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Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (normally numeric addresses) that encode the result of compiler parsing and performing semantic analysis of things like type, scope, and nesting depths of program objects. The name bytecode stems from instruction sets that have one-byte opcodes followed by optional parameters. Intermediate representations such as bytecode may be output by programming language implementations to ease interpretation, or it may be used to reduce hardware and operating system dependence by allowing the same code to run cross-platform, on different devices. Bytecode may often be either directly executed on a virtual machine (a p-code machine, i.e., interpreter), or it may be further compiled into machine code for better performance. Since byt

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The ScalaDyno compiler plugin allows fast prototyping with the Scala programming language, in a way that combines the benefits of both statically and dynamically typed languages. Static name resolution and type checking prevent partially-correct code from being compiled and executed. Yet, allowing programmers to test critical paths in a program without worrying about the consistency of the entire code base is crucial to fast prototyping and agile development. This is where ScalaDyno comes in: it allows partially-correct programs to be compiled and executed, while shifting compile-time errors to program runtime. The key insight in ScalaDyno is that name and type errors affect limited areas of the code, which can be replaced by instructions to output the respective errors at runtime. This allows byte code generation and execution for partially correct programs, thus allowing Python or JavaScript-like fast prototyping in Scala. This is all done without sacrificing name resolution, full type checking and optimizations for the correct parts of the code -- they are still performed, but without getting in the way of agile development. Finally, for release code or sensitive refactoring, runtime errors can be disabled, thus allowing full static name resolution and type checking typical of the Scala compiler.

ACM2014

Miniboxing: Improving the Speed to Code Size Tradeoff in Parametric Polymorphism Translations

Martin Odersky, Cristian Talau, Vlad Ureche

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.

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Scala AST Persistence

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The Scala compiler uses ASTs (abstract syntax trees) as an intermediate representation before generating bytecode. With the development of Scala macros which expand trees at compile time, being able to access, modify and recompose ASTs within the compilation scope is becoming more and more important. One of the common scenarios of using macros is inspecting abstract syntax trees within reach in order to learn more about the code being transformed, to apply more powerful optimizations, etc. However, arguments to macros can depend on third-party libraries, which are precompiled as bytecode and don't have their ASTs available. It would therefore be great to have a way to publish ASTs along with the bytecode. The publishing of those ASTs should be a choice of the programmer and should take as little space as possible in order to be transparent to the user.

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