Re: Xen 4.20 release schedule

[Frediano Ziglio <frediano.ziglio@xxxxxxxxx>]

On Thu, Nov 21, 2024 at 1:40 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 21.11.2024 13:53, Frediano Ziglio wrote:
> > On Wed, Oct 30, 2024 at 1:25 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> > wrote:
> >>
> >> On 21/10/2024 1:02 pm, oleksii.kurochko@xxxxxxxxx wrote:
> >>> Hello everyone,
> >>>
> >>> As there were no objections to the proposed release schedule
> >>> (https://lore.kernel.org/xen-devel/CAMacjJxEi6PThwH2=NwG3He8eQn39aiaxZCw3bQF7i4YcmjuNw@xxxxxxxxxxxxxx/
> >>> ), I've updated the wiki with the schedule for Xen 4.20 release
> >>> (https://wiki.xenproject.org/wiki/Xen_Project_X.YY_Release_Notes), and
> >>> it is now accessible from
> >>> https://xenbits.xen.org/docs/unstable-staging/support-matrix.html.
> >>
> >> I have a blocker to raise (against myself...) and no good idea of how to
> >> proceed.
> >>
> >> The for_each_bit work has a unexpected bug.
> >>
> >>     for_each_bit ( ... )
> >>     {
> >>         if ( ... )
> >>             break;
> >>     }
> >>
> >> will fall into an infinite loop.  This is caused by for_each_bit()
> >> hiding a double for() loop, in order to declare two scope-local
> >> variables of different types.
> >>
> >> The two variables are one copy of the source expression (really quite
> >> important to keep), and one unsigned int iterator (improved optimisation
> >> capability by not using a wider-scope variable).
> >>
> >> Options are (off the top of my head)
> >>
> >> 1) Always take the iterator from outer scope
> >> 2) Iterator always the same type as the source expression
> >> 3) Figure out some way of expressing "once" in the outer loop
> >>
> >> Or anything else that I've missed.
> >>
> >> ~Andrew
> >>
> >
> > Something like
> >
> > #define for_each_set_bit(iter, val)                     \
> >     for ( typeof(val) __v = (val), __c=1; __c; __c=0)   \
> >         for ( unsigned int (iter);                      \
> >               __v && ((iter) = ffs_g(__v) - 1, true);   \
> >               __v &= __v - 1 )
> >
> > ?
>
> Hmm, right, but then we don't even need a 2nd variable, do we?
>
> #define for_each_set_bit(iter, val)                     \
>     for ( typeof(val) __v = (val); __v; __v = 0 )       \
>         for ( unsigned int (iter);                      \
>               __v && ((iter) = ffs_g(__v) - 1, true);   \
>               __v &= __v - 1 )
>
> Jan

In theory it should work too, not sure if the variable aliasing would
compromise the assembly output. We depend on the compiler doing some
optimizations.

Frediano