Abstract. Optimizing compilers for object-oriented languages apply static class analysis and other techniques to try to deduce precise information about the possible classes of the receivers of messages; if successful, dynamicallydispatched messages can be replaced with direct procedure calls and potentially further optimized through inline-expansion. By examining the complete inheritance graph of a program, which we call class hierarchy analysis, the compiler can improve the quality of static class information and thereby improve run-time performance. In this paper we present class hierarchy analysis and describe techniques for implementing this analysis effectively in both statically- and dynamically-typed languages and also in the presence of multi-methods. We also discuss how class hierarchy analysis can be supported in an interactive programming environment and, to some extent, in the presence of separate compilation. Finally, we assess the bottom-line performance improvement due to class hierarchy analysis alone and in combination with two other “competing ” optimizations, profileguided receiver class prediction and method specialization. 1

A fleet of unmanned air vehicles undertakes a mission to disable an enemy airfield. Pre-mission intelligence indicates that the airfield is not defended, and mission planning proceeds accordingly. While the UAVs are en route to the target, new intelligence indicates that a mobile surface-to-air missile launcher now guards the airfield. The UAVs autonomously replan their mission, dividing into two groups—a SAM-suppression unit and an airfield-suppression unit—and proceed to accomplish their objectives. During the flight, specialized algorithms for detecting and recognizing SAM launchers automatically upload and are integrated into the SAM-suppression unit’s software. In this scenario, new software components are dynamically inserted into fielded, heterogeneous systems without requiring system restart, or indeed, any downtime. Mission replanning relies on analyses that include feedback from current performance. Furthermore, such replanning can take place autonomously, can involve multiple, distributed, cooperating planners, and where major changes are demanded and require human approval or guidance, can cooperate with mission analysts. Throughout, system integrity requires the assurance of consistency, correctness, and coordination of changes. Other applications for fleets of UAVs

Squeak is an open, highly-portable Smalltalk implementation whose virtual machine is written entirely in Smalltalk, making it easy to debug, analyze, and change. To achieve practical performance, a translator produces an equivalent C program whose performance is comparable to commercial Smalltalks. Other noteworthy aspects of Squeak include: a compact object format that typically requires only a single word of overhead per object; a simple yet efficient incremental garbage collector for 32-bit direct pointers; efficient bulkmutation of objects; extensions of BitBlt to handle color of any depth and anti-aliased image rotation and scaling; and real-time sound and music synthesis written entirely in Smalltalk. Overview Squeak is a modern implementation of Smalltalk-80 that is available for free via the Internet, at

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We discuss aspects of inlining of virtual method invocations. First, we introduce a new method test to guard inlinings of such invocations, with a different set of tradeoffs from the class-equality tests proposed previously in the literature. Second, we consider the problem of inlining virtual methods directly, with no guarding test, in dynamic languages such as Self or the Java(tm) programming language, whose semantics prohibit a static identification of the complete set of modules that comprise a program. In non-dynamic languages, a whole-program analysis might prove the correctness of a direct virtual inlining. In dynamic languages, however, such analyses can be invalidated by later class loading, and must therefore be treated as assumptions whose later violation must cause recompilation. In the past, such systems have required an on-stack replacement mechanism to update currently-executing invocations of methods containing invalidated inlinings. This paper presents analyses that allow some virtual calls to be inlined directly even if class loading invalidates the inlining for future invocations. This provides the benefits of direct inlining without the need for on-stack replacement, which can be complicated and require space-consuming data structures.

Dynamically-dispatched calls often limit the performance of object-oriented programs since object-oriented programming encourages factoring code into small, reusable units, thereby increasing the frequency of these expensive operations. Frequent calls not only slow down execution with the dispatch overhead per se, but more importantly they hinder optimization by limiting the range and effectiveness of standard global optimizations. In particular, dynamicallydispatched calls prevent standard interprocedural optimizations that depend on the availability of a static call graph. The SELF implementation described here offers two novel approaches to optimization. Type feedback speculatively inlines dynamically-dispatched calls based on profile information that predicts likely receiver classes. Adaptive optimization reconciles optimizing compilation with interactive performance by incrementally optimizing only the frequently-executed parts of a program. When combined, these two techniques result in a system that can execute programs significantly faster than previous systems while retaining much of the interactiveness of an interpreted system.

Abstract: Programming systems should be both responsive (to support rapid development) and efficient (to complete computations quickly). Pure object-oriented languages are harder to implement efficiently since they need optimization to achieve good performance. Unfortunately, optimization conflicts with interactive responsiveness because it tends to produce long compilation pauses, leading to unresponsive programming environments. Therefore, to achieve good responsiveness, existing exploratory programming environments such as the Smalltalk-80 environment rely on interpretation or non-optimizing dynamic compilation. But such systems pay a price for their interactiveness, since they may execute programs several times slower than an optimizing system. SELF-93 reconciles high performance with responsiveness by combining a fast, non-optimizing compiler with a slower, optimizing compiler. The resulting system achieves both excellent performance (two or three times faster than existing Smalltalk systems) and good responsiveness. Except for situations requiring large applications to be (re)compiled from scratch, the system allows for pleasant interactive use with few perceptible compilation pauses. To our knowledge, SELF-93 is the first implementation of a pure object-oriented language achieving both good performance and good responsiveness. When measuring interactive pauses, it is imperative to treat multiple short pauses as one longer pause if the pauses occur in short succession, since they are perceived as one pause by the user. We propose a definition of pause clustering and show that clustering can make an order-of-magnitude difference in the pause time distribution.

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Software systems have been using “just-in-time ” compilation (JIT) techniques since the 1960s. Broadly, JIT compilation includes any translation performed dynamically, after a program has started execution. We examine the motivation behind JIT compilation and constraints imposed on JIT compilation systems, and present a classification scheme for