linux-kernel - Re: [PATCH 2/7] PowerPC: add unlikely() to BUG

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Date:	Thu, 27 Jan 2011 09:57:39 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Coly Li <bosong.ly@...bao.com>
CC:	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Wang Cong <xiyou.wangcong@...il.com>,
	Yong Zhang <yong.zhang0@...il.com>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON()

Why not also CC the PPC maintainers as well?  I am not certain, but I 
think they may be reached at:

linuxppc-dev@...ts.ozlabs.org


On 01/27/2011 04:12 AM, Coly Li wrote:
> Current BUG_ON() arch/powerpc/include/asm/bug.h does not use unlikely(),
> in order to get better branch predict result, source code may have to 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 PPC
> code, callers can use BUG_ON without explicit unlikely() now.
>
> I don't have any PPC hardware to compile and test this fix, any feedback
> of this patch is welcome.
>
> Signed-off-by: Coly Li<bosong.ly@...bao.com>
> Cc: Jeremy Fitzhardinge<jeremy@...p.org>
> Cc: David Daney<ddaney@...iumnetworks.com>
> Cc: Wang Cong<xiyou.wangcong@...il.com>
> Cc: Yong Zhang<yong.zhang0@...il.com>
> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> index 065c590..10889a6 100644
> --- a/arch/powerpc/include/asm/bug.h
> +++ b/arch/powerpc/include/asm/bug.h
> @@ -2,6 +2,7 @@
>   #define _ASM_POWERPC_BUG_H
>   #ifdef __KERNEL__
>
> +#include<linux/compiler.h>
>   #include<asm/asm-compat.h>
>
>   /*
> @@ -71,7 +72,7 @@
>   	unreachable();						\
>   } while (0)
>
> -#define BUG_ON(x) do {						\
> +#define __BUG_ON(x) do {					\
>   	if (__builtin_constant_p(x)) {				\
>   		if (x)						\
>   			BUG();					\
> @@ -85,6 +86,8 @@
>   	}							\
>   } while (0)
>
> +#define BUG_ON(x) __BUG_ON(unlikely(x))
> +

This is the same type of frobbing you were trying to do to MIPS.

I will let the powerpc maintainers weigh in on it, but my opinion is 
that, as with MIPS, BUG_ON() is expanded to a single machine 
instruction, and this unlikely() business will not change the generated 
code in any useful way.  It is thus gratuitous code churn and 
complexification.

David Daney

>   #define __WARN_TAINT(taint) do {				\
>   	__asm__ __volatile__(					\
>   		"1:	twi 31,0,0\n"				\

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