Overview

The Jikes RVM began life as the Jalapeño virtual machine in late 1997. The project had two design goals: 1) support high-performance Java servers running on PowerPC multiprocessors under the AIX operating system, and 2) provide a flexible research platform ``where novel virtual machine ideas can be explored, tested, and evaluated''. Although it was written in the Java programming language, in the initial implementation portability was ``not a design goal: where an obvious performance advantage can be achieved by exploiting the peculiarities of Jalapeño's target architecture ... we feel obliged to take it'' [2].

Jikes RVM does not interpret bytecodes; rather it compiles each method to machine code and executes the machine code natively. In an adaptive Jikes RVM configuration, the baseline compiler performs the initial compilation of a method. Methods that are either frequently executed or computationally intensive are identified via a sampling mechanism and recompiled by the optimizing compiler [4].

The baseline compiler directly mimics the stack machine behavior of the JVM specification. The baseline compiler translates bytecodes to machine code quickly, but the resultant machine code typically runs slowly. The baseline compiler implementation depends heavily on the target instruction set architecture. Much of the work of a functional port lies in constructing a new baseline code generator, more or less from scratch.

The optimizing compiler expends more effort to produce high quality machine code for selected methods. The optimizing compiler implementation far exceeds the baseline compiler in size and complexity. However, most of the optimizing compiler does not depend on the instruction set architecture, reducing porting effort for a second architecture.

The Jikes RVM does not directly map Java threads of an application to operating system threads (POSIX pthreads). Instead, the system creates a virtual processor object for each pthread in use (normally one for each physical CPU). The Jikes RVM thread scheduler multiplexes the application's Java threads and the RVM's daemon threads onto these virtual processors.

Although the Jikes RVM is written in the Java programming language, it must perform actions (e.g. access registers and manipulate raw memory addresses) which cannot be expressed in the Java programming language [8]. The virtual machine provides a VM_Magic class to circumvent these restrictions [3]. The compilers do not translate the bytecodes of VM_Magic methods. Rather, the compilers recognize calls to these methods and inline custom machine code in place of the call.

The original implementation exploits the large PowerPC register set, with 1#1 general-purpose registers and 1#1 floating-point registers.² Adjusting to the register-scarce IA32 architecture presented a major challenge for this port. Differences in the instruction sets led to different calling and stack conventions for the two architectures. Writing in the Java programming language (explicitly big endian) shielded us from many, but not all endian problems³ (For instance, with pre-loaded constants and when atomically updating bit and byte maps). Still, despite our initial indifference to the possibility of an eventual port, large portions of the Jikes RVM more or less worked on Linux/IA32 without modification.

As of February 1, 2002, the source code for the Jikes RVM itself contains approximately 203,000 lines of Java code, 18,000 lines of ``meta'' source files that are the inputs to several code generation tools, 6,000 lines of C++ code to interface with the operating system and to get the RVM started, and about 50 lines of assembly code to effectuate the initial transition from C++ to Java.<sup>4</sup> Most of this source code is independent of the target platform. The Java source files contain 162,000 lines of platform-independent code, 22,000 lines of PowerPC-specific code, and 19,000 lines of IA32-specific code. Approximately 6,000 lines of the ``meta'' source files are platform-independent, 3,600 are PowerPC-specific, and 8,400 are IA32-specific. The optimizing compiler comprises the largest subsystem of Jikes RVM, with 100,000 lines of Java source code; 78,100 are platform-independent, 14,200 lines are PowerPC-specific and 7,700 are IA32-specific. About 900 lines of the C++ operating system interface code are IA32-specific; 1200 are PowerPC-specific (these support both Linux and AIX). The assembly code is completely architecture-dependent. The FullAdaptiveSemispace configuration on Linux/IA32 represents a typical RVM build: it contains 821 Java classes comprising 225,000 lines of code (66,000 are machine generated).