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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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