On Wed, Nov 23, 2022 at 8:40 PM Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Wed, Nov 23, 2022 at 08:28:20PM +0800, liuhongt via Gcc-patches wrote:
> > After supporting real __bf16 type, implementation of mm_cvtsbh_ss went
> > wrong.
> > The patch supports extendbfsf2/truncsfbf2 with pslld/psrld,
> > and then refined the intrinsic with implicit conversion.
>
> This is not correct.
> While using such code for _mm_cvtsbh_ss is fine if it is documented not to
> raise exceptions and turn a sNaN into a qNaN, it is not fine for HONOR_NANS
> (i.e. when -ffast-math is not on), because a __bf16 -> float conversion
> on sNaN should raise invalid exception and turn it into a qNaN.
> We could have extendbfsf2 expander that would FAIL; if HONOR_NANS and
> emit extendbfsf2_1 otherwise.
I see, i'll use target specific builtin and generate psrld just for
the intrinsic, and drop the expander part.
>
> And the truncsfbf2 case isn't correct IMHO even for -ffast-math.
> float -> __bf16 conversion should be properly rounding depending on the
> current rounding mode, while {,v}psrld will always round toward zero.
>
> Jakub
>
--
BR,
Hongtao
