Adding slice-by-4 and slice-by-8 to CRC32

[Sam Russell]

Hi,

First off, this is my first GNU contribution so I have *no idea* what I'm doing, feedback is appreciated.

I've noticed that GZIP trails behind zlib in performance and part of this is down to the fact that zlib is using a more efficient CRC32 implementation. I've written an implementation of this for gnulib based off the Intel paper at https://static.aminer.org/pdf/PDF/000/432/446/a_systematic_approach_to_building_high_performance_software_based_crc.pdf (the code is mine, written based on the paper, the tables are generated by extending the code from RFC 1952 to generate the lookups for partial bitfields, this can be provided on request but it's not my finest work).

The code:

https://github.com/samrussell/gnulib/commit/2d5f5d0e131feea6e04cb48d56589537506f91a8

(yes, I am aware you don't take contributions via github. I have an open ticket with GNU to get my SSH access fixed so I can have non-anonymous access to the repository on Savannah).

Considerations:

- robustness/appropriateness: both the zlib and Linux kernel CRC32 implementations use this slice-by-4 and slice-by-8 algorithm for the performance improvements

- free software/patents: the Intel paper is from 2008; zlib had an independently-discovered version of slice-by-4 from 2002 so it seems unlikely there should be any patents encumbering this. the software is my own design and I am happy for it to be transferred to GNU and licensed accordingly

Requests for help:

- how do I get someone to review my code and then get it added to the codebase? is this done via Savannah, or the mailing list, do we email around PATCH files etc?

- I assume we'll want to gate this functionality based on CPU ability (32-bit arithmetic is presumably fine but 64-bit makes no sense unless running on a system with a native 64-bit bus). What is the convention here for enforcing this at compile-time? I couldn't find the right flag to test for and I'm assuming there's a correct answer that someone knows off the top of their head

Thanks for your time,

Sam