1457256 - TypedArray.prototype.slice is 3x slower than subarray + set

Status: RESOLVED DUPLICATE of bug 1140152

(Core :: JavaScript Engine, defect, P2)

Description

[Markus Stange [:mstange]]

Attachment: testcase

If I have a large Uint8Array and want to copy out a part of it into a new Uint8Array, there is a large performance difference depending on how you do it:

function sliceUsingSlice(originalArray, offset, len) {
  return originalArray.slice(offset, len);
}

function sliceUsingSubarrayAndSet(originalArray, offset, len) {
  const slice = new Uint8Array(len);
  slice.set(originalArray.subarray(offset, len));
  return slice;
}

On a 100MB array, I get times around 250ms on the first and ~75ms on the second.

It looks slice implements the actual copying in self-hosted JS code, so it might be the bounds checks that are hurting us.

>        // Step 14.b.
>        while (k < final) {
>            // Steps 14.b.i-v.
>            A[n++] = O[k++];
>        }

Comment 1

[André Bargull [:anba]]

With the patch attached to bug 1140152 both functions need about the same time for me.

Comment 2

[André Bargull [:anba]]

The issue should be fixed by bug 1140152, but if you still see any major performance differences please reopen. Thanks!

Comment 3

[Markus Stange [:mstange]]

Can confirm. Thanks!

Comment 4

[André Bargull [:anba]]

\o/

Thanks for confirming!