path: root/gcc/ada/sem_attr.ads

------------------------------------------------------------------------------
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------------------------------------------------------------------------------

--  Attribute handling is isolated in a separate package to ease the addition
--  of implementation defined attributes. Logically this processing belongs
--  in chapter 4. See Sem_Ch4 for a description of the relation of the
--  Analyze and Resolve routines for expression components.

--  This spec also documents all GNAT implementation defined pragmas

with Snames; use Snames;
with Types;  use Types;

package Sem_Attr is

   type Attribute_Class_Array is array (Attribute_Id) of Boolean;
   --  Type used to build attribute classification flag arrays

   -----------------------------------------
   -- Implementation Dependent Attributes --
   -----------------------------------------

   --  This section describes the implementation dependent attributes
   --  provided in GNAT, as well as constructing an array of flags
   --  indicating which attributes these are.

   Attribute_Impl_Def : Attribute_Class_Array := Attribute_Class_Array'(

      ------------------
      -- Abort_Signal --
      ------------------

      Attribute_Abort_Signal => True,
      --
      --  Standard'Abort_Signal (Standard is the only allowed prefix)
      --  provides the entity for the special exception used to signal
      --  task abort or asynchronous transfer of control. Normally this
      --  attribute should only be used in the tasking runtime (it is
      --  highly peculiar, and completely outside the normal semantics
      --  of Ada, for a user program to intercept the abort exception).

      ------------------
      -- Address_Size --
      ------------------

      Attribute_Address_Size => True,
      --
      --  Standard'Address_Size (Standard is the only allowed prefix) is
      --  a static constant giving the number of bits in an Address. It
      --  is used primarily for constructing the definition of Memory_Size
      --  in package Standard, but may be freely used in user programs.
      --  This is a static attribute.

      ---------------
      -- Asm_Input --
      ---------------

      Attribute_Asm_Input => True,
      --
      --  Used only in conjunction with the Asm and Asm_Volatile subprograms
      --  in package Machine_Code to construct machine instructions. See
      --  documentation in package Machine_Code in file s-maccod.ads.

      ----------------
      -- Asm_Output --
      ----------------

      Attribute_Asm_Output => True,
      --
      --  Used only in conjunction with the Asm and Asm_Volatile subprograms
      --  in package Machine_Code to construct machine instructions. See
      --  documentation in package Machine_Code in file s-maccod.ads.

      ---------------
      -- AST_Entry --
      ---------------

      Attribute_AST_Entry => True,
      --
      --  E'Ast_Entry, where E is a task entry, yields a value of the
      --  predefined type System.DEC.AST_Handler, that enables the given
      --  entry to be called when an AST occurs. If the name to which the
      --  attribute applies has not been specified with the pragma AST_Entry,
      --  the attribute returns the value No_Ast_Handler, and no AST occurs.
      --  If the entry is for a task that is not callable (T'Callable False),
      --  the exception program error is raised. If an AST occurs for an
      --  entry of a task that is terminated, the program is erroneous.
      --
      --  The attribute AST_Entry is supported only in OpenVMS versions
      --  of GNAT. It will be rejected as illegal in other GNAT versions.

      ---------
      -- Bit --
      ---------

      Attribute_Bit => True,
      --
      --  Obj'Bit, where Obj is any object, yields the bit offset within
      --  the storage unit (byte) that contains the first bit of storage
      --  allocated for the object. The value of this attribute is of the
      --  type Universal_Integer, and is always a non-negative number not
      --  exceeding the value of System.Storage_Unit.
      --
      --  For an object that is a variable or a constant allocated in a
      --  register, the value is zero. (The use of this attribute does not
      --  force the allocation of a variable to memory).
      --
      --  For an object that is a formal parameter, this attribute applies
      --  to either the matching actual parameter or to a copy of the
      --  matching actual parameter.
      --
      --  For an access object the value is zero. Note that Obj.all'Bit is
      --  subject to an Access_Check for the designated object. Similarly
      --  for a record component X.C'Bit is subject to a discriminant check
      --  and X(I).Bit and X(I1..I2)'Bit are subject to index checks.
      --
      --  This attribute is designed to be compatible with the DEC Ada
      --  definition and implementation of the Bit attribute.

      ------------------
      -- Code_Address --
      ------------------

      Attribute_Code_Address => True,
      --
      --  subp'Code_Address, where subp is a subprogram entity, gives the
      --  address of the first generated instruction for a subprogram. This
      --  is often, but not always the same as the 'Address value, which is
      --  the address to be used in a call. The differences occur in the case
      --  of a nested procedure (where Address yields the address of the
      --  trampoline code used to load the static link), and on some systems
      --  which use procedure descriptors (in which case Address yields the
      --  address of the descriptor).

