> 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
