Hi, I was recently playing around with a Kotlin im...

# webassembly

[Alexander Girke]

Hi, I was recently playing around with a Kotlin implementation for the wit-bindgen tool and here are some things that I noticed to be missing during the process:
• @JsName(name = "exports#string-error") fails with error Name contains illegal chars that can't appear in JavaScript identifier -> however, look like such method name should be valid e.g. in WIT definitions: https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md#identifiers
• Writing to Custom Section of WASM file -> looks like this is needed for component model. TeaVM offers an @CustomSection annotation to allow passing data to WASM file (see https://github.com/konsoletyper/teavm/blob/9f349385ec29ced175233cb0c3a247b87719cee[…]main/java/org/teavm/backend/wasm/render/WasmBinaryRenderer.java) is there something similar planned for Kotlin already?
• allocated memory is always 8-byte aligned (https://github.com/JetBrains/kotlin/blob/917f64c8939878821f1bad9b6776238d4f09b675/[…]ibraries/stdlib/wasm/src/kotlin/wasm/unsafe/MemoryAllocation.kt). I see the comment that 8 is the maximum alignment in component model abi. Just for my understanding, does this mean, supporting other alignments is just a matter of optimization? -> to me it seems that other alignments should be possible as well, e.g. at least TeaVM implementation is able to handle that https://github.com/bytecodealliance/wit-bindgen/blob/main/crates/teavm-java/src/lib.rs#L2063.
• In general, the ScopedMemoryAllocator does not exactly fit to the code generated by wit-bindgen, as it doesn’t allow to free memory in a callback - which seems to be the way the component model handles de-allocation (via a generated method prefixed with cabi_post_*) - any chance that there will be an implementation of MemoryAllocator that allows that usage pattern?
I think I’ll be able to share the current state soon to better illustrate the shortcomings.

[Svyatoslav Kuzmich [JB]]

@JsName(name = "exports#string-error") fails with error Name contains illegal chars that can't appear in JavaScript identifier

Annotation @WasmImport("module", "name") should be used for WIT instead, it doesn’t have such restrictions. We’ll also need to implement corresponding @WasmExport

Writing to Custom Section of WASM file

I didn’t know about this, thanks. It looks like we need this indeed, but we don’t have it yet.

allocated memory is always 8-byte aligned. … does this mean, supporting other alignments is just a matter of optimization?

Yes.

ScopedMemoryAllocator does not exactly fit to the code generated by wit-bindgen

Indeed. We need to implement realloc function export.

[Svyatoslav Kuzmich [JB]]

cc: @sdeleuze

[Alexander Girke]

Annotation @WasmImport("module", "name") should be used for WIT instead, it doesn’t have such restrictions. We’ll also need to implement corresponding @WasmExport

Sure, for import I was using @WasmImport, but since there currently is no @WasmExport I’m using @ExperimentalJsExport together with @JsName. How much would it take to remove the restriction for @JsName in the WASM context as a shortcut? Probably implemented faster than introducing the @WasmExport annotation, right?

Regarding the TODOs on Kotlin-side (@WasmExport, Custom Section & realloc function): should I file an issue on Youtrack or will you create an internal ticket for that?

[sdeleuze]

@Alexander Girke Super interesting stuff, do you plan to submit a PR on wit-bindgen side at some point? Interested to see a prototype when you have something sharable.

[sdeleuze]

Also if you want to collaborate on https://github.com/kowasm/kowasm please let me know.

[Alexander Girke]

@sdeleuze yes, will push the code very soon - for a PR it’s probably too early, since the generated code can not be compiled correctly atm due to the above (CustomSection (can be mitigated in tests) & @WasmExport/@JsName restrictions at least) + wasmparser crate not supporting GC proposal code. The latter looks like it’s going to be fixed very soon with https://github.com/bytecodealliance/wasm-tools/pull/1022.

Regarding kowasm, I’d be happy to contribute if you see anything that I can add there.

[sdeleuze]

Glad to see the work Ivan is doing (he is a colleague) is useful.