Re: [llvm-dev] [PATCH] Add optional _Float16 support

[John McCall via Libc-alpha <libc-alpha@sourceware.org>]

On Thu, Jul 29, 2021 at 9:40 AM H.J. Lu <hjl.tools@gmail.com> wrote:

> On Tue, Jul 13, 2021 at 9:24 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Tue, Jul 13, 2021 at 8:41 AM Joseph Myers <joseph@codesourcery.com>
> wrote:
> > >
> > > On Tue, 13 Jul 2021, H.J. Lu wrote:
> > >
> > > > On Mon, Jul 12, 2021 at 8:59 PM Wang, Pengfei <
> pengfei.wang@intel.com> wrote:
> > > > >
> > > > > > Return _Float16 and _Complex _Float16 values in %xmm0/%xmm1
> registers.
> > > > >
> > > > > Can you please explain the behavior here? Is there difference
> between _Float16 and _Complex _Float16 when return? I.e.,
> > > > > 1, In which case will _Float16 values return in both %xmm0 and
> %xmm1?
> > > > > 2, For a single _Float16 value, are both real part and imaginary
> part returned in %xmm0? Or returned in %xmm0 and %xmm1 respectively?
> > > >
> > > > Here is the v2 patch to add the missing _Float16 bits.   The PDF
> file is at
> > > >
> > > > https://gitlab.com/x86-psABIs/i386-ABI/-/wikis/Intel386-psABI
> > >
> > > This PDF shows _Complex _Float16 as having a size of 2 bytes (should be
> > > 4-byte size, 2-byte alignment).
> > >
> > > It also seems to change double from 4-byte to 8-byte alignment, which
> is
> > > wrong.  And it's inconsistent about whether it covers the long double =
> > > double (Android) case - it shows that case for _Complex long double but
> > > not for long double itself.
> >
> > Here is the v3 patch with the fixes.  I also updated the PDF file.
>
> Here is the final patch I checked in.   _Complex _Float16 is changed to
> return
> in XMM0 register.   The new PDF file is at
>
> https://gitlab.com/x86-psABIs/i386-ABI/-/wikis/Intel386-psABI


This should be explicit that the real part is returned in bits 0..15 and
the imaginary part is returned in bits 16..31, or however we conventionally
designate subcomponents of a vector.

John.