Untyped Memory in the Java Virtual Machine

We have implemented a virtual execution environment that executes legacy binary code on top of the type-safe Java Virtual Machine by recompiling native code instructions to type-safe bytecode. As it is essentially impossible to infer static typing into untyped machine code, our system emulates untyped memory on top of Java’s type system. While this approach allows to execute native code on any off-the-shelf JVM, the resulting runtime performance is poor. We propose a set of virtual machine extensions that add type-unsafe memory objects to JVM. We contend that these JVM extensions do not relax Java’s type system as the same functionality can be achieved in pure Java, albeit much less efficiently. 1 Executing Native Machine Code on top of JVM Our research prototype VEELS (Virtual Execution Environment for Legacy Software) is able to execute statically compiled Linux [2] executables for PowerPC, StrongARM, and MIPS on top of any standard Java Virtual Machine [4]. All native code and data is stored within a memory object that emulates untyped memory. A virtual CPU fetches instructions from the memory object and interprets them. Frequently executed code blocks are translated to bytecode by a just-in-time compiler to allow more efficient execution Figure 1. 2 Emulating Untyped Memory The standard implementation of the memory object in VEELS uses a sparse integer array as backing store. Each 4k page of the guest machine is modeled as an array of integers, referenced by a page table. The most common access patterns—reading and writing words from and to memory at an aligned address—are handled efficiently with two memory accesses and a pair of array bounds checks.