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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. Andrea Giammarchi‏ @WebReflection Feb 5
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      Replying to @Zbjorn @matteocollina

      Wouldn't Array(5).fill(0).map(fn) too?

      1 reply 0 retweets 0 likes
    2. Zach Bjornson‏ @Zbjorn Feb 5
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      Replying to @WebReflection @matteocollina

      Yep. AFAIK the only way to avoid elem kind transitions and holes is: const createArray = (len, v) => { if (!len) return []; const a = [v]; // make arr have correct elem kind while (--len) a.push(v); return a; }

      2 replies 0 retweets 3 likes
    3. Andrea Giammarchi‏ @WebReflection Feb 5
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      Replying to @Zbjorn @matteocollina

      cc'ing @mathias here, is that really the only way to avoid holed arrays? It seems like, in an explicit intent from the devs to provide hints to the engine, a possible bug to file in v8

      1 reply 0 retweets 0 likes
    4. Mathias Bynens‏Verified account @mathias Feb 5
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      Replying to @WebReflection @Zbjorn @matteocollina

      Yep, https://v8.dev/blog/elements-kinds … is still correct. It’s not a bug; `new Array(len)` (and `Array(len)`) pre-allocate a sufficiently large backing store, which is beneficial in case of large, extremely sparse arrays.

      3 replies 0 retweets 1 like
    5. Andrea Giammarchi‏ @WebReflection Feb 7
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      Replying to @mathias @Zbjorn @matteocollina

      so, basically all I had to do in order to have PACKED_(XXX_)?ELEMENTS back was to add a `.map($ => $)` after creation 🍻 const createArray = (length, fn) => Array.from( typeof length === 'number' ? {length} : length, typeof fn === 'function' ? fn : () => fn ).map($ => $);

      1 reply 0 retweets 4 likes
    6. Andrea Giammarchi‏ @WebReflection Feb 7
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      Replying to @WebReflection @mathias and

      meaning: every Array .fill(value) should be followed by an identity map in order to have fastest v8 Array optimizations: Array(3).fill(0); // HOLEY_SMI_ELEMENTS Array(3).fill(0).map($ => $); // PACKED_SMI_ELEMENTS now you know (and one more time I miss a Function.identity)

      1 reply 0 retweets 4 likes
    7. Zach Bjornson‏ @Zbjorn Feb 7
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      Replying to @WebReflection @mathias @matteocollina

      That's a very expensive way to get to the end goal (about 70x slower than what I suggested). Would have to have a*ton* of array accesses for the packed optimizations to make up for that.

      1 reply 0 retweets 1 like
    8. Andrea Giammarchi‏ @WebReflection Feb 8
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      Replying to @Zbjorn @mathias @matteocollina

      Not sure I follow ...most answered to my initial tweet "I'd rather use .fill(...)" to which I've replied "if it's packed array you're after, add a .map(identity) after fill". I think fill should promote the array as packed and not holey, the name itself kinda implies that 🤷‍♂️

      1 reply 0 retweets 0 likes
    9. Mathias Bynens‏Verified account @mathias Feb 8
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      Replying to @WebReflection @Zbjorn @matteocollina

      https://bugs.chromium.org/p/v8/issues/detail?id=6892 …

      1 reply 0 retweets 1 like
    10. Andrea Giammarchi‏ @WebReflection Feb 8
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      Replying to @mathias @Zbjorn @matteocollina

      thanks for the pointer, although you mentioned earlier this wasn't a bug, hence I didn't bother filing/searching. It'd be great if that bug gets resolved this year, `Array(length).fill(kind)` looks like the best pattern to create non holey Array with a pre-allocated size 🌈

      1 reply 0 retweets 0 likes
      Mathias Bynens‏Verified account @mathias Feb 8
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      Replying to @WebReflection @Zbjorn @matteocollina

      It’s not a bug — this is more like a feature request. Holey arrays never go back to being non-holey; the issue is asking for an exception to that behavior.

      4:01 AM - 8 Feb 2020
      1 reply 0 retweets 0 likes
        1. Andrea Giammarchi‏ @WebReflection Feb 8
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          Replying to @mathias @Zbjorn @matteocollina

          I don't have enough background knowledge here to understand why arrays can get promoted only in one direction, but fair enough ...it's just weird developers would need user-land libraries to create faster arrays, with a way slower creation time, stuff likehttps://github.com/WebReflection/packed-array#readme …

          0 replies 0 retweets 1 like
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