grt: use uint32_t for the packed coordinate dedup key

The packed (x,y) key shifted a uint16_t left by 16 into a signed int32_t,
which is undefined when the high coordinate has bit 15 set (value >= 2^31).
Use an unsigned 32-bit key (map and pack) to make the shift well-defined.
No behavior change: the same distinct keys and first-match aliasing;
140/140 grt tests pass with byte-identical routing.

Signed-off-by: Saurav Singh <saurav.singh@fermions.co>

Author: saurav-fermions

src/grt/src/fastroute/include/FastRoute.h

@@ -812,7 +812,7 @@ class FastRouteCore
   // Maps a packed (x,y) grid position to the dcor index of the first node
   // inserted at that position, replacing an O(numpoints^2) linear scan with
   // O(1) average lookups. Reused across calls to avoid per-call allocation.
-  std::unordered_map<int32_t, int> tree_node_coord_dedup_;
+  std::unordered_map<uint32_t, int> tree_node_coord_dedup_;
 
   std::vector<FrNet*> nets_;
   std::unordered_map<odb::dbNet*, int> db_net_id_map_;  // db net -> net id

src/grt/src/fastroute/src/utility.cpp

@@ -3018,10 +3018,11 @@ void FastRouteCore::setTreeNodesVariables(const int netID)
     treenodes[d].lID = BIG_INT;
     treenodes[d].status = 0;
 
-    // Pack the int16_t grid coordinates into a single 32-bit key.
-    const int32_t key
-        = (static_cast<int32_t>(static_cast<uint16_t>(treenodes[d].x)) << 16)
-          | static_cast<int32_t>(static_cast<uint16_t>(treenodes[d].y));
+    // Pack the int16_t grid coordinates into a single unsigned 32-bit key.
+    // Unsigned avoids undefined behavior from shifting into the sign bit.
+    const uint32_t key
+        = (static_cast<uint32_t>(static_cast<uint16_t>(treenodes[d].x)) << 16)
+          | static_cast<uint32_t>(static_cast<uint16_t>(treenodes[d].y));
 
     if (d < num_terminals) {
       const int pin_idx = sttrees_[netID].node_to_pin_idx[d];