[vpr][pack] add comment

Author: amin1377

vpr/src/pack/prepack.cpp

@@ -1091,6 +1091,9 @@ static AtomBlockId get_sink_block(const AtomBlockId block_id,
         if (net_id.is_valid()) {
             const auto& net_sinks = atom_nlist.net_sinks(net_id);
             if (is_chain_pattern) {
+                // If the pattern is a chain, allow nets with multiple sinks.
+                // This enables forming chains where the COUT is connected both to 
+                // the next element in the chain and to the block's output pin.
                 for (const auto& sink_pin_id : net_sinks) {
                     auto sink_block_id = atom_nlist.pin_block(sink_pin_id);
                     if (primitive_type_feasible(sink_block_id, to_pb_type)) {
@@ -1102,6 +1105,11 @@ static AtomBlockId get_sink_block(const AtomBlockId block_id,
                     }
                 }
             } else {
+                // For non-chain patterns, we conservatively only consider the sink block 
+                // if the net fanout is 1. To clarify, consider a case where the output of a LUT 
+                // is connected to both a register and an unregistered output that feeds another block.
+                // If the intra-cluster architecture doesn't support having both registered and 
+                // unregistered outputs simultaneously, this could lead to a packing failure.
                 if (net_sinks.size() == 1) {
                     auto sink_pin_id = *(net_sinks.begin());
                     return atom_nlist.pin_block(sink_pin_id);