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

[Jeff Law <law@redhat.com>]

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.

jeff