Ganesh Sittampalam

Adding trace matching with free variables to AspectJ

An aspect observes the execution of a base program; when certain actions occur, the aspect runs some extra code of its own. In the AspectJ language, the observations that an aspect can make are confined to the current action: it is not possible to directly observe the history of a computation.Recently, there have been several interesting proposals for new history-based language features, most notably by Douence et al. and by Walker and Viggers. In this paper, we present a new history-based language feature called tracematches that enables the programmer to trigger the execution of extra code by specifying a regular pattern of events in a computation trace. We have fully designed and implemented tracematches as a seamless extension of AspectJ.A key innovation in our tracematch approach is the introduction of free variables in the matching patterns. This enhancement enables a whole new class of applications in which events can be matched not only by the event kind, but also by the values associated with the free variables. We provide several examples of applications enabled by this feature.After introducing and motivating the idea of tracematches via examples, we present a detailed semantics of our language design, and we derive an implementation from that semantics. The implementation has been realised as an extension of the abc compiler for AspectJ.

abc: an extensible AspectJ compiler

Research in the design of aspect-oriented programming languages requires a workbench that facilitates easy experimentation with new language features and implementation techniques. In particular, new features for AspectJ have been proposed that require extensions in many dimensions: syntax, type checking and code generation, as well as data flow and control flow analyses.The AspectBench Compiler (abc) is an implementation of such a workbench. The base version of abc implements the full AspectJ language. Its frontend is built, using the Polyglot framework, as a modular extension of the Java language. The use of Polyglot gives flexibility of syntax and type checking. The backend is built using the Soot framework, to give modular code generation and analyses.In this paper, we outline the design of abc, focusing mostly on how the design supports extensibility. We then provide a general overview of how to use abc to implement an extension. Finally, we illustrate the extension mechanisms of abc through a number of small, but non-trivial, examples. abc is freely available under the GNU LGPL.

Optimising aspectJ

AspectJ, an aspect-oriented extension of Java, is becoming increasingly popular. However, not much work has been directed at optimising compilers for AspectJ. Optimising AOP languages provides many new and interesting challenges for compiler writers, and this paper identifies and addresses three such challenges.First, compiling around advice efficiently is particularly challenging. We provide a new code generation strategy for around advice, which (unlike previous implementations) both avoids the use of excessive inlining and the use of closures. We show it leads to more compact code, and can also improve run-time performance. Second, woven code sometimes includes run-time tests to determine whether advice should execute. One important case is the cflow pointcut which uses information about the dynamic calling context. Previous techniques for cflow were very costly in terms of both time and space. We present new techniques to minimise or eliminate the overhead of cflow using both intra- and inter-procedural analyses. Third, we have addressed the general problem of how to structure an optimising compiler so that traditional analyses can be easily adapted to the AOP setting.We have implemented all of the techniques in this paper in abc, our AspectBench Compiler for AspectJ, and we demonstrate significant speedups with empirical results. Some of our techniques have already been integrated into the production AspectJ compiler, ajc 1.2.1.

Measuring the dynamic behaviour of AspectJ programs

This paper proposes and implements a rigorous method for studying the dynamic behaviour of AspectJ programs. As part of this methodology several new metrics specific to AspectJ programs are proposed and tools for collecting the relevant metrics are presented. The major tools consist of: (1) a modified version of the AspectJ compiler that tags bytecode instructions with an indication of the cause of their generation, such as a particular feature of AspectJ; and (2) a modified version of the *J dynamic metrics collection tool which is composed of a JVMPI-based trace generator and an analyzer which propagates tags and computes the proposed metrics. This dynamic propagation is essential, and thus this paper contributes not only new metrics, but also non-trivial ways of computing them. We furthermore present a set of benchmarks that exercise a wide range of AspectJs features, and the metrics that we measured on these benchmarks. The results provide guidance to AspectJ users on how to avoid efficiency pitfalls, to AspectJ implementors on promising areas for future optimization, and to tool builders on ways to understand the runtime behaviour of AspectJ.

Generic program transformation

When writing a program, especially in a high level language such as Haskell, the programmer is faced with a tension between abstraction and efficiency. A program that is easy to understand often fails to be efficient, while a more efficient solution often compromises clarity.

Adding open modules to AspectJ

AspectJ does not provide a mechanism to hide implementation details from advice. As a result, aspects are tightly coupled to the implementation of the code they advise, while the behaviour of the base code is impossible to determine without analysing all advice that could modify its behaviour.The concept of open modules is proposed by Aldrich to Solve the problems that arise from unrestricted advice. Defined for a small functional language, it provides an encapsulation construct that allows an implementation to limit the set of points to which external advice may apply.We present an adaptation of open modules for AspectJ. We expand open modules to encompass the full set of pointcut primitives for AspectJ, extend its method of module composition to include the ability to open up a module, and describe the implementation of the design as an extension of the AspectBench compiler. We also provide an example of the use of open modules on a substantial AspectJ program to show how it would fit into existing AspectJ projects.

Higher-order matching for program transformation

We present a simple, practical algorithm for higher-order matching in the context of automatic program transformation. Our algorithm finds more matches than the standard second order matching algorithm of Huet and Lang, but it has an equally simple specification, and it is better suited to the transformation of programs in modern programming languages such as Haskell or ML. The algorithm has been implemented as part of the MAG system for transforming functional programs.

Incremental execution of transformation specifications

We aim to specify program transformations in a declarative style, and then to generate executable program transformers from such specifications. Many transformations require non-trivial program analysis to check their applicability, and it is prohibitively expensive to re-run such analyses after each transformation. It is desirable, therefore, that the analysis information is incrementally updated.We achieve this by drawing on two pieces of previous work: first, Bernhard Steffens proposal to use model checking for certain analysis problems, and second, John Conways theory of language factors. The first allows the neat specification of transformations, while the second opens the way for an incremental implementation. The two ideas are linked by using regular patterns instead of Steffens modal logic: these patterns can be viewed as queries on the set of program paths.

Transforming the .NET intermediate language using path logic programming

Path logic programming is a modest extension of Prolog for the specification of program transformations. We give an informal introduction to this extension, and we show how it can be used in coding standard compiler optimisations, and also a number of obfuscating transformations. The object language is the Microsoft .NET intermediate language (IL).

Efficient trace monitoring

A wealth of recent research involves generating program monitors from declarative specifications. Doing this efficiently has proved challenging, and available implementations often produce infeasibly slow monitors. We demonstrate how to dramatically improve performance -- typically reducing overheads to within an order of magnitude of the programs normal runtime.