Visit runs of 1Q gates before next 2Q.

Qiskit · Aug 16, 2023 · 5be81d0 · 5be81d0
1 parent 213580d
commit 5be81d0
Showing 1 changed file with 20 additions and 10 deletions.
diff --git a/crates/accelerate/src/sabre_swap/mod.rs b/crates/accelerate/src/sabre_swap/mod.rs
@@ -174,19 +174,29 @@ fn populate_extended_set(
     let mut decremented: HashMap<usize, u32> = HashMap::new();
     let mut i = 0;
     while i < to_visit.len() && extended_set.len() < EXTENDED_SET_SIZE {
-        for edge in dag.dag.edges_directed(to_visit[i], Direction::Outgoing) {
-            let successor_node = edge.target();
-            let successor_index = successor_node.index();
-            *decremented.entry(successor_index).or_insert(0) += 1;
-            required_predecessors[successor_index] -= 1;
-            if required_predecessors[successor_index] == 0 {
-                if !dag.node_blocks.contains_key(&successor_index) {
-                    if let [a, b] = dag.dag[successor_node].1[..] {
-                        extended_set.insert(successor_node, &[a, b]);
+        // Visit runs of non-2Q gates fully before moving on to children
+        // of 2Q gates. This way, traversal order is a BFS of 2Q gates rather
+        // than of all gates.
+        let mut visit_now = vec![to_visit[i]];
+        let mut j = 0;
+        while let Some(node) = visit_now.get(j) {
+            for edge in dag.dag.edges_directed(*node, Direction::Outgoing) {
+                let successor_node = edge.target();
+                let successor_index = successor_node.index();
+                *decremented.entry(successor_index).or_insert(0) += 1;
+                required_predecessors[successor_index] -= 1;
+                if required_predecessors[successor_index] == 0 {
+                    if !dag.node_blocks.contains_key(&successor_index) {
+                        if let [a, b] = dag.dag[successor_node].1[..] {
+                            extended_set.insert(successor_node, &[a, b]);
+                            to_visit.push(successor_node);
+                            continue;
+                        }
                     }
+                    visit_now.push(successor_node);
                 }
-                to_visit.push(successor_node);
             }
+            j += 1;
         }
         i += 1;
     }