      -----------------------
      -- Default_Bit_Order --
      -----------------------

      Attribute_Default_Bit_Order => True,
      --
      --  Standard'Default_Bit_Order (Standard is the only permissible prefix),
      --  provides the value System.Default_Bit_Order as a Pos value (0 for
      --  High_Order_First, 1 for Low_Order_First). This is used to construct
      --  the definition of Default_Bit_Order in package System. This is a
      --  static attribute.

      ---------------
      -- Elab_Body --
      ---------------

      Attribute_Elab_Body => True,
      --
      --  This attribute can only be applied to a program unit name. It
      --  returns the entity for the corresponding elaboration procedure
      --  for elaborating the body of the referenced unit. This is used
      --  in the main generated elaboration procedure by the binder, and
      --  is not normally used in any other context, but there may be
      --  specialized situations in which it is useful to be able to
      --  call this elaboration procedure from Ada code, e.g. if it
      --  is necessary to do selective reelaboration to fix some error.

      ---------------
      -- Elab_Spec --
      ---------------

      Attribute_Elab_Spec => True,
      --
      --  This attribute can only be applied to a program unit name. It
      --  returns the entity for the corresponding elaboration procedure
      --  for elaborating the spec of the referenced unit. This is used
      --  in the main generated elaboration procedure by the binder, and
      --  is not normally used in any other context, but there may be
      --  specialized situations in which it is useful to be able to
      --  call this elaboration procedure from Ada code, e.g. if it
      --  is necessary to do selective reelaboration to fix some error.

      ----------------
      -- Elaborated --
      ----------------

      Attribute_Elaborated => True,
      --
      --  Lunit'Elaborated, where Lunit is a library unit, yields a boolean
      --  value indicating whether or not the body of the designated library
      --  unit has been elaborated yet.

      --------------
      -- Enum_Rep --
      --------------

      Attribute_Enum_Rep => True,
      --
      --  For every enumeration subtype S, S'Enum_Rep denotes a function
      --  with the following specification:
      --
      --    function S'Enum_Rep (Arg : S'Base) return universal_integer;
      --
      --  The function returns the representation value for the given
      --  enumeration value. This will be equal to the 'Pos value in the
      --  absence of an enumeration representation clause. This is a static
      --  attribute (i.e. the result is static if the argument is static).

      -----------------
      -- Fixed_Value --
      -----------------

      Attribute_Fixed_Value => True,
      --
      --  For every fixed-point type S, S'Fixed_Value denotes a function
      --  with the following specification:
      --
      --    function S'Fixed_Value (Arg : universal_integer) return S;
      --
      --  The value returned is the fixed-point value V such that
      --
      --    V = Arg * S'Small
      --
      --  The effect is thus equivalent to first converting the argument
      --  to the integer type used to represent S, and then doing an
      --  unchecked conversion to the fixed-point type. This attribute is
      --  primarily intended for use in implementation of the input-output
      --  functions for fixed-point values.

      -----------------------
      -- Has_Discriminants --
      -----------------------

      Attribute_Has_Discriminants => True,
      --
      --  Gtyp'Has_Discriminants, where Gtyp is a generic formal type, yields
      --  a Boolean value indicating whether or not the actual instantiation
      --  type has discriminants.

      ---------
      -- Img --
      ---------

      Attribute_Img => True,
      --
      --  The 'Img function is defined for any prefix, P, that denotes an
      --  object of scalar type T. P'Img is equivalent to T'Image (P). This
      --  is convenient for debugging. For example:
      --
      --     Put_Line ("X = " & X'Img);
      --
      --  has the same meaning as the more verbose:
      --
      --     Put_Line ("X = " & Temperature_Type'Image (X));
      --
      --  where Temperature_Type is the subtype of the object X.

      -------------------
      -- Integer_Value --
      -------------------

      Attribute_Integer_Value => True,
      --
      --  For every integer type S, S'Integer_Value denotes a function
      --  with the following specification:
      --
      --    function S'Integer_Value (Arg : universal_fixed) return S;
      --
      --  The value returned is the integer value V, such that
      --
      --    Arg = V * fixed-type'Small
      --
      --  The effect is thus equivalent to first doing an unchecked convert
      --  from the fixed-point type to its corresponding implementation type,
      --  and then converting the result to the target integer type. This
      --  attribute is primarily intended for use in implementation of the
      --  standard input-output functions for fixed-point values.

      ------------------
      -- Machine_Size --
      ------------------

      Attribute_Machine_Size => True,
      --
      --  This attribute is identical to the Object_Size attribute. It is
      --  provided for compatibility with the DEC attribute of this name.

