[libre-riscv-dev] [isa-dev] Re: Vector Operations Extension (cross, dot, length, lerp, slerp)

[Hendrik Boom]

On Tue, Oct 08, 2019 at 05:43:07AM +0100, Luke Kenneth Casson Leighton wrote:
> On Tue, Oct 8, 2019 at 2:46 AM Hendrik Boom <hendrik at topoi.pooq.com> wrote:
> >
> > On Sun, Oct 06, 2019 at 10:36:37PM -0700, Jacob Lifshay wrote:
> > > On Sat, Oct 5, 2019 at 2:39 AM lkcl <luke.leighton at gmail.com> wrote:
> > > >
> > > > http://bugs.libre-riscv.org/show_bug.cgi?id=142
> > > > (Tracking this extension here)
> > > >
> > > > Found a paper which describes vector normalisation in terms of dotproduct, scalar recipsqrt and then scalar-vector fmul.
> > >
> > > Yeah, that's the standard procedure.
> > >
> > > > With VLENGTH being also expressible as dotproduct followed by scalar sqrt, is it reasonable to have both normalisation as well as VLENGTH as macro op fused sequences?
> >
> > Vector length would presumably involve dotting a vector with itself.
> > The potential advantage I see is that the dot product might be tempted
> > to read that vector twice; whereas the length would only read it once.
> > If some other mechanism eliminates the duplicate read, they would be
> > pretty well equivalent.
> 
> good point - appreciated as something to keep an eye on.  the same
> underlying hardware would be used on a micro-coded vlength however the
> copy of the arguments, read once, would be sent to that hardware,
> rather than read twice.

The self-dot product might be useful on its own in other mathematics, 
but I'm not sure it has much of a use in graphics. 

-- hendrik

> 
> l.
> 
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