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mathias's profile
Mathias Bynens
Mathias Bynens
Mathias Bynens
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@mathias

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Mathias BynensVerified account

@mathias

I work on @ChromeDevTools & @v8js at Google and on ECMAScript through TC39. ♥ JavaScript, HTML, CSS, HTTP, performance, security, Bash, Unicode, i18n, macOS.

Munich, Germany
mths.be
Joined January 2007

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    1. Ryan Boucher‏ @distributedlife 20 Mar 2018
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      @mathias Great talk. Thanks. Does a map on a packed HOLEY array return a PACKED or HOLEY? e.g. Array(3).fill(1).map((_, i) => i) => [1,2,3] //HOLEY_ or _PACKED_

      1 reply 0 retweets 0 likes
    2. Mathias Bynens‏Verified account @mathias 20 Mar 2018
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      Replying to @distributedlife

      Array#map creates a new array of the needed length (see step 5 of https://tc39.github.io/ecma262/#sec-array.prototype.map …) similar to `new Array(length)`, so the result would be HOLEY.

      2 replies 0 retweets 0 likes
    3. Ryan Boucher‏ @distributedlife 20 Mar 2018
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      Replying to @mathias

      Does this mean that I can't avoid HOLEY arrays because a PACKED array is converted to HOLEY upon map.

      1 reply 0 retweets 0 likes
    4. Mathias Bynens‏Verified account @mathias 22 Mar 2018
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      Replying to @distributedlife

      It’s not “converted” — `map` returns a new array. But yeah, if you use `map`, the result is always HOLEY.

      1 reply 0 retweets 0 likes
    5. Ryan Boucher‏ @distributedlife 22 Mar 2018
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      Replying to @mathias

      Are there plans for v8 to have fewer false positives when classifying arrays? The various performance improvements you've made to arrays feel underutilised as most interactions with arrays will downgrade them to the least optimised.

      1 reply 0 retweets 0 likes
    6. Mathias Bynens‏Verified account @mathias 22 Mar 2018
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      Replying to @distributedlife

      HOLEY arrays are still optimized, though! Going HOLEY is not the end of the world. If it makes sense to use `map` in your code, don’t stop doing so.

      1 reply 0 retweets 0 likes
    7. Ryan Boucher‏ @distributedlife 22 Mar 2018
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      Replying to @mathias

      That's why I said least optimised. My experiences lead me to think that not using map and reduce is rare. I'll file this away under how I'd rewrite my js arrays if I need to squeeze more out of my code. Thanks for insights

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    8. Mathias Bynens‏Verified account @mathias 22 Mar 2018
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      Replying to @distributedlife

      I think that’s a good approach! Write idiomatic code that makes the most sense to you, and (only) when you run into a performance issue: 1) let @v8js know so we can fix it, and 2) play around with avoiding holeyness to see if that makes a difference. 👍🏻

      1 reply 0 retweets 0 likes
    9. Ryan Boucher‏ @distributedlife 22 Mar 2018
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      Replying to @mathias @v8js

      If you could pop these into the backlog somewhere: re-evaluating 'element kind' on .fill (as the simple case packs an array with a static type) and .map (somewhat more difficult than .fill) would be useful boons as more arrays would end up being optimised more often. Ta

      1 reply 0 retweets 0 likes
      Mathias Bynens‏Verified account @mathias 22 Mar 2018
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      Replying to @distributedlife @v8js

      Mathias Bynens Retweeted Mathias Bynens

      I brought up the `fill` case myself when I learned about elements kinds. Here’s the counter-argument: https://twitter.com/mathias/status/976531175060615169 … + https://twitter.com/mathias/status/976531391927078912 … It’s likely not worth the effort. That said, I’ll bring it up again once I get back to Munich. Thanks for your feedback!

      Mathias Bynens added,

      Mathias BynensVerified account @mathias
      Replying to @mathias @wSokra @v8js
      …but it would be a major undertaking that’s unlikely to be worth the effort, especially since (again) we can still optimize holey arrays! [2/2]
      3:13 AM - 22 Mar 2018
      1 reply 0 retweets 0 likes
        1. Ryan Boucher‏ @distributedlife 22 Mar 2018
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          Replying to @mathias @v8js

          I agree that fill will not yield sufficient returns to be valuable. I don't use it that often and certainly not at a volume for it to matter. Map on the other hand, it is common and it's not going up the lattice it's starting out at a less than ideal place.

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