Expand comment on orthogonal data fixup.

Signed-off-by: Keith Rothman <[email protected]>

Author: litghost

vpr/src/route/rr_graph_indexed_data.cpp

@@ -115,12 +115,22 @@ void alloc_and_load_rr_indexed_data(const std::vector<t_segment_inf>& segment_in
     load_rr_indexed_data_T_values((CHANX_COST_INDEX_START + num_segment),
                                   num_segment, CHANY, nodes_per_chan, L_rr_node_indices);
 
+    // Scan CHANX/CHANY indexed data and search for uninitialized costs.
+    //
+    // This would occur if a segment ends up only being used as CHANX or a
+    // CHANY, but not both.  If this occurs, then copying the orthogonal
+    // pair's cost data is likely a better choice than leaving it as -1.
+    //
+    // The primary reason for this fixup is to avoid propagating negative
+    // values in cost functions.
     for (int cost_index = CHANX_COST_INDEX_START;
          cost_index < CHANX_COST_INDEX_START + 2 * num_segment; cost_index++) {
         int ortho_cost_index = device_ctx.rr_indexed_data[cost_index].ortho_cost_index;
-        /* If segments doesn't have data (e.g. doesn't exists), check if ortho 
-         * segment does exist. If so, copy that data. */
+
+        // Check if this data is uninitialized, but the orthogonal data is
+        // initialized.
         if (device_ctx.rr_indexed_data[cost_index].T_linear == OPEN && device_ctx.rr_indexed_data[ortho_cost_index].T_linear != OPEN) {
+            // Copy orthogonal data over.
             device_ctx.rr_indexed_data[cost_index].T_linear = device_ctx.rr_indexed_data[ortho_cost_index].T_linear;
             device_ctx.rr_indexed_data[cost_index].T_quadratic = device_ctx.rr_indexed_data[ortho_cost_index].T_quadratic;
             device_ctx.rr_indexed_data[cost_index].C_load = device_ctx.rr_indexed_data[ortho_cost_index].C_load;