linux-kernel - Re: [PATCH] introduce lower_32

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Message-ID: <488A3D95.20108@zytor.com>
Date:	Fri, 25 Jul 2008 16:54:45 -0400
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	joerg.roedel@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] introduce lower_32_bits() macro

Andrew Morton wrote:
>>>   */
>>>  #define upper_32_bits(n) ((u32)(((n) >> 16) >> 16))
>>>  
>>> +/**
>>> + * lower_32_bits - return bits 0-31 of a number
>>> + * @n: the number we're accessing
>>> + */
>>> +#define lower_32_bits(n) ((n) & 0xffffffffULL)
>>> +
>> NAK.  These are assymmetric with regards to type, which is the *last* 
>> thing we want.
> 
> Yes, it will convert a 32-bit expression into a 64-bit one.
> 

Sort of.  upper_32_bits() takes a 32- or 64-bit expression, and delivers 
the upper 32 bits *as a 32-bit number*.  An equivalent expression would be:

	((u32)((u64)(n) >> 32))

... which might actually produce better code, for all I know.

Logically speaking, if we have a lower_32_bits() macro, it should also 
produce a 32-bit type as result.

	-hpa

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