fix(bindgen): fix WASM3/GC transpilation and P3 async stream runtime bugs by gfx · Pull Request #1346 · bytecodealliance/jco

gfx · 2026-03-28T14:20:08Z

Summary

Enable WASM3 and wide arithmetic features so components using GC (rec groups, structs, arrays) can be validated and transpiled
Fix multiple P3 async/stream runtime bugs:
- waitable-set.wait trampoline missing JSPI Suspending wrapper
- waitableSetWait calling undefined task.waitForEvent()
- Missing intrinsic dependencies (LowerFlatResult→LowerFlatVariant, LowerFlatOption→LowerFlatVariant, WaitableSetWait→StoreEventInComponentMemory, StreamRead→AsyncEventCodeEnum)
- _lowerFlatVariant null payload crash and payloadOffset typo
- Stream naming inconsistency (streamEndIdx vs streamEndWaitableIdx)
- Async export void return not falling back to task.completionPromise()
Add globalThis._jcoStreamWriteHook for host stream delivery — without this, host-provided streams (e.g. stdout) deadlock because both the host's stream.next() and the component's stream.write suspend via JSPI and neither can make progress

🤖 Generated with Claude Code

P.S. The wasm in this PR is generated by Wado, a programming language targeted to Wasm CM + WASI-p3.

Currently, jco does not handle any wasm generated by Wado compiler. With this PR, some examples get working.

…bugs Enable WASM3 and wide arithmetic features so components using GC (rec groups, structs, arrays) can be validated and transpiled. Fix multiple P3 async/stream runtime bugs: waitable-set.wait missing JSPI Suspending wrapper, waitableSetWait calling undefined waitForEvent(), missing intrinsic dependencies, _lowerFlatVariant null payload crash and typo, stream naming inconsistency, and async export void return handling. Add a GC + WASI P3 test fixture (hello-gc-p3.wasm) to the P3 transpile test suite. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…very For host-provided streams (e.g. stdout), the host's writeViaStream receives a Stream object and must read from it via stream.next(), while the component writes to the writable end via stream.write. However, both operations suspend via JSPI, creating a deadlock: the host awaits stream.next() while the component awaits stream.write, and neither can make progress because JSPI cannot interleave them. Add a globalThis._jcoStreamWriteHook callback that intercepts stream.write and delivers data directly to the host from linear memory, bypassing the rendezvous mechanism entirely. This is the only workable approach until jco's async runtime can interleave JSPI-suspended host reads with component writes. Add a runtime test that verifies a GC + WASI P3 component outputs "Hello, world!\n" via the stream write hook. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vados-cosmonic · 2026-03-30T00:11:17Z

Hey @gfx thanks for submitting this PR -- I'd like to take the runtime fixes but not the stream write write hook solution here -- if you could split these up that would make this PR much easier to take.

There seem to be at least 3 different changes in here:

Runtime fixes
Introduction of Wasm GC/changing of features
Adding of jco stream write hook for stream delivery

These would be beneficial if split apart -- I will likely also adopt some of these changes in ongoing work on streams/futures in #1343 so either way thanks for the PR!

vados-cosmonic · 2026-03-30T00:27:55Z

crates/js-component-bindgen/src/function_bindgen.rs

+                          if (ret === undefined) {{
+                              const taskResult = await task.completionPromise();
+                              if (taskResult !== undefined) {{ return taskResult; }}


This code isn't quite right -- async exports will go through Instruction::AsyncTaskReturn which will return the completion promise. Did you find that to not work? An examlpe case here would be great to add to the test suite as a regression test if that's the case.

vados-cosmonic · 2026-03-30T00:28:29Z

crates/js-component-bindgen/src/transpile_bindgen.rs

                    self.src.js,
                    r#"
-                    const trampoline{i} = {waitable_set_wait_fn}.bind(null, {{
+                    const trampoline{i} = new WebAssembly.Suspending({waitable_set_wait_fn}.bind(null, {{


This is a great catch!

vados-cosmonic · 2026-03-30T00:29:52Z

packages/jco/test/p3/async.js

+        try {
+            const { instance, cleanup } = await setupAsyncTest({
+                component: {
+                    path: join(P3_COMPONENT_FIXTURES_DIR, "gc/hello-gc-p3.wasm"),


Would you mind including the source of this component? Is it written in Wado? Is there any way it can be written in Rust instead and built with the rest of the test components?

It's also possible to add the wado code + toolchain to testing infrastructure, but that should be under a separate PR if possible.

vados-cosmonic · 2026-03-30T00:31:32Z

Preparing a PR with the changes that I'm ready to take from here -- will open and merge that soon!

This commit enables wasm 3.0 spec features and wide arithmetic features, as introduced by the PR against Jco by Wado compiler maintainers. (see: bytecodealliance#1346) (see https://github.com/WebAssembly/spec/tree/main/specification/wasm-3.0) Co-Authored-By: FUJI Goro <g.psy.va@gmail.com> Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

This commit enables wasm 3.0 spec features and wide arithmetic features, as introduced by the PR against Jco by Wado compiler maintainers. (see: #1346) (see https://github.com/WebAssembly/spec/tree/main/specification/wasm-3.0) Co-Authored-By: FUJI Goro <g.psy.va@gmail.com> Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

gfx and others added 2 commits March 28, 2026 20:44

gfx marked this pull request as ready for review March 29, 2026 00:36

gfx requested a review from vados-cosmonic as a code owner March 29, 2026 00:36

vados-cosmonic reviewed Mar 30, 2026

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vados-cosmonic mentioned this pull request Mar 30, 2026

fix(bindgen): bindgen runtime bugs #1347

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vados-cosmonic mentioned this pull request Mar 30, 2026

feat(bindgen): configure wasmparser for wasm 3.0 support #1348

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fix(bindgen): fix WASM3/GC transpilation and P3 async stream runtime bugs#1346

fix(bindgen): fix WASM3/GC transpilation and P3 async stream runtime bugs#1346
gfx wants to merge 2 commits intobytecodealliance:mainfrom
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gfx commented Mar 28, 2026 •

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