Re: [PATCH] Rewrite NAN and sign handling in frange

[Richard Sandiford <richard.sandiford@arm.com>]

Aldy Hernandez via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> On Thu, Sep 15, 2022 at 9:06 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
>>
>> On Thu, Sep 15, 2022 at 7:41 AM Aldy Hernandez <aldyh@redhat.com> wrote:
>> >
>> > Hi Richard.  Hi all.
>> >
>> > The attatched patch rewrites the NAN and sign handling, dropping both
>> > tristates in favor of a pair of boolean flags for NANs, and nothing at
>> > all for signs.  The signs are tracked in the range itself, so now it's
>> > possible to describe things like [-0.0, +0.0] +NAN, [+0, +0], [-5, +0],
>> > [+0, 3] -NAN, etc.
>> >
>> > There are a lot of changes, as the tristate was quite pervasive.  I
>> > could use another pair of eyes.  The code IMO is cleaner and handles
>> > all the cases we discussed.
>> >
>> > Here is an example of the various ranges and how they are displayed:
>> >
>> >     [frange] float VARYING NAN ;; Varying includes NAN
>> >     [frange] UNDEFINED                      ;; Empty set as always
>> >     [frange] float [] NAN                   ;; Unknown sign NAN
>> >     [frange] float [] -NAN                  ;; -NAN
>> >     [frange] float [] +NAN                  ;; +NAN
>> >     [frange] float [-0.0, 0.0]              ;; All zeros.
>> >     [frange] float [-0.0, -0.0] NAN         ;; -0 or NAN.
>> >     [frange] float [-5.0e+0, -1.0e+0] +NAN  ;; [-5, -1] or +NAN
>> >     [frange] float [-5.0e+0, -0.0] NAN      ;; [-5, -0] or +-NAN
>> >     [frange] float [-5.0e+0, -0.0]          ;; [-5, -0]
>> >     [frange] float [5.0e+0, 1.0e+1]         ;; [5, 10]
>> >
>> > We could represent an unknown sign with +NAN -NAN if preferred.
>>
>> maybe -+NAN or +-NAN?  I prefer to somehow show both signs for clarity
>
> Sure.
>
>>
>> >
>> > Notice the NAN signs are decoupled from the range, so we can represent
>> > a negative range with a positive NAN.  For this range,
>> > frange::known_bit() would return false, as only when the signs of the
>> > NANs and range agree can we be certain.
>> >
>> > There is no longer any pessimization of ranges for intersects
>> > involving NANs.  Also, union and intersect work with signed zeros:
>> >
>> > //   [-0,  x] U [+0,  x] => [-0,  x]
>> > //   [ x, -0] U [ x, +0] => [ x, +0]
>> > //   [-0,  x] ^ [+0,  x] => [+0,  x]
>> > //   [ x, -0] ^ [ x, +0] => [ x, -0]
>> >
>> > The special casing for signed zeros in the singleton code is gone in
>> > favor of just making sure the signs in the range agree, that is
>> > [-0, -0] for example.
>> >
>> > I have removed the idea that a known NAN is a "range", so a NAN is no
>> > longer in the endpoints itself.  Requesting the bound of a known NAN
>> > is a hard fail.  For that matter, we don't store the actual NAN in the
>> > range.  The only information we have are the set of boolean flags.
>> > This way we make sure nothing seeps into the frange.  This also means
>> > it's explicit that we don't track anything but the sign in NANs.  We
>> > can revisit this if we desire to track signalling or whatever
>> > concoction y'all can imagine.
>> >
>> > All in all, I'm quite happy with this.  It does look better, and we
>> > handle all the corner cases we couldn't before.  Thanks for the
>> > suggestion.
>> >
>> > Regstrapped with mpfr tests on x86-64 and ppc64le Linux.  Selftests
>> > were also run with -ffinite-math-only on x86-64.
>> >
>> > At Jakub's suggestion, I built lapack with associated tests.  They
>> > pass on x86-64 and ppc64le Linux with no regressions from mainline.
>> > As a sanity check, I also ran them for -ffinite-math-only on x86 which
>> > (as expected) returned:
>> >
>> >         NaN arithmetic did not perform per the ieee spec
>> >
>> > Otherwise, all tests pass for -ffinite-math-only.
>> >
>> > How does this look?
>>
>> Overall it looks good.
>>
>> Reading ::intersect and ::union I find it less clear to spread out the _nan
>> cases into separate functions.
>
> OK, will inline them.
>
>>
>> Can you add a comment to frange that its representation is
>> a single value-range specified by m_type, m_min, m_max
>> unioned with the set of { -NaN, +NaN }?  Because somehow
>> the ::undefined_p vs. m_type == VR_UNDEFINED checks are
>> a bit confusing to the occasional reader can we instead use
>> ::nan_p to complement ::undefined_p?
>
> Wouldn't that just make nan_p the same as known_nan?  Speaking of
> which, I'm not a big fan of known_nan.  Perhaps we should rename all
> the known_foo variants to foo_p variants?  Or...maybe even:
>
>   // fpclassify like API
>   bool isfinite () const;
>   bool isinf () const;
>   bool maybe_isinf () const;
>   bool isnan () const;
>   bool maybe_isnan () const;
>   bool signbit_p (bool &signbit) const;
>
> That would make it clear how they map to the fpclassify API.  And the
> signbit_p() follows what we do for singleton_p(tree *).
>
> isnan() would be your nan_p suggestion.

FWIW, the reason I didn't do this with the poly_int stuff is that
it makes negative conditions harder to reason about.  It's easy for
tired eyes to read:

   !isfinite()

as meaning "is infinite", especially since there isn't a separate
isinfinite() query.  But if isfinite() is equivalent to known_isfinite()
then !isfinite() instead means "might be infinite".  !known_isfinite()
IMO makes that more explicit.

Thanks,
Richard