Add more default trials to sabre layout (#13360)

* Add more default trials to sabre layout Right now sabre layout uses n random trials (as specified by the user or defaulting to num_cpus) and since #12453 one additional trial taking the qubits of the densest subgraph as a starting point. There are also a couple of other trivial examples to try which may or may not produce better results depending on the circuit, a trivial layout and a reverse trivial layout. In the case of a hardware efficient circuit the trivial layout will map exactly and would always be picked. When running in a preset pass manager sabre should never be called in this scenario because VF2Layout will find the mapping, but the incremental cost of adding the trial is minimal. Similarly the cost of a reversed trivial layout (where virtual qubit 0 -> n, 1 -> n - 1, etc.) is trivial and may in some scenarios produce a better results. * Update layouts for failing tests * Add ring layouts for common connectivity graphs * Add release notes and docs * Apply suggestions from code review Co-authored-by: Elena Peña Tapia <[email protected]> * Fix lint --------- Co-authored-by: Elena Peña Tapia <[email protected]>
Qiskit · Nov 6, 2024 · a18ef02 · a18ef02
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diff --git a/crates/accelerate/src/sabre/layout.rs b/crates/accelerate/src/sabre/layout.rs
@@ -62,6 +62,70 @@ pub fn sabre_layout_and_routing(
         &target,
         run_in_parallel,
     ));
+    starting_layouts.push(
+        (0..target.neighbors.num_qubits() as u32)
+            .map(Some)
+            .collect(),
+    );
+    starting_layouts.push(
+        (0..target.neighbors.num_qubits() as u32)
+            .rev()
+            .map(Some)
+            .collect(),
+    );
+    // This layout targets the largest ring on an IBM eagle device. It has been
+    // shown to have good results on some circuits targeting these backends. In
+    // all other cases this is no different from an additional random trial,
+    // see: https://xkcd.com/221/
+    if target.neighbors.num_qubits() == 127 {
+        starting_layouts.push(
+            [
+                0, 1, 2, 3, 4, 5, 6, 15, 22, 23, 24, 25, 34, 43, 42, 41, 40, 53, 60, 59, 61, 62,
+                72, 81, 80, 79, 78, 91, 98, 99, 100, 101, 102, 103, 92, 83, 82, 84, 85, 86, 73, 66,
+                65, 64, 63, 54, 45, 44, 46, 47, 35, 28, 29, 27, 26, 16, 7, 8, 9, 10, 11, 12, 13,
+                17, 30, 31, 32, 36, 51, 50, 49, 48, 55, 68, 67, 69, 70, 74, 89, 88, 87, 93, 106,
+                105, 104, 107, 108, 112, 126, 125, 124, 123, 122, 111, 121, 120, 119, 118, 110,
+                117, 116, 115, 114, 113, 109, 96, 97, 95, 94, 90, 75, 76, 77, 71, 58, 57, 56, 52,
+                37, 38, 39, 33, 20, 21, 19, 18, 14,
+            ]
+            .into_iter()
+            .map(Some)
+            .collect(),
+        );
+    } else if target.neighbors.num_qubits() == 133 {
+        // Same for IBM Heron 133 qubit devices. This is the ring computed by using rustworkx's
+        // max(simple_cycles(graph), key=len) on the connectivity graph.
+        starting_layouts.push(
+            [
+                108, 107, 94, 88, 89, 90, 75, 71, 70, 69, 56, 50, 51, 52, 37, 33, 32, 31, 18, 12,
+                11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 15, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28,
+                29, 36, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38, 53, 57, 58, 59, 60, 61, 62, 63,
+                64, 65, 66, 67, 74, 86, 85, 84, 83, 82, 81, 80, 79, 78, 77, 76, 91, 95, 96, 97,
+                110, 116, 117, 118, 119, 120, 111, 101, 102, 103, 104, 105, 112, 124, 125, 126,
+                127, 128, 113, 109,
+            ]
+            .into_iter()
+            .map(Some)
+            .collect(),
+        );
+    } else if target.neighbors.num_qubits() == 156 {
+        // Same for IBM Heron 156 qubit devices. This is the ring computed by using rustworkx's
+        // max(simple_cycles(graph), key=len) on the connectivity graph.
+        starting_layouts.push(
+            [
+                136, 123, 122, 121, 116, 101, 102, 103, 96, 83, 82, 81, 76, 61, 62, 63, 56, 43, 42,
+                41, 36, 21, 22, 23, 16, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 19, 35, 34,
+                33, 32, 31, 30, 29, 28, 27, 26, 25, 37, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55,
+                59, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 77, 85, 86, 87, 88, 89, 90, 91, 92,
+                93, 94, 95, 99, 115, 114, 113, 112, 111, 110, 109, 108, 107, 106, 105, 117, 125,
+                126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 139, 155, 154, 153, 152, 151,
+                150, 149, 148, 147, 146, 145, 144, 143,
+            ]
+            .into_iter()
+            .map(Some)
+            .collect(),
+        );
+    }
     let outer_rng = match seed {
         Some(seed) => Pcg64Mcg::seed_from_u64(seed),
         None => Pcg64Mcg::from_entropy(),

