WebAssembly Runtime

Refer https://developer.mozilla.org/en-US/docs/WebAssembly for WebAssembly overview.

Execution environment

• Main thread

  This can do all the DOM operations.

  The jsaddle JS code run on the main thread and it communicates with WebWorker thread using message passing with channel (https://developer.mozilla.org/en-US/docs/Web/API/Channel_Messaging_API/Using_channel_messaging)

• WebWorker thread.

  The wasm code (kernel + haskell runtime) runs on a WebWorker thread. The functionality available in these threads are limited (https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Functions_and_classes_available_to_workers), therefore one cannot execute arbitrary JS in them.

• Communication between Wasm and JavaScript

  • Through syscall import/export APIs

    The kernel APIs like syscall are implemented in the webabi JS code, and they are imported in the wasm code. Using these syscall APIs all the communication between wasm and JS can be implemented by reading and writing to unix style devices.

    We have currently chosen this approach over importing arbitrary user-defined JS APIs.

Memory

The memory allocated to a WebAssembly executable is a linear array (with index starting from 0), which is accessible from both JavaScript and WebAssembly code.

The memory is just a plain continuous block of byte array, which has to be used by the runtime as either Heap or stack.

The memory block only grows, there is no concept of freeing the memory.

The pointer values are simply the indices of the array.

Accessing memory from JavaScript

// Example C
myfun (int n, int* iPtr, int** iSPtr) {
  int i = 0;
  for (; i < n; i++) {
    int j = iPtr[i];
    iSPtr[i][j] = i * j;
  }
}

// JS
myfun (n, iPtr_, iSPtr_) {
  var iPtr = iPtr_ / 4;
  var iSPtr = iSPtr_ / 4;
  for (var i = 0; i < n; i++) {
    var j = heap_uint32[iPtr + i];
    var ptr_ = heap_uint32[iSPtr + i];
    var ptr = ptr_ / 4;
    heap_uint32[ptr + j] = i * j;
  }
}

Kernel

Overview of kernel Sys calls http://man7.org/linux/man-pages/dir_section_2.html

Input / Output

FileSystem

Concurrency

This looks promising https://github.com/lars-t-hansen/parlib-simple

https://github.com/kripken/emscripten/blob/incoming/src/pthread-main.js

Timer/Interrupts