Deterministic parallel router #3203
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… from memory and queries it using distances. If the cost map contains the minimum delay and minimum congestion costs per distance, then the lookahead is admissible.
… for floating point errors. If the lookahead is admissible and we account for the floating point arithmetic errors, the parallel router is deterministic.
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Jul 20, 2025
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This PR provides everything necessary to make the parallel router deterministic without having to offset the lookahead costs.
Note: the vtr::isclose() function cannot used because it checks the absolute difference (it is symmetric), whereas we can only tolerate error on the left hand side (assymetric). Using this function makes the parallel router non-deterministic!
By using appropriate (admissible) cost maps, both of these changes allow the parallel router to be deterministic. Attached are admissible cost maps for the flagship VTR architecture (
k6_frac_N10_frac_chain_mem32K_40nm) devices.vtr_devices_admissible_lookahead_maps.zip