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Subject: Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc,
	and flags with xxhash
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To: Paolo Bonzini <pbonzini@redhat.com>, "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Sergey Fedorov <serge.fdrv@gmail.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Peter Crosthwaite <crosthwaite.peter@gmail.com>

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~