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jedisct1's profile
Frank
Frank
Frank
@jedisct1

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Frank

@jedisct1

France
Joined November 2008

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    1.  👻 🎃 Status Quo  🎃 👻‏ @KardOnIce 9 Oct 2019
      • Report Tweet
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      Replying to @KardOnIce @saleemrash1d and

      > 1.5 << 2 4 Javascript truncates the number to an integer, and then applies the shift. There's no reason for low-level, C mask hacks for bitshifts.

      1 reply 0 retweets 0 likes
    2. signal eleven‏ @widdr 9 Oct 2019
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      Replying to @KardOnIce @saleemrash1d and

      All of JavaScript's bitwise operators first coerce the value to a signed 32 bit int, then convert it back. This makes it possible to optimize chains of these operations; most js engines even have an int32 array specialization.

      2 replies 0 retweets 0 likes
    3. signal eleven‏ @widdr 9 Oct 2019
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      Replying to @widdr @KardOnIce and

      This is how webassembly was first done; you can `|0` every value after every operation (like addition) to keep them in int32 form, and Mozilla made programs of that form optimize aggressively to show off. None of this to say JS isn't bad, mind.

      1 reply 0 retweets 0 likes
    4.  👻 🎃 Status Quo  🎃 👻‏ @KardOnIce 9 Oct 2019
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      Replying to @widdr @saleemrash1d and

      My point is to say: float-to-int conversions => expensive bitmask for shift => cheap logical check => not that expensive

      2 replies 0 retweets 0 likes
    5. signal eleven‏ @widdr 9 Oct 2019
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      Replying to @KardOnIce @saleemrash1d and

      float to int is a lot cheaper than you might think! like, really very cheap indeed: on the semi-recent Haswell arch, cvtsi2sd has about 1-per-clock throughput and 4-clock latency. you can nitpick exactly how expensive it is in real terms, but in the worst case it is NEGLIGIBLE.

      1 reply 2 retweets 1 like
    6. signal eleven‏ @widdr 9 Oct 2019
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      Replying to @widdr @KardOnIce and

      logical checks are usually much more expensive than this on average, as they can lead to mispredictions. as the double -> int conversion is hardware optimized it's almost guaranteed to look pretty lousy by comparison.

      1 reply 0 retweets 0 likes
    7. signal eleven‏ @widdr 9 Oct 2019
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      Replying to @widdr @KardOnIce and

      ref:https://stackoverflow.com/a/28668349 

      2 replies 0 retweets 0 likes
    8.  👻 🎃 Status Quo  🎃 👻‏ @KardOnIce 9 Oct 2019
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      Replying to @widdr @saleemrash1d and

      It's true on newer architectures, but there's ways of making the logical check a bit cheaper. Also, ~7 latency for older architectures is about what I know.

      1 reply 0 retweets 0 likes
    9.  👻 🎃 Status Quo  🎃 👻‏ @KardOnIce 9 Oct 2019
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      Replying to @KardOnIce @widdr and

      Anyway, something like this magically runs faster on my PC than just the straight SHR (it has worse codegen though, so there are damned lies and then benchmarks): inline int32_t shr(int32_t x, int32_t n) { if (likely(n < 32)) { return x >> n; } else { return 0; } }

      2 replies 0 retweets 0 likes
    10. signal eleven‏ @widdr 9 Oct 2019
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      Replying to @KardOnIce @saleemrash1d and

      yeah that sounds right. I forget if JS bitshifts sign-extend, though...

      2 replies 0 retweets 1 like
      Frank‏ @jedisct1 9 Oct 2019
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      Replying to @widdr @KardOnIce and

      Depends on wether you use >> or >>>

      5:11 PM - 9 Oct 2019 from Paris, France
      • 2 Likes
      • signal eleven 👻🎃 Status Quo 🎃👻
      1 reply 0 retweets 2 likes
        1. New conversation
        2.  👻 🎃 Status Quo  🎃 👻‏ @KardOnIce 9 Oct 2019
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          Replying to @jedisct1 @widdr and

          Always fun: > -1 >>> 3 536870911

          1 reply 0 retweets 1 like
        3. signal eleven‏ @widdr 9 Oct 2019
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          Replying to @KardOnIce @jedisct1 and

          oh man i forgot js has java-style bitshift ops

          0 replies 0 retweets 1 like
        4. End of conversation

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