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    SubjectRe: [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@linux-foundation.org> wrote:

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