linux-kernel - Re: [PATCH v3] powerpc: Implement csum_ipv6

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Message-ID: <3848a4ad-2c0e-691f-e98f-347cfe3484e8@c-s.fr>
Date:   Thu, 24 May 2018 08:20:16 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
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 v3] powerpc: Implement csum_ipv6_magic in assembly



Le 23/05/2018 à 20:34, Segher Boessenkool a écrit :
> On Tue, May 22, 2018 at 08:57:01AM +0200, Christophe Leroy wrote:
>> The generic csum_ipv6_magic() generates a pretty bad result
> 
> <snip>
> 
> Please try with a more recent compiler, what you used is pretty ancient.
> It's not like recent compilers do great on this either, but it's not
> *that* bad anymore ;-)
> 
>> --- a/arch/powerpc/lib/checksum_32.S
>> +++ b/arch/powerpc/lib/checksum_32.S
>> @@ -293,3 +293,36 @@ dst_error:
>>   	EX_TABLE(51b, dst_error);
>>   
>>   EXPORT_SYMBOL(csum_partial_copy_generic)
>> +
>> +/*
>> + * static inline __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
>> + *				      const struct in6_addr *daddr,
>> + *				      __u32 len, __u8 proto, __wsum sum)
>> + */
>> +
>> +_GLOBAL(csum_ipv6_magic)
>> +	lwz	r8, 0(r3)
>> +	lwz	r9, 4(r3)
>> +	lwz	r10, 8(r3)
>> +	lwz	r11, 12(r3)
>> +	addc	r0, r5, r6
>> +	adde	r0, r0, r7
>> +	adde	r0, r0, r8
>> +	adde	r0, r0, r9
>> +	adde	r0, r0, r10
>> +	adde	r0, r0, r11
>> +	lwz	r8, 0(r4)
>> +	lwz	r9, 4(r4)
>> +	lwz	r10, 8(r4)
>> +	lwz	r11, 12(r4)
>> +	adde	r0, r0, r8
>> +	adde	r0, r0, r9
>> +	adde	r0, r0, r10
>> +	adde	r0, r0, r11
>> +	addze	r0, r0
>> +	rotlwi	r3, r0, 16
>> +	add	r3, r0, r3
>> +	not	r3, r3
>> +	rlwinm	r3, r3, 16, 16, 31
>> +	blr
>> +EXPORT_SYMBOL(csum_ipv6_magic)
> 
> Clustering the loads and carry insns together is pretty much the worst you
> can do on most 32-bit CPUs.

Oh, really ? __csum_partial is written that way too.

Right, now I tried interleaving the lwz and adde. I get no improvment at 
all on a 885, but I get a 15% improvment on a 8321.

Christophe

> 
> 
> Segher
>