ChanServ changed the topic of #yosys to: Yosys Open SYnthesis Suite: https://github.com/YosysHQ/yosys/ | Channel logs: https://libera.irclog.whitequark.org/yosys/
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<corecode> oh duh, of course
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<corecode> LUT4 are just not so good for MUXes, it seems
<corecode> xdot complains about error: unknown xdot opcode '0]' for this line that yosys generates: n4 [ shape=diamond, label="$0\\counter[23:0]" ];
<corecode> seems it is related to the \\
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<corecode> oh it's a bug in graphviz.
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<corecode> it's a bug that was fixed in graphviz, but xdot didn't release yet since.
<corecode> it would be nice if show could collapse busses with the same structure
<corecode> oh cool i found a segfault in yosys
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<whitequark> flowmap isn't... very functional
<whitequark> (i wrote flowmap)
<whitequark> there's a few issues with flowmap -relax it that basically require a complete rewrite
<whitequark> without -relax it should work but i'm not surprised there are other bugs in it
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<corecode> hi
<corecode> yea i saw experimental, but i figured it shouldn't crash with what's likely an over/underflow
<Sarayan> flowmap is what, a synthesis methodology?
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<lofty> Sarayan: it's a LUT mapper
<lofty> I've actually written a few, and I am kind of surprised how bloody difficult it is to get right
<Sarayan> must be fun
<Sarayan> if a pain
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<corecode> lofty: difficult not to make mistakes?
<lofty> corecode: there are a lot of invariants that are difficult to articulate in such a way that one can meaningfully test them
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<corecode> interesting
<corecode> i would have thought that transforming a set of logic should be easy to test for equivalence
<lofty> Oh, it is
<lofty> But equivalent does not mean bug free
<lofty> This is not strictly relevant to FlowMap because it has a different working methodology, but ABC needs "reference counters", and these represent how many best LUTs at a node use this node as an input
<lofty> ... Sorry, I should rephrase that: these represent how many nodes use this node as an input to their best LUT
<lofty> When a LUT is selected as the best at a node, it increments the reference counters of the inputs to that LUT
<lofty> When the mapper changes its mind, it has to decrement the reference counters of the inputs to its previous-best
<lofty> Now, suppose you have a bug somewhere where you decrement a reference counter when you shouldn't
<lofty> You'll only discover that there was a mistake when you decrement the reference counter when you should have
<lofty> Which means failure cases are highly nonlocal
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<corecode> i see
<corecode> thanks
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