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SubjectRe: [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@lists.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@taobao.com>
> Cc: Jeremy Fitzhardinge<jeremy@goop.org>
> Cc: David Daney<ddaney@caviumnetworks.com>
> Cc: Wang Cong<xiyou.wangcong@gmail.com>
> Cc: Yong Zhang<yong.zhang0@gmail.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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