Chandler Carruth cc9b4516cb Rewrite LLVM's generalized support library for hashing to follow the API ... of the proposed standard hashing interfaces (N3333), and to use a modified and tuned version of the CityHash algorithm. Some of the highlights of this change: -- Significantly higher quality hashing algorithm with very well distributed results, and extremely few collisions. Should be close to a checksum for up to 64-bit keys. Very little clustering or clumping of hash codes, to better distribute load on probed hash tables. -- Built-in support for reserved values. -- Simplified API that composes cleanly with other C++ idioms and APIs. -- Better scaling performance as keys grow. This is the fastest algorithm I've found and measured for moderately sized keys (such as show up in some of the uniquing and folding use cases) -- Support for enabling per-execution seeds to prevent table ordering or other artifacts of hashing algorithms to impact the output of LLVM. The seeding would make each run different and highlight these problems during bootstrap. This implementation was tested extensively using the SMHasher test suite, and pased with flying colors, doing better than the original CityHash algorithm even. I've included a unittest, although it is somewhat minimal at the moment. I've also added (or refactored into the proper location) type traits necessary to implement this, and converted users of GeneralHash over. My only immediate concerns with this implementation is the performance of hashing small keys. I've already started working to improve this, and will continue to do so. Currently, the only algorithms faster produce lower quality results, but it is likely there is a better compromise than the current one. Many thanks to Jeffrey Yasskin who did most of the work on the N3333 paper, pair-programmed some of this code, and reviewed much of it. Many thanks also go to Geoff Pike Pike and Jyrki Alakuijala, the original authors of CityHash on which this is heavily based, and Austin Appleby who created MurmurHash and the SMHasher test suite. Also thanks to Nadav, Tobias, Howard, Jay, Nick, Ahmed, and Duncan for all of the review comments! If there are further comments or concerns, please let me know and I'll jump on 'em. llvm-svn: 151822		2012-03-01 18:55:25 +00:00
..
llvm	Rewrite LLVM's generalized support library for hashing to follow the API	2012-03-01 18:55:25 +00:00
llvm-c	[unwind removal] Remove all of the code for the dead 'unwind' instruction. There	2012-02-06 21:44:22 +00:00