[libre-riscv-dev] [Bug 132] SIMD-like nmigen signal for partitioning

[bugzilla-daemon at libre-riscv.org]

http://bugs.libre-riscv.org/show_bug.cgi?id=132

--- Comment #35 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #34)
> (In reply to Jacob Lifshay from comment #32)
> 
> > > here's a simple example:
> > > 
> > > -        with m.If(self.i.product[-1]):
> > > -            comb += p.eq(self.i.product)
> > > -        with m.Else():
> > > -            # get 1 bit of extra accuracy if the mantissa top bit is zero
> > > -            comb += p.eq(self.i.product<<1)
> > > -            comb += self.o.z.e.eq(self.i.z.e-1)
> > > +        msb = Signal(reset_less=True)
> > > +        e = self.o.z.e
> > > +        comb += msb.eq(self.i.product[-1])
> > > +        comb += p.eq(Mux(msb, self.i.product, self.i.product<<1))
> > > +        comb += e.eq(Mux(msb, self.i.z.e, self.i.z.e-1))
> > > 
> > > that would do alright, wouldn't it? 
> > 
> > that looks fine to me.
> 
> yeh.  oh, except i realised: arrays.  the next bit of the code is as follows:
> 
>         mw = self.o.z.m_width
>         comb += [
>             self.o.z.m.eq(p[mw+2:]),            # mantissa
>             self.o.of.m0.eq(p[mw+2]),           # copy of LSB
>             self.o.of.guard.eq(p[mw+1]),        # guard
>             self.o.of.round_bit.eq(p[mw]),      # round
>             self.o.of.sticky.eq(p[0:mw].bool()) # sticky
>         ]
> 
> the assumption here is that the mantissa width is a fixed parameter.
> 
> we'll need a P-variant of "Part" which takes a *list* of partition-selectable
> widths/offsets - is that making any sense?

yup, makes sense to me.

> 
> this is going to be quite intrusive, however, ultimately, i think if we
> tried to write an "explicit" partitioned version of this code (with for-loops
> etc.) it would quickly become not just unreadable but also apparent that
> the process is extremely regular and monotonous.

yeah. it's only good as a backup option, since it would still work despite the
verbosity.

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