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

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Date:	Fri, 28 Jan 2011 18:41:41 +0800
From:	Coly Li <i@...y.li>
To:	David Daney <ddaney@...iumnetworks.com>
CC:	Coly Li <bosong.ly@...bao.com>, 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()

On 2011年01月28日 01:50, David Daney Wrote:
> 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.
>

Since unlikely() in arch implemented BUG_ON() is gratuitous, using unlikely() in kernel code like BUG_ON(unlikely(...)) 
for arch implemented BUG_ON is unwelcome neither.

The NAK makes sense, thanks for your reply.

Coly

-- 
Coly Li
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