path: root/gcc/ada/a-tasatt.adb

------------------------------------------------------------------------------
--                                                                          --
--                        GNAT RUN-TIME COMPONENTS                          --
--                                                                          --
--                  A D A . T A S K _ A T T R I B U T E S                   --
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--             Copyright (C) 1991-1994, Florida State University            --
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-- unit, or you link  this unit with other files  to produce an executable, --
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-- covered  by the  GNU  General  Public  License.  This exception does not --
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------------------------------------------------------------------------------

--  The following notes are provided in case someone decides the implementation
--  of this package is too complicated, or too slow. Please read this before
--  making any "simplifications".

--  Correct implementation of this package is more difficult than one might
--  expect. After considering (and coding) several alternatives, we settled on
--  the present compromise. Things we do not like about this implementation
--  include:

--  - It is vulnerable to bad Task_Id values, to the extent of possibly
--     trashing memory and crashing the runtime system.

--  - It requires dynamic storage allocation for each new attribute value,
--     except for types that happen to be the same size as System.Address, or
--     shorter.

--  -  Instantiations at other than the library level rely on being able to
--     do down-level calls to a procedure declared in the generic package body.
--     This makes it potentially vulnerable to compiler changes.

--  The main implementation issue here is that the connection from task to
--  attribute is a potential source of dangling references.

--  When a task goes away, we want to be able to recover all the storage
--  associated with its attributes. The Ada mechanism for this is finalization,
--  via controlled attribute types. For this reason, the ARM requires
--  finalization of attribute values when the associated task terminates.

--  This finalization must be triggered by the tasking runtime system, during
--  termination of the task. Given the active set of instantiations of
--  Ada.Task_Attributes is dynamic, the number and types of attributes
--  belonging to a task will not be known until the task actually terminates.
--  Some of these types may be controlled and some may not. The RTS must find
--  some way to determine which of these attributes need finalization, and
--  invoke the appropriate finalization on them.

--  One way this might be done is to create a special finalization chain for
--  each task, similar to the finalization chain that is used for controlled
--  objects within the task. This would differ from the usual finalization
--  chain in that it would not have a LIFO structure, since attributes may be
--  added to a task at any time during its lifetime. This might be the right
--  way to go for the longer term, but at present this approach is not open,
--  since GNAT does not provide such special finalization support.

--  Lacking special compiler support, the RTS is limited to the normal ways an
--  application invokes finalization, i.e.

--  a) Explicit call to the procedure Finalize, if we know the type has this
--     operation defined on it. This is not sufficient, since we have no way
--     of determining whether a given generic formal Attribute type is
--     controlled, and no visibility of the associated Finalize procedure, in
--     the generic body.

--  b) Leaving the scope of a local object of a controlled type. This does not
--     help, since the lifetime of an instantiation of Ada.Task_Attributes
--     does not correspond to the lifetimes of the various tasks which may
--     have that attribute.

--  c) Assignment of another value to the object. This would not help, since
--     we then have to finalize the new value of the object.

--  d) Unchecked deallocation of an object of a controlled type. This seems to
--     be the only mechanism available to the runtime system for finalization
--     of task attributes.

--  We considered two ways of using unchecked deallocation, both based on a
--  linked list of that would hang from the task control block.

--  In the first approach the objects on the attribute list are all derived
--  from one controlled type, say T, and are linked using an access type to
--  T'Class. The runtime system has an Ada.Unchecked_Deallocation for T'Class
--  with access type T'Class, and uses this to deallocate and finalize all the
--  items in the list. The limitation of this approach is that each
--  instantiation of the package Ada.Task_Attributes derives a new record
--  extension of T, and since T is controlled (RM 3.9.1 (3)), instantiation is
--  only allowed at the library level.

--  In the second approach the objects on the attribute list are of unrelated
--  but structurally similar types. Unchecked conversion is used to circument
--  Ada type checking. Each attribute-storage node contains not only the
--  attribute value and a link for chaining, but also a pointer to descriptor
--  for the corresponding instantiation of Task_Attributes. The instantiation
--  descriptor contains pointer to a procedure that can do the correct
--  deallocation and finalization for that type of attribute. On task
--  termination, the runtime system uses the pointer to call the appropriate
--  deallocator.

