Re: Fix optimize_mask_stores profile update

[Jan Hubicka <hubicka@ucw.cz>]

> On Mon, Jul 17, 2023 at 12:36 PM Jan Hubicka via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > Hi,
> > While looking into sphinx3 regression I noticed that vectorizer produces
> > BBs with overall probability count 120%.  This patch fixes it.
> > Richi, I don't know how to create a testcase, but having one would
> > be nice.
> >
> > Bootstrapped/regtested x86_64-linux, commited last night (sorry for
> > late email)
> 
> This should trigger with sth like
> 
>   for (i)
>     if (cond[i])
>       out[i] = 1.;
> 
> so a masked store and then using AVX2+.  ISTR we disable AVX masked
> stores on zen (but not AVX512).

OK, let me see if I can get a testcase out of that.
> >        efalse = make_edge (bb, store_bb, EDGE_FALSE_VALUE);
> >        /* Put STORE_BB to likely part.  */
> >        efalse->probability = profile_probability::unlikely ();
> > +      e->probability = efalse->probability.invert ();
> >        store_bb->count = efalse->count ();
> 
> isn't the count also wrong?  Or rather efalse should be likely().   We're
> testing doing
> 
>   if (!mask all zeros)
>     masked-store
> 
> because a masked store with all zero mask can end up invoking COW page fault
> handling multiple times (because it doesn't actually write).

Hmm, I only fixed the profile, efalse was already set to unlikely, but
indeed I think it should be likely. Maybe we can compute some bound on
actual probability by knowing if(cond[i]) probability.
If the loop always does factor many ones or zeros, the probability would
remain the same.
If that is p and they are all independent, the outcome would be
(1-p)^factor

sp we know the conditoinal shoul dbe in ragne (1-p)^factor....(1-p),
right?

Honza

> 
> Note -Ofast allows store data races and thus does RMW instead of a masked store.
> 
> >        make_single_succ_edge (store_bb, join_bb, EDGE_FALLTHRU);
> >        if (dom_info_available_p (CDI_DOMINATORS))