Anyref toolchain story?

[kripken]

I don't think we have a full plan for anyref yet. One issue is how to implement it in LLVM - do we need a new LLVM IR type? There are also questions about how source code for using it would be written in source langues like C, C++, and Rust. Opening this issue for more discussion on this topic.

The use case I'm most familiar with is the glue code in emscripten, like the WebGL glue: Compiled C does a glDrawArrays or other GL call, which goes into the JS glue which holds on to WebGL JS objects like the context, textures, etc., and it does the WebGL call using those, after mapping the C texture index (an integer) into the JS object, etc. In that use case, I don't think we have immediate plans to use anyref - wasm+anyref can't do all the stuff the current JS glue does (like, say, subarray-ing a Typed Array).

But for glue code that could be done in wasm (which eventually should be all of it, but that may take a while), I'm not sure we necessarily need clang and LLVM support. It would be nice, but if it's hard, another option might be to write such code in AssemblyScript or another close-to-wasm language. It's easy and natural to express anyrefs there. Then that would be compiled to wasm and linked to the LLVM output.

Curious to hear of more use cases, and whether there is a more immediate goal for using anyrefs in emscripten, LLVM, clang, etc. (for binaryen and wabt, there is the obvious immediate goal of having full anyrefs support).

cc @Keno @wingo @dcodeIO @aardappel

[aardappel]

I agree that writing code that uses anyrefs in small linkable files externally to LLVM, and then calling into these from C/C++/Rust could really simplify implementing support for them.. but I bet if you have more complex patterns of using anyref, then having direct support in the source language may be essential.

Certainly this approach can be tried in parallel with trying to add them directly to LLVM. Both can be useful.

[Keno]

One issue is how to implement it in LLVM - do we need a new LLVM IR type?

My suggestion for this would be a non-integral address space ptr. That should have all the right semantics and the optimizer treats them mostly as opaque references. The biggest question is what to do about the table/get set intrinsics. One option thing would be to just use load/store from a global of appropriate element type (i.e. a pointer in that address space). Of course that raises the question of what the pointer type of that table pointer is. It probably has to be non-integral also (so getelementptrs get preserved through the backend). It would probably be fine to use the same address space also and pattern match in the backend or we could use a different address space. Another option would be wasm-specific intrinsics for table get/set.

On the julia side, I'm planning to just have the codegen emit anyref whenever it has an unboxed [1] JSObject [2]. That shouldn't be too hard, because we already have this notion. I'll leave other frontends to people who actually work on them ;).

From our perspective, while we do need anyref support in LLVM, I don't think we care about anyref support in clang, since our runtime operates exclusively on boxed objects. Any shims we'd probably be happy to write in LLVM IR or some other sufficiently low level representation.

[1] From the julia perspective, obviously it'll still be a boxed reference to the JS heap unless the VM can prove something else later.
[2] https://github.com/JuliaLang/julia/pull/32791/files#diff-ffbd55360045dbded14e935c2dd0580eR8

[tschneidereit]

cc @sunfishcode @fitzgen @alexcrichton

[dcodeIO]

On the AssemblyScript side, I imagine an explicit anyref type that works like any other native Wasm type (very similar to what one would write in wat), plus potentially combining multiple imports (i.e. with anyref this arguments) to something that looks and feels like a normal class for convenience (what WebIDL bindings will eventually do). One interesting experiment could be to create bindings for the majority of the JS standard library, incl. strings and whatnot, and see how making programs with just these (essentially no AS stdlib) turns out.

[alexcrichton]

I can speak to at least a Rust perspective on this issue of anyref, and I think the Rust story ends up being very similar to C/C++ in terms of language semantics at least.

I've brainstormed with @fitzgen and we've come up with possible semantics to actually represent an anyref value as a first-class type/value in Rust-the-language (this part isn't so applicable to C/C++), but it's a pretty massive change for Rust and is pretty unlikely to happen unless it has extremely strong motivation to pursue it. This has basically led us to the conclusion that even if LLVM were to have support for anyref, we wouldn't really be able to use it because we don't have a satisfactory answer in Rust at least to actually expressing something usable to developers. C/C++ I think are in a similar-ish boat where dealing with something that's an anyref in the same way you've got an int that can be passed anywhere seems unfortunately a bit far-fetched right now. We'd love to be wrong on this point and figure something out that works everywhere!

Failing first-class language support, the next-best-thing we could think of was the model we have implemented today in wasm-bindgen. All "anyref values" are represented in the host language as an integer index into a table. Only when you call functions (maybe internally at some point but currently only via exports/imports in the wasm module) does that index actually get translated to an anyref value. These boundaries are currently managed by the wasm-bindgen CLI tool, but I could imagine that eventually this boundary logic of translating indices to values could happen in LLVM at some point too.

