linux-kernel - Re: [PATCH] introduce lower_32

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Date:	Fri, 25 Jul 2008 17:39:21 -0400
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Roland Dreier <rdreier@...co.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>, joerg.roedel@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] introduce lower_32_bits() macro

Roland Dreier wrote:
>  > There's some readability benefit.  Sometimes it is hard to understand
>  > why some random open-coded cast was used.  But I seem to recall that
>  > there was another reason why we decided we needed this.  I forget, and
>  > so apparently did the changelog author ;)
> 
> The guy who added the macro explained it in a comment ;)
> 
>  * A basic shift-right of a 64- or 32-bit quantity.  Use this to suppress
>  * the "right shift count >= width of type" warning when that quantity is
>  * 32-bits.

That was for upper_32_bits() -- and can also be done with an internal 
(u64) cast; it would be interesting to find out if that would generate 
better code.

	-hpa
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