V8 optimizes array operations based on the array’s Elements Kind. Explanation + JavaScript performance tips: https://v8project.blogspot.com/2017/09/elements-kinds-in-v8.html …pic.twitter.com/tmVDOiuXKH
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I work on @ChromeDevTools & @v8js at Google and on ECMAScript through TC39.
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V8 optimizes array operations based on the array’s Elements Kind. Explanation + JavaScript performance tips: https://v8project.blogspot.com/2017/09/elements-kinds-in-v8.html …pic.twitter.com/tmVDOiuXKH
Is it possible to recognize patterns like `new Array(N).fill(null)` to preallocate packed elements, or it isn't worth it?
You mean that Array#fill would turn the array into PACKED mode?
No, when it is instantiated and immediately filled with null value. For ex: preallocate arrays for children normalization in vdom libs.
The optimizing compiler could recognize it, but for the interpreter it's cumbersome. And producing different hidden classes in different...
...tiers would probably lead to more polymorphism in the code following it (and unnecessary deopts).
Thanks, also curious if this is still relevant (pretransitioning) https://research.google.com/pubs/pub43823.html … ? And will it lead to polymorphism?
Yes, @v8js is still using that, but exactly to *avoid* polymorphism.
Just thought that if preallocating can cause deopts, pretransitioning can also deopt code that used different repr before actual transition
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