Easy 10x performance boost for TypedArray.prototype.fill()? (in special cases)

[Ashton Six]

Hi,

I noticed `new Int8Array(int32.buffer).fill(-1)` is equivalent to `int32.fill(-1)` but 10x faster, since [Uint, Int]8.fill() uses memset whereas every other implementation of TypedArray.fill() uses a for loop (https://github.com/llvm/llvm-project/blob/8bb1dbbf7544eaac3afab8d1f91b71f383dab903/libcxx/include/algorithm#L1987-L2005). Benchmark: https://jsbench.me/1zl3y8q6q7/1.

Is it worth making this optimisation part of v8 for TypedArray.fill(0), Int16Array.fill(-1) and Int32Array.fill(-1)?

Regards,
Ashton

[Marja Hölttä]

Interesting, thanks for making us / me aware of this.

It sounds like a generally reasonable optimization; at least in the fill(0) case. Not sure how common fill(-1) is but if it doesn't increase the code complexity too much, why not.

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[Leszek Swirski]

I guess we could do it for anything where all two/four bytes of the fill value are identical? Probably (aside from 0) an unusual case like you say, but the complexity of checking it should be low enough.

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[Jakob Kummerow]

I guess 0 is by far the most common case.

A patch is easy enough: https://chromium-review.googlesource.com/c/v8/v8/+/3687700

Locally, I'm seeing 1.5x - 2x improvement, far from 10x but clearly measurable for large arrays.

[Ashton Six]

Cool, glad this has worked out.

I decided to play around a bit more and figured out a faster implementation of TypedArray.prototype.fill() for arbitrary values BTW (3x improvement for me):

```
function fill(arr, value) {
  arr[0] = value
  for (let i = 1; i < arr.length; i *= 2) {
    arr.copyWithin(i, 0, i)
  }
  return arr
}
```

This works out because `TypedArray.prototype.copyWithin()` uses memmove. Maybe worth doing for 16/32/64-bit typed arrays, perhaps even for plain old javascript arrays?

Updated benchmark: https://jsbench.me/1zl3y8q6q7/2

[Claudia]

Have you all also considered using `rep stos{b,w,d,q}` on x86 for this? While it's highly architecture-specific (ARM's also got a variant on the way, but it's not here yet), it might be able to work for similar effect as a universal optimization.