Slide: 1

The Implementation of Ada 2005 Synchronized Interfaces in the GNAT Compiler

Javier MirandaHristian KirtchevEdmond Schonberg

Presentation cover page EU

www.adacore.com

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Interfaces Types

• Interfaces in Ada 2005

• Implementation of regular interfaces

• Limited, Synchronized, Protected, and Task interfaces in Ada 2005

• Basic implementation model: Corresponding records

• Dispatching calls in selective waits

• Status

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Interface Types in Ada 2005

type Root is tagged record ... ; . . .

type DT_1 is new Root and I1 and I2 with record . . . ; -- Must provide the implementation of P and R

type DT_2 is new DT_1 with record . . . ; -- Inherits all the primitives and interfaces of the ancestor

type I1 is interface;function P (X : I1) return Natural is abstract;

type I2 is interface and I1; procedure Q (X : I2) is null; procedure R (X : I2) is abstract;

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procedure Dispatch_I1_Call (Obj : I1'Class) isbegin -- Dispatch call to user -- defined subprogram . . := P (Obj);

-- Dispatch call to predefined -- stream operation I1'Write (Stream, Obj);end Dispatch_I1_Call;

procedure Dispatch_Call (Obj : Root'Class) isbegin -- Membership test if Obj in I2'Class then R (I2'Class (Obj));

end if;end Dispatch_Call;

type I1 is interface;function P (X : I1) return Natural is abstract;

type I2 is interface and I1; procedure Q (X : I2) is null; procedure R (X : I2) is abstract;

Interface Types in Ada 2005

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GNAT Implementation

• Main features:– Constant-time dispatching calls through interfaces– Compatible with the C++ ABI Layout

http://www.codesourcery.com/cxx-abi

• J.Miranda, E.Schonberg, G.Dismukes. The Implementation of Ada 2005 Interfaces Types in the

GNAT Compiler. AdaEurope’2005, York. Lecture Notes in Computer Science. Eds: T.Vardanega,

A.Wellings. Vol. 3555, pages 208-219. June, 2005. Springer-Verlag.

ISSN 0302-9743 ISBN 3-540-26286-5

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GNAT Object Layout: Example

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Primary and secondary dispatch tables

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C++ structure with abstract classes

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Interface Dispatching Call

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procedure P (Obj : DT; << formals >>) is ... . . .end P;

procedure I1_P_Thunk (Obj_Addr : Address; << formals>>) is M : constant := DT.I1'Position; Obj_Base : constant := Obj_Addr – Storage_Offset (M);begin P (Obj_Base, << formals >>); -- In practice not a call but a jumpend I1_P_Thunk;

Thunk Code

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Interface categories

• Regular interfaces– Implemented by tagged record types

• Task interfaces– Implemented by task types

• Protected interfaces– Implemented by protected types

• Synchronized interfaces– Implemented by tasks or protected types

• Limited interfaces– Implemented by record, task, or protected types.

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Run-time representation of Synchronized types

• Similar discriminated records

• Protected types– Private data

– Lock structure

– Entry queues (discriminated component)

– Finalization queues• Task types

– Pointer to task control block

– Lock structures

– Size, priority

– Task info• Target-independent layout

• Simpler structure for Ravenscar profile

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Implementing a synchronized interface with a task

type I1 is limited interface; procedure P (Obj : in out I1) is abstract; procedure Q (Obj : in out I1) is null;

type I2 is synchronized interface and I1; procedure R (Obj : in out I2; Data : Integer) is abstract;

task type Tsk is new I2 with entry P; -- Implements P of I1 entry R (Data : Integer); -- Implements R of I2 end Tsk;

procedure Q (T : in out Tsk); -- Implements Q of I1

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Implementation steps• Make corresponding records tagged (they are already limited)

– Interface dispatching data structures are as for regular interfaces

– Conformance rules must handle first argument specially

• Create wrappers for entries and protected operations– Wrappers are primitive operations of the corresponding record

– Interface dispatching calls require one additional call

• For entries, wrapper contains an entry call

• For protected operations, wrapper contains a call to the protected operation– Protected operation locks object, calls unprotected version

– Unprotected subprog contains code of operation (direct use in internal calls)

• Dispatching calls on a synchronized interface handle tasks and protected types uniformly

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Interface dispatching in selective wait and ATC

type I1 is limited interface; procedure P (X in out I1) is abstract;

protected type Prot1 is I1 with… type Lim is new I1 with record … end record;

procedure Dispatch (X: in out I1’class) is begin select X.P; -- protected or regular operation? else delay 0.1; end select; end Dispatch;

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Need additional Run-Time operationsto determine kind of target

• Operations on all types that implement limited interfaces:– Get_Prim_Op_Kind

– Record, task, or protected type operation? Procedure or entry?

