place: added comments on the sub tile compatibility map

Signed-off-by: Alessandro Comodi <[email protected]>

Author: acomodi

vpr/src/place/compressed_grid.cpp

@@ -29,13 +29,21 @@ std::vector<t_compressed_block_grid> create_compressed_block_grids() {
         for (const auto& physical_tile : logical_block.equivalent_tiles) {
             std::vector<int> compatible_sub_tiles;
 
+            // Build a vector to store all the sub tile indices that are compatible with the
+            // (physical_tile, logical_block) pair.
+            //
+            // Given that a logical block can potentially be compatible with only a subset of the
+            // sub tiles of a physical tile, we need to ensure which sub tile locations are part of
+            // this subset.
             for (int capacity = 0; capacity < physical_tile->capacity; capacity++) {
                 if (is_sub_tile_compatible(physical_tile, &logical_block, capacity)) {
                     compatible_sub_tiles.push_back(capacity);
                 }
             }
 
-            compressed_block_grid.compatible_sub_tiles_map.insert({physical_tile->index, compatible_sub_tiles});
+            // For each of physical tiles compatible with the current logical block, create add an entry
+            // to the compatible sub tiles map with the physical tile index and the above generated vector.
+            compressed_block_grid.compatible_sub_tiles_for_tile_map.insert({physical_tile->index, compatible_sub_tiles});
         }
 
         compressed_type_grids[logical_block.index] = compressed_block_grid;

vpr/src/place/compressed_grid.h

@@ -33,7 +33,11 @@ struct t_compressed_block_grid {
     //This makes it easy to check whether there exist
     std::vector<vtr::flat_map2<int, t_type_loc>> grid;
 
-    std::unordered_map<int, std::vector<int>> compatible_sub_tiles_map;
+    //The sub type compatibility for a given physical tile and a compressed block grid
+    //corresponding to the possible placement location for a given logical block
+    //  - key: physical tile index
+    //  - value: vector of compatible sub tiles for the physical tile/logical block pair
+    std::unordered_map<int, std::vector<int>> compatible_sub_tiles_for_tile_map;
 };
 
 //Compressed grid space for each block type

vpr/src/place/move_utils.cpp

@@ -639,7 +639,7 @@ bool find_to_loc_uniform(t_logical_block_type_ptr type,
 
     //Each x/y location possibly contains multiple sub tiles, so we need to pick
     //a z location within a compatible sub tile.
-    auto& compatible_sub_tiles = compressed_block_grid.compatible_sub_tiles_map.at(to_type->index);
+    auto& compatible_sub_tiles = compressed_block_grid.compatible_sub_tiles_for_tile_map.at(to_type->index);
     to.sub_tile = compatible_sub_tiles[vtr::irand((int)compatible_sub_tiles.size() - 1)];
 
     VTR_ASSERT_MSG(is_tile_compatible(to_type, type), "Type must be compatible");