On Thu, Jun 20, 2013 at 11:43 PM, Blair Zajac <bl...@orcaware.com> wrote:
> On 06/20/2013 02:40 PM, stef...@apache.org wrote: > >> Author: stefan2 >> Date: Thu Jun 20 21:40:50 2013 >> New Revision: 1495204 >> >> URL: http://svn.apache.org/r1495204 >> Log: >> Follow-up to r1495063: integer overflows result in an inefficient hash >> function and reduce cache effectiveness. >> >> * subversion/libsvn_fs_fs/tree.c >> (cache_lookup): prevent unnecessary integer overflows in hash function >> > > Hi Stefan, > > I've been reading up on hash functions and am curious about this how > overflow can reduce cache effectiveness. Do you have any links or can > offer an explanation? > > This illustrates what happened before the patch: char c = 99; unsigned hash = 0; hash |= c << 8; /* c << 8 is often 0, actually it's undefined */ On a more general note, you don't want to make it easy for parts of the input to cancel each other out. So, adding (potentially) negative values is a bad thing (strategically). -- Stefan^2.
