Software developer Matt Keeter has leveraged a new feature in Rust’s nightly build to create a high-performance interpreter for the Uxn CPU architecture. By utilizing the recently added `become` keyword, Keeter successfully implemented tail-call optimization, a technique that has gained significant traction in systems programming for its ability to streamline instruction dispatch.
Keeter’s project focuses on emulating the Uxn CPU, a compact stack machine used within the Hundred Rabbits ecosystem. Previous attempts to optimize this emulator involved writing thousands of lines of manual ARM64 and x86-64 assembly. While these assembly implementations provided a 40% to 100% performance boost over initial Rust versions, they introduced significant safety risks, including memory corruption vulnerabilities.
Moving away from unsafe assembly
The new Rust-based approach allows for the benefits of "threaded code"—a method where instructions jump directly to one another—without the dangers associated with manual assembly management. Keeter noted that the tail-call implementation is "surprisingly pleasant" to write and offers a viable alternative to his previous hand-coded backend. While the new Rust backend incurs a minor performance penalty compared to the assembly version, it eliminates the memory safety issues that previously caused segmentation faults in his fuzzer.
Keeter emphasized that his latest work is entirely human-authored, distancing it from recent experiments he conducted using Large Language Models to assist in porting assembly code. The transition to the `become` keyword allows the compiler to handle 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 step in a multi-year project to improve Uxn emulation efficiency. Keeter’s work provides 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.
