查找第 300 行的长度*慢
首先，我已阅读 Why is my WebAssembly function slower than the JavaScript equivalent? 的答案
但这并没有说明问题，而且我已经投入了大量时间，这很可能是靠墙的黄色东西。
我不使用全局变量，我不使用任何内存。我有两个简单的函数，它们可以找到线段的长度，并将它们与普通旧 Javascript 中的相同内容进行比较。我有 4 个参数 3 个本地人，并返回一个浮点数或 double 数。
在 Chrome 上，Javascript 比 webAssembly 快 40 倍，在 Firefox 上，wasm 几乎比 Javascript 慢 300 倍。
jsPref 测试用例。
我在 jsPref WebAssembly V Javascript math 中添加了一个测试用例
我究竟做错了什么？
任何一个

Re-use existing code by targeting WebAssembly, embedded in a larger JavaScript / HTML application. This could be anything from simple helper libraries, to compute-oriented task offload.

// setup and associated functions
    const setOf = (count, callback) => {var a = [],i = 0; while (i < count) { a.push(callback(i ++)) } return a };
    const rand  = (min = 1, max = min + (min = 0)) => Math.random() * (max - min) + min;
    const a = setOf(100009,i=>rand(-100000,100000));
    var bigCount = 0;




    function len(x,y,x1,y1){
        var nx = x1 - x;
        var ny = y1 - y;
        return Math.sqrt(nx * nx + ny * ny);
    }
    function lenSlow(x,y,x1,y1){
        var nx = x1 - x;
        var ny = y1 - y;
        return Math.hypot(nx,ny);
    }
    function lenEmpty(x,y,x1,y1){
        return x;
    }


// Test functions in same scope as above. None is in global scope
// Each function is copied 4 time and tests are performed randomly.
// c += length(...  to ensure all code is executed. 
// bigCount += c to ensure whole function is executed.
// 4 lines for each function to reduce a inlining skew
// all values are randomly generated doubles 
// each function call returns a different result.

tests : [{
        func : function (){
            var i,c=0,a1,a2,a3,a4;
            for (i = 0; i < 10000; i += 1) {
                a1 = a[i];
                a2 = a[i+1];
                a3 = a[i+2];
                a4 = a[i+3];
                c += length(a1,a2,a3,a4);
                c += length(a2,a3,a4,a1);
                c += length(a3,a4,a1,a2);
                c += length(a4,a1,a2,a3);
            }
            bigCount = (bigCount + c) % 1000;
        },
        name : "length64",
    },{
        func : function (){
            var i,c=0,a1,a2,a3,a4;
            for (i = 0; i < 10000; i += 1) {
                a1 = a[i];
                a2 = a[i+1];
                a3 = a[i+2];
                a4 = a[i+3];
                c += lengthF(a1,a2,a3,a4);
                c += lengthF(a2,a3,a4,a1);
                c += lengthF(a3,a4,a1,a2);
                c += lengthF(a4,a1,a2,a3);
            }
            bigCount = (bigCount + c) % 1000;
        },
        name : "length32",
    },{
        func : function (){
            var i,c=0,a1,a2,a3,a4;
            for (i = 0; i < 10000; i += 1) {
                a1 = a[i];
                a2 = a[i+1];
                a3 = a[i+2];
                a4 = a[i+3];                    
                c += len(a1,a2,a3,a4);
                c += len(a2,a3,a4,a1);
                c += len(a3,a4,a1,a2);
                c += len(a4,a1,a2,a3);
            }
            bigCount = (bigCount + c) % 1000;
        },
        name : "length JS",
    },{
        func : function (){
            var i,c=0,a1,a2,a3,a4;
            for (i = 0; i < 10000; i += 1) {
                a1 = a[i];
                a2 = a[i+1];
                a3 = a[i+2];
                a4 = a[i+3];                    
                c += lenSlow(a1,a2,a3,a4);
                c += lenSlow(a2,a3,a4,a1);
                c += lenSlow(a3,a4,a1,a2);
                c += lenSlow(a4,a1,a2,a3);
            }
            bigCount = (bigCount + c) % 1000;
        },
        name : "Length JS Slow",
    },{
        func : function (){
            var i,c=0,a1,a2,a3,a4;
            for (i = 0; i < 10000; i += 1) {
                a1 = a[i];
                a2 = a[i+1];
                a3 = a[i+2];
                a4 = a[i+3];                    
                c += lenEmpty(a1,a2,a3,a4);
                c += lenEmpty(a2,a3,a4,a1);
                c += lenEmpty(a3,a4,a1,a2);
                c += lenEmpty(a4,a1,a2,a3);
            }
            bigCount = (bigCount + c) % 1000;
        },
        name : "Empty",
    }
],

