The State of JSC

• Moved from LLVM to B3/Air
  • Transition is complete
  • FTL Jit in 64-bit platforms
  • Big Jetstream improvements
  • Big Kraken improvements

• Moved from Tracing to Sampling profiler
  • Performance is an order of magnitude better in Web Inspector
  • 10-20x faster with the sampling profiler

• ES6 Feature Complete
  • Block Scoping
  • Generator Functions

• Current work
  • ES6 Sample Bench
    • Fast Generators
    • Fast Map/Set
    • gotten 20% faster
    • Runs 2 different tests
    • Takes geometric mean of tests
    • Steady state
  • Concurrent Garbage Collection
    • Gonna mean pauses in JSC
    • Pauses won’t be nearly as long
    • Execute JS during GC
    • There will be points where you pause the world but it won’t be in high milliseconds
  • Working on WebAssembly
    • It’s being implemented
  • DOM JIT
    • Creating an abstraction for the JSC for operations on the DOM
    • Be aware of getter accessors of being compiled
  • Integrating ES6 Modules in HTML
    • <script type=“module”>
  • Almost done ES2017
    • Support everything buy async/await
    • Patches on async/await

• Demo of command line:
  • ./Tools/Scripts/run-jsc —release —sample ~/test.js
  • Shows sampling rate
  • and the hottest byte code samples
  • The hash identifies the code blocks and just
  • These tools would be nice for WebContent process
    • Too difficult to use with the sample profiles
  • Would be cool to add byte code information in inspector
  • At a high level a Heat Map would be nice (being worked on)
  • If it is in inspector, it can impact performance and throw off the profile
  • JSC_dumpDFGDisassembly=1 ./Tools/Scripts/run-jsc —release —sample ~/test.js

• How to async/await
  • async func with await is a shorthand for promises
  • await automatically wraps the given function in a Promise and returns the promise

• WebAssembly
  • Work is underway
  • There is a trapping semantic in WebAssembly
  • Cant export a function that takes 64-bit integer
  • Internally in WebAssembly you can handle 64-bit integers
  • You have to check that the memory is in bounds, if not then you have to unwind the webassembly
  • Does the exception flatten the webassembly?
    • The plan is to have each WebAssembly frame visible
  • You can have import and export in WebAssembly to call from WebAssembly to JS
  • Developers would likely have WebAssembly that calls to a JS helper to modify DOM since you can’t modify the DOM from WebAssembly
  • There is a lot of debate still happening
  • You can use the JS GC for whatever language your using
  • How fast is WebAssembly?
    • Significantly faster for common cases of compiling C code
    • WASM is quite slow compared to native codes
    • WebAssembly has more info about how things work that allow for more optimizations
    • Close to native performance
  • How do I compile to WebAssembly?
    • Compile through clang, that pass it to LLVM IR then pass it back to WebAssembly
    • It will likely get easier as tools are created