CLBs Positioned Far from IOs

[WindFrank]

Hi VTR team,
I am striving to achieve a design solution with the shortest possible slack. Despite setting the goal for the solution to 'slack_timing', VPR continues to output a design where several CLBs and multipliers are noticeably placed far from the IOs.

Expected Behaviour

By setting the command options:
--alpha_clustering 1
--place_quench_algorithm slack_timing
--noc_placement_weighting 0
--noc_latency_weighting 0
I expected the worst slack to be the primary consideration for VPR. I utilized VPR to process the netlist from condition_program_first4984.pre-vpr.blif, anticipating a design solution optimized for low slack.

Current Behaviour

Contrary to my expectations, the solution I received was atypical. The multipliers and CLBs were centrally placed within the architecture rather than near the IOs, which are located at the sides. What’s more, a detour path (105,228)->(104,84)->…->(105,228)->(102,65)->(105,229) occurred in the design. The worst slack observed was also below my expectations.

Possible Solution

It is evident that the devices were positioned in inappropriate locations. This issue might stem from an underlying flaw in VTR that needs addressing, or perhaps more explicit operations to prioritize slack in the initial design phase should be implemented.

Steps to Reproduce

Please run the following command:
vpr EArch_fixed_160_230_no_power.xml condition_program_first4984 --circuit_file condition_program_first4984.pre-vpr.blif --device fixed --alpha_clustering 1.0 --noc_placement_weighting 0 --noc_latency_weighting 0 --place_quench_algorithm slack_timing --route_chan_width 100 --noc_swap_percentage 0 --timing_report_detail detailed --timing_report_npaths 100000 --max_router_iterations 150

Context

I am attempting to obtain a high-performance P&R solution but encountered an unexpected detour situation. I would greatly appreciate a prompt response.

Your Environment

• VTR revision used: FCCM 2023 version
• Operating System and version: Ubuntu 20.04
• Compiler version: gcc 9.4.0

CLBs_Positioned_Far_from_IOs.zip

I have observed a similar situation with another netlist condition_program_first4681. In this netlist's solution, the register is positioned far from the IOs. I am not sure if these instances are related, so please could you help to clarify? Thank you!

condition_program_first4681.zip