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).

Richard,
if we know probability of if (cond[i]) to be p,
then we know that the combined conditional is somewhere between
  low = p      (the strategy packing true and falses into VF sized
  		blocks)
and
  high = min (p*vf,1)
  	       (the stragegy doing only one true per block if possible)
Likely value is

  likely = 1-pow(1-p, vf)

I wonder if we can work out p at least in common cases. 
Making store unlikely as we do right now will place it offline with
extra jump.  Making it likely is better unless p is very small.

I think if p is close to 0 or 1 which may be common case the analysis
above may be useful. If range [low...high] is small, we can use likely
and keep it as reliable.
If it is high, we can probably just end up with guessed value close but
above 50% so the store stays inline.

Honza