– Get_Entry_Index– If entry-call, position of entry queue in object– Could be combined with previous operation

– Get_Offset_Index– Index in primary table corresponding to interface operation in

secondary table

• Corresponding information must be added to the Run-Time Type-Specific Data

• Operations are not dispatching

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New Dispatching Operations

• Expansion of selective waits depends critically on whether trigger is task or protected operation: no possible factorization

• Primitive operations for selective waits– Dispatching_Timed_Select

– Dispatching_Conditional_Select

• Primitive operations for ATC– Dispatching_Asynchronous_Select

– Dispatching_Get_Prim_Op_Kind

• Primitive operations for task interfaces– Dispatching_Get_Task_Id (for task abortion and attributes ‘Callable and

‘Terminated)

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Dispatch Table Structure

type Lim_Iface is limited interface;procedure A (Obj : in out Lim_Iface) is abstract;

type Synch_Iface is synchronized interface;procedure B (Obj : in out Synch_Iface) is abstract;

protected type Prot_Typ is new Lim_Iface and Synch_Ifacewith procedure A; entry B;end Prot_Typ;

P : Prot_Typ;

Offset_To_TopOSD_Ptr : Predefined : thunks : tableThunk_A’address

Secondary DT

Offset_To_TopOSD_Ptr : Predefined : thunks : tableThunk_B’address

Secondary DT

_Tag_DT1_DT2…Queues

POffset_To_TopTSD_Ptr : Predefined : primitives : table A’address B’address

Primary DT

Inheritance depthAccess levelExpanded nameExternal tag. . . SSD_PtrAncestor tagsInterface tags

Type Specific Data

B’Pos in the Primary DT

Offset Specific Data

A Prot.Proc -B Prot.Entry 1

Select Specific Data

A’Pos in the Primary DT

Offset Specific Data

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with DT_Pkg; use DT_Pkg;

procedure Simple_Dispatching is procedure Dispatch_On_A (Obj : in out Lim_Iface’Class) is begin

end Dispatch_On_A;

P : Prot_Typ;

begin

end Simple_Dispatching;

Interface dispatching call

_Tag_DT1_DT2…Queues

POffset_To_TopTSD_Ptr : Predefined : primitives : tableA’addressB’address

Primary DT

Offset_To_TopOSD_Ptr : Predefined : thunks : tableThunk_A’address

Secondary DT

Offset_To_TopOSD_Ptr : Predefined : thunks : tableThunk_B’address

Secondary DT

Inheritance depthAccess levelExpanded nameExternal tag. . .SSD_PtrAncestor tagsInterface tags

Type Specific Data

B’Pos in the primary DT

Offset Specific Data

A Prot_Proc -B Prot_Entry 1

Select Specific Data

declare Temp_Tag : Tag := Tag (Obj); Temp_Addr : Address := Temp_Tag.all.prim_tbl (A’Pos); begin Temp_Addr (Obj); end;

Obj.A;

Dispatch_On_A (Lim_Iface (P._DT1)); Dispatch_On_A (P);

PP._DT1Obj

A’Pos in the primary DT

Offset Specific Data

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Dispatching in Selective waits

with Ada.Text_IO; use Ada.Text_IO;with DT_Pkg; use DT_Pkg;

procedure Select_Dispatching is procedure Select_On_B (Obj : in out Synch_Iface’Class) is begin declare B : Boolean := False; C : Prim_Op_Kind; D : Duration := To_Duration (1.0); K : Tagged_Kind := Get_Tagged_Kind (Tag (Obj)); M : Integer := 1; P : Parameters := (Param1 … ParamN); S : Integer; begin if K = TK_Limited_Tagged then Obj.B; else S := Get_Offset_Index ( Tag (Obj), DT_Position (B));