diff --git a/qiskit/transpiler/passes/layout/sabre_layout.py b/qiskit/transpiler/passes/layout/sabre_layout.py
@@ -148,7 +148,9 @@ def __init__(
                 (and ``routing_pass`` is not set) then the number of local
                 physical CPUs will be used as the default value. This option is
                 mutually exclusive with the ``routing_pass`` argument and an error
-                will be raised if both are used.
+                will be raised if both are used. An additional 3 or 4 trials
+                depending on the ``coupling_map`` value are run with common layouts
+                on top of the random trial count specified by this value.
             skip_routing (bool): If this is set ``True`` and ``routing_pass`` is not used
                 then routing will not be applied to the output circuit.  Only the layout
                 will be set in the property set. This is a tradeoff to run custom

diff --git a/releasenotes/notes/add-more-sabre-trials-9b421f05d2f48d18.yaml b/releasenotes/notes/add-more-sabre-trials-9b421f05d2f48d18.yaml
@@ -0,0 +1,11 @@
+---
+features_transpiler:
+  - |
+    The :class:`.SabreLayout` transpiler pass has been updated to run an
+    additional 2 or 3 layout trials by default independently of the
+    ``layout_trials`` keyword argument's value. A trivial
+    layout and its reverse are included for all backends, just like the :class:`.DenseLayout`
+    trial that was added in 1.2.0. In addition to this, the largest rings on 
+    an IBM backend heavy hex connectivity graph are added if the backends are 127,
+    133, or 156 qubits. This can provide a good starting point for some circuits on these commonly run
+    backends, while for all others it's just an additional "random trial".
diff --git a/test/python/transpiler/test_sabre_layout.py b/test/python/transpiler/test_sabre_layout.py
@@ -207,7 +207,7 @@ def test_layout_with_classical_bits(self):
         self.assertIsInstance(res, QuantumCircuit)
         layout = res._layout.initial_layout
         self.assertEqual(
-            [layout[q] for q in qc.qubits], [11, 19, 18, 16, 26, 8, 21, 1, 5, 15, 3, 12, 14, 13]
+            [layout[q] for q in qc.qubits], [2, 0, 5, 1, 7, 3, 14, 6, 9, 8, 10, 11, 4, 12]
         )
 
     # pylint: disable=line-too-long
@@ -271,7 +271,7 @@ def test_layout_many_search_trials(self):
         self.assertIsInstance(res, QuantumCircuit)
         layout = res._layout.initial_layout
         self.assertEqual(
-            [layout[q] for q in qc.qubits], [22, 7, 2, 12, 1, 5, 14, 4, 11, 0, 16, 15, 3, 10]
+            [layout[q] for q in qc.qubits], [0, 12, 7, 3, 6, 11, 1, 10, 4, 9, 2, 5, 13, 8]
         )
 
     def test_support_var_with_rust_fastpath(self):