linux-kernel - Re: [PATCH 1/7] MIPS: add unlikely() to BUG

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Message-ID: <4D41B06F.8070804@caviumnetworks.com>
Date:	Thu, 27 Jan 2011 09:50:39 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Coly Li <bosong.ly@...bao.com>
CC:	linux-kernel@...r.kernel.org, Wang Cong <xiyou.wangcong@...il.com>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-mips <linux-mips@...ux-mips.org>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH 1/7] MIPS: add unlikely() to BUG_ON()

Please Cc: linux-mips@...ux-mips.org for MIPS patches.

On 01/27/2011 04:12 AM, Coly Li wrote:
> Current BUG_ON() in arch/mips/include/asm/bug.h does not use unlikely(),
> in order to get better branch predict result, source code should call
> BUG_ON() with unlikely() explicitly. This is not a suggested method to
> use BUG_ON().
>
> This patch adds unlikely() inside BUG_ON implementation on MIPS code,
> callers can use BUG_ON without explicit unlikely() now.
>
> I have no usable MIPS hardware to build and test the fix, any test result
> of this patch is welcome.
>
> Signed-off-by: Coly Li<bosong.ly@...bao.com>
> Cc: David Daney<ddaney@...iumnetworks.com>
> Cc: Wang Cong<xiyou.wangcong@...il.com>
> Cc: Yong Zhang<yong.zhang0@...il.com>
> ---
>   arch/mips/include/asm/bug.h |    2 +-
>   1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/arch/mips/include/asm/bug.h b/arch/mips/include/asm/bug.h
> index 540c98a..6771c07 100644
> --- a/arch/mips/include/asm/bug.h
> +++ b/arch/mips/include/asm/bug.h
> @@ -30,7 +30,7 @@ static inline void  __BUG_ON(unsigned long condition)
>   			     : : "r" (condition), "i" (BRK_BUG));
>   }
>
> -#define BUG_ON(C) __BUG_ON((unsigned long)(C))
> +#define BUG_ON(C) __BUG_ON((unsigned long)unlikely(C))
>

NAK.

__BUG_ON() expands to a single instruction.  Frobbing about with 
unlikely() will have no effect on the generated code and is thus 
gratuitous code churn.

David Daney
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