Our current thinking is that this is likely good enough for the near (and possibly far?) future. We can't figure out a compelling use case where actually passing around anyref values by value would result in a measurable increase in performance (relative to passing integers and dealing with the overhead of bounds checks on stores/loads at the fringes). This also sort of plays into the first part about how we found first-class language support to be extremely difficult (albeit at least plausible). The difficulty doesn't currently seem worth the effort necessary to fully implement everything.

All that's to say that I think convention-wise it'd be great if C/C++/Rust could all use the same strategy for managing these table indices that translate to values at boundaries. (If C/C++ folks agree that this is a reasonable strategy to take there as well of course). We haven't really put any thought into what it might look like to actually stabilize or implement this functionality in an official manner, it's pretty ad-hoc today. I think that we've got a good grasp on the goals to solve, just not how to model it in LLVM IR for example :)

One final thing we've thought about is that tools like binaryen/wabt/etc as mentioned will all have full support for anyref, and we're sort of assuming that binaryen would basically clean up the mess that we generate for anyref table indices and such. That would perhaps go even further to reducing the overhead of our current anyref transformation passes. Perhaps in an ideal world we could even teach binaryen about various anyref intrinsics (or something like that) used to manage these boundaries and they could help binaryen optimize even further and get us real close to an anyref-value-storing world perhaps. (this is particularly far-future though)

[kripken]

@alexcrichton Yeah, that basically matches my own thinking about C/C++ - to use integer indexes, and that anything more may be very hard and of unclear benefit. The wasm-bindgen approach you mention is basically the same approach emscripten uses too (e.g. the WebGL bindings have tables for Textures, Buffers, etc.); and all of this is basically the same as how file resources are typically handled (integer file descriptor), GPU resources in OpenGL (again, integer id), etc. - so I think table indexes are a natural model for languages using linear memory like C/C++/Rust.

(I do think non-linear memory languages can do better here! Both in terms of full cycle collection and avoiding table indirection overhead. But that's a separate issue.)

I agree that binaryen could help optimize away some of the table overhead using inlining + specific table-aware passes. I have some vague ideas about this.

All that's to say that I think convention-wise it'd be great if C/C++/Rust could all use the same strategy for managing these table indices that translate to values at boundaries.

I'm curious what you see as the benefit to using the same strategy in these languages? E.g. right now emscripten uses a separate table for different WebGL resources, but other use cases may want a single table for everything, etc. Another difference is emscripten doesn't plan to replace much of the WebGL bindings with wasm+anyref anytime soon, since that glue code needs to do things like subarray a buffer and other JS stuff, but other use cases may be in wasm. Overall, it's hard for me to see a single approach being enough?

[wingo]

I think with the only-table-on-the-side approach we are missing some of the advantages of anyref.

1. We lose a bit of perf: with anyref, you have not only the cost of bounds-checking, but also the memory traffic of getting function arguments / return values in and out of tables, of tracking the latest index for storing in the table, of clearing stale entries when you're done with them...
2. We make no progress towards fixing the JS<->Wasm cycle issue. I think only with an ability to allocate traceable C++ memory can we do this, and that means allocating on the GC or JS heap, which would need to be represented to wasm as a reference type. But if you then squirrel these managed allocations away in a table of roots, you have to manually GC over these roots, whereas the whole point of a managed allocation approach to cycle collection is to let the GC do this for us.

Rooting has a place for limited-lifetime references, but we should have some solution for data that has indefinite lifetime and which may participate in cycles across the JS/wasm boundary.

For me, a robust solution for a C++ library with "interesting" JS interaction (e.g. DOM-like) looks like this. I think these arguments apply to Rust as well, fwiw.

1. Core of the application in portable C++.
2. Code for values which may participate in JS cycles written in "C++ plus reference types", like C++/CLI. LLVM compiles allocations of these types as callouts to runtime routines that return anyref. Initialization/access to fields also via run-time callouts in an MVP; in the future with gc types, the allocations and field accesses can happen directly via wasm. In C++ source, reference types annotated via LLVM address spaces.
3. References to C++ reference types from the core are via handles / rooting.

This approach complements the always-handles approach, as you need handles anyway. It is also similar to @kripken's original thoughts above. But from a where-are-we-going point of view I think we should keep the broader horizon in view, in order to have a reasonable answer to the cycle problem. It's not clear that we'll be able to make it there but I think we should try.

[alexcrichton]

@kripken

I'm curious what you see as the benefit to using the same strategy in these languages?

Oh sorry I should probably clarify what I mean by this. Right now in Rust we have no way to use anyref for raw binaries coming out of rustc itself. Only if you use the wasm-bindgen CLI tool can you actually produce a wasm binary that uses the anyref type. That's a bummer and something we'd like to fix in the long run.

