linux-kernel - Re: [PATCH 2/2] The new jhash implementation

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Date:	Thu, 25 Nov 2010 21:55:35 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org,
	Linus Torvalds <torvalds@...l.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 2/2] The new jhash implementation

On Thu, Nov 25, 2010 at 9:49 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le jeudi 25 novembre 2010 à 14:15 +0100, Jozsef Kadlecsik a écrit :
>> The current jhash.h implements the lookup2() hash function by Bob Jenkins.
>> However, lookup2() is outdated as Bob wrote a new hash function called
>> lookup3(). The new hash function
>>
>> - mixes better than lookup2(): it passes the check that every input bit
>>   changes every output bit 50% of the time, while lookup2() failed it.
>> - performs better: compiled with -O2 on Core2 Duo, lookup3() 20-40% faster
>>   than lookup2() depending on the key length.
>>
>> The patch replaces the lookup2() implementation of the 'jhash*'
>> functions with that of lookup3().
>>
>> You can read a longer comparison of the two and other hash functions at
>> http://burtleburtle.net/bob/hash/doobs.html.
>>
>> Signed-off-by: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
>> ---
>>  include/linux/jhash.h |  136 +++-----------------------------------------
>>  lib/Makefile          |    2 +-
>>  lib/jhash.c           |  153 +++++++++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 162 insertions(+), 129 deletions(-)
>>  create mode 100644 lib/jhash.c
>>
> ...
>
> I agree jhash() should be not be inlined.
>
> I am not sure for other variants.
>
>> +u32 jhash(const void *key, u32 length, u32 initval)
>> +{
>> +     u32 a, b, c;
>> +     const u8 *k = key;
>> +
>> +     /* Set up the internal state */
>> +     a = b = c = JHASH_INITVAL + length + initval;
>> +
>> +     /* All but the last block: affect some 32 bits of (a,b,c) */
>> +     while (length > 12) {
>> +             a += k[0] + ((u32)k[1]<<8) + ((u32)k[2]<<16) + ((u32)k[3]<<24);
>
> disassembly code on x86_32 for the previous line :
>
>  26:   66 90                   xchg   %ax,%ax
>  28:   0f b6 72 01             movzbl 0x1(%edx),%esi
>  2c:   0f b6 4a 02             movzbl 0x2(%edx),%ecx
>  30:   c1 e6 08                shl    $0x8,%esi
>  33:   c1 e1 10                shl    $0x10,%ecx
>  36:   8d 0c 0e                lea    (%esi,%ecx,1),%ecx
>  39:   0f b6 32                movzbl (%edx),%esi
>  3c:   8d 34 31                lea    (%ecx,%esi,1),%esi
>  3f:   0f b6 4a 03             movzbl 0x3(%edx),%ecx
>  43:   c1 e1 18                shl    $0x18,%ecx
>  46:   8d 0c 0e                lea    (%esi,%ecx,1),%ecx
>
> or (CONFIG_CC_OPTIMIZE_FOR_SIZE=y) :
>
>  1b:   0f b6 7b 01             movzbl 0x1(%ebx),%edi
>  1f:   c1 e7 08                shl    $0x8,%edi
>  22:   89 7d f0                mov    %edi,-0x10(%ebp)
>  25:   0f b6 7b 02             movzbl 0x2(%ebx),%edi
>  29:   c1 e7 10                shl    $0x10,%edi
>  2c:   03 7d f0                add    -0x10(%ebp),%edi
>  2f:   89 7d f0                mov    %edi,-0x10(%ebp)
>  32:   0f b6 3b                movzbl (%ebx),%edi
>  35:   03 7d f0                add    -0x10(%ebp),%edi
>  38:   89 7d f0                mov    %edi,-0x10(%ebp)
>  3b:   0f b6 7b 03             movzbl 0x3(%ebx),%edi
>  3f:   c1 e7 18                shl    $0x18,%edi
>  42:   03 7d f0                add    -0x10(%ebp),%edi
>
>
> I suggest :
>
> #include <linux/unaligned/packed_struct.h>
> ...
>        a += __get_unaligned_cpu32(k);
>        b += __get_unaligned_cpu32(k+4);
>        c += __get_unaligned_cpu32(k+8);
>
> Fits nicely in registers.
>

I think you mean get_unaligned_le32().

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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