Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash

[Richard Henderson]

On 04/05/2016 08:48 AM, Paolo Bonzini wrote:
> I think it's fine to use the struct.  The exact size of the struct
> varies from 3 to 5 32-bit words, so it's hard to write nice
> size-dependent code for the hash.

I don't think it is.  We have 3 integers.  It is trivial to create a simple 
function of 2 multiplies, two adds, and a remainder.

Take the primes from the xxhash.h, for example:

  (phys_pc * PRIME32_2 + pc * PRIME32_3 + flags)
  % PRIME32_1
  & (CODE_GEN_PHYS_HASH_SIZE - 1)

Obviously, some bucket measurements should be taken, but I can well imagine 
that this might perform just as well as the fully generic hasher.


r~