_DTS (Obj, S, P’Address, D, M, C, B);

if C = POK_Protected_Entry or else C = POK_Task_Entry then

with Ada.Text_IO; use Ada.Text_IO;with DT_Pkg; use DT_Pkg;

procedure Select_Dispatching is procedure Select_On_B (Obj : in out Synch_Iface’Class) is begin select Obj.B; or delay 1.0; Put_Line (“Delayed”); end select; end Select_On_A;

P : Prot_Typ;begin Select_On_B (P);end Select_Dispatching;

Param1 := P.Param1; . . . ParamN := P.ParamN; end if;

if B then if C = POK_Proc or else C = POK_Protected_Proc or else C = POK_Task_Proc then Obj.B; end if; <triggering-statements> else <delay-statements> end if; end if; end; end Select_On_B;

P : Prot_Typ;

procedure _DTS (T : Prot_TypV; S : Integer; P : System.Address; D : Duration; M : Integer;

C : out Prim_Op_Kind; B : out Boolean) is I : Integer; begin C := Get_Prim_Op_Kind (T._Tag, S); if C = POK_Proc or else C = POK_Protected_Proc or else C = POK_Task_Proc then B := True; return; end if; I := Get_Entry_Index (T._Tag, S);

-- If Prot_Typ is a task type, the -- following expansion has a call -- to Timed_Task_Entry_Call

System.Timed_Protected_Entry_Call (T._Object’Unchecked_Access, Protected_Entry_Index (I), P, D, M, B); end _DTS;

begin Select_On_B (Synch_Iface (P._DT2));end Select_Dispatching;

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with Ada.Text_IO; use Ada.Text_IO;with DT_Pkg; use DT_Pkg;

procedure Select_Dispatching is procedure Select_On_B (Obj : in out Synch_Iface’Class) is begin declare B : Boolean := False; C : Prim_Op_Kind; D : Duration := To_Duration (1.0); K : Tagged_Kind := Get_Tagged_Kind (Tag (Obj)); M : Integer := 1; P : Parameters := (Param1 … ParamN); S : Integer ; begin if K = TK_Limited_Tagged then Obj.B; else S := Get_Offset_Index ( Tag (Obj), DT_Position (B));

_DTS (Obj, S, P’Address, D, M, C, B);

Dispatching in Selective waits: Example

_Tag_DT1_DT2…Queues

Prot_TypVOffset_To_TopTSD_Ptr 01 Predefined : primitives : table16 A’address17 B’address

Primary DT

Offset_To_TopOSD_Ptr01 Predefined : thunks : table16 Thunk of A

Secondary DT

Offset_To_TopOSD_Ptr01 Predefined : thunks : table16 Thunk of B

Secondary DT

Inheritance depthAccess levelExpanded nameExternal tag. . .SSD_PtrAncestor tagsInterface tags

Type Specific Data

A’Pos = 16Offset Specific Data

B’Pos = 17Offset Specific Data

A Prot_Proc -B Prot_Entr 1

Select Specific Data is I : Integer; begin C := Get_Prim_Op_Kind (T._Tag, S);

if C = POK_Proc or else C = POK_Protected_Proc or else C = POK_Task_Proc then B := True; return; end if;

I := Get_Entry_Index (T._Tag, S);

System.Timed_Protected_Entry_Call (T._Object’Unchecked_Access, Protected_Entry_Index (I), P, D, M, B); end _DTS;

begin Select_On_B (Synch_Iface (P._DT2));end Select_Dispatching;

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GAP 2005 Release

Abstract Interface Types (including task, protected and synchronized interfaces) available in GAP-2005

All 2005 enhancements available to GNAT Pro users under -gnat05 switch.

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The Implementation of Ada 2005 synchronized Interfaces in the GNAT Compiler

Javier MirandaHristian KirtchevEdmond Schonberg

End of talk

Presentation cover page EU

www.adacore.com

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GNAT Implementation ApproachesPermutation Maps versus C++ Model

• Constant-time dispatching calls through interfaces• Compatibility with the C++ ABI Layout

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Basic dispatch table structure

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Primary dispatch table

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