Some observations while writing a collection of optimized PRNGs in JS

Perf. optimization

  • JS engines don’t like unsigned integers. Unsigned integers are usually slower than signed.
  • Things that are fast in JIT engines may be slow in jitless, and vice versa.
  • Functional JavaScript is a slow as hell. Ramda is slow as hell. In JITs, in jitless, it’s always super S-L-O-O-O-W. All-my-homies-hate-Ramda.jpeg
  • How to write optimized JS for JITs:
    1. Write it in asm.js.
    2. Validate.
    3. Fix warnings.
    4. Remove the use asm pragma.
    5. Move the code around as you want.
    6. But never erase the type hints.
    7. asm.js is usually slower than the same code without the use asm pragma.
  • How to write optimized JS for jitless engines:
    1. As less JavaScript operations as possible. Not the instructions a corresponding C code could be compiled into with optimizing compiler.
    2. Also take in account algorithms defined in the spec for each operation.
    3. Don’t assume anything like it’s easy to prove the invariants and replace the code (even interpreted bytecode) with faster instructions.
    4. Certain things that are slow in JITs (like generators and other way-too-high-level sugar) are usually fast in jitless engines.
    5. asm.js-like code that is fast in JITs may be slow in jitless.
    6. Don’t do this unless you absolutely need this.
  • WASM is fast, but the interop cost is too high, so calling WASM functions from JS side can be much slower than writing the same code in plain JS and calling it with no interop overhead.
  • SpiderMonkey is slower than V8 and JSCore.
  • Jitless SpiderMonkey is slower than jitless V8 and QuickJS.
  • Jitless V8 is the fastest of jitless engines.
  • QuickJS is quick (but still slower than jitless V8).
  • crypto.getRandomValues() is not slow (when cached).
  • Closures and closure-based objects are slower than class-based objects.
    • Even if methods are defined as arrow functions (i.e. closures).
  • Oh, bigints are slooooow (as expected).


  • Javascripters don’t know how to write optimized code. Well, with some notable exceptions.
  • Javascripters don’t know how to write good PRNGs. And introduce questionable bullshit like, ahem, G🤬bson’s UHEPRNG.
  • Sometimes javascripters know how to write optimized code, but don’t know how to construct high-quality RNGs. And introduce things like Alea.


Alea is really fast. In jitless engines (in JITs it’s even slower than [optimized] MT19937). It’s based on Marsaglia’s MWC algorithms, but the state is too smol, the constants are too unclear, and the hash it uses to seed the state is “Oh dear what is mash() even doing…..” according to Appleby. Well, I could test it with TestU01 and PractRand, but I’m too lazy to do this.

  • comment

No comment found.

Add a comment

You must sign in to post a comment.