--  While this gets around the limitation that instantations be at the library
--  level, it relies on an implementation feature that may not always be safe,
--  i.e. that it is safe to call the Deallocate procedure for an instantiation
--  of Ada.Task_Attributes that no longer exists. In general, it seems this
--  might result in dangling references.

--  Another problem with instantiations deeper than the library level is that
--  there is risk of storage leakage, or dangling references to reused storage.
--  That is, if an instantiation of Ada.Task_Attributes is made within a
--  procedure, what happens to the storage allocated for attributes, when the
--  procedure call returns? Apparently (RM 7.6.1 (4)) any such objects must be
--  finalized, since they will no longer be accessible, and in general one
--  would expect that the storage they occupy would be recovered for later
--  reuse. (If not, we would have a case of storage leakage.) Assuming the
--  storage is recovered and later reused, we have potentially dangerous
--  dangling references. When the procedure containing the instantiation of
--  Ada.Task_Attributes returns, there may still be unterminated tasks with
--  associated attribute values for that instantiation. When such tasks
--  eventually terminate, the RTS will attempt to call the Deallocate procedure
--  on them. If the corresponding storage has already been deallocated, when
--  the master of the access type was left, we have a potential disaster. This
--  disaster is compounded since the pointer to Deallocate is probably through
--  a "trampoline" which will also have been destroyed.

--  For this reason, we arrange to remove all dangling references before
--  leaving the scope of an instantiation. This is ugly, since it requires
--  traversing the list of all tasks, but it is no more ugly than a similar
--  traversal that we must do at the point of instantiation in order to
--  initialize the attributes of all tasks. At least we only need to do these
--  traversals if the type is controlled.

--  We chose to defer allocation of storage for attributes until the Reference
--  function is called or the attribute is first set to a value different from
--  the default initial one. This allows a potential savings in allocation,
--  for attributes that are not used by all tasks.

--  For efficiency, we reserve space in the TCB for a fixed number of direct-
--  access attributes. These are required to be of a size that fits in the
--  space of an object of type System.Address. Because we must use unchecked
--  bitwise copy operations on these values, they cannot be of a controlled
--  type, but that is covered automatically since controlled objects are too
--  large to fit in the spaces.

--  We originally deferred initialization of these direct-access attributes,
--  just as we do for the indirect-access attributes, and used a per-task bit
--  vector to keep track of which attributes were currently defined for that
--  task. We found that the overhead of maintaining this bit-vector seriously
--  slowed down access to the attributes, and made the fetch operation non-
--  atomic, so that even to read an attribute value required locking the TCB.
--  Therefore, we now initialize such attributes for all existing tasks at the
--  time of the attribute instantiation, and initialize existing attributes for
--  each new task at the time it is created.

--  The latter initialization requires a list of all the instantiation
--  descriptors. Updates to this list, as well as the bit-vector that is used
--  to reserve slots for attributes in the TCB, require mutual exclusion. That
--  is provided by the Lock/Unlock_RTS.

--  One special problem that added complexity to the design is that the per-
--  task list of indirect attributes contains objects of different types. We
--  use unchecked pointer conversion to link these nodes together and access
--  them, but the records may not have identical internal structure. Initially,
--  we thought it would be enough to allocate all the common components of
--  the records at the front of each record, so that their positions would
--  correspond. Unfortunately, GNAT adds "dope" information at the front
--  of a record, if the record contains any controlled-type components.
--
--  This means that the offset of the fields we use to link the nodes is at
--  different positions on nodes of different types. To get around this, each
--  attribute storage record consists of a core node and wrapper. The core
--  nodes are all of the same type, and it is these that are linked together
--  and generally "seen" by the RTS. Each core node contains a pointer to its
--  own wrapper, which is a record that contains the core node along with an
--  attribute value, approximately as follows:

--    type Node;
--    type Node_Access is access all Node;
--    type Wrapper;
--    type Access_Wrapper is access all Wrapper;
--    type Node is record
--       Next    : Node_Access;
--       ...
--       Wrapper : Access_Wrapper;
--    end record;
--    type Wrapper is record
--       Dummy_Node : aliased Node;
--       Value      : aliased Attribute;  --  the generic formal type
--    end record;

