[Libre-soc-bugs] [Bug 413] DIV "trial" blocks are too large

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Thu Jul 2 20:15:46 BST 2020


--- Comment #3 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #1)
> (In reply to Luke Kenneth Casson Leighton from comment #0)
> > the "trial" blocks used in the div pipelines create 2.1 megabyte VHDL
> > files each, during layout, and there are 270 of them.  this results
> > in a jump in the size of the BLIF file from 10 megabytes to 100 megabytes,
> > and a jump from 85,000 blocks (which coriolis2 can cope with)
> > to 500,000 (which it cannot).
> yeah, I'm aware the div pipeline is currently too big. I'm planning on
> rewriting it to shrink it by a large factor. For now, we can change the
> number of bits calculated per pipeline stage to 1, which should reduce it to
> 3/8 the current size.

it really needs to be a factor of 10 reduction.  i'll cancel the current
build and see what happens when radix=1.

> The downside is that takes 64 compute stages (32
> pipeline stages), so, for now we'll just have to live with not being able to
> keep the div pipeline full due to limited fu count.

that's fine - it was always never going to be full until we have 8-12
parallel Function Units dedicated to DIV.

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