Design Doc: Wasm Worker Pthread Compatibility

• Status: Completed
• Bug: https://github.com/emscripten-core/emscripten/issues/26631

Context

Wasm Workers in Emscripten are a lightweight alternative to pthreads. They use the same memory and can use the same synchronization primitives, but they do not have a full struct pthread and thus many pthread-based APIs (like pthread_self()) currently do not work when called from a Wasm Worker.

This is not an issue in pure Wasm Workers programs but we also support hybrid programs that run both pthreads and Wasm Workers. In this cases the pthread API is available, but will fail in undefined ways if called from Wasm Workers.

This document describes the implementation to improve the hybrid mode by adding the pthread metadata (struct pthread) to each Wasm Worker, allowing the pthread API (or at least some subset of it) APIs to used from Wasm Workers.

Implemented Changes

1. Memory Layout

Normally, Wasm Workers allocate space for only TLS and stack: [TLS data] [Stack]. For hybrid mode (when pthreed are enabled as well as Wasm Workers) we changed this to also include pthread-specific data: [struct pthread] [TSD pointers] [TLS data] [Stack].

The struct pthread is located at the very beginning of the allocated memory block for each Wasm Worker.

2. struct pthread Initialization

The struct pthread is initialized by the creator thread in emscripten_create_wasm_worker (or emscripten_malloc_wasm_worker). This includes:

• Zero-initializing the structure.
• Setting the self pointer to the start of the struct pthread.
• Initializing essential fields like tid.

On the worker thread side, initialization is completed by calling __set_thread_state (via JS ___set_thread_state in libwasm_worker.js) to set the thread pointer, making it available to __get_tp.

3. __get_tp Support

We will modify system/lib/pthread/emscripten_thread_state.S to provide a __get_tp implementation for Wasm workers that returns the address of the struct pthread. This will allow __pthread_self() and other related functions to work correctly.

4. Supported Pthread API Subset

We intend to support a subset of the pthread API within Wasm workers:

• pthread_self(): Returns the worker's struct pthread pointer.
• pthread_equal(): Works normally.
• pthread_getspecific() / pthread_setspecific(): TSD (Thread Specific Data) should work if tsd field in struct pthread is initialized.
• pthread_mutex_*: Mutexes will work as they rely on struct pthread for owner tracking.
• pthread_cond_*: Condition variables will work as they rely on struct pthread for waiter tracking.
• Low-level synchronization primitives that use struct pthread (e.g., some internal locks).

APIs that will NOT be supported (or will have limited support):

• pthread_create() / pthread_join() / pthread_detach(): Wasm workers have their own creation and lifecycle management.
• pthread_cancel(): Not supported in Wasm workers.
• pthread_kill(): Not supported in Wasm workers.

Implementation Details

1. Modified emscripten_create_wasm_worker in system/lib/wasm_worker/library_wasm_worker.c to account for sizeof(struct pthread) in memory allocation and initialize the structure.
2. Updated $_wasmWorkerInitializeRuntime in src/lib/libwasm_worker.js to call ___set_thread_state to set the thread pointer.
3. Verified that essential pthread functions (like pthread_self()) work in Wasm workers in hybrid mode.

Verification

• New tests added to ensure pthread_self() and low level synchronization APIs work when called from a Wasm Worker.
• Verify that existing Wasm worker tests still pass.
• Verify no extra overhead for regular (non-hybrid) Wasm Worker builds.