[Pack] Estimating Inter-Cluster Delay Before Packing

[AlexandreSinger]

There is code that occurs just before packing which estimates the inter-cluster delay as a single floating point number. This is used in timing analysis to better inform the cost of going between clusters:

vtr-verilog-to-routing/vpr/src/pack/pack.cpp

Lines 498 to 544 in 23b3aa3

	static float approximate_inter_cluster_delay(const t_arch& arch,
	const t_det_routing_arch& routing_arch,
	const std::string& device_layout) {
	/* If needed, estimate inter-cluster delay. Assume the average routing hop goes out of
	* a block through an opin switch to a length-4 wire, then through a wire switch to another
	* length-4 wire, then through a wire-to-ipin-switch into another block. */
	constexpr int wire_segment_length = 4;
	/* We want to determine a reasonable fan-in to the opin, wire, and ipin switches, based
	* on which the intercluster delays can be estimated. The fan-in of a switch influences its
	* delay.
	*
	* The fan-in of the switch depends on the architecture (unidirectional/bidirectional), as
	* well as Fc_in/out and Fs */
	int opin_switch_fanin, wire_switch_fanin, ipin_switch_fanin;
	get_intercluster_switch_fanin_estimates(arch, routing_arch, device_layout, wire_segment_length, &opin_switch_fanin,
	&wire_switch_fanin, &ipin_switch_fanin);
	float Tdel_opin_switch, R_opin_switch, Cout_opin_switch;
	float opin_switch_del = get_arch_switch_info(arch.Segments[0].arch_opin_switch, opin_switch_fanin,
	Tdel_opin_switch, R_opin_switch, Cout_opin_switch);
	float Tdel_wire_switch, R_wire_switch, Cout_wire_switch;
	float wire_switch_del = get_arch_switch_info(arch.Segments[0].arch_wire_switch, wire_switch_fanin,
	Tdel_wire_switch, R_wire_switch, Cout_wire_switch);
	float Tdel_wtoi_switch, R_wtoi_switch, Cout_wtoi_switch;
	float wtoi_switch_del = get_arch_switch_info(routing_arch.wire_to_arch_ipin_switch, ipin_switch_fanin,
	Tdel_wtoi_switch, R_wtoi_switch, Cout_wtoi_switch);
	float Rmetal = arch.Segments[0].Rmetal;
	float Cmetal = arch.Segments[0].Cmetal;
	/* The delay of a wire with its driving switch is the switch delay plus the
	* product of the equivalent resistance and capacitance experienced by the wire. */
	float first_wire_seg_delay = opin_switch_del
	+ (R_opin_switch + Rmetal * (float)wire_segment_length / 2)
	* (Cout_opin_switch + Cmetal * (float)wire_segment_length);
	float second_wire_seg_delay = wire_switch_del
	+ (R_wire_switch + Rmetal * (float)wire_segment_length / 2)
	* (Cout_wire_switch + Cmetal * (float)wire_segment_length);
	/* multiply by 4 to get a more conservative estimate */
	return 4 * (first_wire_seg_delay + second_wire_seg_delay + wtoi_switch_del);
	}

This is using relatively complicated RRGraph related computations; however, it is hard to get an estimate of this number this early on. This code can probably be simplified without any affect on QoR.

See PR #2881 for more info. Specifcally, Vaughn's comment here: #2881 (comment)