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Date: Tue, 06 Mar 2007 10:39:36 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: David Howells <dhowells@redhat.com>
CC: torvalds@osdl.org, akpm@osdl.org, benh@kernel.crashing.org,
       linux-kernel@vger.kernel.org, johannes@sipsolutions.net
Subject: Re: [PATCH] Fix get_order() [try #2]
References: <20070306183426.10768.51800.stgit@warthog.cambridge.redhat.com>
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David Howells wrote:
>  
>  /**
> + * ilog2_up - rounded up log of base 2 of 32-bit or a 64-bit unsigned value
> + * @n - parameter
> + *
> + * constant-capable log of base 2 calculation
> + * - this can be used to initialise global variables from constant data, hence
> + *   the massive ternary operator construction
> + * - the result is rounded up
> + * - the result is undefined when n < 1
> + *
> + * selects the appropriately-sized optimised version depending on sizeof(n)
> + */
> +#define ilog2_up(n) ((n) == 1 ? 0 : ilog2((n) - 1) + 1)
> +

Why not just make it ((n) < 1 ? 0 : ...) and make it well-defined for
n == 0?

> +/**
>   * roundup_pow_of_two - round the given value up to nearest power of two
>   * @n - parameter
>   *
> @@ -159,8 +175,8 @@ unsigned long __roundup_pow_of_two(unsigned long n)
>  #define roundup_pow_of_two(n)			\
>  (						\
>  	__builtin_constant_p(n) ? (		\
> -		(n == 1) ? 0 :			\
> -		(1UL << (ilog2((n) - 1) + 1))	\
> +		(n == 1) ? 1 :			\
> +		(1UL << ilog2_up(n))		\
>  				   ) :		\

... then this can be just "1UL << ilog2_up(n)".

[Sorry about previous email.  I got confused about what 
roundup_pow_of_two() actually does.]

	-hpa