Reiterating some discussion with bmeurer@ I had this morning:

My main concern is making sure we have a usable **and performant** "vector" type in V8 extras. Currently we are only using "push", but at points in the past we used: pop, push, shift, unshift, splice, slice. I want to ensure that if we remove InternalPackedArray, we can still recover performant versions of those operations in the future.

It seems like this could work just by creating our own "internal-ish Array" via nulling out the prototype of an array and transferring the methods from Array.prototype, since they operate generically. I am a little worried that we'll hit deopts inside pop/push/shift/unshift/splice/slice because such an array doesn't look like a normal array to V8, but I don't know V8's code in that area. So I'd be happy to try it out and report back and remove InternalPackedArray usage if the results work well for us.

(Concretely, I think this means creating a local benchmark that reverts to the earlier version of the queue, and running it with InternalPackedArray vs. a homegrown null-proto array.)

#2: In my opinion, shift() and unshift() are a trap because it's easy to create code that looks performant but which falls off a cliff in a case you didn't test for. Indeed, this already happened in  issue chromium:681493 .

splice() may be similarly problematic depending on how it is used.

I have been doing some rough benchmarking on M-59 and there's no statistically significant difference between array.push(undefined) and array[array.length] = undefined;. The standard deviation is huge, so it would probably be better to start a fresh instance of d8 for each run, rather than trying to benchmark in the noisy environment of the browser.

I was reminded that even if InternalPackedArray goes away, InternalArray does not. I had not realized that at the time of my initial comments. So we can just switch to InternalArray; that should be a one-line patch in V8.

I think however we need to do the following to avoid breaking any builds:

- Land a patch to V8 that exports InternalArray to V8 extras
- Land a patch to Chromium that switches usage of InternalPackedArray to InternalArray
- Land a patch to V8 that removes the exporting of InternalPackedArray

If there is a smarter way to land these sort of breaking changes to V8, let me know.

Sounds good to me. InternalArray is a lot less dangerous than InternalArray.

...than InternalPackedArray.

Since we plan to stop using V8 Extras for Streams within a few months, we will probably just wait to remove InternalPackedArray from V8 until after we remove the V8 Extras-based implementation of Streams from Blink.