linux-kernel - Re: [PATCH 02/34] AMD IOMMU: add header file for driver data structures and defines

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Message-Id: <20080709193601.347134bf.akpm@linux-foundation.org>
Date:	Wed, 9 Jul 2008 19:36:01 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Joerg Roedel <joerg.roedel@....com>, tglx@...utronix.de,
	mingo@...hat.com, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org, bhavna.sarathy@....com,
	Sebastian.Biemueller@....com, robert.richter@....com,
	joro@...tes.org
Subject: Re: [PATCH 02/34] AMD IOMMU: add header file for driver data
 structures and defines

On Wed, 9 Jul 2008 18:50:55 -0700 Arjan van de Ven <arjan@...radead.org> wrote:

> On Wed, 9 Jul 2008 18:38:23 -0700
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Thu, 26 Jun 2008 21:27:38 +0200 Joerg Roedel
> > <joerg.roedel@....com> wrote:
> > 
> > > +/* helper macros */
> > > +#define LOW_U32(x) ((x) & ((1ULL << 32)-1))
> > > +#define HIGH_U32(x) (LOW_U32((x) >> 32))
> > 
> > Please avoid putting general-purpose helpers into private header
> > files.
> 
> especially broken helpers.
> 
> A >> 32 on something that may be a 32 bit entry is bad; int32 >> 32...
> gcc can (and does!) optimize that out.
> 
> (because it first gets translated into a SHR x86 instruction which then
> notices it's encoded as a zero shift.. which then gets deleted)
> 

Well yeah.  upper_32_bits() gets it all corect.  We could do a
lower_32_bits() I suppose, if it's useful.

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