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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    1. Wojciech Muła‏ @pshufb 5 Oct 2016

      AVX512F strlen is faster nearly 5 times than the library function, and strchr nearly 4.5 times. https://github.com/WojciechMula/toys/tree/master/avx512-string … #avx512

      2 replies 8 retweets 16 likes
      Rich Felker‏ @RichFelker 5 Oct 2016
      Replying to @pshufb @solardiz

      But it makes context switch 5x slower by requiring an extra 1.5+ kB to be saved/restored, right?

      10:04 AM - 5 Oct 2016
      • 1 Like
      • Daniel Lemire
      3 replies 0 retweets 1 like
        1. Wojciech Muła‏ @pshufb 5 Oct 2016
          Replying to @RichFelker

          I have no idea about context switching overhead. It is a really good question. (KNL are clocked @ 1.2GHz, BTW) @solardiz

          0 replies 0 retweets 0 likes
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        1. New conversation
        2. Wojciech Muła‏ @pshufb 5 Oct 2016
          Replying to @RichFelker

          Not to mention that my avx512 procedures are unsafe. :) @solardiz

          1 reply 0 retweets 1 like
        3. Jykke‏ @okkejytila 7 Oct 2016
          Replying to @pshufb @RichFelker @solardiz

          you mean overreading data by up to 63 bytes in e.g. strlen implementation?

          1 reply 0 retweets 0 likes
        4. Wojciech Muła‏ @pshufb 7 Oct 2016
          Replying to @okkejytila

          Yes, this might crash program. Can be avoided by reading aligned data, like here https://github.com/WojciechMula/sse2string/blob/master/src/strlen.S#L38 … @RichFelker @solardiz

          1 reply 0 retweets 0 likes
        5. Jykke‏ @okkejytila 7 Oct 2016
          Replying to @pshufb @RichFelker @solardiz

          but the string can be at the end of page boundary, it can segfault if you read one byte too many

          1 reply 0 retweets 0 likes
        6. Wojciech Muła‏ @pshufb 7 Oct 2016
          Replying to @okkejytila

          True for my current implementation, but the SSE2 version I linked to has no this flaw. @RichFelker @solardiz

          0 replies 0 retweets 0 likes
        7. End of conversation
        1. New conversation
        2. always @ ( * )‏ @rzidane360 5 Oct 2016
          Replying to @RichFelker @pshufb @solardiz

          related question - how does the OS know which SIMD registers (if any) to save when preempting a thread?

          1 reply 0 retweets 0 likes
        3. CopperheadOS‏ @CopperheadOS 5 Oct 2016
          Replying to @rzidane360 @RichFelker and

          It seems like it must have to save the SSE/AVX context regardless of whether you use them.

          2 replies 0 retweets 0 likes
        4. always @ ( * )‏ @rzidane360 5 Oct 2016
          Replying to @CopperheadOS @RichFelker and

          that's what I thought. So it'll save all sse/sse2/avx/avx512 registers? That seems like a lot.

          2 replies 0 retweets 0 likes
        5. Wojciech Muła‏ @pshufb 5 Oct 2016
          Replying to @rzidane360 @CopperheadOS and

          remember that SSE & AVX registers map to lower parts of AVX512 regs, like AL, AX, EAX

          1 reply 0 retweets 2 likes
        6. always @ ( * )‏ @rzidane360 5 Oct 2016
          Replying to @pshufb @CopperheadOS and

          true but that's still 1.5 KB to save every time another user thread needs to run?

          1 reply 0 retweets 0 likes
        7. CopperheadOS‏ @CopperheadOS 5 Oct 2016
          Replying to @rzidane360 @pshufb and

          https://events.linuxfoundation.org/sites/events/files/slides/LinuxCon_NA_2014.pdf … does seem to imply that it somehow knows if it wasn't used yet.

          0 replies 0 retweets 1 like
        8. End of conversation

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