The author details their process of creating a WebAssembly (Wasm) virtual machine (VM) written entirely in C. Driven by a desire for a lightweight, embeddable Wasm runtime for resource-constrained environments, they built the VM from scratch, implementing core features like the stack-based execution model, linear memory, and basic WebAssembly System Interface (WASI) support. The project focused on simplicity and understandability over performance, serving primarily as a learning exercise and a platform for experimentation with Wasm. The post walks through key aspects of the VM's design and implementation, including parsing the Wasm binary format, handling function calls, and managing memory. It also highlights the challenges faced and lessons learned during the development process.
In a detailed blog post titled "I Wrote a WebAssembly VM in C," the author chronicles their journey of creating a WebAssembly (Wasm) virtual machine from scratch using the C programming language. Their primary motivation stemmed from a desire to deeply understand the inner workings of Wasm, moving beyond simply utilizing existing tools and libraries. This hands-on approach allowed them to grasp the intricacies of the Wasm specification and the challenges involved in its implementation.
The post begins by outlining the core components of a Wasm VM, including the stack, memory, and execution environment. The author then meticulously describes the process of parsing and interpreting Wasm bytecode, explaining how each instruction is handled by the VM. They delve into the complexities of implementing the stack-based virtual machine architecture, covering topics such as operand evaluation, function calls, and local variable management. Specific instructions, like i32.add and local.get, are used as examples to illustrate the execution flow and data manipulation within the VM.
The development process involved several iterative steps. The author started with a basic framework capable of executing simple arithmetic operations and gradually expanded its functionality to support more complex features like function calls and control flow instructions. They emphasized the importance of rigorous testing throughout the development cycle, using carefully crafted test cases to ensure the correctness of their implementation.
The author acknowledges that their implementation is not fully compliant with the complete Wasm specification, focusing primarily on a subset of core instructions. However, this simplified approach served their educational purpose of gaining a foundational understanding of Wasm execution. The post concludes with a reflection on the lessons learned during the project and a discussion of potential future enhancements, including adding support for more advanced Wasm features and optimizing the VM's performance. The author's code, written entirely in C, is available publicly for others to explore and learn from, offering a tangible resource for anyone interested in diving into the world of WebAssembly virtual machines.
Summary of Comments ( 43 )
https://news.ycombinator.com/item?id=42918524
Hacker News users generally praised the author's clear writing style and the educational value of the post. Several commenters discussed the project's performance, noting that it's not optimized for speed and suggesting potential improvements like just-in-time compilation. Some shared their own experiences with WASM interpreters and related projects, including comparisons to other implementations and alternative approaches like using a stack machine. Others appreciated the detailed explanation of the parsing and execution process, finding it helpful for understanding WASM internals. A few users pointed out minor corrections or areas for potential enhancement in the code, demonstrating active engagement with the technical details.
The Hacker News post "I Wrote a WebAssembly VM in C" (https://news.ycombinator.com/item?id=42918524) generated a moderate amount of discussion, with several commenters engaging with the project and offering insights or related experiences.
A recurring theme was admiration for the author's undertaking, with several commenters acknowledging the complexity and difficulty of writing a Wasm VM. One commenter pointed out the educational value of such projects, emphasizing the deep understanding of Wasm's internals that one gains through implementation. They also noted that while Wasm is often perceived as a compilation target, understanding its runtime environment is equally crucial.
Another user shared a personal anecdote of a similar project, where they wrote a Wasm interpreter in Rust. They explained that their motivation stemmed from a need to run Wasm in a constrained embedded environment lacking a JIT compiler. This comment highlighted a practical use case for Wasm interpreters, contrasting with the more common JIT-based implementations.
A discussion unfolded about the performance characteristics of interpreted Wasm versus compiled Wasm. One commenter questioned the practical applicability of interpreters, speculating that their performance limitations might restrict their usefulness. Another user countered this by suggesting potential niche applications, such as debugging or educational purposes, where raw performance is less critical than other features like understandability and control. They also mentioned the possibility of using an interpreter as a fallback mechanism when JIT compilation is unavailable.
The author of the Wasm VM chimed in to address some of these questions. They clarified that the project was primarily an educational exercise, not intended for production use. They acknowledged the performance limitations of interpretation and confirmed they had no plans to add a JIT compiler. They also engaged with other commenters, discussing technical details of their implementation, such as the handling of garbage collection.
Finally, one comment drew a parallel between the author's project and the early days of Java, where interpreted execution was common before JIT compilation became prevalent. This comparison highlighted the potential evolution of Wasm runtimes, suggesting that interpreters might play a more significant role in the future, particularly in resource-constrained environments.