--  Another interesting problem is with the initialization of the instantiation
--  descriptors. Originally, we did this all via the Initialize procedure of
--  the descriptor type and code in the package body. It turned out that the
--  Initialize procedure needed quite a bit of information, including the size
--  of the attribute type, the initial value of the attribute (if it fits in
--  the TCB), and a pointer to the deallocator procedure. These needed to be
--  "passed" in via access discriminants. GNAT was having trouble with access
--  discriminants, so all this work was moved to the package body.

--  Note that references to objects declared in this package body must in
--  general use 'Unchecked_Access instead of 'Access as the package can be
--  instantiated from within a local context.

with System.Error_Reporting;
with System.Storage_Elements;
with System.Task_Primitives.Operations;
with System.Tasking;
with System.Tasking.Initialization;
with System.Tasking.Task_Attributes;

with Ada.Exceptions;
with Ada.Unchecked_Conversion;
with Ada.Unchecked_Deallocation;

pragma Elaborate_All (System.Tasking.Task_Attributes);
--  To ensure the initialization of object Local (below) will work

package body Ada.Task_Attributes is

   use System.Error_Reporting,
       System.Tasking.Initialization,
       System.Tasking,
       System.Tasking.Task_Attributes,
       Ada.Exceptions;

   package POP renames System.Task_Primitives.Operations;

   ---------------------------
   -- Unchecked Conversions --
   ---------------------------

   --  The following type corresponds to Dummy_Wrapper,
   --  declared in System.Tasking.Task_Attributes.

   type Wrapper;
   type Access_Wrapper is access all Wrapper;

   pragma Warnings (Off);
   --  We turn warnings off for the following To_Attribute_Handle conversions,
   --  since these are used only for small attributes where we know that there
   --  are no problems with alignment, but the compiler will generate warnings
   --  for the occurrences in the large attribute case, even though they will
   --  not actually be used.

   function To_Attribute_Handle is new Ada.Unchecked_Conversion
     (System.Address, Attribute_Handle);
   function To_Direct_Attribute_Element is new Ada.Unchecked_Conversion
     (System.Address, Direct_Attribute_Element);
   --  For reference to directly addressed task attributes

   type Access_Integer_Address is access all
     System.Storage_Elements.Integer_Address;

   function To_Attribute_Handle is new Ada.Unchecked_Conversion
     (Access_Integer_Address, Attribute_Handle);
   --  For reference to directly addressed task attributes

   pragma Warnings (On);
   --  End warnings off region for directly addressed attribute conversions

   function To_Access_Address is new Ada.Unchecked_Conversion
     (Access_Node, Access_Address);
   --  To store pointer to list of indirect attributes

   pragma Warnings (Off);
   function To_Access_Wrapper is new Ada.Unchecked_Conversion
     (Access_Dummy_Wrapper, Access_Wrapper);
   pragma Warnings (On);
   --  To fetch pointer to actual wrapper of attribute node. We turn off
   --  warnings since this may generate an alignment warning. The warning can
   --  be ignored since Dummy_Wrapper is only a non-generic standin for the
   --  real wrapper type (we never actually allocate objects of type
   --  Dummy_Wrapper).

   function To_Access_Dummy_Wrapper is new Ada.Unchecked_Conversion
     (Access_Wrapper, Access_Dummy_Wrapper);
   --  To store pointer to actual wrapper of attribute node

   function To_Task_Id is new Ada.Unchecked_Conversion
     (Task_Identification.Task_Id, Task_Id);
   --  To access TCB of identified task

   type Local_Deallocator is access procedure (P : in out Access_Node);

   function To_Lib_Level_Deallocator is new Ada.Unchecked_Conversion
     (Local_Deallocator, Deallocator);
   --  To defeat accessibility check

   ------------------------
   -- Storage Management --
   ------------------------

   procedure Deallocate (P : in out Access_Node);
   --  Passed to the RTS via unchecked conversion of a pointer to permit
   --  finalization and deallocation of attribute storage nodes.

