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

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Date:   Mon, 8 Mar 2021 16:52:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Tiezhu Yang' <yangtiezhu@...ngson.cn>,
        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()

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.

	David

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