That's to say that when I say that C/C++/Rust should probably all converge around a similar strategy I mean we should take the common denominator (LLVM + LLD) and make sure it's supported there somehow. We could add a rustc-specific pass and Clang (and/or Emscripten) could do similar, but it'd be awesome if we could centralize the implementaiton in LLVM+LLD which everything would share.

I would suspect that the language level support and even the intrinsics used across the languages would probably different, so I definitely don't think we should try to shoehorn everything into the same hole, just shoehorning into the same code generator!

[kripken]

@wingo

We lose a bit of perf: with anyref

Agreed, yeah. I have some hope of optimizations in Binaryen being able to help (figuring out that an anyref is loaded more than once from the table, and reusing it, sending it as a param, etc.), but as mentioned above this is speculative.

Rooting has a place for limited-lifetime references, but we should have some solution for data that has indefinite lifetime and which may participate in cycles across the JS/wasm boundary.

I agree. I'm working with @aardappel atm on one approach for solving that, that may be useful under the assumption that cross-VM cycles are rare and do not need urgent collection. I think we can do that with JS WeakRefs and no other new APIs in JS or wasm (but with significant work in the compiled VM). I hope to have a prototype soon.

Otherwise I think your approach is very interesting - I mostly worry about the complexity of adding anyref to LLVM and to C. But you may know more than me about the difficulty there.

@alexcrichton

Thanks, I see now. Yeah, agreed we should share as much code in a central place as possible!

[wingo]

I agree that binaryen can claw a fair amount of the performance back, if the rooting-table management happens on the wasm side and not a JS wrapper. Many uses of anyref will be ephemeral and will never need to be rooted, and binaryen should be able to find that out.

I look forward to seeing the WeakRef work :) Sounds interesting. Many manual solutions with WeakRef can work, but a robust general mechanism wasn't apparent to my ignorant eyes.

[tschneidereit]

I'm working with @aardappel atm on one approach for solving that, that may be useful under the assumption that cross-VM cycles are rare and do not need urgent collection.

We'll probably only really know once there's usage in the field, but I'm curious: what's the reasoning leading to this assumption?

[aardappel]

It seems to me the design of anyref so far was under the assumption we can make it a somewhat first class feature in LLVM, and that languages can use it directly too. But if whatever practical implementation we are going to end up with for most languages (that solves the cycle problem to the best of our abilities) is "indices all the way", then I wonder how that informs what anyref really should be.

I guess we really need to turn this upside down and first ask: what does it look like for a C++ or Rust program to have general collectable cycles with a JS program (again, other languages may be involved, but this is a prototypical example). Once we agree there is a solid implementation possible there, we can see which parts of toolchain and engine should represent these these references, and how.

This of course becomes more interesting/complicated if the C++/Rust program actually contains an implementation for a language that does GC or other memory management in linear memory, which is something me & @kripken have been looking at.

[jgravelle-google]

It seems to me the design of anyref so far was under the assumption we can make it a somewhat first class feature in LLVM, and that languages can use it directly too. But if whatever practical implementation we are going to end up with for most languages (that solves the cycle problem to the best of our abilities) is "indices all the way", then I wonder how that informs what anyref really should be.

Not sure if it was designed that way. IIRC it originated from the GC proposal, as a Top type for the whole type hierarchy. We then split it out early, not because linear memory-based languages would be able to use it easily and naturally, but so linear memory languages would be able to reason about external references at all.

So even if C++/Rust can't make natural use of it, anyref still makes sense N years from now with the full GC proposal, in the same way anyfunc isn't deprecated by typed funcrefs.

[KronicDeth]

This of course becomes more interesting/complicated if the C++/Rust program actually contains an implementation for a language that does GC or other memory management in linear memory, which is something me & @kripken have been looking at.
-- @aardappel

That covers our Erlang/Elixir implementation too - we're using Rust to do the implementation and some LLVM IR directly, but we're doing our own GC on top, so we don't get anyref in our GC-language without it being supported in LLVM and Rust.

[wingo]

I know this is a thread about LLVM, but @aardappel seems to be questioning the value of the anyref feature at all. Allow me to chime in with a defense and a use case :)

It is perfectly possible to fix the cycle problem now with anyref. You simply represent the parts of your data that should be garbage-collected using garbage-collected memory from the host, and allow the host to GC. The Schism compiler does this and I am confident that it will never leak memory, even in the presence of cycles.

Currently this solution is less than optimal given that anyref values are opaque and so you need to call out to the runtime for field access, constructors, and type predicates, but that will be fixed in the mid-term with the GC proposal, which grants you access to all these from wasm. I think it's going to be the long-term solution for all languages that need garbage collection.

If in the short-term, we can't get LLVM to have this property -- I think we agree there -- then a rooting table is a perfectly good stopgap for the general case, for C++ or Rust. But a table of handles is not a great plan for languages with GC, whether they use LLVM or not.