   --------------------------
   -- Instantiation Record --
   --------------------------

   Local : aliased Instance;
   --  Initialized in package body

   type Wrapper is record
      Dummy_Node : aliased Node;

      Value : aliased Attribute := Initial_Value;
      --  The generic formal type, may be controlled
   end record;

   --  A number of unchecked conversions involving Wrapper_Access sources are
   --  performed in this unit. We have to ensure that the designated object is
   --  always strictly enough aligned.

   for Wrapper'Alignment use Standard'Maximum_Alignment;

   procedure Free is
      new Ada.Unchecked_Deallocation (Wrapper, Access_Wrapper);

   procedure Deallocate (P : in out Access_Node) is
      T : Access_Wrapper := To_Access_Wrapper (P.Wrapper);
   begin
      Free (T);
   end Deallocate;

   ---------------
   -- Reference --
   ---------------

   function Reference
     (T    : Task_Identification.Task_Id := Task_Identification.Current_Task)
      return Attribute_Handle
   is
      TT            : constant Task_Id := To_Task_Id (T);
      Error_Message : constant String  := "Trying to get the reference of a ";

   begin
      if TT = null then
         Raise_Exception (Program_Error'Identity, Error_Message & "null task");
      end if;

      if TT.Common.State = Terminated then
         Raise_Exception (Tasking_Error'Identity,
           Error_Message & "terminated task");
      end if;

      --  Directly addressed case

      if Local.Index /= 0 then

         --  Return the attribute handle. Warnings off because this return
         --  statement generates alignment warnings for large attributes
         --  (but will never be executed in this case anyway).

         pragma Warnings (Off);
         return
           To_Attribute_Handle (TT.Direct_Attributes (Local.Index)'Address);
         pragma Warnings (On);

      --  Not directly addressed

      else
         declare
            P       : Access_Node := To_Access_Node (TT.Indirect_Attributes);
            W       : Access_Wrapper;
            Self_Id : constant Task_Id := POP.Self;

         begin
            Defer_Abort (Self_Id);
            POP.Lock_RTS;

            while P /= null loop
               if P.Instance = Access_Instance'(Local'Unchecked_Access) then
                  POP.Unlock_RTS;
                  Undefer_Abort (Self_Id);
                  return To_Access_Wrapper (P.Wrapper).Value'Access;
               end if;

               P := P.Next;
            end loop;

            --  Unlock the RTS here to follow the lock ordering rule
            --  that prevent us from using new (i.e the Global_Lock) while
            --  holding any other lock.

            POP.Unlock_RTS;
            W := new Wrapper'
                  ((null, Local'Unchecked_Access, null), Initial_Value);
            POP.Lock_RTS;

            P := W.Dummy_Node'Unchecked_Access;
            P.Wrapper := To_Access_Dummy_Wrapper (W);
            P.Next := To_Access_Node (TT.Indirect_Attributes);
            TT.Indirect_Attributes := To_Access_Address (P);
            POP.Unlock_RTS;
            Undefer_Abort (Self_Id);
            return W.Value'Access;

         exception
            when others =>
               POP.Unlock_RTS;
               Undefer_Abort (Self_Id);
               raise;
         end;
      end if;

      pragma Assert (Shutdown ("Should never get here in Reference"));
      return null;

   exception
      when Tasking_Error | Program_Error =>
         raise;

      when others =>
         raise Program_Error;
   end Reference;

   ------------------
   -- Reinitialize --
   ------------------

   procedure Reinitialize
     (T : Task_Identification.Task_Id := Task_Identification.Current_Task)
   is
      TT            : constant Task_Id := To_Task_Id (T);
      Error_Message : constant String  := "Trying to Reinitialize a ";

   begin
      if TT = null then
         Raise_Exception (Program_Error'Identity, Error_Message & "null task");
      end if;

      if TT.Common.State = Terminated then
         Raise_Exception (Tasking_Error'Identity,
           Error_Message & "terminated task");
      end if;

      if Local.Index /= 0 then
         Set_Value (Initial_Value, T);
      else
         declare
            P, Q    : Access_Node;
            W       : Access_Wrapper;
            Self_Id : constant Task_Id := POP.Self;

         begin
            Defer_Abort (Self_Id);
            POP.Lock_RTS;
            Q := To_Access_Node (TT.Indirect_Attributes);

            while Q /= null loop
               if Q.Instance = Access_Instance'(Local'Unchecked_Access) then
                  if P = null then
                     TT.Indirect_Attributes := To_Access_Address (Q.Next);
                  else
                     P.Next := Q.Next;
                  end if;

