Bug 760

Summary: calculate signal values in a Topological Ordering
Product: Libre-SOC's second ASIC Reporter: Jacob Lifshay <programmerjake>
Component: source codeAssignee: wielgusmikolaj
Status: CONFIRMED ---    
Severity: enhancement CC: libre-soc-bugs, lkcl, wielgusmikolaj
Priority: Lowest    
Version: unspecified   
Hardware: Other   
OS: Linux   
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Description Jacob Lifshay 2021-12-23 18:33:19 GMT 
https://libre-soc.org/irclog/%23libre-soc.2021-12-23.log.html#t2021-12-23T18:22:12

if you just have a single combinatorial circuit without feedback loops, you should be able to calculate a topological ordering of the signals, such that you don't need a simulate loop cuz it can always calculate all signal values in a 	single step by calculating them in that specific ordering. this should greatly simplify the produced c code and make it run faster cuz you don't need the whole signal change tracking system.

https://en.wikipedia.org/wiki/Topological_sort

that would also help locate combinatorial loops (which is something not done at the moment, at all, in nmigen Simulation, and it's a pain)

the only thing being a pain in the neck, that sort takes place across an entire swathe of modules/fragments/processes
Comment 1 Luke Kenneth Casson Leighton 2021-12-23 19:14:29 GMT 
nice idea, bear in mind it is an optimisation (priority set to low)
even detecting feedback loops is slightly problematic when SR latches
are involved.