[Libre-soc-bugs] [Bug 784] Implement cl* instructions for carry-less operations

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=784

Luke Kenneth Casson Leighton <lkcl at lkcl.net> changed:

           What    |Removed                     |Added
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                 CC|                            |lkcl at lkcl.net

--- Comment #4 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #3)
> I started adding CLDivRem, but ran into a yosys bug:
> https://github.com/YosysHQ/yosys/issues/3268

intriguing.  relying on complex switch/case, not so hot, eh? :)

> I added TestEqualLeadingZeroCount which will be used in the cldivrem
> algorithm for testing if two GF(2) polynomials have equal degrees. 

hmm hmm i was looking at this and thought, actually, finding the
minimum cntlz of two things might be more useful?

https://git.libre-soc.org/?p=nmigen-gf.git;a=blob;f=gf_reference/gfpinv.py;h=45b6dbbdd4cb19bdbfa0538fb9dfcb79981655fe;hb=926798b6e232f09a36773be07de2d90e3fd431a4

> It should
> have a similar complexity to a binary addition (it uses binary addition
> internally, the extraneous xor gates from addition are optimized out by
> yosys).

are or should be? relying on complex low-level capability of yosys is
not... wise.

> https://git.libre-soc.org/?p=nmigen-gf.git;a=blob;f=src/nmigen_gf/hdl/
> cldivrem.py;h=9a89c43046e2535fb272f47efefe51fb256db482;
> hb=423b3a3279e3637614a614ec7038ebaa7cabd6c0

nextpnr-ecp5 doesn't have carry-out, and both symbiflow and
nextpnr-xilinx's use of CARRY4 are borked at the moment.
also the balancing of carry propagation is not very good in
yosys.

spot the common theme here... :)

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