Implementing jalapeño in Java

Abstract

Jalapeno is a virtual machine for JavaTM servers written in Java.A running Java program involves four layers of functionality: the user code, the virtual-machine, the operating system, and the hardware.By drawing the Java / non-Java boundary below the virtual machine rather than above it, Jalapeiio reduces the boundary-crossing overhead and opens up more opportunities for optimization. To get Jalapeno started, a boot image of a working Jalapeiio virtual machine is concocted and written to a file. Later, this file can be loaded into memory and executed. Because the boot image consists entirely of Java objects, it can be concocted by a Java program that runs in any JVM. This program uses reflection to convert the boot image into Jalapeno's object format.A special MAGIC class allows unsafe casts and direct access to the hardware.Methods of this class are recognized by Jalapeno's three compilers, which ignore their bytecodes and emit special-purpose machine code. User code will not be allowed to call MAGIC methods so Java's integrity is preserved.A small non-Java program is used to start up a boot image and as an interface to the operating system. Java's programming features -object orientation, type safety, automatic memory management -greatly facilitated development of Jalapeno. However, we also discovered some of the langauge's limitations.

Snippets

“…Therefore, snippets need a raw pointer type. Metacircular Java VMs such as the Jikes RVM [Alpern et al 1999[Alpern et al , 2000Rogers and Grove 2009;Frampton et al 2009] and the Maxine VM [Wimmer et al 2013] pioneered this idea of low-level programming in Java. Similar to these VMs, we define a class Word to represent raw machine words.…”

“…However, metacircular VMs such as the Klein VM [Ungar et al 2005], the Jikes RVM [Alpern et al 1999[Alpern et al , 2000Rogers and Grove 2009], and the Maxine VM [Wimmer et al 2013] have demonstrated the feasibility of using a high-level language instead. High-level languages bring significant benefits: better modularity, memory safety, and automatic memory management.…”

“…The dynamic compiler in the Jikes RVM [Alpern et al 1999[Alpern et al , 2000Rogers and Grove 2009], the first metacircular Java VM, inlines these methods without formalization and without an abstraction layer between compiler and VM. The original dynamic compiler in the metacircular Maxine VM [Wimmer et al 2013], called CPS, first introduced the term snippet [Mathiske 2008;Simon 2009] to formalize the compiler-VM interface and to control inlining of Maxine snippets 1 via a bootstrapping process.…”

Dynamic linking on a shared-memory multiprocessor

“…If the class designated by the FieldRef is not yet a part of the running image, a synchronized link method loads, resolves, and instantiates the class. 3 Synchronization is required to ensure that this happens exactly once (the link method returns if the class has already been instantiated). Notice that the link method is only called the first time a dynamic link site for a class is executed, most dynamic link sites are backpatched without any call to a synchronized method.…”

“…This paper shows how the Jalapeño JVM [2,3,4,7] overcomes the difficulties inherent in concurrent backpatching on PowerPC multiprocessors [14]. Section 2 details Java's requirements for dynamic linking.…”