dpl: make optimize_mirroring deterministic

net_box_map_ is an unordered_map, so equal-hpwl boxes sorted in
heap-address order (ranges::sort is not stable), making the mirroring
result and the written db non-deterministic. Tie-break by net id.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

src/dpl/src/OptMirror.cpp

@@ -63,10 +63,14 @@ void OptimizeMirroring::run()
   }
 
   // Sort net boxes by net hpwl.
-  std::ranges::sort(sorted_boxes,
-                    [](NetBox* net_box1, NetBox* net_box2) -> bool {
-                      return net_box1->hpwl() > net_box2->hpwl();
-                    });
+  std::ranges::sort(
+      sorted_boxes, [](NetBox* net_box1, NetBox* net_box2) -> bool {
+        if (net_box1->hpwl() != net_box2->hpwl()) {
+          return net_box1->hpwl() > net_box2->hpwl();
+        }
+        // net_box_map_ is unordered; tie-break for stable output.
+        return net_box1->getNet()->getId() < net_box2->getNet()->getId();
+      });
 
   std::vector<odb::dbInst*> mirror_candidates
       = findMirrorCandidates(sorted_boxes);