nakengelhardt
commented
1 year ago I'm not sure that the number of instances makes for a good heuristic in the place where flattening is done currently (at the very beginning of the flow). Is this the script you are currently using to pick what to keep? That seems to be based on the number of gates in the synthesized module. If you want to make the decision based on the size in gates of the design, there isn't really a choice but to synthesize to gate level first and then decide. You might be able to save a little time by selectively flattening the in-memory design at that point and re-running the optimization directly on the fine-grain (and you wouldn't need any modification to flatten or synth for that, if you're using tcl, you could directly add the keep_hierarchy attributes to the design).
Have you experimented a bit with replacing "synth" with its component passes and inserting your selective flattening heuristic in various places to see what the effects on the results are?
maliberty
widlarizer
Feature Description
In OpenRoad-flow-scripts we use yosys calling synth --flatten. This has worked well but we would like to keep the hierarchy in the final result. If we simply remove --flatten then QOR takes a hit as optimization across module boundaries can't happen.
Therefore we would like to flatten small modules while keeping larger modules to minimize QOR loss while retaining the useful hierarchy information (for display, clustering, UPF, hierarchical SDC, etc.). While the user could do this manually we would like to have an automated default.
The current approach is to run yosys hierarchically, decide what to keep, and re-run with keep_hierarchy settings. This works but requires two full runs which isn't great. I would like to enhance flatten to have some option(s) to control flattening (and perhaps add them to synth).
Would something like "flatten -cell_threshold" make sense?
I'm glad to prepare a PR but I don't want to spend the time if this isn't the right approach and it wouldn't be accepted. Of course I'm glad for someone else to do it too :)