linux-kernel - Re: [PATCH] page: get

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Date:	Fri, 1 Apr 2011 21:40:16 +0200
From:	"Christophe JAILLET" <christophe.jaillet@...adoo.fr>
To:	linux-kernel@...r.kernel.org
Cc:	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] page: get_order() optimization

Hi,

why not have something like :
> + order = 0;
> + if (size != 0)
> + order = (__builtin_clzl(size) ^ (BITS_PER_LONG - 1));

we avoid the "complex" computation when not necessary.
No ?

Best regards,
Christophe JAILLET

"Maksym Planeta" <mcsim.planeta@...il.com> a écrit dans le message de
news:1301685493-2567-1-git-send-email-mcsim.planeta@...il.com...
> Loop was repalaced with __builtin_clz(). This still allows to precompute
> constants, but on some architectures it uses special instruction to
> calculate order.
>
> Signed-off-by: Maksym Planeta <mcsim.planeta@...il.com>
> ---
>  include/asm-generic/getorder.h |    8 +++-----
>  1 files changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/include/asm-generic/getorder.h
b/include/asm-generic/getorder.h
> index 67e7245..fe8020c 100644
> --- a/include/asm-generic/getorder.h
> +++ b/include/asm-generic/getorder.h
> @@ -11,11 +11,9 @@ static inline __attribute_const__ int
get_order(unsigned long size)
>   int order;
>
>   size = (size - 1) >> (PAGE_SHIFT - 1);
> - order = -1;
> - do {
> - size >>= 1;
> - order++;
> - } while (size);
> + order = (__builtin_clzl(size) ^ (BITS_PER_LONG - 1));
> + if (size == 0)
> + order = 0;
>   return order;
>  }
>
> -- 
> 1.7.2.3
>



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