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Re: The 8-Byte Two-Step

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On 21/09/2014 6:41 AM, Rich wrote:
> http://zinascii.com/2014/the-8-byte-two-step.html
>
>     While studying the Illumos queue implementation I came across a line of
>     code that left me quite perplexed.
>
>     #define MQ_ALIGNSIZE 8 /* 64-bit alignment */ ... temp = (mqhp->mq_maxsz
>     + MQ_ALIGNSIZE - 1) & ~(MQ_ALIGNSIZE - 1);
>
>     I knew it had something to do with alignment on a 64-bit boundary, but
>     just how the hell was it doing it? Why is it subtracting one from the
>     alignment size? Why is it negating the expression on the right? Why is
>     it adding to the max message size? What the hell is going on?
>
>     At first I attempted to derive the pattern in my head but kept losing my
>     place; forced to start over multiple times. After placing my pride on
>     the bookshelf next to me, I wrote some C to help decode the mystery. The
>     program breaks the expression into its three parts, printing the base-2
>     representation of each step. Here are some test runs with different
>     sizes:
>
>     ...
>

The thinking behind the caculation is just this: Add an amount that is 
guaranteed to take you to or beyond the next multiple of the alignment 
size, and then throw away the remainder by subtracting the result modulo 
the alignment size. In this case, since the alignment size is a power of 
two, the latter step can be achieved using a mask rather than the much 
more expensive division.

Sylvia

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The 8-Byte Two-Step Rich <rich@example.invalid> - 2014-09-20 20:41 +0000
  Re: The 8-Byte Two-Step Bob Eager <news0002@eager.cx> - 2014-09-20 21:14 +0000
    Re: The 8-Byte Two-Step Marko Rauhamaa <marko@pacujo.net> - 2014-09-21 00:48 +0300
      Re: The 8-Byte Two-Step Bob Eager <news0002@eager.cx> - 2014-09-21 08:27 +0000
        Re: The 8-Byte Two-Step Marko Rauhamaa <marko@pacujo.net> - 2014-09-21 14:14 +0300
          Re: The 8-Byte Two-Step Marko Rauhamaa <marko@pacujo.net> - 2014-09-23 11:19 +0300
          Re: The 8-Byte Two-Step Christian Neukirchen <chneukirchen@gmail.com> - 2014-09-23 18:26 +0200
      Re: The 8-Byte Two-Step mm0fmf <none@mailinator.com> - 2014-09-21 20:45 +0100
  Re: The 8-Byte Two-Step Sylvia Else <sylvia@not.at.this.address> - 2014-09-21 13:42 +1000

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