linux-kernel - Re: [PATCH 1/1] core-kernel: use multiply instead of shifts in hash

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Message-ID: <CADroS=5Cy=a0_AzkpwpsWZSf1FL+gLNbsMWLW1WNcQn++hpgUw@mail.gmail.com>
Date:	Thu, 12 Jul 2012 13:51:50 -0700
From:	Andrew Hunter <ahh@...gle.com>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] core-kernel: use multiply instead of shifts in hash_64

On Tue, Jul 10, 2012 at 6:35 AM, Michael Tokarev <mjt@....msk.ru> wrote:
> On 03.07.2012 00:25, Andrew Hunter wrote:
>> diff --git a/include/linux/hash.h b/include/linux/hash.h
>> index b80506b..daabc3d 100644
>> --- a/include/linux/hash.h
>> +++ b/include/linux/hash.h
>> @@ -34,7 +34,9 @@
>>  static inline u64 hash_64(u64 val, unsigned int bits)
>>  {
>>       u64 hash = val;
>> -
>> +#if BITS_PER_LONG == 64
>> +     hash *= GOLDEN_RATIO_PRIME_64;
>> +#else
>>       /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
>
> Hmm.  Does this comment make sense here now?
>

I haven't checked what output gcc provides for 32-bit kernels with
this or a literal multiply.  It's not even clear what optimization is
_asked_ for here (possibly the reduction of strength that we probably
don't even want.)
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