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

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Date:   Sun, 14 Mar 2021 21:49:19 +0100 (CET)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        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 Tue, 9 Mar 2021, Tiezhu Yang wrote:

> 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

 This looks much better to me, but I'd keep the variable names unchanged 
as `sum_in' isn't used beyond the initial assignment anyway (you'll have 
to update the references with asm operands accordingly of course).

 Have you verified that code produced by GCC remains the same with your 
change in place as it used to be up to commit 198688edbf77?  I can see no
such information in the commit description whether here or in the said 
commit.

  Maciej