Re: [COMMITTED] Be even more conservative in intersection of NANs.

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

On Mon, Sep 5, 2022 at 12:24 PM Aldy Hernandez <aldyh@redhat.com> wrote:
>
> On Mon, Sep 5, 2022 at 11:53 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > On Mon, Sep 5, 2022 at 11:41 AM Aldy Hernandez <aldyh@redhat.com> wrote:
> > >
> > > On Mon, Sep 5, 2022 at 11:18 AM Richard Biener
> > > <richard.guenther@gmail.com> wrote:
> > > >
> > > > On Mon, Sep 5, 2022 at 11:12 AM Aldy Hernandez <aldyh@redhat.com> wrote:
> > > > >
> > > > > On Mon, Sep 5, 2022 at 11:06 AM Jakub Jelinek <jakub@redhat.com> wrote:
> > > > > >
> > > > > > On Mon, Sep 05, 2022 at 11:00:54AM +0200, Richard Biener wrote:
> > > > > > > On Mon, Sep 5, 2022 at 8:24 AM Aldy Hernandez via Gcc-patches
> > > > > > > <gcc-patches@gcc.gnu.org> wrote:
> > > > > > > >
> > > > > > > > Intersecting two ranges where one is a NAN is keeping the sign bit of
> > > > > > > > the NAN range.  This is not correct as the sign bits may not match.
> > > > > > > >
> > > > > > > > I think the only time we're absolutely sure about the intersection of
> > > > > > > > a NAN and something else, is when both are a NAN with exactly the same
> > > > > > > > properties (sign bit).  If we're intersecting two NANs of differing
> > > > > > > > sign, we can decide later whether that's undefined or just a NAN with
> > > > > > > > no known sign.  For now I've done the latter.
> > > > > > > >
> > > > > > > > I'm still mentally working on intersections involving NANs, especially
> > > > > > > > if we want to keep track of signbits.  For now, let's be extra careful
> > > > > > > > and only do things we're absolutely sure about.
> > > > > > > >
> > > > > > > > Later we may want to fold the intersect of [NAN,NAN] and say [3,5]
> > > > > > > > with the posibility of NAN, to a NAN, but I'm not 100% sure.
> > > > > > >
> > > > > > > The intersection of [NAN, NAN] and [3, 5] is empty.  The intersection
> > > > > > > of [NAN, NAN] and VARYING is [NAN, NAN].
> > > > > >
> > > > > > I think [3.0, 5.0] printed that way currently means U maybe NAN,
> > > > > > it would be [3.0, 5.0] !NAN if it was known not to be NAN.
> > > > >
> > > > > Right.  I don't print any of the "maybe" properties, just if they're
> > > > > definitely set or definitely clear.  I'm open to suggestions as to how
> > > > > to display them.  Perhaps NAN, !NAN, ?NAN.
> > > >
> > > > There's no NAN tristate.  Your "definitely NAN" would be simply
> > > > ][ NAN, that is, the value range only contains NAN.  Your !NAN
> > > > is <whatever range> and non NAN.  Likewise for the sign, the
> > > > range either includes -NAN and NAN or one or none of those.
> > > > For signed zeros you either have [-0, upper-bound] or [0, upper-bound]
> > > > where it either includes both -0 and 0 or just one of them
> > >
> > > But there is a tristate.  We may definitely have a NAN, definitely not
> > > have a NAN, or the state of the NAN is unknown.
> >
> > Sure.  But we are talking about sets of values a variable can have
> > (a value "range" where "range" is a bit misleading for something
> > like a NAN).  The set of possible values either includes
> > NAN (or -NAN and +NAN) or it doesn't include NAN (or -NAN and +NAN).
> > A set cannot include or not include a "maybe NAN".
> >
> > >  Say [3,5] ?NAN.
> > > That's [3,5] with the possibility of a NAN.  On the true side of x >=
> > > 5.0, we'd have [5.0, INF] !NAN.  On the false side we'd have [-INF,
> > > 5.0] ?NAN.
> >
> > On the true side of x >= 5.0 the set of values is described by
> > the [5., +INF] range.  On the false side the set is described
> > by the union of the range [-INF, 5.0] and the { -NAN, +NAN }
> > set.
> >
> > There's no may-NAN.  There's also no ?4.0, the range either
> > includes 4.0 or it doesn't.
>
> Ah, ok.  I see where the confusion lies.  You're missing that we don't
> have sub-ranges like we do for irange.  We only have two endpoints and
> a set of flags.  So we can't represent [3,4] U NAN "elegantly".
> However, we can do it with [3,4] ?NAN.  This is by design, but not
> permanent.  I don't have infinite time to work on frange on this cycle
> (I have other things like wide-ints conversion, prange, removal of
> legacy, etc etc), so I wanted something that worked with endpoints,
> signs, and NANs, that's about it.  If at a later time we decide to go
> full throttle with the ability to represent sub-ranges, we can do so.
> Heck, you're welcome to try-- just let me finish the initial
> implementation and get it working correctly first.
>
> It is more important right now to get the usage than the
> representation right.  We could always add sub-ranges, or change the
> representation altogether.  What is very important we agree on is the
> usage, so your suggestions about the FP classification functions below
> are golden.  I'll look into that.
>
> Does that make sense?

Not really.  I didn't ask for sub-ranges for NAN, but even with a "flag"
it should still semantically be [3, 4] U NAN or [3, 4].  It's not necessary
but confusing to leave the notion of a SET here.

> BTW, [NAN, NAN] is a special case.  It doesn't behave like a
> singleton.  Both endpoints must match.  We assert this much.  We don't
> propagate it.  We can't do equality to it.  The fact that it lives in
> the endpoints is just an implementation detail.

And even here, having [NAN, NAN] but [3, 4] with maybe-NAN flag
is just inconsistent.  Why's that necessary?  Is there no "empty range"
(aka UNDEFINED) for frange?

>
> Aldy
>
> >
> > Note the frange class should probably have APIs that match
> > the FP classification functions isfinite(), isnormal(),
> > isnan(), isinf () and signbit() likewise compares like
> > isunordered() .  Note isnormal () exposes that FP numbers can
> > be denormal, not sure if that's worth tracking.
> >
> > > With this representation we can fold __builtin_isnan() even on an
> > > unknown value... say on the true side of x == y we know that both x
> > > and y cannot be NANs...but on the false side we know nothing so there
> > > is the possibility of a NAN.
> > >
> > > I do like your idea for signed zeros.  I think I could make it work
> > > and get rid of the sign bit.
> > >
> > > Aldy
> > >
> > > >
> > > > > I'm mostly worried about removing a NAN from the IL that was going to
> > > > > signal, or some such.  While I agree with you Richard, I just want to
> > > > > make real sure, because getting something wrong in the frange or
> > > > > range-ops bowels means the problem becomes pervasive to all of ranger
> > > > > ...and threader...and loop ch...and vrp, etc etc.  I just want to take
> > > > > more time to test things.  I promise it won't stay varying too long.
> > > >
> > > > There's sNANs and qNANs, but I think for value-ranges we should
> > > > concern ourselves only with qNANs for now and leave sNANs VARYING.
> > > > All operations only ever produce qNANs (loads can produce sNANs).
> > > >
> > > > Richard.
> > > >
> > > > > Aldy
> > > > >
> > > >
> > >
> >
>