commit 53b178be6f9c46ab74eaa0718196a381c72bbec1
author Keith Rothman <537074+litghost@users.noreply.github.com> Mon Sep 16 13:40:12 2019 -0700
committer kmurray <kmurray@users.noreply.github.com> Wed Sep 18 16:39:56 2019 -0400
tree 075a2761d933c8518dca51f1dae33c6b2692ce2b
parent 17728adf21b505ebcfdaa52362dcc13b3bccd45b

Expand comment on orthogonal data fixup.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>

vpr/src/route/rr_graph_indexed_data.cpp

diff --git a/vpr/src/route/rr_graph_indexed_data.cpp b/vpr/src/route/rr_graph_indexed_data.cpp
index cdfb687..0c2e520 100644
--- a/vpr/src/route/rr_graph_indexed_data.cpp
+++ b/vpr/src/route/rr_graph_indexed_data.cpp
@@ -115,12 +115,22 @@
     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;