Join the super secret V8 club and become a JavaScriptbender

@polesdesfetes (Private)

Staff engineer at Data Dog.

V8 Engine

• Takes JS as input and executes it
• Createed by Google, open source, C++
  • https://chromium.googlesource.com/v8/v8.git
• good resource
• document is not defined in V8
• Doesn't just interpret JS
  • Parse to AST
  • Ignition: bytecode generation
  • Sparkplug: fast codegen
  • Turbofan: optimized codegen
  • reminder you can define numbers as 1_000_000 in js
    • underscore in numbers
  • objects have a Map, a hidden class.
  • what is % in fromnt of functions? It's "Native syntaxes". console.log(%MaxSmi())
  • You can set V8 flages with node using require('v8').setFlags (?) ('--flag')
  • Native syntax: undocumented V8 APIs.
  • Benchmark JS for testing optimizations... optimizations DO matter, up to 2x faster for a simple add function.
  • V8OptimizationStoatus() to get the status of optimization
  • How do debuggers work?
    • Chrome Debug Protocol
      • Has a Heap profiler and CPU profiler
        • CPU profiling is done with polling, similar to the top program (?)
  • See the Torque language which compiles directly into v8 bytecode. torque

Notes by kevin

• javascript engine

  • multiple engines: v8, spidermonkey, apple jscriptcore

• v8 executes js in context

  • ecmascript web specs not part of v8
  • chromium embeds v8 and provides DOM apis
  • node embeds v8 and provides system apis
  • ruby racer embeds v8 to call js from ruby

• v8 does not just interpret js

  • many parts
    • parse, optimization, codegen
    • turbofan: part that optimize code

• optimizing

  • what v8 mainly optimizes
    • inlinable functions
    • function with stable signature

• functions example

  • #todo (Private)

• objects example

  • getObject 1e6 times
  • to optimize object, requires
    • same fields with same type
    • types crated in same order
  • calling same object initialized with object of different order will stop optimizations

• native funciton syntax %

  • run v8 with --allow-native-syntax

    TIP: you can set v8 flags inside nodejs

  • native syntax are undocumented v8 apis, used for testing and debugging
    • you can find the v8 source
    • docs are in v8 tests
  • can used to manipulate optimization (eg. never optimize this function)

• what can i do with native functions?

• question1: are optimized functions faster?

  • benchmarkjs to test

• question2: check optimization status of a function within code

  • print to debug
  • if you keep opt/de-opt function, v8 will give up on your code

• question3: can i manipulate memory related stuff

  • yes, but please do it

• how does devtools work

  • chrome debug protocol
    • websockets
    • communicate with v8/chromium
    • access debug interface:
      • debugger
      • heap profiler
      • cpu profiling/code coverage
      • dom (browser only)
      • eg. playwright uses this
      • eg. remote debug node
    • gives access to everything in node process

• debug protocol

  • cpu profiling is polling based
  • control polling rate from protocol (not available in dev tools)

• heap profiler

  • show everything in heap
  • eg. find memory leak

• why

  • remote debugging
  • atuomated diagnostics
  • script debugging
    • eg. add breakpoint
    • execute code and check heap
    • canbe used in extensions

• other topics

  • v8 inlining
  • torque language
  • v8 treats web assembly
  • how v8 manages strings
  • etc
  • most of this is on https://v8.dev/

Notes by aminamos

• v8 is a JS engine
  • the engine takes JS code as an input and executes it
• v8 is one of many engines
• DOM manipulation APIs are not defined in v8
• node embeds v8 and has JS APIs
• v8 is embedded (ruby, deno examples)
• turbofan, sparkplug, ignition
• [discussion around code optimiziation, de-optimization, re-optimization]
• are optimized functions better? yes
• can I check optimization status of a function in the code? yes
• chrome debug protocol convo, CPU profiling is polling based (and this rate is configurable)
• can get heap dump, then dig into info in dev tools (all objects, all streams that are currently alive)
• other toppics
  • v8 code inilining, WebAssembly, string management, torque language, v8 snapshots
  • check out https://v8.dev/