On Tue, Nov 8, 2022 at 10:11 AM Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Tue, Nov 08, 2022 at 09:44:40AM -0800, Andrew Waterman wrote:
> > On Tue, Nov 8, 2022 at 3:20 AM Jakub Jelinek via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> > >
> > > On Mon, Nov 07, 2022 at 04:41:23PM +0100, Aldy Hernandez wrote:
> > > > As suggested upthread, I have also adjusted update_nan_sign() to drop
> > > > the NAN sign to VARYING if both operands are NAN. As an optimization
> > > > I keep the sign if both operands are NAN and have the same sign.
> > >
> > > For NaNs this still relies on something IEEE754 doesn't guarantee,
> > > as I cited, after a binary operation the sign bit of the NaN is
> > > unspecified, whether there is one NaN operand or two.
> > > It might be that all CPUs handle it the way you've implemented
> > > (that for one NaN operand the sign of NaN result will be the same
> > > as that NaN operand and for two it will be the sign of one of the two
> > > NaNs operands, never something else), but I think we'd need to check
> > > more than one implementation for that (I've only tried x86_64 and thus
> > > SSE behavior in it), so one would need to test i387 long double behavior
> > > too, ARM/AArch64, PowerPC, s390{,x}, RISCV, ...
> > > The guarantee given by IEEE754 is only for those copy, negate, abs,
> > > copySign
> > > operations, so copying values around, NEG_EXPR, ABS_EXPR, __builtin_fabs*,
> > > __builtin_copysign*.
> >
> > FWIW, RISC-V canonicalizes NaNs by clearing the sign bit; the signs of
> > the input NaNs do not factor in.
>
> Just for binary operations and some unary, or also the ones that
> IEEE754 spells out (moves, negations, absolute value and copysign)?
I should've been more specific in my earlier email: I was referring to
the arithmetic operators. Copysign and friends do not canonicalize
NaNs.
>
> Jakub
>
