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KG.cxx 💻

@aikathg.bsky.social did:plc:tmyif2lansps6apqfgleo4rc

Humano semi-funcional. As vezes posto coisas sobre programação, matemática, xadrez, músicas japonesas, etc. Coshuler, curso técnico em Informática e estudante de CC 🏳️‍🌈🏳️‍⚧️

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Zusammenfassung

KG.cxx 💻 (@aikathg.bsky.social) ist seit 29 August 2024 auf Bluesky. Das Konto hat 196 Follower, folgt 136 Konten und hat 1.340 Posts veröffentlicht.

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  • Andrew J. Bromage@pseudonym73.bsky.social

    For the array search part, don't forget that vector instructions exist these days. With SSE2, for example, you can do four 32-bit float comparisons in parallel, _mm_movemask to put the result in an integer register, then bsr/bsf on that. ARM Neon has all this too.

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  • Andrew J. Bromage@pseudonym73.bsky.social

    If you have a lot of floats and you need to binary search, you can use this variant: unsigned logl = ceil_log_2(y.size()); int i = 0; for (unsigned k = 1 << (logl-1); k > 4; k >>= 1) { int t = i + k; if (yt >= y[t]) i = t; // Conditional move instruction }

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  • Andrew J. Bromage@pseudonym73.bsky.social

    After this, i is now pointing at an aligned subsequence of four floats that you can do the vector comparison trick on. This is the fastest way I've found to do this kind of search on modern hardware, assuming that you can't exploit the distribution of y somehow (e.g. bucketing).

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  • Jeff Silksong Coelho@jeffquesado.ulivre.dev

    Unfortunately I guess there is none. Perhaps a binary search can help, but I can't think of a O(1) algorithm

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  • Calle Svensson@zetatwo.bsky.social

    I think this is a very sound approach. The rule of thumb I got from my friend who builds fuzzers is that for lists up to ~1000 elements, the fastest thing you can do is a linear search so a good initial guess plus that sounds optimal.

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