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    SubjectRe: BUG: GCC-4.4.x changes the function frame on some functions

    On Fri, 20 Nov 2009, Thomas Gleixner wrote:
    > While testing various kernel configs we found out that the problem
    > comes and goes. Finally I started to compare the gcc command line
    > options and after some fiddling it turned out that the following
    > minimal deltas change the code generator behaviour:
    > Bad: -march=pentium-mmx -Wa,-mtune=generic32
    > Good: -march=i686 -mtune=generic -Wa,-mtune=generic32
    > Good: -march=pentium-mmx -mtune-generic -Wa,-mtune=generic32
    > I'm not supposed to understand the logic behind that, right ?

    Are you sure it's just the compiler flags?

    There's another configuration portion: the size of the alignment itself.
    That's dependent on L1_CACHE_SHIFT, which in turn is taken from the kernel
    config CONFIG_X86_L1_CACHE_SHIFT.

    Maybe that value matters too - for example maybe gcc will not try to align
    the stack if it's big?

    [ Btw, looking at that, why are X86_L1_CACHE_BYTES and X86_L1_CACHE_SHIFT
    totally unrelated numbers? Very confusing. ]

    The compiler flags we use are tied to some of the same choices that choose
    the cache shift, so the correlation you found while debugging this would
    still hold.


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      Last update: 2009-11-20 02:03    [W:0.021 / U:13.484 seconds]
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