/*
=======================================
Performance test. : WebAssm V Javascript
Use strict....... : true
Data view........ : false
Duplicates....... : 4
Cycles........... : 147
Samples per cycle : 100
Tests per Sample. : undefined
---------------------------------------------
Test : 'length64'
Mean : 12736µs ±69µs (*) 3013 samples
---------------------------------------------
Test : 'length32'
Mean : 13389µs ±94µs (*) 2914 samples
---------------------------------------------
Test : 'length JS'
Mean : 728µs ±6µs (*) 2906 samples
---------------------------------------------
Test : 'Length JS Slow'
Mean : 23374µs ±191µs (*) 2939 samples   << This function use Math.hypot 
                                            rather than Math.sqrt
---------------------------------------------
Test : 'Empty'
Mean : 79µs ±2µs (*) 2928 samples
-All ----------------------------------------
Mean : 10.097ms Totals time : 148431.200ms 14700 samples
(*) Error rate approximation does not represent the variance.

*/

   
// declare func the < indicates export name, the param with types and return type
func <lengthF(float x, float y, float x1, float y1) float {
    float nx, ny, dist;  // declare locals float is f32
    nx = x1 - x;
    ny = y1 - y;
    dist = sqrt(ny * ny + nx * nx);
    return dist;
}
// and as double
func <length(double x, double y, double x1, double y1) double {
    double nx, ny, dist;
    nx = x1 - x;
    ny = y1 - y;
    dist = sqrt(ny * ny + nx * nx);
    return dist;
}

(module
(func 
    (export "lengthF")
    (param f32 f32 f32 f32)
    (result f32)
    (local f32 f32 f32)
    get_local 2
    get_local 0
    f32.sub
    set_local 4
    get_local 3
    get_local 1
    f32.sub
    tee_local 5
    get_local 5
    f32.mul
    get_local 4
    get_local 4
    f32.mul
    f32.add
    f32.sqrt
)
(func 
    (export "length")
    (param f64 f64 f64 f64)
    (result f64)
    (local f64 f64 f64)
    get_local 2
    get_local 0
    f64.sub
    set_local 4
    get_local 3
    get_local 1
    f64.sub
    tee_local 5
    get_local 5
    f64.mul
    get_local 4
    get_local 4
    f64.mul
    f64.add
    f64.sqrt
)
)

    // hex of above without the name section
    const asm = `0061736d0100000001110260047d7d7d7d017d60047c7c7c7c017c0303020001071402076c656e677468460000066c656e67746800010a3b021c01037d2002200093210420032001932205200594200420049492910b1c01037c20022000a1210420032001a122052005a220042004a2a09f0b`
    const bin = new Uint8Array(asm.length >> 1);
    for(var i = 0; i < asm.length; i+= 2){ bin[i>>1] = parseInt(asm.substr(i,2),16) }
    var length,lengthF;

    WebAssembly.compile(bin).then(module => {
        const wasmInstance = new WebAssembly.Instance(module, {});
        lengthF = wasmInstance.exports.lengthF;
        length = wasmInstance.exports.length;
    });
    // test values are const (same result if from array or literals)
    const a1 = rand(-100000,100000);
    const a2 = rand(-100000,100000);
    const a3 = rand(-100000,100000);
    const a4 = rand(-100000,100000);

    // javascript version of function
    function len(x,y,x1,y1){
        var nx = x1 - x;
        var ny = y1 - y;
        return Math.sqrt(nx * nx + ny * ny);
    }

 tests : [{
        func : function (){
            var i;
            for (i = 0; i < 100000; i += 1) {
               length(a1,a2,a3,a4);

            }
        },
        name : "length64",
    },{
        func : function (){
            var i;
            for (i = 0; i < 100000; i += 1) {
                lengthF(a1,a2,a3,a4);
             
            }
        },
        name : "length32",
    },{
        func : function (){
            var i;
            for (i = 0; i < 100000; i += 1) {
                len(a1,a2,a3,a4);
             
            }
        },
        name : "lengthNative",
    }
]

 /*
=======================================
Performance test. : WebAssm V Javascript
Use strict....... : true
Data view........ : false
Duplicates....... : 4
Cycles........... : 34
Samples per cycle : 100
Tests per Sample. : undefined
---------------------------------------------
Test : 'length64'
Mean : 26359µs ±128µs (*) 1128 samples
---------------------------------------------
Test : 'length32'
Mean : 27456µs ±109µs (*) 1144 samples
---------------------------------------------
Test : 'lengthNative'
Mean : 106µs ±2µs (*) 1128 samples
-All ----------------------------------------
Mean : 18.018ms Totals time : 61262.240ms 3400 samples
(*) Error rate approximation does not represent the variance.
*/

最佳答案

Andreas 描述了 JavaScript 实现是 initially observed to be x300 faster 的一些充分理由。 .但是，您的代码还有许多其他问题。

Whats the point of WebAssembly if it does not optimise

关于javascript - 为什么 webAssembly 函数比相同的 JS 函数慢近 300 倍，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/48173979/