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

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Oct 31, 2018 at 7:30 AM bin.cheng <bin.cheng@linux.alibaba.com> 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?

I'll defer on the emit_store_flag_force change, likewise for the zero
handling in
compares - I don't think zeros of different qualities should compare equal.
Would compares against ::always() not have the very same issue?
Likewise ::even(),
::likely(), etc.?  Those always get guessed quality.

The invert change looks OK to me.  The related change to the always() API would
suggest to replace guessed_always() with always (guessed) and also do similar
changes throughout the whole API...

Honza?

Thanks,
Richard.


> 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.