      -----------------------
      -- Maximum_Alignment --
      -----------------------

      Attribute_Maximum_Alignment => True,
      --
      --  Standard'Maximum_Alignment (Standard is the only permissible prefix)
      --  provides the maximum useful alignment value for the target. This
      --  is a static value that can be used to specify the alignment for an
      --  object, guaranteeing that it is properly aligned in all cases. The
      --  time this is useful is when an external object is imported and its
      --  alignment requirements are unknown. This is a static attribute.

      --------------------
      -- Mechanism_Code --
      --------------------

      Attribute_Mechanism_Code => True,
      --
      --  function'Mechanism_Code yeilds an integer code for the mechanism
      --  used for the result of function, and subprogram'Mechanism_Code (n)
      --  yields the mechanism used for formal parameter number n (a static
      --  integer value, 1 = first parameter). The code returned is:
      --
      --     1 = by copy (value)
      --     2 = by reference
      --     3 = by descriptor (default descriptor type)
      --     4 = by descriptor (UBS  unaligned bit string)
      --     5 = by descriptor (UBSB aligned bit string with arbitrary bounds)
      --     6 = by descriptor (UBA  unaligned bit array)
      --     7 = by descriptor (S    string, also scalar access type parameter)
      --     8 = by descriptor (SB   string with arbitrary bounds)
      --     9 = by descriptor (A    contiguous array)
      --    10 = by descriptor (NCA  non-contiguous array)

      --------------------
      -- Null_Parameter --
      --------------------

      Attribute_Null_Parameter => True,
      --
      --  A reference T'Null_Parameter denotes an (imaginary) object of
      --  type or subtype T allocated at (machine) address zero. The
      --  attribute is allowed only as the default expression of a formal
      --  parameter, or as an actual expression of a subporgram call. In
      --  either case, the subprogram must be imported.
      --
      --  The identity of the object is represented by the address zero
      --  in the argument list, independent of the passing mechanism
      --  (explicit or default).
      --
      --  The reason that this capability is needed is that for a record
      --  or other composite object passed by reference, there is no other
      --  way of specifying that a zero address should be passed.

      -----------------
      -- Object_Size --
      -----------------

      Attribute_Object_Size => True,
      --
      --  Type'Object_Size is the same as Type'Size for all types except
      --  fixed-point types and discrete types. For fixed-point types and
      --  discrete types, this attribute gives the size used for default
      --  allocation of objects and components of the size. See section
      --  in Einfo ("Handling of type'Size values") for further details.

      -------------------------
      -- Passed_By_Reference --
      -------------------------

      Attribute_Passed_By_Reference => True,
      --
      --  T'Passed_By_Reference for any subtype T returns a boolean value
      --  that is true if the type is normally passed by reference and
      --  false if the type is normally passed by copy in calls. For scalar
      --  types, the result is always False and is static. For non-scalar
      --  types, the result is non-static (since it is computed by Gigi).

      ------------------
      -- Range_Length --
      ------------------

      Attribute_Range_Length => True,
      --
      --  T'Range_Length for any discrete type T yields the number of
      --  values represented by the subtype (zero for a null range). The
      --  result is static for static subtypes. Note that Range_Length
      --  applied to the index subtype of a one dimensional array always
      --  gives the same result as Range applied to the array itself.
      --  The result is of type universal integer.

      ------------------
      -- Storage_Unit --
      ------------------

      Attribute_Storage_Unit => True,
      --
      --  Standard'Storage_Unit (Standard is the only permissible prefix)
      --  provides the value System.Storage_Unit, and is intended primarily
      --  for constructing this definition in package System (see note above
      --  in Default_Bit_Order description). The is a static attribute.

      ----------------
      -- To_Address --
      ----------------

      Attribute_To_Address => True,
      --
      --  System'To_Address (Address is the only permissible prefix)
      --  is a function that takes any integer value, and converts it into
      --  an address value. The semantics is to first convert the integer
      --  value to type Integer_Address according to normal conversion
      --  rules, and then to convert this to an address using the same
      --  semantics as the System.Storage_Elements.To_Address function.
      --  The important difference is that this is a static attribute
      --  so it can be used in initializations in preealborate packages.

