Add swap_trials argument to SabreLayout

This commit adds a swap_trials argument to the SabreLayout pass so that
users can control how many trials to run in SabreSwap internally. This
is necessary for reproducibility between systems for the same reason
it's required on SabreSwap.

qiskit/transpiler/passes/layout/sabre_layout.py

@@ -49,20 +49,42 @@ class SabreLayout(AnalysisPass):
     `arXiv:1809.02573 <https://arxiv.org/pdf/1809.02573.pdf>`_
     """
 
-    def __init__(self, coupling_map, routing_pass=None, seed=None, max_iterations=3):
+    def __init__(
+        self, coupling_map, routing_pass=None, seed=None, max_iterations=3, swap_trials=None
+    ):
         """SabreLayout initializer.
 
         Args:
             coupling_map (Coupling): directed graph representing a coupling map.
             routing_pass (BasePass): the routing pass to use while iterating.
+                This is mutually exclusive with the ``swap_trials`` argument and
+                if both are set an error will be raised.
             seed (int): seed for setting a random first trial layout.
             max_iterations (int): number of forward-backward iterations.
+            swap_trials (int): The number of trials to run of
+                :class:`~.SabreSwap` for each iteration. This is equivalent to
+                the ``trials`` argument on :class:`~.SabreSwap`. If this is not
+                specified (and ``routing_pass`` isn't set) by default the number
+                of physical CPUs on your local system will be used. For
+                reproducibility between environments it is best to set this
+                to an explicit number because the output will potentially depend
+                on the number of trials run. This option is mutually exclusive
+                with the ``routing_pass`` argument and an error will be raised
+                if both are used.
+
+        Raises:
+            TranspilerError: If both ``routing_pass`` and ``swap_trials`` are
+                specified
         """
         super().__init__()
         self.coupling_map = coupling_map
+        if routing_pass is not None and swap_trials is not None:
+            raise TranspilerError("Both routing_pass and swap_trials can't be set at the same time")
         self.routing_pass = routing_pass
         self.seed = seed
         self.max_iterations = max_iterations
+        self.trials = swap_trials
+        self.swap_trials = swap_trials
 
     def run(self, dag):
         """Run the SabreLayout pass on `dag`.
@@ -86,7 +108,9 @@ def run(self, dag):
         initial_layout = Layout({q: dag.qubits[i] for i, q in enumerate(physical_qubits)})
 
         if self.routing_pass is None:
-            self.routing_pass = SabreSwap(self.coupling_map, "decay", seed=self.seed, fake_run=True)
+            self.routing_pass = SabreSwap(
+                self.coupling_map, "decay", seed=self.seed, fake_run=True, trials=self.swap_trials
+            )
         else:
             self.routing_pass.fake_run = True
 

qiskit/transpiler/preset_passmanagers/level0.py

@@ -84,7 +84,9 @@ def _choose_layout_condition(property_set):
     elif layout_method == "noise_adaptive":
         _choose_layout = NoiseAdaptiveLayout(backend_properties)
     elif layout_method == "sabre":
-        _choose_layout = SabreLayout(coupling_map, max_iterations=1, seed=seed_transpiler)
+        _choose_layout = SabreLayout(
+            coupling_map, max_iterations=1, seed=seed_transpiler, swap_trials=5
+        )
     else:
         raise TranspilerError("Invalid layout method %s." % layout_method)
 
@@ -97,7 +99,7 @@ def _choose_layout_condition(property_set):
     elif routing_method == "lookahead":
         routing_pass = LookaheadSwap(coupling_map, search_depth=2, search_width=2)
     elif routing_method == "sabre":
-        routing_pass = SabreSwap(coupling_map, heuristic="basic", seed=seed_transpiler)
+        routing_pass = SabreSwap(coupling_map, heuristic="basic", seed=seed_transpiler, trials=5)
     elif routing_method == "toqm":
         HAS_TOQM.require_now("TOQM-based routing")
         from qiskit_toqm import ToqmSwap, ToqmStrategyO0, latencies_from_target

qiskit/transpiler/preset_passmanagers/level1.py

@@ -142,7 +142,9 @@ def _vf2_match_not_found(property_set):
     elif layout_method == "noise_adaptive":
         _improve_layout = NoiseAdaptiveLayout(backend_properties)
     elif layout_method == "sabre":
-        _improve_layout = SabreLayout(coupling_map, max_iterations=2, seed=seed_transpiler)
+        _improve_layout = SabreLayout(
+            coupling_map, max_iterations=2, seed=seed_transpiler, swap_trials=5
+        )
     else:
         raise TranspilerError("Invalid layout method %s." % layout_method)
 

qiskit/transpiler/preset_passmanagers/level2.py

@@ -125,7 +125,9 @@ def _vf2_match_not_found(property_set):
     elif layout_method == "noise_adaptive":
         _choose_layout_1 = NoiseAdaptiveLayout(backend_properties)
     elif layout_method == "sabre":
-        _choose_layout_1 = SabreLayout(coupling_map, max_iterations=2, seed=seed_transpiler)
+        _choose_layout_1 = SabreLayout(
+            coupling_map, max_iterations=2, seed=seed_transpiler, swap_trials=10
+        )
     else:
         raise TranspilerError("Invalid layout method %s." % layout_method)
 

qiskit/transpiler/preset_passmanagers/level3.py

@@ -132,7 +132,9 @@ def _vf2_match_not_found(property_set):
     elif layout_method == "noise_adaptive":
         _choose_layout_1 = NoiseAdaptiveLayout(backend_properties)
     elif layout_method == "sabre":
-        _choose_layout_1 = SabreLayout(coupling_map, max_iterations=4, seed=seed_transpiler)
+        _choose_layout_1 = SabreLayout(
+            coupling_map, max_iterations=4, seed=seed_transpiler, swap_trials=20
+        )
     else:
         raise TranspilerError("Invalid layout method %s." % layout_method)
 

releasenotes/notes/multiple-parallel-rusty-sabres-32bc93f79ae48a1f.yaml

@@ -9,3 +9,12 @@ features:
     now run the algorithm with ``trials`` number of random seeds and pick the
     best (with the fewest swaps inserted). If ``trials`` is not specified the
     pass will default to use the number of physical CPUs on the local system.
+  - |
+    The :class:`~.SabreLayout` transpiler pass has a new keyword argument on
+    its constructor, ``swap_trials``. The ``swap_trials`` argument is used
+    to specify how many random seed trials to run on the :class:`~.SabreSwap`
+    pass internally. It corresponds to the ``trials`` arugment on the
+    :class:`~.SabreSwap` pass. When set For each iteration of
+    :class:`~.SabreLayout` :class:`~.SabreSwap` will be run internally with
+    ``swap_trials``random seed trials. If ``swap_trials`` is not specified
+    the will default to use the number of physical CPUs on the local system.

test/python/transpiler/test_sabre_layout.py

@@ -52,7 +52,7 @@ def test_5q_circuit_20q_coupling(self):
         circuit.cx(qr[1], qr[2])
 
         dag = circuit_to_dag(circuit)
-        pass_ = SabreLayout(CouplingMap(self.cmap20), seed=0)
+        pass_ = SabreLayout(CouplingMap(self.cmap20), seed=0, swap_trials=32)
         pass_.run(dag)
 
         layout = pass_.property_set["layout"]