Add support for wasm32-wasi-threads

[theoparis]

See WebAssembly/wasi-sdk#274

I tried building something that requires cmake and pthreads and it failed to compile with zig;cc as the compiler and wasm32-wasi. If I try to use the wasm32-wasi-threads target I get this:

error: UnknownApplicationBinaryInterface

[andrewrk]

If WebAssembly supports threads now, we can start by changing this logic:

zig/src/target.zig

Lines 210 to 212 in aa3405a

	pub fn isSingleThreaded(target: std.Target) bool {
	return target.isWasm();
	}

Are WebAssembly threads supported in browsers?

[jakehemmerle]

Are WebAssembly threads supported in browsers?

It looks like Wasm threads have been supported in major browsers for a year or two now.

Be aware, there are two Wasm threading specs to be aware of: Threads and Atomics (which is in phase3/5 for the core Wasm spec), and wasi-threads (which is phase 1/5 for the WASI spec).

What's the difference? wasi-threads intro explains:

That Threads and Atomics proposal provides the primitives necessary for shared memory, atomic operations, and wait/notify. This wasi-threads proposal solely provides a mechanism for spawning threads. Any other thread-like operations (thread joining, locking, etc.) will use primitives from the Threads and Atomics proposal.

Some background: browsers already have a mechanism for spawning threads — Web Workers — and the Threads and Atomics proposal avoided specifying how thread spawning should occur. This allows other uses of WebAssembly — i.e., outside the browser — to specify their own mechanism for spawning threads.

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Regarding support for wasi-threads: wasmtime seems to be the defacto Wasm runtime and it indicates both full support for Threads and Atomics and full support for wasm-threads.

[andrewrk]

Done:

• WASI: Implement experimental threading support #16207
• wasm-linker: finish shared-memory & TLS implementation #16439