Difference between revisions of "Generating WebAssembly with LDC"

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m (→‎Examples)
m (→‎Test in HTML page: format JS code)
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   <head>
 
   <head>
 
     <script>
 
     <script>
−
 
+
      const request = new XMLHttpRequest();
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const request = new XMLHttpRequest();
+
      request.open('GET', 'wasm.wasm');
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request.open('GET', 'wasm.wasm');
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      request.responseType = 'arraybuffer';
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request.responseType = 'arraybuffer';
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      request.onload = () => {
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request.onload = () => {
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        console.log('response received');
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  console.log('response received');
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        const bytes = request.response;
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  const bytes = request.response;
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        const importObject = {};
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  const importObject = {};
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        WebAssembly.instantiate(bytes, importObject).then(result => {
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  WebAssembly.instantiate(bytes, importObject).then(result => {
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          console.log('instantiated');
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    console.log('instantiated');
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          const { exports } = result.instance;
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    const { exports } = result.instance;
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          // finally, call the add() function implemented in D:
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    // finally, call the add() function implemented in D:
+
          const r = exports.add(42, -2.5);
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    const r = exports.add(42, -2.5);
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          console.log('r = ' + r);
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    console.log('r = ' + r);
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        });
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  });
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      };
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};
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      request.send();
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request.send();
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      console.log('request sent');
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console.log('request sent');
 
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     </script>
 
     </script>
 
   </head>
 
   </head>

Revision as of 15:02, 22 January 2019

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: 

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

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
instantiated
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:

wasm.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
instantiated
callback from D: 42 + -2.5 = 39.5
r = 39.5

Examples

Retrieved from "https://wiki.dlang.org/?title=Generating_WebAssembly_with_LDC&oldid=9500"