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    SubjectRe: [rfc 37/45] x86_64: Support for fast per cpu operations
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    From: "H. Peter Anvin" <hpa@zytor.com>
    Date: Mon, 19 Nov 2007 18:00:23 -0800

    > Christoph Lameter wrote:
    > > Support fast cpu ops in x86_64 by providing a series of functions that
    > > generate the proper instructions. Define CONFIG_FAST_CPU_OPS so that core code
    > > can exploit the availability of fast per cpu operations.
    > >
    > > Signed-off-by: Christoph Lameter <clameter@sgi.com>
    >
    > There was, at some point, discussion about using the gcc TLS mechanism,
    > which should permit even better code to be generated. Unfortunately, it
    > would require gcc to be able to reference %gs instead of %fs (and vice
    > versa for i386), which I don't think is available in anything except
    > maybe the most cutting-edge version of gcc.

    You can't use __thread because GCC will cache __thread computed
    addresses across context switches and cpu changes.

    It's been tried before on powerpc, it doesn't work.
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