                  W := To_Access_Wrapper (Q.Wrapper);
                  Free (W);
                  POP.Unlock_RTS;
                  Undefer_Abort (Self_Id);
                  return;
               end if;

               P := Q;
               Q := Q.Next;
            end loop;

            POP.Unlock_RTS;
            Undefer_Abort (Self_Id);

         exception
            when others =>
               POP.Unlock_RTS;
               Undefer_Abort (Self_Id);
               raise;
         end;
      end if;

   exception
      when Tasking_Error | Program_Error =>
         raise;

      when others =>
         raise Program_Error;
   end Reinitialize;

   ---------------
   -- Set_Value --
   ---------------

   procedure Set_Value
     (Val : Attribute;
      T   : Task_Identification.Task_Id := Task_Identification.Current_Task)
   is
      TT            : constant Task_Id := To_Task_Id (T);
      Error_Message : constant String  := "Trying to Set the Value of a ";

   begin
      if TT = null then
         Raise_Exception (Program_Error'Identity, Error_Message & "null task");
      end if;

      if TT.Common.State = Terminated then
         Raise_Exception (Tasking_Error'Identity,
           Error_Message & "terminated task");
      end if;

      --  Directly addressed case

      if Local.Index /= 0 then

         --  Set attribute handle, warnings off, because this code can generate
         --  alignment warnings with large attributes (but of course will not
         --  be executed in this case, since we never have direct addressing in
         --  such cases).

         pragma Warnings (Off);
         To_Attribute_Handle
            (TT.Direct_Attributes (Local.Index)'Address).all := Val;
         pragma Warnings (On);
         return;
      end if;

      --  Not directly addressed

      declare
         P       : Access_Node := To_Access_Node (TT.Indirect_Attributes);
         W       : Access_Wrapper;
         Self_Id : constant Task_Id := POP.Self;

      begin
         Defer_Abort (Self_Id);
         POP.Lock_RTS;

         while P /= null loop

            if P.Instance = Access_Instance'(Local'Unchecked_Access) then
               To_Access_Wrapper (P.Wrapper).Value := Val;
               POP.Unlock_RTS;
               Undefer_Abort (Self_Id);
               return;
            end if;

            P := P.Next;
         end loop;

         --  Unlock RTS here to follow the lock ordering rule that prevent us
         --  from using new (i.e the Global_Lock) while holding any other lock.

         POP.Unlock_RTS;
         W := new Wrapper'((null, Local'Unchecked_Access, null), Val);
         POP.Lock_RTS;
         P := W.Dummy_Node'Unchecked_Access;
         P.Wrapper := To_Access_Dummy_Wrapper (W);
         P.Next := To_Access_Node (TT.Indirect_Attributes);
         TT.Indirect_Attributes := To_Access_Address (P);

         POP.Unlock_RTS;
         Undefer_Abort (Self_Id);

      exception
         when others =>
            POP.Unlock_RTS;
            Undefer_Abort (Self_Id);
            raise;
      end;

   exception
      when Tasking_Error | Program_Error =>
         raise;

      when others =>
         raise Program_Error;
   end Set_Value;

   -----------
   -- Value --
   -----------

   function Value
     (T : Task_Identification.Task_Id := Task_Identification.Current_Task)
      return Attribute
   is
      TT            : constant Task_Id := To_Task_Id (T);
      Error_Message : constant String  := "Trying to get the Value of a ";

   begin
      if TT = null then
         Raise_Exception (Program_Error'Identity, Error_Message & "null task");
      end if;

      if TT.Common.State = Terminated then
         Raise_Exception
           (Program_Error'Identity, Error_Message & "terminated task");
      end if;

