[Packer] Load The ClusteredNetlist Directly From The ClusterLegalizer

[AlexandreSinger]

The current behaviour of the Packer is as follows:

1. It performs prepacking on the atoms to create pack molecules (using the Prepacker)
2. It uses these molecules to create LegalizationClusters (using the ClusterLegalizer)
3. It writes these LegalizationClusters to a .net file (using output clustering)
   

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

   Lines 271 to 273 in e38c0f3

   	//check clustering and output it
   	check_and_output_clustering(cluster_legalizer, *packer_opts, is_clock, arch);

4. It then destroys all of the LegalizationClusters
5. Then it reads back in this .net file to generate the ClusteredNetlist (using load packing)
   

   vtr-verilog-to-routing/vpr/src/base/vpr_api.cpp

   Lines 594 to 599 in e38c0f3

   	//TODO: to be consistent with placement/routing vpr_pack should really
   	// load the netlist data structures itself, instead of re-loading
   	// the netlist from the .net file
   	//Load the result from the .net file
   	vpr_load_packing(vpr_setup, arch);

This is very inefficient since a lot of the information from the clustering state is being regenerated again, which wastes a lot of time. It also means that a .net file MUST be made to run VPR, even though it is not necessary.

Instead, the ClusterLegalizer could have a method to convert all of its LegalizationClusters into a valid ClusteredNetlist. The write_packing_results_to_xml method could then be used to write the ClusteredNetlist directly to a file (instead of generating it from the LegalizationClusters), if the .net file is needed.

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

Lines 720 to 743 in e38c0f3

	/********************************************************************
	* A useful API to output packing results to a XML file
	* This function is a wrapper for the function output_clustering()
	* but remove all the requirements on input data structures that
	* have to be built with other APIs
	*
	* As such, this function is expected to be a standard API
	* which can be called anytime and anywhere after packing is finished.
	********************************************************************/
	void write_packing_results_to_xml(const bool& global_clocks,
	const std::string& architecture_id,
	const char* out_fname) {
	std::unordered_set<AtomNetId> is_clock = alloc_and_load_is_clock();
	// Since the cluster legalizer is not being used to output the clustering
	// (from_legalizer is false), passing in nullptr.
	output_clustering(nullptr,
	global_clocks,
	is_clock,
	architecture_id,
	out_fname,
	false, /*skip_clustering*/
	false /*from_legalizer*/);
	}

This should be a straightforward change, but the biggest thing is making sure that the ClusteredNetlist generated from the ClusterLegalizer EXACTLY matches the ClusteredNetlist generated from reading a .net file. A unit test should be added which compares these two ClusteredNetlists and ensures they exactly match.