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

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Date:   Tue, 9 Mar 2021 10:22:49 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     David Laight <David.Laight@...LAB.COM>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>
Subject: Re: [PATCH] MIPS: Check __clang__ to avoid performance influence with
 GCC in csum_tcpudp_nofold()

On 03/09/2021 12:52 AM, David Laight wrote:
> From: Tiezhu Yang
>> Sent: 08 March 2021 12:50
>>
>> 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 | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/arch/mips/include/asm/checksum.h b/arch/mips/include/asm/checksum.h
>> index 1e6c135..64d353e 100644
>> --- a/arch/mips/include/asm/checksum.h
>> +++ b/arch/mips/include/asm/checksum.h
>> @@ -130,7 +130,9 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
>>   					__u32 len, __u8 proto,
>>   					__wsum sum)
>>   {
>> +#ifdef __clang__
>>   	unsigned long tmp = (__force unsigned long)sum;
>> +#endif
> What happens if you make the above:
> #ifdef __clang__
> 	unsigned long tmp = (__force unsigned long)sum;
> #else
> 	__wsum tmp = sum;
> #endif
> 	
> and then leave the rest of the function the same for both compilers.
> Maybe do s/sum/sum_in/,s/tmp/sum/ to reduce the changes.

Hi David,

Thank you very much.

As you suggested, the following changes looks much better,
I will test it and then send v2 later.

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__
+       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

Thanks,
Tiezhu

>
> 	David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)