JSIR: A High-Level IR for JavaScript
discourse.llvm.org
discourse.llvm.org
My interpretation: If the JSIR project can successfully prove bi-directional source to MLIR transformation, it could lead to a new crop of source to source compilers across different languages (as long as they can be lowered to MLIR and back).
Imagine transmorphing Rust to Swift and back. Of course you’d still need to implement or shim any libraries used in the source language. This might help a little bit with C++ to Rust conversions - as more optimizations and analysis would now be possible at the MLIR level. Though I won’t expect unsafe code to magically become safe without some manual intervention.
JSIR is optimizing for round-trips back to JavaScript source. But since in language to language conversion teh consumer is a backend emitter (C# in my case), instead of preserving source structure perfectly, my IR preserves resolved semantic facts: types, generic substitutions, overload decisions, package/binding resolution, and other lowering-critical decisions.
I could be wrong, but I suspect transpilers are easier to build if it's lowering oriented (for specific targets).
We have been exploring what an IR looks like when the author is an AI and the consumer is a compiler, and no human needs to read the output at all. ARIA (aria-ir.org) goes the other direction from JSIR. No source round-trip, no ergonomic abstractions, but first-class intent annotations, declared effects verified at compile time, and compile-time memory safety.
The use cases are orthogonal. JSIR is the right tool when you need to understand and transform code humans wrote. ARIA is the right tool when you want the AI to skip the human-readable layer entirely.
The JSIR paper on combining Gemini and JSIR for deobfuscation is a good example of where the two worlds might intersect. Curious whether you have thought about what properties an IR should have to make that LLM reasoning more reliable.
This seems like a big bet on the assumption that fully autonomous codegen without humans in the loop is imminent if not already present - frankly, I hope you are wrong.
Even if that comes to pass in some cases, I also find it hard to believe that an LLM will ever be able to generate code in any new language at the same level with which it can generate stack overflow-shaped JavaScript and python, because it’ll never have as robust of a training set for new languages.
We don't have real AI & no one is anywhere near anything that can consistently generate code of moderate complexity w/o bugs or accidental issues like deleting files during basic data processing (something I ran into recently while writing a local semantic search engine for some of my PDFs using open source neural networks).
I go subsystem by subsystem.
Writing the interpreter for a stack vm is as simple as it gets.
"Trend"?
This was always the best practice. It's not a "trend".
Compiler engineers for mostly linear-memory languages tend to only think in terms of SSA, and assume it's the only reasonable way to perform optimizations. That transpires in this particular article: the only difference between an AST and what they call IR is that the latter is SSA-based. So it's like for them something that's not SSA is not a "serious" data structure in which you can perform optimizations, i.e., it can't be an IR.
On the other side, you actually have a bunch of languages, typically GC-based for some reason, whose compilers use expression-based structures. Either in the form of an AST or stack-based IR. These compilers don't lack any optimization opportunities compared to SSA-based ones. However it often happens that compiler authors for those (I am one of them) don't always realize all the optimization set that SSA compilers do, although they could very well be applied in their AST/stack-based IR as well.
You usually compile from SSA to WASM bytecode, and then immediately JIT (Cranelift) by reconstructing an SSA-like graph IR. If you look at the flow, it's basically:
Graph IR -> WASM (stack-based bytecode) -> Graph IR
So the stack-based IR is used as a kind of IR serialization layer. Then I realized that this works well because a stack-based IR is just a linearized encoding of a dataflow graph. The data dependencies are implicit in the stack discipline, but they can be recovered mechanically. Once you see that, the blindness mostly disappears, since the difference between SSA/graph IRs and expression/stack-based IRs is about how the dataflow (mostly around def-use chains) is represented rather than about what optimizations are possible.
Fom there it becomes fairly obvious that graph IR techniques can be applied to expression-based structures as well, since the underlying information is the same, just represented differently.
Didn't look close enough to JSIR, but from looking around (and from building a restricted Source <-> Graph IR on JS for some code transforms), it basically shows you have at least a homomorphic mapping between expression-oriented JS and graph IR, if not even a proper isomorphism (at least in a structured and side-effect-constrained subsets).
My broader point is that for SSA-based pipelines targeting Wasm, translation between SSA/graph IR and stack-based IR is largely mechanical and efficient. Whether a compiler uses SSA as an intermediary or goes straight from an AST to Wasm, the fact remains that you can round-trip between a SSA-like IR and a stack-based IR without losing the underlying dataflow information.
Yeah, mapping is not canonical and some non-semantic structure is not preserved (evaluation order, materialization points, join encoding, CFG reshaping for structured control and probably some more structure I'm not familiar with), but optimization power is unaffected.
And JSIR seems to be based on an even stronger assumption.
Would appreciate corrections if you see things differently.
Here's the repo: https://github.com/google/jsir (it seems not everything is public).
Here's a presentation about it: https://www.youtube.com/watch?v=SY1ft5EXI3I (linked in from the repo)
And my dumb brain still don't understand how IR is "better" than AST after reading this post. Current AST based JS tools working reasonably well, and it's not clear to me how introducing this JSIR helps tool authors or downstream users, when there are all those roadblocks mentioned at the end.
It has become a liability in the build process and people are getting rid of it.
You could use it as an intermediate form in a JS->C# pipeline, but you still have to define a subset of JavaScript that lowers cleanly to your target C# runtime and implementing the IR->C# lowering yourself.
I'd imagine the hard part is not the IR, but aligning the JavaScript semantics (object model, closures, prototypes etc.) with C# (static type system, different execution model..).
I do think aligning the semantics will be the easier part, honestly, because I'm only trying to transpile the supported source for the game engine. Since that's all written in typescript and I'm not guaranteeing full parity if you are trying to transpile arbitrary ts/js (only the source that can be parsed the same way the game engine is parsed), I'm expecting it to be a near 1-to-1 conversion. I started writing everything in C# and copied the structure to JS, knowing that this was the eventual plan, so the JS can actually be re-written as C# with a pretty simple regex tokenizer.
My hope, here, is that by having the code morphed into an IR, that the IR would be some kind of well-known IR that - for instance - C# could also be morphed into and - therefore - would allow automatic parsing back and forth. From what you're saying, though, it sounds like IRs don't use a common structure for describing code (I'm guessing because of the semantic misalignment you mention between a wide variety of different paradigms?), so this would only work if I made the map from IR to C# which would be just as complex (or more so) than just regexing my JS into C#. If I've got that right, that's a bummer, but understandable. If I'm wrong, though, happy to learn more!
Btw, JS doesn't even have an official bytecode. The spec is defined at the language semantics level, so each engine/toolchain invents its own internal representation.
CLR already has multiple Language front-ends (C#, F#, VB, IronPython)
That seems a bit disingenuous given this is not a source-preserving IR! All comments and nonstandard spacing would be completely removed from your code if you gave it a round trip through this format. That doesn't sound like 99.9% source recovery to me...
Also, if you use a nonstandard spacing, I'd say that's on you to preserve a mechanical Source-AST mapping if you want to use any tool that mentions dataflow analysis & transforms.
As a side note, comments are much trickier than non-standard spacing if their positioning is semantic.