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<agg>
i've been playing with running lots of nextpnr seeds and noticed that even seemingly insignificant/very small changes to my design can lead to large shifts in the nextpnr timing distribution, as though yosys/abc is either finding a bad seed or (assuming it's all deterministic) has very high sensitivity to initial conditions; are there any knobs i can tweak inside synthesis?
<agg>
for example, I have a design with 300 luts and might get 80+-5MHz from 500 nextpnr runs, and swapping the order of a small priority encoder drops the whole distribution 10MHz, same number of cells used
<agg>
I don't have an exact example case to share yet (and mostly I'm going through nmigen too, which adds another layer), but I've noticed it happening several times while trying to improve timing on this design
<agg>
(e.g. in nmigen I have an if/elif that drives a registered signal, and just swapping their ordering can cause this huge change in timing distribution, and then making another small change elsewhere restores it)
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