Bug 164531 – Spread(ArrayUse:@child) should participate in CSE

• Status: NEW
• Component: JavaScriptCore

[Saam Barati]

After I land https://bugs.webkit.org/show_bug.cgi?id=164258, we will emit Spread(ArrayUse:@child) only when we know that the iterator protocol is proven to be non-observable.
This means we can CSE two Spread(ArrayUse:@child) together when we've proven that nobody changes @child in between the two Spreads.

[Saam Barati]

It's important to also note that Spread will allocate a JSFixedArray, but JSFixedArray is a VM internal cell, it's not observable to user code. Therefore, it's ok to simply drop the second allocation and use the first one.

[David Kilzer (:ddkilzer)]

Since the comment mentioning this bug was removed in r227229, can this be changed to RESOLVED/WONTFIX?

[Saam Barati]

(In reply to David Kilzer (:ddkilzer) from comment #2)
> Since the comment mentioning this bug was removed in r227229, can this be
> changed to RESOLVED/WONTFIX?

I think it's worth keeping this. We could theoretically do this if we speculated in some particular way. Or if we proved the input to Spread doesn't have a symbol iterator.