis of an attribute. Note that the
 544    --  parser has already checked that type returning attributes appear only
 545    --  in appropriate contexts (i.e. in subtype marks, or as prefixes for
 546    --  other attributes).
 547
 548    procedure Resolve_Attribute (N : Node_Id; Typ : Entity_Id);
 549    --  Performs type resolution of attribute. If the attribute yields
 550    --  a universal value, mark its type as that of the context. On
 551    --  the other hand, if the context itself is universal (as in
 552    --  T'Val (T'Pos (X)), mark the type as being the largest type of
 553    --  that class that can be used at run-time. This is correct since
 554    --  either the value gets folded (in which case it doesn't matter
 555    --  what type of the class we give if, since the folding uses universal
 556    --  arithmetic anyway) or it doesn't get folded (in which case it is
 557    --  going to be dealt with at runtime, and the largest type is right).
 558
 559 end Sem_Attr;