[libre-riscv-dev] Instruction sorta-prefixes for easier high-register access

[Jacob Lifshay]

On Wed, Jan 23, 2019, 00:06 Luke Kenneth Casson Leighton <lkcl at lkcl.net
wrote:

> ---
> crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
>
> On Wed, Jan 23, 2019 at 3:26 AM Jacob Lifshay <programmerjake at gmail.com>
> wrote:
> >
> > On Tue, Jan 22, 2019 at 3:10 PM Luke Kenneth Casson Leighton <
> lkcl at lkcl.net>
> > wrote:
> >
> > > btw jacob i'm not saying "no" to modified instructions (non-RV),
> > > especially given that the priority is llvm, where yes we have to
> > > maintain a custom version of that: gcc and binutils can be deferred.
> > >
> > > what i'm saying is: if there's any other way (such as the implicit
> > > 0b00-prefix => scalar idea) i'd prefer that to be prioritised.
> > >
> > Ok, what do you think about requiring the starting register for vectors
> to
> > be aligned by 2?
>
>  yeah that would work.  also, i noticed that the 48-bit prefix is
> 7-bits long.  why?  that's a precious bit that could be used.
>
In case any other extensions want to have 48-bit instructions. Otherwise we
are using the entire 48-bit address space. If you think we should use all
of the 48-bit address space anyway, I recommend using the new bit to expand
the vlp fields by 1 bit, allowing us to specify more predication and vector
length combinations. We can use my proposal for requiring aligned register
numbers to encode scalar/vector. Also, using the register number fields to
encode scalar/vector allows us to have reduce operations by having a scalar
destination. For operations where reduction doesn't make sense (fld for
example), we just have the vectorization length be 1.

For operations where a vector length multiplier is selected, scalar
arguments/results are vectors of the length you'd get if the VL register
were 1. This greatly simplifies vectorizing code like:

vec4 color = ...;
color = mix(0.5, vec4(1.0, 0.0, 1.0, 1.0), color);
// mix color with magenta

which would compile to:

.rodata
c0.5_0.0_0.5_0.5: ds 0.5, 0.0, 0.5, 0.5
c0.5_0.5_0.5_0.5: ds 0.5, 0.5, 0.5, 0.5
.text
; color in f32...
li a0, c0.5_0.0_0.5_0.5
fld.w f0(vector), a0, len=4
li a1, c0.5_0.5_0.5_0.5
fld.w f4(vector), a1, len=4
fmadd.s f32(vector), f32(vector), f4(scalar), f0(scalar), len=VL*4

Jacob