Outline

What is WASM?

• virtual machine
• execution environment
• nearly native code in browser

VM

isn't that slow?

• good ratio of vm instructions to native clock cycles
• many programs are currently run in VMs
  language-oriented VMs: Java, JS, .Net
  platform-oriented VMs: VirtualBox, VMWare, WINE

Yet another VM?

the right VM for the job

Why

• Faster
• Good compilation target
  leverage decades of compiler geekery!

Faster

Current use

two C compilation tools

• LLVM (and sort of GCC)
• emscripten
  (wrapper for LLVM + APIs for e.g. WebGL)

other langs

• Rust
• Kotlin

Using

• emscripten

  emcc fib.c -s WASM=1 -o fib.html

  gives you .html which references .js:

  <script async type="text/javascript" src="hello.js"></script>

  which references .wasm:

  WebAssembly.instantiateStreaming(fetch(wasmBinaryFile), info)
  .then(instAndModule => {
    instAndModule.instance.exports.fib(10)
  })

• LLVM compiler (C code)

  clang -emit-llvm --target=wasm32 -Oz fib.c -c -o fib.bc
  llc -asm-verbose=false -o fib.s fib.bc
          s2wasm fib.s > fib.wast

• s-expression assembler

  (module
    (func $addTwo (param i32 i32) (result i32)
      get_local 0
      get_local 1
      i32.add)
    (export "addTwo" (func $addTwo)))

W3C process

• stable specifications
• tests
• community review
• coherent constellation of REC-track documents

WASM Process

• stabilize in the CG
  many forks, rapid incubation
• standardize in the WG
  one fork, careful integration

The WG plans to publishe minimum viable product (MVP) followed by many additions.
The WebAssembly W3C Process doc describes the project management to get there.

Feature phases

• 0 Pre-Proposal [CG] - individual's idea
• 1 Feature Proposal [CG] - group interest
• 2 Proposed Spec Text [CG+WG]
• 3 Implementation Phase [CG+WG]
• 4 Standardize (two implementations)
• 5 Merge into master (in WG)

18 in flight proposals

Future features

SIMD

Exceptions

currently cost 5-10X

• common in C++
  two styles:
  • standard operations (e.g. EOF)
  • "real" exceptions (network failure, nuclear strike)
• slow in emscripten
  can disable with DISABLE_EXCEPTION_CATCHING
• discussion: e.g. expressivity needed for catch

shared memory

getting good performance with concurrency

• Workers use (slow) message passing.
• Processors design for shared memory.
• SharedArrayBuffer was removed for SPECTRE.
• Site isolation in critical path for restoration.

WebGL can consume {,Shared}ArrayBuffer.

WASM buffers are growable.

JS-like

• move epic JS boilerplate into WASM
• will enable deadcode elimination
• weakrefs -- finalization signal to call C++ destructor
  less bookkeeping in the javascript caller
• Ecmascript modules interface

  <script src="fib" type="module"/>

  import "fib"

• optimize calling boundaries
  eliminate the ~seven thunk layers with typechecks, fallbacks etc, required to call e.g. WebGL from WASM.

Dev tools

source-level debugging

• improve sourcemaps for e.g. variable introspecting
  analogous to minimalist ELF's DWARF
  performance before breakpoint is bad
• share crash report API with Javascript (level 0)
• best in ff right now

Pushing phat audio