Re: [PATCH AutoFDO/2]Treat ZERO as common profile probability/count

["Bin.Cheng" <amker dot cheng at gmail dot com>]

On Wed, Oct 31, 2018 at 10:36 PM Jeff Law <law@redhat.com> wrote:
>
> On 10/31/18 12:30 AM, bin.cheng wrote:
> > Hi,
> > In new profile probability/count infra, we have different precision quality categories,
> > and probabilities/counts of different categories are not supposed to be compared or
> > calculated.  Though in general is an improvement, it introduces unexpected behavior.
> > Specifically, class profile_probablity and profile_count themselves are implemented
> > by comparing probabilities/counts against profile_count::zero().  while zero() is of
> > profile_precision category, it's always compared different to zero of other precision
> > categories including afdo.
> >
> > I can see two ways fixing this: 1) Treat zero as a common probability/count regardless
> > of its category; 2) Provide an "is_zero" method rather than relying on "==" comparison
> > against probability_count::zero().  2) requires lots of code changes so I went with 1)
> > in this patch set.  This patch doesn't handle "always" but it might be.
> >
> > This patch also corrects a minor issue where we try to invert an uninitialized value.
> >
> > Bootstrap and test on x86_64 in patch set.  Is it OK?
> >
> > Thanks,
> > bin
> >
> > 2018-10-31  Bin Cheng  <bin.cheng@linux.alibaba.com>
> >
> >       * expmed.c (emit_store_flag_force): Use profile_probability::always.
> >       * profile-count.h (profile_probability::always): Add parameter.
> >       (profile_probability::operator==, profile_count::operator==): Treat
> >       ZERO as common probability/count regardless of its quality.
> >       (profile_probability::invert): Don't invert uninitialized probability.
> >
>
> I'm really not sure the emit_store_flag_force change is right --
> essentially without external information I can't see how we can pass in
> any probability here other than "we don't know" which is
> profile_probability::uninitialized IIUC.
>
> You could potentially make an argument that a 50-50 probability is
> reasonable here.  That's profile_probability::even.  But I just don't
> see how profile_probability::always is right here.
Thanks, I realized I mis-spotted that piece of code.  Will discard the part.

Thanks,
bin
>
> jeff