      --  Directly addressed case

      if Local.Index /= 0 then

         --  Get value of attribute. We turn Warnings off, because for large
         --  attributes, this code can generate alignment warnings. But of
         --  course large attributes are never directly addressed so in fact
         --  we will never execute the code in this case.

         pragma Warnings (Off);
         return To_Attribute_Handle
           (TT.Direct_Attributes (Local.Index)'Address).all;
         pragma Warnings (On);
      end if;

      --  Not directly addressed

      declare
         P       : Access_Node;
         Result  : Attribute;
         Self_Id : constant Task_Id := POP.Self;

      begin
         Defer_Abort (Self_Id);
         POP.Lock_RTS;
         P := To_Access_Node (TT.Indirect_Attributes);

         while P /= null loop
            if P.Instance = Access_Instance'(Local'Unchecked_Access) then
               Result := To_Access_Wrapper (P.Wrapper).Value;
               POP.Unlock_RTS;
               Undefer_Abort (Self_Id);
               return Result;
            end if;

            P := P.Next;
         end loop;

         POP.Unlock_RTS;
         Undefer_Abort (Self_Id);
         return Initial_Value;

      exception
         when others =>
            POP.Unlock_RTS;
            Undefer_Abort (Self_Id);
            raise;
      end;

   exception
      when Tasking_Error | Program_Error =>
         raise;

      when others =>
         raise Program_Error;
   end Value;

--  Start of elaboration code for package Ada.Task_Attributes

begin
   --  This unchecked conversion can give warnings when alignments are
   --  incorrect, but they will not be used in such cases anyway, so the
   --  warnings can be safely ignored.

   pragma Warnings (Off);
   Local.Deallocate := To_Lib_Level_Deallocator (Deallocate'Access);
   pragma Warnings (On);

   declare
      Two_To_J : Direct_Index_Vector;
      Self_Id  : constant Task_Id := POP.Self;
   begin
      Defer_Abort (Self_Id);

      --  Need protection for updating links to per-task initialization and
      --  finalization routines, in case some task is being created or
      --  terminated concurrently.

      POP.Lock_RTS;

      --  Add this instantiation to the list of all instantiations

      Local.Next := System.Tasking.Task_Attributes.All_Attributes;
      System.Tasking.Task_Attributes.All_Attributes :=
        Local'Unchecked_Access;

      --  Try to find space for the attribute in the TCB

      Local.Index := 0;
      Two_To_J := 1;

      if Attribute'Size <= System.Address'Size then
         for J in Direct_Index_Range loop
            if (Two_To_J and In_Use) = 0 then

               --  Reserve location J for this attribute

               In_Use := In_Use or Two_To_J;
               Local.Index := J;

               --  This unchecked conversion can give a warning when the
               --  alignment is incorrect, but it will not be used in such
               --  a case anyway, so the warning can be safely ignored.

               pragma Warnings (Off);
               To_Attribute_Handle (Local.Initial_Value'Access).all :=
                 Initial_Value;
               pragma Warnings (On);

               exit;
            end if;

            Two_To_J := Two_To_J * 2;
         end loop;
      end if;

      --  Attribute goes directly in the TCB

      if Local.Index /= 0 then
         --  Replace stub for initialization routine that is called at task
         --  creation.

         Initialization.Initialize_Attributes_Link :=
           System.Tasking.Task_Attributes.Initialize_Attributes'Access;

         --  Initialize the attribute, for all tasks

         declare
            C : System.Tasking.Task_Id := System.Tasking.All_Tasks_List;
         begin
            while C /= null loop
               C.Direct_Attributes (Local.Index) :=
                 To_Direct_Attribute_Element
                   (System.Storage_Elements.To_Address (Local.Initial_Value));
               C := C.Common.All_Tasks_Link;
            end loop;
         end;

      --  Attribute goes into a node onto a linked list

      else
         --  Replace stub for finalization routine called at task termination

         Initialization.Finalize_Attributes_Link :=
           System.Tasking.Task_Attributes.Finalize_Attributes'Access;
      end if;

      POP.Unlock_RTS;
      Undefer_Abort (Self_Id);
   end;
end Ada.Task_Attributes;