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    SubjectRe: [PATCH 09/30] x86_64: ifdef out struct thread_struct::ip
    On Fri, Apr 10, 2009 at 11:19:31AM +0200, Ingo Molnar wrote:
    >
    > * Matt Helsley <matthltc@us.ibm.com> wrote:
    >
    > > On Fri, Apr 10, 2009 at 06:35:22AM +0400, Alexey Dobriyan wrote:
    > > > struct thread_struct::ip isn't used on x86_64, struct pt_regs::ip is used
    > > > instead.
    > > >
    > > > kgdb should be reading 0, but I can't check it.
    > > >
    > > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
    > > > ---
    > > >
    > > > arch/x86/include/asm/processor.h | 2 ++
    > > > arch/x86/kernel/kgdb.c | 2 +-
    > > > 2 files changed, 3 insertions(+), 1 deletion(-)
    > > >
    > > > --- a/arch/x86/include/asm/processor.h
    > > > +++ b/arch/x86/include/asm/processor.h
    > > > @@ -421,7 +421,9 @@ struct thread_struct {
    > > > unsigned short fsindex;
    > > > unsigned short gsindex;
    > > > #endif
    > > > +#ifdef CONFIG_X86_32
    > > > unsigned long ip;
    > > > +#endif
    > > > #ifdef CONFIG_X86_64
    > > > unsigned long fs;
    > > > #endif
    > >
    > > Do these make struct thread_struct behave better in cachelines
    > > (smaller, less aliasing)? Can we really fit more in the slab du
    > > jour?
    > >
    > > Otherwise it seems like we're littering these structs with #ifdefs
    > > and not really saving anything. [...]
    >
    > Removing fields always saves memory (even if it does not show up
    > currently due to allocators cache-aligning sizes).
    >
    > But the #ifdef ugliness is a real worry.

    You should have thought about it when i386/x86_64 unification was
    introduced.

    pagefault code was full of ifdefs (it's less now), and this is trivial
    ifdef in a header.

    > > [...] If these #ifdefs don't save any
    > > space why not just put in a comment:
    > >
    > > > unsigned long ip; /* Used only on i386 */
    >
    > Yes.
    >
    > > Or maybe even:
    > >
    > > union {
    > > unsigned long ip; /* Used only on i386 */
    > > unsigned long fs; /* Used only on x86_64 */
    > > };
    >
    > Maybe. If this ever gets misunderstood somewhere in platform code we
    > will get ugly failure modes and zero compiler help.


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