Re: undefined reference to __udivdi3 (gcc-4.3)

[Jeremy Fitzhardinge]

Segher Boessenkool wrote:
> > I assume it's one or both of these loops in arch/x86/xen/time.c 
> > do_stolen_accounting() that are being optimized into a divide which 
> > generates a libgcc call:
> >
> >     while (stolen >= NS_PER_TICK) {
> >         ticks++;
> >         stolen -= NS_PER_TICK;
> >     }
> >
> > or
> >
> >     while (blocked >= NS_PER_TICK) {
> >         ticks++;
> >         blocked -= NS_PER_TICK;
> >     }
>
> That looks plausible.

Yep.  Probably both.

> > Not sure if that is a sustainable fix, though..
>
> It should be.  The asm() arg tells GCC that the asm() could modify
> "ns" in some way, so GCC cannot optimise away the loop, since it
> doesn't have the required info about the induction variable to do
> that. 

Yep, it's guaranteed to work.  But it's an ugly hack to work around an 
over-enthusiastic compiler, and so is an inherent maintainability burden.

I think the correct fix here is to introduce an iter_div_rem() function 
which contains this hack, so we can avoid scattering it all over the 
place.  I'll cook up a patch.

   J
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