D32 Code generation

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General Overview

The code generation activity has been undertaken at the University of Southampton. This has been a new line of work for DEPLOY that was not identified in the original Description of Work for the project. The development of the approach, and the tools to support, it involved a number of team members at Southampton; and also at other institutions. This work draws on our recent experience with technologies such as Shared Event Decomposition , and the EMF Framework for Event-B . There was collaboration at an early stage with Newcastle University, where we explored the commonalities between their flow plug-in and the algorithmic structures used in our approach. Collaboration with the University of York was also established since we chose to use their Epsilon model-to-model transformation technology.

Motivations

The decision was taken in 2009 to include code generation as a project goal . It had been recognised that support for generation of code from refined Event-B models would be an important factor in ensuring eventual deployment of the DEPLOY approach within their organisations. This was especially true for Bosch and Space Systems Finland (SSF). After receiving more detailed requirements from Bosch and SSF, it became clear we should focus our efforts on supporting the generation of code for typical real-time embedded control software.

Choices / Decisions

Strategic Overview

During the last year we have focussed on supporting the generation of code for typical real-time embedded control software. To this end we have evolved a multi-tasking approach which is conceptually similar to that of the Ada tasking model. Individual tasks are treated as sequential programs; these tasks are modelled by an extension to Event-B, called Tasking Machines. Tasks have mutually exclusive access to state variables through the use of protected resources. The protected resources correspond to Event-B machines. For real-time control, periodic and one-shot activation is currently supported; and it is planned to support aperiodic tasks in the near future. Tasks have priorities to ensure appropriate responsiveness of the control software. For the DEPLOY project, it was regarded as sufficient to support construction of programs with a fixed number of tasks and a fixed number of shared variables – no dynamic creation of processes or objects has been accommodated.

Our main goal this year has been to devise an approach for, and provide tool support for, code generation. In accord with the resources available during the year it was decided to limit the provision of tool support to that of a demonstrator tool. The tool is a proof-of-concept only, and lacks the productivity enhancements expected in a more mature tool. Nevertheless much insight has been gained in undertaking this work; it lays a foundation for future research, and will be useful since it will allow interested parties to explore the approach.

The Tasking Extension for Event-B

The following text can be read in conjunction with the slides from the Deploy Plenary Meeting - Zurich 2010.

Tasking Event-B can be viewed as an extension of the existing Event-B language. We use the existing approaches of refinement and decomposition to structure a development that is suitable for construction of a Tasking Development. At some point during the modelling phase parameters may have to be introduced to facilitate decomposition. This constitutes a natural part of the refinement process as it moves towards decomposition and on to the implementation level. During decomposition parameters form part of the interface that enables event synchronization. We make use of this interface and add information (see #Events For Tasking) to facilitate code generation.

A Tasking Development is generated programmatically, at the direction of the user; the Tasking Development consists of a number of machines (and perhaps associated contexts). In our approach we make use of the Event-B EMF extension mechanism which allows addition of new constructs to a model. The tasking extension consists of the constructs in the following table.

Construct	Options
Machine Type	DeclaredTask, AutoTask, SharedMachine
Control	Sequence, Loop, Branch, EventSynch
Task Type	Periodic(n), Triggered, Repeating, OneShot
Priority	-
Event Type	Branch, Loop, ProcedureDef, ProcedureSynch
Parameter Type	ActualIn, ActualOut, FormalIn, FormalOut

The machines in the Tasking Development are extended with the constructs shown in the table, and may be viewed as keywords in a textual representation of the language. With extensions added, a Tasking Development can be translated to a common language model for mapping to implementation source code. There is also a translator that constructs new machines/contexts modelling the implementation, and these should refine/extend the existing elements of the Event-B project.

Tasking Machines

The following constructs relate only to Tasking Machines, and provide implementation details. Timing of periodic tasks is not modelled formally. Tasking Machines map to the concept of a process/thread/task. These can be implemented by Ada tasks, use of the pthread library in C, or Java threads.

• Tasking Machines may be characterised by the following types:
  • AutoTasks - Singleton Tasks.
  • Declared tasks - (Not currently used) A task template relating to an Ada tasktype declaration.
  • TaskType - Defines the scheduling, cycle and lifetime of a task. i.e. one-shot periodic or triggered.
  • Priority - An integer value is supplied, the task with the highest value priority takes precedence when being scheduled.

Shared Machines

Shared Machines map to the concept of a protected resource, or monitor. They may be implemented in Ada as a protected object, in C using a mutex lock, or Java monitor.

• Applied to the Shared Machine we have:
  • A SharedMachine keyword that identifies a machine as a Shared Machine.

More Details

Control Constructs

Each Tasking Machine has a task body which contains the flow control, or algorithmic, constructs.

• We have the following constructs available in the Tasking Machine body:
  • Sequence - for imposing an order on events.
  • Branch - choice between a number of mutually exclusive events.
  • Loop - event repetition while it's guard remains true.
  • Event Synchronisation - synchronization between an event in a Tasking Machine and an event in a Shared Machine.
  • Event-wrappers - The synchronization construct is contained in an event wrapper. The wrapper may also contain a single event which may belong to the Tasking Machine, or the Shared Machine.

Events In Tasking Developments

Event implementation. Branch, Loop, ProcedureSych, ProcedureDef

Event parameter types. FormalIn FormalOut, ActualIn, ActualOut

• In Shared Machines:
  • events can only be designated as ProcedureDef or ProcedureSynch.
  • parameters of ProcedureSynch can only be FormalIn or FormalOut

• In procedureDef
  • parameters are not allowed.

Other Technical Issues

Meta-models: The use of Epsilon for translation.

The Deliverable

The demonstrator tool was released on 30 November 2010, and is available as an update site, or bundled Rodin package from:

https://sourceforge.net/projects/codegenerationd/files 

Sources are available from:

https://codegenerationd.svn.sourceforge.net/svnroot/codegenerationd

The tool is based on a build of Rodin 1.3.1 (not Rodin 2.0.0 due to dependency conflicts).

• The Code Generation tool consists of,
  • a Tasking Development Generator.
  • a Tasking Development Editor (Based on an EMF Tree Editor).
  • a translator, from Tasking Development to Common Language Model (IL1).
  • a translator, from the Tasking Development to Event-B model of the implementation.
  • a pretty-printer for the Tasking Development.
  • a pretty-printer for Common Language Model, which generates Ada Source Code.

Available Documentation

Much insight was gained during the work on code generation reported in the thesis Providing Concurrent Implementations for Event-B Developments

Tooling issues were reported in a paper Tool Support for Event-B Code Generation which was presented at Workshop on Tool Building in Formal Methods, http://abzconference.org/

There are technical notes available , that give more precise details of the approach and the mapping between Event-B and the common language meta-model, and its corresponding Event-B model.

For users

There is a wiki page at http://wiki.event-b.org/index.php/Code_Generation_Activity

There is a tutorial at http://wiki.event-b.org/index.php/Code_Generation_Tutorial

Planning

This paragraph shall give a timeline and current status (as of 28 Jan 2011).