[Webkit-unassigned] [Bug 156404] STP TypedArray.subarray 5x slowdown compared to 9.1

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=156404

--- Comment #4 from Keith Miller <keith_miller at apple.com> ---
(In reply to comment #3)
> (In reply to comment #2)
> > (In reply to comment #1)
> > > Fascinating.  That's a pretty serious regression.  Keith, do you remember
> > > what we've done with subarray?
> > > 
> > 
> > This regression is not particularly surprising. The ES6 spec changes the
> > behavior of the subarray function substantially. I expect that issue here is
> > that the subarray function now uses species constructors, which require
> > several property lookups. Namely, the constructor and the Symbol.species
> > properties. Since the subarray function is in C++ (where we don't cache the
> > property lookups) and the subarrays you are making are very small (4
> > elements). The cost of the species construction will dominate the cost of
> > the element copying.
> 
> Why not perform the property accesses in JavaScript and then call a helper
> function to do the allocation?
> 
> I don't think we want to consider a 5X regression unsurprising.

I agree, unsurprising was the wrong word choice. I think the a JS wrapper would work. I just didn't bother implementing it as I was unsure how popular the subarray function was in practice. If the subarray function is popular then creating a wrapper is something we should consider doing.

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