[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 20:22:57 BST 2020


--- Comment #4 from Jacob Lifshay <programmerjake at gmail.com> ---
(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).

one additional way that the size might be unnecessarily large is if yosys for
some reason doesn't do the almost-trivial constant propagation on the
DivPipeCoreOperation, since I'm relying on that to discard the dead code for
the sqrt/rsqrt portions of the div pipe. I didn't check if yosys discards that.
It probably needs to be flattened for yosys to optimize properly.

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