Re: [PATCH] [PR68097] frange::set_nonnegative should not contain -NAN.

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

On Mon, Sep 19, 2022 at 2:52 PM Aldy Hernandez <aldyh@redhat.com> wrote:
>
> On Mon, Sep 19, 2022 at 10:14 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > On Mon, Sep 19, 2022 at 9:59 AM Aldy Hernandez <aldyh@redhat.com> wrote:
> > >
> > > ISTM that a specifically nonnegative range should not contain -NAN,
> > > otherwise signbit_p() would return false, because we'd be unsure of the
> > > sign.
> > >
> > > Do y'all agree?
> >
> > what tree_expr_nonnegative_p actually means isn't 100% clear.  For REAL_CST
> > it actually looks at the sign-bit but we have
> >
> > (simplify
> >  /* copysign(x,y) -> fabs(x) if y is nonnegative.  */
> >  (COPYSIGN_ALL @0 tree_expr_nonnegative_p@1)
> >  (abs @0))
> >
> > is abs (@0) OK for sNaNs and -NaN/+NaN?
>
> At least for real_value's, ABS_EXPR works on NAN's.  There's no
> special code dealing with them.  We just clear the sign bit:
>
> real_arithmetic:
>     case ABS_EXPR:
>       *r = *op0;
>       r->sign = 0;
>       break;
>
> >
> > And we have
> >
> > /* Convert abs[u] (X)  where X is nonnegative -> (X).  */
> > (simplify
> >  (abs tree_expr_nonnegative_p@0)
> >  @0)
> >
> > where at least sNaN -> qNaN would be dropped?
> >
> > And of course
> >
> > (simplify
> >  /* signbit(x) -> 0 if x is nonnegative.  */
> >  (SIGNBIT tree_expr_nonnegative_p@0)
> >  { integer_zero_node; })
> >
> > that is, is tree_expr_nonnegative_p actually tree_expr_sign or
> > does tree_expr_nonnegative (x) mean x >= (typeof(X)) 0
> > or !(x < (typeof(X))0)?
>
> I have no idea, but I'm happy to have frange::set_nonnegative() do
> whatever you agree on.
>
> Actually TBH, ranger only uses set_nonnegative for call's, not much else:
>
>   if (range_of_builtin_call (r, call, src))
>     ;
>   else if (gimple_stmt_nonnegative_warnv_p (call, &strict_overflow_p))
>     r.set_nonnegative (type);
>   else if (gimple_call_nonnull_result_p (call)
>        || gimple_call_nonnull_arg (call))
>     r.set_nonzero (type);
>   else
>     r.set_varying (type);
>
> but I guess it's good we do the right thing for correctness sake, and
> if it ever gets used by someone else.
>
> >
> > That said, 'set_nonnegative' could be interpreted to be without
> > NaNs?
>
> Sounds good to me.  How's this?

Hmm, I merely had lots of questions above so I think to answer them
we at least should document what 'set_nonnegative' implies in the
abstract vrange class.

It's probably safer to keep NaN in.  For example unfolded copysign (x, 1.)
will return true for x == -NaN but the result will be a NaN.

Richard.

>
> Aldy