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• 句法
• 描述
• Polyfill
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

Math.imul() function returns the result of the C-like 32-bit multiplication of the two parameters.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

句法

var product = Math.imul(a, b);
					

参数

a

First number.

b

Second number.

返回值

The result of the C-like 32-bit multiplication of the given arguments.

描述

Math.imul() allows for 32-bit integer multiplication with C-like semantics. This feature is useful for projects like Emscripten . Because imul() is a static method of Math , you always use it as Math.imul() , rather than as a method of a Math object you created ( Math is not a constructor). If you use normal JavaScript floating point numbers in imul, you will experience a degrade in performance. This is because of the costly conversion from a floating point to an integer for multiplication, and then converting the multiplied integer back into a floating point. The reason imul exists is because it is faster in only one (so far) circumstance: AsmJS. AsmJS allows for JIST-optimizers to more easily implement internal integers in JavaScript. Multiplying two numbers stored internally as integers (which is only possible with AsmJS) with imul is the only potential circumstance where Math.imul may prove performant in current browsers.

Polyfill

This can be emulated with the following function:

if (!Math.imul) Math.imul = function(a, b) {
  var aHi = (a >>> 16) & 0xffff;
  var aLo = a & 0xffff;
  var bHi = (b >>> 16) & 0xffff;
  var bLo = b & 0xffff;
  // the shift by 0 fixes the sign on the high part
  // the final |0 converts the unsigned value into a signed value
  return ((aLo * bLo) + (((aHi * bLo + aLo * bHi) << 16) >>> 0) | 0);
};
					

However, the following function is more performant because it is likely that browsers in which this polyfill would be used do not optimize with an internal integer type in javascript, instead using floating points for all numbers.

if (!Math.imul) Math.imul = function(opA, opB) {
  opB |= 0; // ensure that opB is an integer. opA will automatically be coerced.
  // floating points give us 53 bits of precision to work with plus 1 sign bit
  // automatically handled for our convienence:
  // 1. 0x003fffff /*opA & 0x000fffff*/ * 0x7fffffff /*opB*/ = 0x1fffff7fc00001
  //    0x1fffff7fc00001 < Number.MAX_SAFE_INTEGER /*0x1fffffffffffff*/
  var result = (opA & 0x003fffff) * opB;
  // 2. We can remove an integer coersion from the statement above because:
  //    0x1fffff7fc00001 + 0xffc00000 = 0x1fffffff800001
  //    0x1fffffff800001 < Number.MAX_SAFE_INTEGER /*0x1fffffffffffff*/
  if (opA & 0xffc00000 /*!== 0*/) result += (opA & 0xffc00000) * opB |0;
  return result |0;
};
					

范例

使用 Math.imul()

Math.imul(2, 4);          // 8
Math.imul(-1, 8);         // -8
Math.imul(-2, -2);        // 4
Math.imul(0xffffffff, 5); // -5
Math.imul(0xfffffffe, 5); // -10
					

规范

规范
ECMAScript (ECMA-262) The definition of 'Math.imul' in that specification.

浏览器兼容性

The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
imul	Chrome 28	Edge 12	Firefox 20	IE No	Opera 16	Safari 7	WebView Android ≤37	Chrome Android 28	Firefox Android 20	Opera Android 15	Safari iOS 7	Samsung Internet Android 1.5	nodejs 0.12

图例

完整支持

完整支持

不支持

不支持

另请参阅

• Emscripten

元数据

• 最后修改： May 20, 2020

相关话题

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