netdev - Re: [PATCH 8/9] powerpc: simplify csum_add(a, b) in case a or b is constant 0

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Date:	Mon, 29 Feb 2016 08:26:42 +0100
From:	Christophe Leroy <christophe.leroy@....fr>
To:	Scott Wood <scottwood@...escale.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 8/9] powerpc: simplify csum_add(a, b) in case a or b is
 constant 0



Le 23/10/2015 05:33, Scott Wood a écrit :
> On Tue, 2015-09-22 at 16:34 +0200, Christophe Leroy wrote:
>> Simplify csum_add(a, b) in case a or b is constant 0
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>> ---
>>   arch/powerpc/include/asm/checksum.h | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/arch/powerpc/include/asm/checksum.h
>> b/arch/powerpc/include/asm/checksum.h
>> index 56deea8..f8a9704 100644
>> --- a/arch/powerpc/include/asm/checksum.h
>> +++ b/arch/powerpc/include/asm/checksum.h
>> @@ -119,7 +119,13 @@ static inline __wsum csum_add(__wsum csum, __wsum
>> addend)
>>   {
>>   #ifdef __powerpc64__
>>        u64 res = (__force u64)csum;
>> +#endif
>> +     if (__builtin_constant_p(csum) && csum == 0)
>> +             return addend;
>> +     if (__builtin_constant_p(addend) && addend == 0)
>> +             return csum;
>>
>> +#ifdef __powerpc64__
>>        res += (__force u64)addend;
>>        return (__force __wsum)((u32)res + (res >> 32));
>>   #else
> How often does this happen?
>
>
In the following patch (9/9), csum_add() is used to implement 
csum_partial() for small blocks.
In several places in the networking code, csum_partial() is called with 
0 as initial sum.

Christophe