add comments to pick_from_highly_critical_block()

Author: soheilshahrouz

vpr/src/place/move_utils.cpp

@@ -623,12 +623,14 @@ ClusterBlockId pick_from_highly_critical_block(ClusterNetId& net_from,
     std::pair<ClusterNetId, int> crit_pin = highly_crit_pins[rng.irand(highly_crit_pins.size() - 1)];
     ClusterBlockId b_from = cluster_ctx.clb_nlist.net_driver_block(crit_pin.first);
 
-    //Check if picked block type matches with the blk_type specified, and it is not fixed
-    //blk_type from propose move doesn't account for the EMPTY type
     auto b_from_type = cluster_ctx.clb_nlist.block_type(b_from);
+
+    // check if the type of the picked block matches with the specified block type
+    // when a block type is specified, i.e. when logical_blk_type_index >= 0
     if (b_from_type->index == logical_blk_type_index || logical_blk_type_index < 0) {
+        // ensure that the selected block is not fixed
         if (block_locs[b_from].is_fixed) {
-            return ClusterBlockId::INVALID(); //Block is fixed, cannot move
+            return ClusterBlockId::INVALID();   // a fixed block can't be moved
         }
 
         net_from = crit_pin.first;

vpr/src/place/move_utils.h

@@ -200,8 +200,14 @@ ClusterBlockId pick_from_block(int logical_blk_type_index, vtr::RngContainer& rn
 /**
  * @brief Find a highly critical block with a specific block type to be swapped with another block.
  *
+ * @param net_from The clustered net id of the critical connection of the selected block by this function.
+ * To be filled by this function.
+ * @param pin_from The pin id of the critical connection of the  selected block by this function.
+ * To be filled by this function.
  * @param logical_blk_type_index The logical type of the moving block. If a negative value is passed,
  * the block is selected randomly from all movable blocks and not from a specific type.
+ * @param placer_state Used to access the current placement's info, e.g. block locations and if they are fixed.
+ * @param placer_criticalities Holds the clustered netlist connection criticalities.
  * @param rng A random number generator used to select a random highly critical block.
  * 
  * @return BlockId of the selected block, ClusterBlockId::INVALID() if no block with specified block type found.