Re: [PATCH] [range-ops] Implement sqrt.

[Aldy Hernandez <aldyh@redhat.com>]

On Sun, Nov 13, 2022 at 9:39 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Sun, Nov 13, 2022 at 09:05:53PM +0100, Aldy Hernandez wrote:
> > It seems SQRT is relatively straightforward, and it's something Jakub
> > wanted for this release.
> >
> > Jakub, what do you think?
> >
> > p.s. Too tired to think about op1_range.
>
> That would be multiplication of the same value twice, i.e.
> fop_mult with trio that has op1_op2 () == VREL_EQ?
> But see below, as sqrt won't be always precise, we need to account for
> some errors.
>
> > gcc/ChangeLog:
> >
> >       * gimple-range-op.cc (class cfn_sqrt): New.
> >       (gimple_range_op_handler::maybe_builtin_call): Add cases for sqrt.
>
> Yes, I'd like to see SQRT support in.
> The only thing I'm worried is that unlike {+,-,*,/}, negation etc. typically
> implemented in hardware or precise soft-float, sqrt is often implemented
> in library using multiple floating point arithmetic functions.  And different
> implementations have different accuracy.
>
> So, I wonder if we don't need to add a target hook where targets will be
> able to provide upper bound on error for floating point functions for
> different floating point modes and some way to signal unknown accuracy/can't
> be trusted, in which case we would give up or return just the range for
> VARYING.
> Then, we could write some tests that say in a loop constructs random
> floating point values (perhaps sanitized to be non-NAN), calls libm function
> and the same mpfr one and return maximum error in ulps.
> And then record those, initially for glibc and most common targets and
> gradually maintainers could supply more.
>
> If we add an infrastructure for that within a few days, then we could start
> filling the details.  One would hope that sqrt has < 10ulps accuracy if not
> already the 0.5ulp one, but for various other functions I think it can be

I don't know what would possess me to think that sqrt would be easy
;-).  Sure, I can sink a few days to flesh this out if you're willing
to review it.

Aldy