Binary AST Vs Bytecode Understanding The Misconceptions

by Sharif Sakr 56 views

Hey guys! Let's dive into a fascinating discussion about binary ASTs and bytecode. There's been some talk about why we don't just ship bytecode instead of other formats, and some of the reasoning might be a bit misleading. Let's break it down and see what's really going on.

Why Not Ship Bytecode? The Common Arguments

One of the key points often raised is that shipping bytecode on the web is a no-go. The reasons usually given are:

  • Engines don’t want to be tied to a bytecode version.
  • It’s harder to verify bytecode, as bytecode is more expressive than syntax.
  • It runs the risk of bifurcating the language, as bytecode may be more expressive than structured JavaScript source.
  • Designing a new bytecode is an even more ambitious undertaking.

These points make some sense on the surface, but let's dig a little deeper and see if they hold up under scrutiny.

The Heart of the Matter Binary ASTs and Bytecode

The core argument we're tackling today is that saying a binary AST isn't bytecode is, well, inaccurate. Think about it this way: a binary AST, especially if it's represented in reverse Polish notation (where the operator comes after the operands), can very much be interpreted as stack machine bytecode. This is a crucial point because it challenges the common perception that bytecode is somehow fundamentally different from a binary AST.

Reverse Polish Notation A Quick Primer

For those not familiar, reverse Polish notation (RPN), also known as postfix notation, is a way of writing expressions where the operator follows its operands. For example, the expression 2 + 3 in infix notation becomes 2 3 + in RPN. This format is naturally suited for stack-based machines, where operands are pushed onto a stack, and operators then pop the necessary operands, perform the operation, and push the result back onto the stack.

The Connection WebAssembly as a Case Study

To illustrate this, let's look at WebAssembly (Wasm). Wasm's bytecode was intentionally designed to be a context-sensitive AST encoding from the very beginning. Even after evolving into a stack machine, Wasm still maintains a structure that's almost entirely representable as an AST. This is a powerful example of how bytecode can indeed be closely tied to an AST representation.

The Flexibility of AST-like Bytecode

The beauty of this approach is its flexibility. If you allow non-value-returning nodes to exist at the beginning of any block and after any value-returning node, you can read the entire structure as if it were an AST. This means that the bytecode can be interpreted in a way that preserves the high-level structure of the code, making it easier to optimize and verify.

Deeper Dive Into the Misconceptions

Let's address the initial arguments against shipping bytecode one by one, keeping in mind our understanding of binary ASTs and their bytecode-like nature.

Engines Don’t Want to Be Tied to a Bytecode Version

This is a valid concern, but it's not insurmountable. The key is to design a bytecode format that's flexible and evolvable. By focusing on a high-level, AST-like representation, we can create a bytecode that's less tied to specific engine implementations and more adaptable to future language changes. Think of it as defining a standard intermediate representation (IR) that different engines can target.

It’s Harder to Verify Bytecode

This argument often stems from the perception that bytecode is a low-level, complex format. However, if our bytecode is essentially a binary AST, verification becomes much more manageable. ASTs are inherently structured and easier to reason about than unstructured bytecode. We can apply well-established techniques for AST analysis and verification to ensure the safety and correctness of the code.

Risk of Bifurcating the Language

The worry here is that bytecode might introduce features or behaviors not present in the source language, leading to fragmentation. Again, this is a design issue. If the bytecode is a faithful representation of the AST, and if the AST closely reflects the source language, then this risk is significantly reduced. The bytecode should be a serialization format for the AST, not a separate language.

Designing a New Bytecode Is Ambitious

No one's saying it's a walk in the park, but it's not as daunting as it sounds, especially if we leverage the principles of AST encoding. We don't need to invent a completely new virtual machine or instruction set. Instead, we can focus on creating an efficient and verifiable binary representation of the existing language's AST. WebAssembly has already paved the way, showing that it's possible to design a bytecode format that's both performant and amenable to formal verification.

The Advantages of a Binary AST Approach

So, why even bother with a binary AST or bytecode? What are the potential benefits?

  • Performance: Binary formats are generally more compact and faster to parse than text-based formats like JavaScript. This can lead to significant improvements in load times and overall application performance.
  • Security: A well-designed binary AST can be easier to verify and sandbox, reducing the risk of malicious code execution.
  • Standardization: A standard binary format could facilitate interoperability between different JavaScript engines and tools.

Conclusion It's All About Perspective

In conclusion, the idea that a binary AST cannot be considered a form of bytecode is a misleading oversimplification. By framing the discussion in terms of AST encodings and stack machine interpretations, we can see that the line between the two is quite blurry. The challenges associated with shipping bytecode are real, but they're not insurmountable. By embracing the principles of AST-based bytecode design, we can potentially unlock significant performance and security benefits for the web.

What do you guys think? Is it time to reconsider the role of binary ASTs and bytecode in the future of JavaScript?