Inlining of Virtual Methods

Abstract

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™ 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, while ensuring that invocations in progress may complete safely 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.

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