Re: [PATCH 02/34] AMD IOMMU: add header file for driver datastructures and defines

[Arjan van de Ven]

On Wed, 9 Jul 2008 18:38:23 -0700
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Thu, 26 Jun 2008 21:27:38 +0200 Joerg Roedel
> <joerg.roedel@xxxxxxx> 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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