ColinEberhardt · January 16, 2018 22:04
diff --git a/.block b/.block
 license: mit
diff --git a/README.md b/README.md
diff --git a/index.html b/index.html
 <!DOCTYPE html>
 <head>
  <meta charset="utf-8">
  <script src="script.js"></script>
 </head>
 <body>
 </body>
diff --git a/script.js b/script.js
 // test.wasm is the following:

 // (module
 //   (func $div_s (param i32) (param i32)  (result i32)
 //    (get_local 0)
 //    (get_local 1)
 //    (i32.div_s)
 //   )
 //   (func $div_u (param i32) (param i32)  (result i32)
 //    (get_local 0)
 //    (get_local 1)
 //    (i32.div_u)
 //   )
 //   (func $echo (param i32) (result i32)
 //    (get_local 0)
 //   )
 //   (export "echo" (func $echo))
 //   (export "div_s" (func $div_s))
 //   (export "div_u" (func $div_u))
 // )


 fetch('test.wasm').then(response =>
  response.arrayBuffer()
 ).then(bytes =>
  WebAssembly.instantiate(bytes)
 ).then(module => {
  
  const {div_s, div_u, echo} = module.instance.exports;
  
  // As per the wasm spec, the ToWebAssemblyValue algoritm coerces a JS value
  // into a wasm value
  // https://webassembly.github.io/spec/js-api/index.html#exported-function-exotic-objects
  // for i32, this is the ToInt32 as described here:
  // https://tc39.github.io/ecma262/#sec-toint32
  const positiveMod = (a, n) => {
    const remain = a % n;
    return remain > 0 ? remain : remain + n;
  };
  const toWebAssemblyValue = value => {
    const sign = value > 0 ? 1 : -1;
    const ints = sign * Math.floor(Math.abs(value));
    const int32bit = positiveMod(ints, Math.pow(2, 32));
    return int32bit >= Math.pow(2, 31) ? int32bit - Math.pow(2, 32) : int32bit;
  };
  
  // The inverse algorithm, ToJSValue, is slighty more vaguelly described 
  // as " the Number value for i32"
  // my guess is that this is the following conversion
  const toJSValue = value => value | 0;
  
  // testing the conversion to wasm and back to JS, via a simple echo function
  // that simply returns the input argument
  const value = 335991768761;
  console.log("echo wasm", m.exports.echo(value));
  console.log("echo js", toJSValue(toWebAssemblyValue(value)));
  
  // this works just fine!
  
  // unsigned division - here's a problem case (found through random number testing)
  const a = 335991768761;
  const b = 77022195821; 

  console.log("div_u wasm", m.exports.div_u(a, b));
  console.log("div_u js",
    toJSValue(div_u(toWebAssemblyValue(a), toWebAssemblyValue(b)))
  );
  
  // these give a different result :-(
 });
diff --git a/test.wasm b/test.wasm
 asm``echodiv_sdiv_u
  m  n
	<!DOCTYPE html>
	<head>
	<meta charset="utf-8">
	<script src="script.js"></script>
	</head>
	<body>
	</body>
	// test.wasm is the following:

	// (module
	// (func $div_s (param i32) (param i32) (result i32)
	// (get_local 0)
	// (get_local 1)
	// (i32.div_s)
	// )
	// (func $div_u (param i32) (param i32) (result i32)
	// (get_local 0)
	// (get_local 1)
	// (i32.div_u)
	// )
	// (func $echo (param i32) (result i32)
	// (get_local 0)
	// )
	// (export "echo" (func $echo))
	// (export "div_s" (func $div_s))
	// (export "div_u" (func $div_u))
	// )


	fetch('test.wasm').then(response =>
	response.arrayBuffer()
	).then(bytes =>
	WebAssembly.instantiate(bytes)
	).then(module => {

	const {div_s, div_u, echo} = module.instance.exports;

	// As per the wasm spec, the ToWebAssemblyValue algoritm coerces a JS value
	// into a wasm value
	// https://webassembly.github.io/spec/js-api/index.html#exported-function-exotic-objects
	// for i32, this is the ToInt32 as described here:
	// https://tc39.github.io/ecma262/#sec-toint32
	const positiveMod = (a, n) => {
	const remain = a % n;
	return remain > 0 ? remain : remain + n;
	};
	const toWebAssemblyValue = value => {
	const sign = value > 0 ? 1 : -1;
	const ints = sign * Math.floor(Math.abs(value));
	const int32bit = positiveMod(ints, Math.pow(2, 32));
	return int32bit >= Math.pow(2, 31) ? int32bit - Math.pow(2, 32) : int32bit;
	};

	// The inverse algorithm, ToJSValue, is slighty more vaguelly described
	// as " the Number value for i32"
	// my guess is that this is the following conversion
	const toJSValue = value => value \| 0;

	// testing the conversion to wasm and back to JS, via a simple echo function
	// that simply returns the input argument
	const value = 335991768761;
	console.log("echo wasm", m.exports.echo(value));
	console.log("echo js", toJSValue(toWebAssemblyValue(value)));

	// this works just fine!

	// unsigned division - here's a problem case (found through random number testing)
	const a = 335991768761;
	const b = 77022195821;

	console.log("div_u wasm", m.exports.div_u(a, b));
	console.log("div_u js",
	toJSValue(div_u(toWebAssemblyValue(a), toWebAssemblyValue(b)))
	);

	// these give a different result :-(
	});