Software developer Matt Keeter has harnessed a new feature in Rust’s nightly build to develop a high-performance interpreter for the Uxn CPU architecture. By utilizing the recently introduced `become` keyword, Keeter successfully implemented tail-call optimization—a technique gaining significant momentum in systems programming for its ability to streamline instruction dispatch.
Keeter’s project centers on emulating the Uxn CPU, a compact stack machine central to the Hundred Rabbits ecosystem. Previous efforts to optimize this emulator required writing thousands of lines of manual ARM64 and x86-64 assembly. While those assembly implementations delivered a 40% to 100% performance boost over the initial Rust versions, they also introduced serious safety risks, including memory corruption vulnerabilities.
Moving away from unsafe assembly
The new Rust-based approach provides the benefits of "threaded code"—a method where instructions jump directly to one another—without the hazards of manual assembly management. Keeter noted that the tail-call implementation is "surprisingly pleasant" to write and serves as a viable alternative to his previous hand-coded backend. Although the new Rust backend incurs a slight performance penalty compared to the assembly version, it eliminates the memory safety issues that previously triggered segmentation faults in his fuzzer.
Keeter emphasized that his latest work is entirely human-authored, distinguishing it from recent experiments where he used Large Language Models to assist in porting assembly code. The shift to the `become` keyword allows the compiler to manage the underlying machine state, effectively replacing complex, manual assembly logic with safer, high-level code that maintains competitive execution speeds.
This implementation marks the latest milestone in a multi-year project to enhance Uxn emulation efficiency. Keeter’s work serves as a case study for how modern language features can bridge the gap between high-level safety and low-level performance requirements in virtual machine design.
