linux-kernel - Re: [PATCH 8/9] powerpc: simplify csum_add(a, b) in case a or b is constant 0

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Date:	Thu, 22 Oct 2015 22:33:27 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Christophe Leroy <christophe.leroy@....fr>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	<linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
	<netdev@...r.kernel.org>
Subject: Re: [PATCH 8/9] powerpc: simplify csum_add(a, b) in case a or b is
 constant 0

On Tue, 2015-09-22 at 16:34 +0200, Christophe Leroy wrote:
> Simplify csum_add(a, b) in case a or b is constant 0
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>  arch/powerpc/include/asm/checksum.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/checksum.h 
> b/arch/powerpc/include/asm/checksum.h
> index 56deea8..f8a9704 100644
> --- a/arch/powerpc/include/asm/checksum.h
> +++ b/arch/powerpc/include/asm/checksum.h
> @@ -119,7 +119,13 @@ static inline __wsum csum_add(__wsum csum, __wsum 
> addend)
>  {
>  #ifdef __powerpc64__
>       u64 res = (__force u64)csum;
> +#endif
> +     if (__builtin_constant_p(csum) && csum == 0)
> +             return addend;
> +     if (__builtin_constant_p(addend) && addend == 0)
> +             return csum;
>  
> +#ifdef __powerpc64__
>       res += (__force u64)addend;
>       return (__force __wsum)((u32)res + (res >> 32));
>  #else

How often does this happen?

-Scott

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