Java bytecode

Summary

In computing, Java bytecode is the bytecode-structured instruction set of the Java virtual machine (JVM), a virtual machine that enables a computer to run programs written in the Java programming language and several other programming languages, see List of JVM languages. Relation to Java A Java programmer does not need to be aware of or understand Java bytecode at all. However, as suggested in the IBM developerWorks journal, "Understanding bytecode and what bytecode is likely to be generated by a Java compiler helps the Java programmer in the same way that knowledge of assembly helps the C or C++ programmer." Instruction set architecture The JVM is both a stack machine and a register machine. Each frame for a method call has an "operand stack" and an array of "local variables". The operand stack is used for operands to computations and for receiving the return value of a called method, while local variables serve the same purpose as registers and are also us

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Miniboxing: An Encoding for Specialization

Cristian Talau

In the presence of parametric polymorphism, erasure-based languages such as Java and Scala handle primitives (boolean values, integers and floating point numbers) in a suboptimal way: in order to provide a uniform representation on the low level, all primitive values are stored in heap objects, in a process known as boxing. This leads to access overheads, wasteful usage of the heap space and broken cache locality. Specialization enables Scala to optimally handle primitive values in the context of generic classes, but this is done at the expense of duplicating classes up to 10 times for each type parameter, for each of the 9 Scala primitive types and heap objects. This prevents specialization of key classes in the Scala library, such as Function2, List, Map and so on. This project aims at testing the hypothesis that encoding several primitive types into a larger stack-based primitive type can maintain the performance of specialized code, while dramatically decreasing the generated bytecode size.

2012

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Parametric polymorphism enables code reuse and type safety. Underneath the uniform interface exposed to programmers, however, its low level implementation has to cope with inherently non-uniform data: value types of different sizes and semantics (bytes, integers, floating point numbers) and reference types (pointers to heap objects). On the Java Virtual Machine, parametric polymorphism is currently translated to bytecode using two competing approaches: homogeneous and heterogeneous. Homogeneous translation requires boxing, and thus introduces indirect access delays. Heterogeneous translation duplicates and adapts code for each value type individually, producing more bytecode. Therefore bytecode speed and size are at odds with each other. This paper proposes a novel translation that significantly reduces the bytecode size without affecting the execution speed. The key insight is that larger value types (such as integers) can hold smaller ones (such as bytes) thus reducing the duplication necessary in heterogeneous translations. In our implementation, on the Scala compiler, we encode all primitive value types in long integers. The resulting bytecode approaches the performance of monomorphic code, matches the performance of the heterogeneous translation and obtains speedups of up to 22x over the homogeneous translation, all with modest increases in size.

2013

State Exploration of Scala Actor Programs

Mirco Dotta

The growing use of multicore and networked computing systems is increasing the importance of developing reliable parallel and distributed code. Unfortunately, developing and testing such code is notoriously hard, especially for shared-memory models of programming. The actor model offers a promising alternative paradigm based on message passing. Essentially, an actor is an autonomous concurrent object which interacts with other actors only by exchanging messages. In actor-based systems, the key source of non-determinism is the order in which messages are delivered to—and processed by—the actors. Bugs can still occur in actor programs as the interleaving of messages may be incorrect, or the sequential code within an actor can have bugs. We developed a general framework, called SEJAP, for exploring possible message schedules in actor systems compiled to the Java bytecode. Specifically, in this dissertation, we present one instantiation of SEJAP for the actor library of the Scala programming language. To the best of our knowledge, this is the first framework that allows systematic exploration of Scala actor programs. We also present two optimizations that alleviate the state explosion problem typical for such exploration, and thus speed up the overall exploration of actor programs in SEJAP. We have implemented our framework, Scala instantiation, and optimizations in Java PathFinder, a widely used model checker for Java bytecode developed by NASA. Preliminary results show that SEJAP can effectively explore executions of actor programs. Further, our use of SEJAP already discovered a previously unknown bug in the sample code from a popular site for Scala.

2009