Webassembly & Fallout2

How it went

It begun with fallout2-ce project

• Someone used IDA pro for reverse-engineering
• Working C code

Webassembly

Webassembly.Memory ~ ArrayBuffer

JavaScript

Webassembly

js_func1()

wa_func2()

js_func3()

Emscripten

LLVM code

Emscripten output

JS

WASM

HTML

libc / SDL / etc

JS event loop

• call init
• call emscripten_set_main_loop

JS

Webassembly

requestAnimationFrame()

call_me_on_each_frame()

Real-life example

more than 2 loops = “many”

First try: thread

​

PROXY_TO_PTHREAD || --proxy-to-worker == did not work with SDL & opengl

Main thread

Game thread

<waits in Atomics>

postMessage

Another try: post ImageBuffer

Very slow

Main thread

Game thread

postMessage with ImageBuffer

OffscreenCanvas?

WebGL commits on event loop

Main thread

Game thread

Canvas

OffscreenCanvas

Asyncify

Unwinds / rewinds call stack

“Pause and Resume WebAssembly with Asyncify, Alon Zakai”

Binaryen & JS

js_run_main()

wa_main()

main_loop()

render_frame()

get_time()

SDL_sleep()

sleep()

js_sleep()

return Date.now()

GLOBAL STATE = normal | unwinding | rewinding

char asyncify_stack[1000];

move_enemies()

It works!

Asyncfetchfs

Usual way is to preload everything

With Asyncify we can have async filesystem

​

Emscripten

JS

Asyncfetchfs

JS

__syscall_openat

WASM

stdlib

open()

WASM

fallout2-ce

fopen()

Issue with random freezing

___syscall_openat creates new stream everytime it runs

Emscripten

JS

WASM

Asyncfetchfs

JS

__syscall_openat

Here is side effect

open()

fd_read

In theory possible too

Emscripten

JS

WASM

wasi fd_read

Asyncfetchfs

JS

read()

Final FS layout

/ = MEMFS with no preloaded files

/app = asyncfetchfs

/app/data/SAVEGAME = IDBFS

/app/<temp1> = MEMFS

/app/<temp2> = MEMFS

Mostly no changes to original code

• Remove spin waits
• Call stack exceeded when pathfinding - solved via “user space” stack
• Call fsync to trigger IDBFS to save data

Spinwait for sound

while (! is_sound_buf_empty()) {

SDL_Sleep(1); // Add this line!

}

WebAudio scriptProcessorNode

read_data_from_SDL_buf()

Javascript code

• Download/upload savegames
• Use service worker for engine
• Save downloaded file into `caches`

Thanks!

Questions?