demo · v130

Wasm JS String Builtins benchmark

Instantiate two real Wasm modules — one using imported wasm:js-string built-ins, one calling back into JS — and benchmark a string-heavy loop. The first should be measurably faster because the calls inline.

JS-callback (slow path)

—
ms

wasm:js-string (fast path)

—
ms

speedup

—
× faster

the code

// Wasm module declares an import against "wasm:js-string":
(module
  (import "wasm:js-string" "length" (func $len (param externref) (result i32)))
  (import "wasm:js-string" "charCodeAt"
    (func $cca (param externref i32) (result i32)))
  (func (export "sum") (param $s externref) (result i32)
    ;; sum char codes — calls inline into native string ops
    …))

// JS side passes { builtins: ["js-string"] } at compile time:
const mod = await WebAssembly.compileStreaming(fetch("m.wasm"),
  { builtins: ["js-string"] });

see also