[Libre-soc-isa] [Bug 1071] add parallel prefix sum remap mode

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1071

--- Comment #20 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #19)
> (In reply to Luke Kenneth Casson Leighton from comment #15)
> > (In reply to Jacob Lifshay from comment #14)
> 
> > > for i in range(0, length, chunk_size):
> > >     sum += reduce_chunk(VL=chunk_size)
> > > sum += reduce_tail(VL=length % chunk_size)  # dynamic VL here
> > 
> > drat, you're right.
> > 
> > of course. okaaay sigh so this involves creating a new Form, with Parallel
> > Reduction similar to svshape2 "carving out" its own niche... urrr...
> 
> ah. realised there's a complication. VL and MAXVL determines the number
> of operations carried out, not the width of the reduction.
> 
> for now the tail will have to be done by recursive macros using
> immediates manually.

that's why I'm proposing a svshape with GPR/VL input, so it can calculate the
proper VL (which is a little less than 2x bigger but doesn't follow any simple
formula I could find).

MAXVL would be adjusted following the same algorithm but using MAXVL as the
input rather than VL or a GPR.

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