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 10:24:02 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     Alexander Lobakin <alobakin@...me>,
        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()

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?

> +	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