On Wed, Apr 06, 2016 at 12:39:42 +0100, Alex Bennée wrote: > > Emilio G. Cota <c...@braap.org> writes: > > > xxhash is a fast, high-quality hashing function. The appended > > brings in the 32-bit version of it, with the small modification that > > it assumes the data to be hashed is made of 32-bit chunks; this increases > > speed slightly for the use-case we care about, i.e. tb-hash. > > > > The original algorithm, as well as a 64-bit implementation, can be found at: > > https://github.com/Cyan4973/xxHash > > > > Signed-off-by: Emilio G. Cota <c...@braap.org> > > --- > > include/qemu/xxhash.h | 106 > > ++++++++++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 106 insertions(+) > > create mode 100644 include/qemu/xxhash.h > > > > diff --git a/include/qemu/xxhash.h b/include/qemu/xxhash.h > > new file mode 100644 > > index 0000000..a13a665 > > --- /dev/null > > +++ b/include/qemu/xxhash.h > > @@ -0,0 +1,106 @@ > <snip> > > + > > +/* u32 hash of @n contiguous chunks of u32's */ > > +static inline uint32_t qemu_xxh32(const uint32_t *p, size_t n, uint32_t > > seed) > > +{ > > What is the point of seed here? I looked on the original site to see if > there was any guidance on tuning seed but couldn't find anything. I > appreciate the compiler can inline the constant away but perhaps we > should #define it and drop the parameter if we are not intending to > modify it?
The seed value would only matter if we needed consistent hashes (we don't). Any seed value should give similar performance, given xxhash's quality. I'll set a define for this seed if it is used in more than one place. > Also it might be helpful to wrap the call to avoid getting the > boilerplate sizing wrong: > > #define qemu_xxh32(s) qemu_xxh32_impl((const uint32_t *)s, > sizeof(*s)/sizeof(uint32_t), 1) > > Then calls become a little simpler for the user: > > return qemu_xxh32(&k); > > Do we need to include a compile time check for structures that don't > neatly divide into uint32_t chunks? I'll give the original byte-sized function a shot and come back to this if necessary. Thanks, E.