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    SubjectRe: BUG: GCC-4.4.x changes the function frame on some functions
    On Thu, 19 Nov 2009, Linus Torvalds wrote:
    > 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?

    I first captured the command line with V=1 and created a script of
    it. Then I changed the -march -mtune options in that script and
    compiled just that single file manually w/o changing .config or
    invoking the kernel make magic.

    The good ones produce:

    650: 55 push %ebp
    651: 89 e5 mov %esp,%ebp
    653: 83 e4 f0 and $0xfffffff0,%esp

    The bad one:

    000005f0 <timer_stats_update_stats>:
    5f0: 57 push %edi
    5f1: 8d 7c 24 08 lea 0x8(%esp),%edi
    5f5: 83 e4 f0 and $0xfffffff0,%esp
    5f8: ff 77 fc pushl -0x4(%edi)
    5fb: 55 push %ebp
    5fc: 89 e5 mov %esp,%ebp

    > 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?

    That does not change any of the compiler options, but yes it could
    have some effect via the various include magics, but all I have seen
    so far is linkage.h which should not affect the compiler. And the
    manual compile did not change any of this.

    > [ 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.

    Digging further tomorrow when my brain is more awake.



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