这里是素数生成算法，一个带有“ use asm” ，另一个没有。 在实时代码段的末尾有统计信息，看起来 asm.js 的运行速度比纯js慢4倍，为什么？
asm.js
function asmPrimes(stdlib,foreign,heap) {
'use asm';
var array = new stdlib.Int32Array(heap);
function asmPrimes1(elementsCount) {
elementsCount = elementsCount | 0;
var number = 0;
var idx = 0;
var j = 0;
var isPrimeflag = 1;
for (number = 2; (idx | 0) < (elementsCount | 0); number = (number + 1) | 0) {
isPrimeflag = 1;
for (j = 0; (j | 0) < (idx | 0); j = (j + 1) | 0) {
if (+(number | 0) % +(array[j << 2 >> 2] | 0) == +0) {
isPrimeflag = 0;
break;
}
}
if (isPrimeflag) {
array[idx << 2 >> 2] = number;
idx = (idx + 1) | 0;
}
}
return 0;
}
return asmPrimes1;
}
“只是” JS
function getPrimes(elementsCount) {
let idx = 0;
const array = [];
let number = 2;
while (idx < elementsCount) {
let isPrime = true;
for (let j = 0; j < array.length - 1; j++) {
if (!(number % array[j])) {
isPrime = false;
break;
}
}
if (isPrime) {
array.push(number);
idx++;
}
number++;
}
return array;
}
function asmPrimes(stdlib,heap) {
'use asm';
var array = new stdlib.Int32Array(heap);
function asmPrimes1(elementsCount) {
elementsCount = elementsCount | 0;
var number = 0;
var idx = 0;
var j = 0;
var isPrimeflag = 1;
for (number = 2; (idx | 0) < (elementsCount | 0); number = (number + 1) | 0) {
isPrimeflag = 1;
for (j = 0; (j | 0) < (idx | 0); j = (j + 1) | 0) {
if (+(number | 0) % +(array[j << 2 >> 2] | 0) == +0) {
isPrimeflag = 0;
break;
}
}
if (isPrimeflag) {
array[idx << 2 >> 2] = number;
idx = (idx + 1) | 0;
}
}
return 0;
}
return asmPrimes1;
}
function getPrimes(elementsCount) {
let idx = 0;
const array = [];
let number = 2;
while (idx < elementsCount) {
let isPrime = true;
for (let j = 0; j < array.length - 1; j++) {
if (!(number % array[j])) {
isPrime = false;
break;
}
}
if (isPrime) {
array.push(number);
idx++;
}
number++;
}
return array;
}
var start;
var MIN_SIZE = 1024; // Uint32Array won't create size is not X*1024
var PRIMES_AMOUNT_TO_FIND = MIN_SIZE * 4;
start = window.performance.now();
getPrimes(PRIMES_AMOUNT_TO_FIND);
write(`<pre>${'getPrimes 1'} ${(window.performance.now() - start).toFixed(2)}ms</pre>`);
start = window.performance.now();
var primes = getPrimes(PRIMES_AMOUNT_TO_FIND);
write(`<pre>${'getPrimes 2'} ${(window.performance.now() - start).toFixed(2)}ms</pre>`);
write(`<i>last 3 ${primes.slice(PRIMES_AMOUNT_TO_FIND - 3,PRIMES_AMOUNT_TO_FIND).join(',')}</i>`);
var array = new Int32Array(0x10000);
var asmPrimesCompiled = asmPrimes({ Int32Array },{},array.buffer);
start = window.performance.now();
asmPrimesCompiled(PRIMES_AMOUNT_TO_FIND);
write(`<pre>${'asm getPrimes 1'} ${(window.performance.now() - start).toFixed(2)}ms</pre>`);
start = window.performance.now();
asmPrimesCompiled(PRIMES_AMOUNT_TO_FIND);
write(`<pre>${'asm getPrimes 2'} ${(window.performance.now() - start).toFixed(2)}ms</pre>`);
write(`<i>last 3 ${array.slice(PRIMES_AMOUNT_TO_FIND - 3,')}</i>`);
function write(text){
document.body.innerHTML += text;
}<h2>First 4048 prime numbers js vs asm.js</h2>