为什么将Uint8Array复制到Uint8ClampedArray比将Uint8Array复制到新Uint8Array慢？

Question

将Uint8Array复制到Uint8ClampedArray中似乎比克隆Uint8Array并使用其底层ArrayBuffer慢得多

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const foo = new Uint8Array(0x10000000); // 256MiB
console.time('Copy into Uint8ClampedArray');
const bar = new Uint8ClampedArray(foo);
console.timeEnd('Copy into Uint8ClampedArray');

​

上面的代码在我的机器(Chrome，MacBook v96 )上的运行时间约为160ms。

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const foo = new Uint8Array(0x10000000); // 256MiB
console.time('Clone, then use ArrayBuffer');
const bar = new Uint8ClampedArray(new Uint8Array(foo).buffer);
console.timeEnd('Clone, then use ArrayBuffer');

​

上面的代码在我的机器上以大约70ms的速度计时。

Firefox给出了类似的统计数据(140ms vs 60ms)，而Safari则显示出更极端的差异(550ms vs 30ms)。

Answer 1

我在浏览器或NodeJs中看不到这些时间差

foo = new Uint8Array(256 * 1024**2); // 256MiB
console.time(label = 'Copy into Uint8ClampedArray');
bar = new Uint8ClampedArray(foo);
console.timeEnd(label);

foo = new Uint8Array(256 * 1024**2); // 256MiB
console.time(label = 'Clone, then use ArrayBuffer');
bar = new Uint8ClampedArray(new Uint8Array(foo).buffer);
console.timeEnd(label);

$ node -v && node test.js
v17.2.0
Copy into Uint8ClampedArray: 174.049ms
Clone, then use ArrayBuffer: 193.819ms

$ node -v && node test.js
v17.2.0
Copy into Uint8ClampedArray: 170.152ms
Clone, then use ArrayBuffer: 192.41ms

$ node -v && node test.js
v17.2.0
Copy into Uint8ClampedArray: 168.555ms
Clone, then use ArrayBuffer: 209.525ms