dpl: reduce memory used by Pixel::padding_reserved_by field

Signed-off-by: Osama <[email protected]>

Author: osamahammad21

src/dpl/src/Opendp.cpp

@@ -271,7 +271,7 @@ void Opendp::setFixedGridCells()
     if (cell->getType() == Node::CELL && cell->isFixed()) {
       grid_->visitCellPixels(*cell, true, [&](Pixel* pixel, bool padded) {
         if (padded) {
-          pixel->padding_reserved_by.insert(cell.get());
+          pixel->padding_reserved_by = cell.get();
         } else {
           setGridCell(*cell, pixel);
         }

src/dpl/src/PlacementDRC.cpp

@@ -312,10 +312,8 @@ bool PlacementDRC::checkPadding(const Node* cell,
       if (hasPaddingConflict(cell, pixel->cell)) {
         return false;
       }
-      for (auto padding_cell : pixel->padding_reserved_by) {
-        if (hasPaddingConflict(cell, padding_cell)) {
-          return false;
-        }
+      if (hasPaddingConflict(cell, pixel->padding_reserved_by)) {
+        return false;
       }
     }
   }

src/dpl/src/infrastructure/Grid.cpp

@@ -437,9 +437,8 @@ void Grid::erasePixel(Node* cell)
       }
 
       // Clear padding reservations made by this cell
-      if (pixel->padding_reserved_by.find(cell)
-          != pixel->padding_reserved_by.end()) {
-        pixel->padding_reserved_by.erase(cell);
+      if (pixel->padding_reserved_by == cell) {
+        pixel->padding_reserved_by = nullptr;
       }
     }
   }
@@ -491,7 +490,7 @@ void Grid::paintCellPadding(Node* cell,
       if (pixel == nullptr) {
         continue;
       }
-      pixel->padding_reserved_by.insert(cell);
+      pixel->padding_reserved_by = cell;
     }
   }
 
@@ -502,7 +501,7 @@ void Grid::paintCellPadding(Node* cell,
       if (pixel == nullptr) {
         continue;
       }
-      pixel->padding_reserved_by.insert(cell);
+      pixel->padding_reserved_by = cell;
     }
   }
 }

src/dpl/src/infrastructure/Grid.h

@@ -45,8 +45,8 @@ struct Pixel
   bool is_valid = false;     // false for dummy cells
   bool is_hopeless = false;  // too far from sites for diamond search
   uint8_t blocked_layers = 0;
-  // Cells that reserved this pixel for padding
-  std::unordered_set<Node*> padding_reserved_by;
+  // Cell that reserved this pixel for padding
+  Node* padding_reserved_by = nullptr;
 };
 
 // Return value for grid searches.