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

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Message-ID: <20080709185055.3b47c4db@infradead.org>
Date:	Wed, 9 Jul 2008 18:50:55 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.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 18:38:23 -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)



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