changed the subtile selection in the initial_placement to prevent

falling into random macro placement while we have available
subtiles in the selected location.

Author: haydar-c

vpr/src/place/initial_placement.cpp

@@ -597,7 +597,23 @@ static bool try_centroid_placement(const t_pl_macro& pl_macro,
         const auto& compressed_block_grid = g_vpr_ctx.placement().compressed_block_grids[block_type->index];
         const auto& type = device_ctx.grid.get_physical_type({centroid_loc.x, centroid_loc.y, centroid_loc.layer});
         const auto& compatible_sub_tiles = compressed_block_grid.compatible_sub_tile_num(type->index);
-        centroid_loc.sub_tile = compatible_sub_tiles[rng.irand((int)compatible_sub_tiles.size() - 1)];
+        
+        //filter out occupied subtiles to prevent falling to try_place_macro_randomly while we have available subtiles already.
+        const GridBlock& grid_blocks = blk_loc_registry.grid_blocks();
+        std::vector<int> available_sub_tiles;
+        for (int sub_tile : compatible_sub_tiles) {
+            t_pl_loc pos = {centroid_loc.x, centroid_loc.y, sub_tile, centroid_loc.layer};
+            if (!grid_blocks.block_at_location(pos)) {
+                available_sub_tiles.push_back(sub_tile);
+            }
+        }
+
+        //if no available subtile is found, we don't need to try_place_macro.
+        if (available_sub_tiles.empty()) {
+            return false;
+        }
+
+        centroid_loc.sub_tile = available_sub_tiles[rng.irand((int)available_sub_tiles.size() - 1)];
     }
     int width_offset = device_ctx.grid.get_width_offset({centroid_loc.x, centroid_loc.y, centroid_loc.layer});
     int height_offset = device_ctx.grid.get_height_offset({centroid_loc.x, centroid_loc.y, centroid_loc.layer});