Enable BF16 support (Please ignore my former email)

H.J. Lu hjl.tools@gmail.com
Fri Apr 12 15:24:00 GMT 2019


On Fri, Apr 12, 2019 at 3:19 AM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Fri, Apr 12, 2019 at 11:03 AM Hongtao Liu <crazylht@gmail.com> wrote:
> >
> > On Fri, Apr 12, 2019 at 3:30 PM Uros Bizjak <ubizjak@gmail.com> wrote:
> > >
> > > On Fri, Apr 12, 2019 at 9:09 AM Liu, Hongtao <hongtao.liu@intel.com> wrote:
> > > >
> > > > Hi :
> > > >     This patch is about to enable support for bfloat16 which will be in Future Cooper Lake, Please refer to https://software.intel.com/en-us/download/intel-architecture-instruction-set-extensions-programming-reference
> > > > for more details about BF16.
> > > >
> > > > There are 3 instructions for AVX512BF16: VCVTNE2PS2BF16, VCVTNEPS2BF16 and DPBF16PS instructions, which are Vector Neural Network Instructions supporting:
> > > >
> > > > -       VCVTNE2PS2BF16: Convert Two Packed Single Data to One Packed BF16 Data.
> > > > -       VCVTNEPS2BF16: Convert Packed Single Data to Packed BF16 Data.
> > > > -       VDPBF16PS: Dot Product of BF16 Pairs Accumulated into Packed Single Precision.
> > > >
> > > > Since only BF16 intrinsics are supported, we treat it as HI for simplicity.
> > >
> > > I think it was a mistake declaring cvtps2ph and cvtph2ps using HImode
> > > instead of HFmode. Is there a compelling reason not to introduce
> > > corresponding bf16_format supporting infrastructure and declare these
> > > intrinsics using half-binary (HBmode ?) mode instead?
> > >
> > > Uros.
> >
> > Bfloat16 isn't IEEE standard which we want to reserve HFmode for.
>
> True.
>
> > The IEEE 754 standard specifies a binary16 as having the following format:
> > Sign bit: 1 bit
> > Exponent width: 5 bits
> > Significand precision: 11 bits (10 explicitly stored)
> >
> > Bfloat16 has the following format:
> > Sign bit: 1 bit
> > Exponent width: 8 bits
> > Significand precision: 8 bits (7 explicitly stored), as opposed to 24
> > bits in a classical single-precision floating-point format
>
> This is why I proposed to introduce HBmode (and corresponding
> bfloat16_format) to distingush between ieee HFmode and BFmode.
>

Unless there is BF16 language level support,  HBmode has no advantage
over HImode.   We can add HBmode when we gain BF16 language support.

-- 
H.J.



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