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.
>
> Otherwise LGTM (but would be nice to get into GCC13 not just
> +, but also -, *, /, sqrt at least).
>
> Jakub
>
