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

If using DUB:

dub build --compiler=ldc2 --arch=wasm32-unknown-unknown-wasm

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

// import a function "callback" from default import module name "env"
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

To import functions from other modules or rename imported functions, we use LDC's @llvmAttr:

import ldc.attributes;

extern(C): // disable D mangling

// import a function "add" from module name "math" and rename it to "add_numbers"
@llvmAttr("wasm-import-module", "math") @llvmAttr("wasm-import-name", "add") { 
	int add_numbers(int a, int b);

// export a function "hello"
export int hello(int a, int b, int c)
	int s1 = add_numbers(a, b);
	int s2 = add_numbers(s1, c);
	return s2;

void _start() {}

Calling conventions


When calling an external function from web assembly, arrays are passed as two separate arguments: the length followed by the pointer (offset into linear memory) to the array data. For example, if an external function is declared as:

void externalFunc(int a, string b, float c);

The corresponding Javascript function should be declared this way:

function externalFunc(a, bLength, bOffset, c) { ... }

The array data can be accessed from Javascript by creating the appropriate typed array over the web assembly module's linear memory. For example, a passed string can be retrieved in Javascript like this:

function externalFunc(a, bLength, bOffset, c) {
   // result is the object passed by WebAssembly.instantiate to the callback.
   bytes = new Uint8Array(result.instance.memory.buffer, bOffset, bLength);
   str = TextDecoder('utf8').decode(bytes);

Non-POD parameters

Non-POD values are passed by reference to external functions. I.e., the external function will receive a pointer (offset) to the value, and must read the linear memory at that offset to retrieve the data.

Non-POD return values

Non-POD return values are implemented by passing a pointer as the first argument of the external function. I.e.,

struct MyStruct { int x; }
MyStruct externalFunc(int x);

translates to the Javascript function:

function MyStruct(resultOffset, x) {
   // caller expects the return value to be stored in linear memory at offset resultOffset.
   // No return statement (behaves like a void function).

Additional LLVM wasm features

You can list the supported LLVM features for WASM with your LDC version like SIMD or exception handling using

ldc2 -mtriple=wasm32-unknown-unknown-wasm -mattr=help

Example output: (may be outdated, run the above command yourself to get the supported features for your LDC version)

Targeting wasm32. Available CPUs for this target:

  bleeding-edge - Select the bleeding-edge processor.
  generic       - Select the generic processor.
  mvp           - Select the mvp processor.

Available features for this target:

  atomics             - Enable Atomics.
  bulk-memory         - Enable bulk memory operations.
  exception-handling  - Enable Wasm exception handling.
  multivalue          - Enable multivalue blocks, instructions, and functions.
  mutable-globals     - Enable mutable globals.
  nontrapping-fptoint - Enable non-trapping float-to-int conversion operators.
  reference-types     - Enable reference types.
  sign-ext            - Enable sign extension operators.
  simd128             - Enable 128-bit SIMD.
  tail-call           - Enable tail call instructions.

Use +feature to enable a feature, or -feature to disable it.
For example, llc -mcpu=mycpu -mattr=+feature1,-feature2

Conditional compilation

The version identifier version(WebAssembly) is defined when compiling for a web assembly target. This can be used for conditional compilation involving web assembly targets.