Use Sabre by default for optimization levels 1 and 2

qiskit/transpiler/passes/routing/sabre_swap.py

@@ -150,14 +150,7 @@ def __init__(self, coupling_map, heuristic="basic", seed=None, fake_run=False, t
         if coupling_map is not None:
             self._neighbor_table = NeighborTable(retworkx.adjacency_matrix(self.coupling_map.graph))
 
-        if heuristic == "basic":
-            self.heuristic = Heuristic.Basic
-        elif heuristic == "lookahead":
-            self.heuristic = Heuristic.Lookahead
-        elif heuristic == "decay":
-            self.heuristic = Heuristic.Decay
-        else:
-            raise TranspilerError("Heuristic %s not recognized." % heuristic)
+        self.heuristic = heuristic
 
         if seed is None:
             ii32 = np.iinfo(np.int32)
@@ -191,6 +184,15 @@ def run(self, dag):
         if len(dag.qubits) > self.coupling_map.size():
             raise TranspilerError("More virtual qubits exist than physical.")
 
+        if self.heuristic == "basic":
+            heuristic = Heuristic.Basic
+        elif self.heuristic == "lookahead":
+            heuristic = Heuristic.Lookahead
+        elif self.heuristic == "decay":
+            heuristic = Heuristic.Decay
+        else:
+            raise TranspilerError("Heuristic %s not recognized." % self.heuristic)
+
         self.dist_matrix = self.coupling_map.distance_matrix
 
         # Preserve input DAG's name, regs, wire_map, etc. but replace the graph.
@@ -229,7 +231,7 @@ def run(self, dag):
             sabre_dag,
             self._neighbor_table,
             self.dist_matrix,
-            self.heuristic,
+            heuristic,
             self.seed,
             layout,
             self.trials,

qiskit/transpiler/preset_passmanagers/builtin_plugins.py

@@ -218,7 +218,7 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
             )
         if optimization_level == 1:
             routing_pass = SabreSwap(
-                coupling_map, heuristic="lookahead", seed=seed_transpiler, trials=5
+                coupling_map, heuristic="decay", seed=seed_transpiler, trials=5
             )
             return common.generate_routing_passmanager(
                 routing_pass,

qiskit/transpiler/preset_passmanagers/level1.py

@@ -71,9 +71,9 @@ def level_1_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     inst_map = pass_manager_config.inst_map
     coupling_map = pass_manager_config.coupling_map
     initial_layout = pass_manager_config.initial_layout
-    layout_method = pass_manager_config.layout_method or "dense"
     init_method = pass_manager_config.init_method
-    routing_method = pass_manager_config.routing_method or "stochastic"
+    layout_method = pass_manager_config.layout_method or "sabre"
+    routing_method = pass_manager_config.routing_method or "sabre"
     translation_method = pass_manager_config.translation_method or "translator"
     optimization_method = pass_manager_config.optimization_method
     scheduling_method = pass_manager_config.scheduling_method

qiskit/transpiler/preset_passmanagers/level2.py

@@ -74,8 +74,8 @@ def level_2_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     coupling_map = pass_manager_config.coupling_map
     initial_layout = pass_manager_config.initial_layout
     init_method = pass_manager_config.init_method
-    layout_method = pass_manager_config.layout_method or "dense"
-    routing_method = pass_manager_config.routing_method or "stochastic"
+    layout_method = pass_manager_config.layout_method or "sabre"
+    routing_method = pass_manager_config.routing_method or "sabre"
     translation_method = pass_manager_config.translation_method or "translator"
     optimization_method = pass_manager_config.optimization_method
     scheduling_method = pass_manager_config.scheduling_method

releasenotes/notes/sabres-for-everyone-3148ccf2064ccb0d.yaml

@@ -0,0 +1,17 @@
+---
+upgrade:
+  - |
+    The preset pass managers for levels 1 and 2, which will be used when
+    ``optimization_level=1`` or ``optimization_level=2`` with
+    :func:`~.transpile` or :func:`~.generate_preset_pass_manager` and output
+    from :func:`~.level_1_pass_manager` and :func:`~.level_2_pass_manager`,
+    will now use :class:`~.SabreLayout` and :func:`~SabreSwap` by default
+    instead of the previous defaults :class:`~.DenseLayout` and
+    :class:`~.StochasticSwap`. This change was made to improve the output
+    quality of the transpiler, the :class:`~.SabreLayout` and
+    :func:`~SabreSwap` combination typically results in fewer
+    :class:`~.SwapGate` objects being inserted into the output circuit.
+    If you would like to use the previous default passes you can set
+    ``layout_method='dense'`` and ``routing_method='stochastic'`` on
+    :func:`~.transpile` or :func:`~.generate_preset_pass_manager to
+    leverage :class:`~.DenseLayout` and :class:`~.StochasticSwap` respectively.

