[Bug 1742941] Re: zlib: improve crc32 performance on P8

[Andrew Cloke]

** Also affects: ubuntu-power-systems
   Importance: Undecided
       Status: New

** Changed in: ubuntu-power-systems
   Importance: Undecided => Low

** Changed in: ubuntu-power-systems
     Assignee: (unassigned) => David Britton (davidpbritton)

** Tags added: triage-g

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https://bugs.launchpad.net/bugs/1742941

Title:
  zlib: improve  crc32 performance on P8

Status in The Ubuntu-power-systems project:
  New
Status in zlib package in Ubuntu:
  New

Bug description:
  Calculate the checksum of data that is 16 byte aligned and a multiple
  of  16 bytes.

  The first step is to reduce it to 1024 bits. We do this in 8 parallel
   chunks in order to mask the latency of the vpmsum instructions. If we
   have more than 32 kB of data to checksum we repeat this step multiple
   times, passing in the previous 1024 bits.

   The next step is to reduce the 1024 bits to 64 bits. This step adds
   32 bits of 0s to the end - this matches what a CRC does. We just
   calculate constants that land the data in this 32 bits.

   We then use fixed point Barrett reduction to compute a mod n over GF(2)
   for n = CRC using POWER8 instructions. We use x = 32.

   http://en.wikipedia.org/wiki/Barrett_reduction

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