      ----------------
      -- Type_Class --
      ----------------

      Attribute_Type_Class => True,
      --
      --  T'Type_Class for any type or subtype T yields the value of the
      --  type class for the full type of T. If T is a generic formal type,
      --  then the value is the value for the corresponding actual subtype.
      --  The value of this attribute is of type System.Aux_DEC.Type_Class,
      --  which has the following definition:
      --
      --    type Type_Class is
      --      (Type_Class_Enumeration,
      --       Type_Class_Integer,
      --       Type_Class_Fixed_Point,
      --       Type_Class_Floating_Point,
      --       Type_Class_Array,
      --       Type_Class_Record,
      --       Type_Class_Access,
      --       Type_Class_Task,
      --       Type_Class_Address);
      --
      --  Protected types yield the value Type_Class_Task, which thus
      --  applies to all concurrent types. This attribute is designed to
      --  be compatible with the DEC Ada attribute of the same name.
      --
      --  Note: if pragma Extend_System is used to merge the definitions of
      --  Aux_DEC into System, then the type Type_Class can be referenced
      --  as an entity within System, as can its enumeration literals.

      -----------------
      -- UET_Address --
      -----------------

      Attribute_UET_Address => True,
      --
      --  Unit'UET_Address, where Unit is a program unit, yields the address
      --  of the unit exception table for the specified unit. This is only
      --  used in the internal implementation of exception handling. See the
      --  implementation of unit Ada.Exceptions for details on its use.

      ------------------------------
      -- Universal_Literal_String --
      ------------------------------

      Attribute_Universal_Literal_String => True,
      --
      --  The prefix of 'Universal_Literal_String must be a named number.
      --  The static result is the string consisting of the characters of
      --  the number as defined in the original source. This allows the
      --  user program to access the actual text of named numbers without
      --  intermediate conversions and without the need to enclose the
      --  strings in quotes (which would preclude their use as numbers).
      --  This is used internally for the construction of values of the
      --  floating-point attributes from the file ttypef.ads, but may
      --  also be used by user programs.

      -------------------------
      -- Unrestricted_Access --
      -------------------------

      Attribute_Unrestricted_Access => True,
      --
      --  The Unrestricted_Access attribute is similar to Access except that
      --  all accessibility and aliased view checks are omitted. This is very
      --  much a user-beware attribute. Basically its status is very similar
      --  to Address, for which it is a desirable replacement where the value
      --  desired is an access type. In other words, its effect is identical
      --  to first taking 'Address and then doing an unchecked conversion to
      --  a desired access type. Note that in GNAT, but not necessarily in
      --  other implementations, the use of static chains for inner level
      --  subprograms means that Unrestricted_Access applied to a subprogram
      --  yields a value that can be called as long as the subprogram is in
      --  scope (normal Ada 95 accessibility rules restrict this usage).

      ---------------
      -- VADS_Size --
      ---------------

      Attribute_VADS_Size => True,
      --
      --  Typ'VADS_Size yields the Size value typically yielded by some
      --  Ada 83 compilers. The differences between VADS_Size and Size
      --  is that for scalar types for which no Size has been specified,
      --  VADS_Size yields the Object_Size rather than the Value_Size.
      --  For example, while Natural'Size is typically 31, the value of
      --  Natural'VADS_Size is 32. For all other types, Size and VADS_Size
      --  yield the same value.

      ----------------
      -- Value_Size --
      ----------------

      Attribute_Value_Size => True,
      --
      --  Type'Value_Size is the number of bits required to represent a
      --  value of the given subtype. It is the same as Type'Size, but,
      --  unlike Size, may be set for non-first subtypes. See section
      --  in Einfo ("Handling of type'Size values") for further details.

      ---------------
      -- Word_Size --
      ---------------

      Attribute_Word_Size => True,
      --
      --  Standard'Word_Size (Standard is the only permissible prefix)
      --  provides the value System.Word_Size, and is intended primarily
      --  for constructing this definition in package System (see note above
      --  in Default_Bit_Order description). This is a static attribute.

      others => False);

   -----------------
   -- Subprograms --
   -----------------

   procedure Analyze_Attribute (N : Node_Id);
   --  Performs bottom up semantic analysis of an attribute. Note that the
   --  parser has already checked that type returning attributes appear only
   --  in appropriate contexts (i.e. in subtype marks, or as prefixes for
   --  other attributes).

   procedure Resolve_Attribute (N : Node_Id; Typ : Entity_Id);
   --  Performs type resolution of attribute. If the attribute yields
   --  a universal value, mark its type as that of the context. On
   --  the other hand, if the context itself is universal (as in
   --  T'Val (T'Pos (X)), mark the type as being the largest type of
   --  that class that can be used at run-time. This is correct since
   --  either the value gets folded (in which case it doesn't matter
   --  what type of the class we give if, since the folding uses universal
   --  arithmetic anyway) or it doesn't get folded (in which case it is
   --  going to be dealt with at runtime, and the largest type is right).

end Sem_Attr;