src/sabre_swap/neighbor_table.rs

@@ -36,38 +36,48 @@ pub struct NeighborTable {
 #[pymethods]
 impl NeighborTable {
     #[new]
-    pub fn new(adjacency_matrix: PyReadonlyArray2<f64>) -> Self {
-        let adj_mat = adjacency_matrix.as_array();
+    pub fn new(adjacency_matrix: Option<PyReadonlyArray2<f64>>) -> Self {
         let run_in_parallel = getenv_use_multiple_threads();
-        let build_neighbors = |row: ArrayView1<f64>| -> Vec<usize> {
-            row.iter()
-                .enumerate()
-                .filter_map(
-                    |(row_index, value)| {
-                        if *value == 0. {
-                            None
-                        } else {
-                            Some(row_index)
-                        }
-                    },
-                )
-                .collect()
-        };
-        if run_in_parallel {
-            NeighborTable {
-                neighbors: adj_mat
-                    .axis_iter(Axis(0))
-                    .into_par_iter()
-                    .map(|row| build_neighbors(row))
-                    .collect(),
-            }
-        } else {
-            NeighborTable {
-                neighbors: adj_mat
-                    .axis_iter(Axis(0))
-                    .map(|row| build_neighbors(row))
-                    .collect(),
+        let neighbors = match adjacency_matrix {
+            Some(adjacency_matrix) => {
+                let adj_mat = adjacency_matrix.as_array();
+                let build_neighbors = |row: ArrayView1<f64>| -> Vec<usize> {
+                    row.iter()
+                        .enumerate()
+                        .filter_map(
+                            |(row_index, value)| {
+                                if *value == 0. {
+                                    None
+                                } else {
+                                    Some(row_index)
+                                }
+                            },
+                        )
+                        .collect()
+                };
+                if run_in_parallel {
+                    adj_mat
+                        .axis_iter(Axis(0))
+                        .into_par_iter()
+                        .map(|row| build_neighbors(row))
+                        .collect()
+                } else {
+                    adj_mat
+                        .axis_iter(Axis(0))
+                        .map(|row| build_neighbors(row))
+                        .collect()
+                }
             }
-        }
+            None => Vec::new(),
+        };
+        NeighborTable { neighbors }
+    }
+
+    fn __getstate__(&self) -> Vec<Vec<usize>> {
+        self.neighbors.clone()
+    }
+
+    fn __setstate__(&mut self, state: Vec<Vec<usize>>) {
+        self.neighbors = state
     }
 }

test/python/compiler/test_transpiler.py

@@ -716,7 +716,7 @@ def test_move_measurements(self):
         circ = QuantumCircuit.from_qasm_file(os.path.join(qasm_dir, "move_measurements.qasm"))
 
         lay = [0, 1, 15, 2, 14, 3, 13, 4, 12, 5, 11, 6]
-        out = transpile(circ, initial_layout=lay, coupling_map=cmap)
+        out = transpile(circ, initial_layout=lay, coupling_map=cmap, routing_method="stochastic")
         out_dag = circuit_to_dag(out)
         meas_nodes = out_dag.named_nodes("measure")
         for meas_node in meas_nodes:

test/python/transpiler/test_preset_passmanagers.py

@@ -701,29 +701,6 @@ def test_layout_tokyo_fully_connected_cx(self, level):
             19: ancilla[14],
         }
 
-        dense_layout = {
-            11: qr[0],
-            6: qr[1],
-            5: qr[2],
-            10: qr[3],
-            15: qr[4],
-            0: ancilla[0],
-            1: ancilla[1],
-            2: ancilla[2],
-            3: ancilla[3],
-            4: ancilla[4],
-            7: ancilla[5],
-            8: ancilla[6],
-            9: ancilla[7],
-            12: ancilla[8],
-            13: ancilla[9],
-            14: ancilla[10],
-            16: ancilla[11],
-            17: ancilla[12],
-            18: ancilla[13],
-            19: ancilla[14],
-        }
-
         sabre_layout = {
             11: qr[0],
             17: qr[1],
@@ -748,8 +725,8 @@ def test_layout_tokyo_fully_connected_cx(self, level):
         }
 
         expected_layout_level0 = trivial_layout
-        expected_layout_level1 = dense_layout
-        expected_layout_level2 = dense_layout
+        expected_layout_level1 = sabre_layout
+        expected_layout_level2 = sabre_layout
         expected_layout_level3 = sabre_layout
 
         expected_layouts = [