linux-kernel - Re: [PATCH v2] MIPS: Check __clang__ to avoid performance influence with GCC in csum_tcpudp

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Date:   Mon, 15 Mar 2021 20:10:32 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     Alexander Lobakin <alobakin@...me>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        David Laight <David.Laight@...LAB.COM>
Subject: Re: [PATCH v2] MIPS: Check __clang__ to avoid performance influence
 with GCC in csum_tcpudp_nofold()

On 03/15/2021 06:24 PM, Alexander Lobakin wrote:
> From: Tiezhu Yang <yangtiezhu@...ngson.cn>
> Date: Tue, 9 Mar 2021 12:18:13 +0800
>
>> The asm code in csum_tcpudp_nofold() is performance-critical, I am sorry
>> for the poorly considered implementation about the performance influence
>> with GCC in the commit 198688edbf77 ("MIPS: Fix inline asm input/output
>> type mismatch in checksum.h used with Clang").
>>
>> With this patch, we can build successfully by both GCC and Clang,
>> at the same time, we can avoid the potential performance influence
>> with GCC.
>>
>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>> ---
>>   arch/mips/include/asm/checksum.h | 12 ++++++++----
>>   1 file changed, 8 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/mips/include/asm/checksum.h b/arch/mips/include/asm/checksum.h
>> index 1e6c135..80eddd4 100644
>> --- a/arch/mips/include/asm/checksum.h
>> +++ b/arch/mips/include/asm/checksum.h
>> @@ -128,9 +128,13 @@ static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
>>
>>   static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
>>   					__u32 len, __u8 proto,
>> -					__wsum sum)
>> +					__wsum sum_in)
>>   {
>> -	unsigned long tmp = (__force unsigned long)sum;
>> +#ifdef __clang__
> Why not rely on CONFIG_CC_IS_CLANG here?

Hi,

Thanks for your suggestion, I once considered that way:
https://lore.kernel.org/patchwork/patch/1371666/#1587127

But it still occurs build error under CC_IS_GCC when
make M=samples/bpf which used with Clang compiler,
so use __clang__ is better.

Thanks,
Tiezhu

>
>> +	unsigned long sum = (__force unsigned long)sum_in;
>> +#else
>> +	__wsum sum = sum_in;
>> +#endif
>>
>>   	__asm__(
>>   	"	.set	push		# csum_tcpudp_nofold\n"
>> @@ -159,7 +163,7 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
>>   	"	addu	%0, $1		\n"
>>   #endif
>>   	"	.set	pop"
>> -	: "=r" (tmp)
>> +	: "=r" (sum)
>>   	: "0" ((__force unsigned long)daddr),
>>   	  "r" ((__force unsigned long)saddr),
>>   #ifdef __MIPSEL__
>> @@ -169,7 +173,7 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
>>   #endif
>>   	  "r" ((__force unsigned long)sum));
>>
>> -	return (__force __wsum)tmp;
>> +	return (__force __wsum)sum;
>>   }
>>   #define csum_tcpudp_nofold csum_tcpudp_nofold
>>
>> --
>> 2.1.0
> Al