Generating WebAssembly with LDC

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Starting with v1.11, LDC supports compiling and linking directly to WebAssembly. This page shows how to get started.

Building WebAssembly

Let's generate a .wasm file for this D code (wasm.d):

extern(C): // disable D mangling

double add(double a, double b) { return a + b; }

// seems to be the required entry point
void _start() {}

Build wasm.wasm:

ldc2 -mtriple=wasm32-unknown-unknown-wasm -betterC wasm.d

In case LDC errors out (e.g., with unsupported -link-internally), try an official prebuilt release package.

Test in HTML page

Let's test it with a little HTML page, loading and invoking the WebAssembly via JavaScript. Generate an .html file in the same directory as the .wasm file, with the following contents:


const request = new XMLHttpRequest();'GET', 'wasm.wasm');
request.responseType = 'arraybuffer';
request.onload = () => {
  console.log('response received');
  const bytes = request.response;
  const importObject = {};
  WebAssembly.instantiate(bytes, importObject).then(result => {
    const { exports } = result.instance;
    // finally, call the add() function implemented in D:
    const r = exports.add(42, -2.5);
    console.log('r = ' + r);
console.log('request sent');

    Test page

Note that fetch() doesn't work for files in the local filesystem (file://), but XMLHttpRequest does in Firefox (not in Chrome though IIRC).

Open the HTML page; the JavaScript console should show:

request sent
response received
r = 39.5

Calling external functions

The minimal example above only calls in one direction, from JavaScript to WebAssembly. Here's how to call external functions in D:


extern(C): // disable D mangling

void callback(double a, double b, double c);

double add(double a, double b)
    const c = a + b;
    callback(a, b, c);
    return c;

void _start() {}

Add -L-allow-undefined as linker flag to the LDC command line, otherwise LLD refuses to link due to undefined callback().

Implement the callback() function in JavaScript and specify it in importObject.env:

const callback = (a, b, c) => {
  console.log(`callback from D: ${a} + ${b} = ${c}`);

// ...

  const importObject = {
    env: { callback }

The log should now show:

request sent
response received
callback from D: 42 + -2.5 = 39.5
r = 39.5