Filter/ignore qubits in Target without any operations

Building off Qiskit#9840 which adds full path support in all the preset pass
managers for targetting backends with a disconnected coupling graph,
this commit adds support for ignoring qubits that do not support any
operations. When a Target is generated from Qiskit#9911 with `filter_faulty`
set to `True` this will potentially result in qubits being present in
the `Target` without any supported operations. In these cases the
layout passes in the transpiler might inadvertently use these qubits
only to fail in the basis translator because there are no instructions
available. This commit adds filtering of connected components from the
list of output connected components if the `Target` does have any
supported instructions on a qubit.

This works by building a copy of the coupling map's internal graph
that removes the nodes which do not have any supported operations.
Then when we compute the connected components of this graph it will
exclude any components of isolated qubits without any operations
supported. A similar change is made to the coupling graph we pass to
rustworkx.vf2_mapping() inside the vf2 layout family of passes.

qiskit/transpiler/coupling.py

@@ -420,7 +420,7 @@ def largest_connected_component(self):
         """Return a set of qubits in the largest connected component."""
         return max(rx.weakly_connected_components(self.graph), key=len)
 
-    def connected_components(self) -> List["CouplingMap"]:
+    def connected_components(self, target=None) -> List["CouplingMap"]:
         """Separate a :Class:`~.CouplingMap` into subgraph :class:`~.CouplingMap`
         for each connected component.
 
@@ -463,20 +463,38 @@ def connected_components(self) -> List["CouplingMap"]:
 
         will print ``3`` as index ``0`` in the second component is qubit 3 in the original cmap.
 
+        Args:
+            target (Target): If used the provided :class:`~.Target` will be used to filter any
+                qubits that do not have any supported operations so that those qubits
+                will not be included as a connected component in the output list. When not
+                specified these qubits without any supported operations will be included in
+                the output as a connected component of a single qubit.
+
         Returns:
             list: A list of :class:`~.CouplingMap` objects for each connected
                 components. The order of this list is deterministic but
                 implementation specific and shouldn't be relied upon as
                 part of the API.
         """
+        graph = self.graph
+        if target is not None:
+            qargs = target.qargs
+            graph = self.graph.copy()
+            for index in graph.node_indices():
+                for qargs in target.qargs:
+                    if index in qargs:
+                        break
+                else:
+                    graph.remove_node(index)
+
         # Set payload to index
-        for node in self.graph.node_indices():
-            self.graph[node] = node
-        components = rx.weakly_connected_components(self.graph)
+        for node in graph.node_indices():
+            graph[node] = node
+        components = rx.weakly_connected_components(graph)
         output_list = []
         for component in components:
             new_cmap = CouplingMap()
-            new_cmap.graph = self.graph.subgraph(list(sorted(component)))
+            new_cmap.graph = graph.subgraph(list(sorted(component)))
             output_list.append(new_cmap)
         return output_list
 

qiskit/transpiler/passes/layout/dense_layout.py

@@ -69,7 +69,10 @@ def run(self, dag):
                 "A coupling_map or target with constrained qargs is necessary to run the pass."
             )
         layout_components = disjoint_utils.run_pass_over_connected_components(
-            dag, self.coupling_map, self._inner_run
+            dag,
+            self.coupling_map,
+            self._inner_run,
+            self.target,
         )
         layout_mapping = {}
         for component in layout_components:

qiskit/transpiler/passes/layout/disjoint_utils.py

@@ -13,7 +13,7 @@
 """This module contains common utils for disjoint coupling maps."""
 
 from collections import defaultdict
-from typing import List, Callable, TypeVar, Dict
+from typing import List, Callable, TypeVar, Dict, Optional
 import uuid
 
 import rustworkx as rx
@@ -22,6 +22,7 @@
 from qiskit.dagcircuit.dagcircuit import DAGCircuit
 from qiskit.dagcircuit.dagnode import DAGOutNode
 from qiskit.transpiler.coupling import CouplingMap
+from qiskit.transpiler.target import Target
 from qiskit.transpiler.exceptions import TranspilerError
 from qiskit.transpiler.passes.layout import vf2_utils
 
@@ -32,11 +33,17 @@ def run_pass_over_connected_components(
     dag: DAGCircuit,
     coupling_map: CouplingMap,
     run_func: Callable[[DAGCircuit, CouplingMap], T],
+    target: Optional[Target] = None,
 ) -> List[T]:
     """Run a transpiler pass inner function over mapped components."""
-    cmap_components = coupling_map.connected_components()
+    cmap_components = coupling_map.connected_components(target=target)
     # If graph is connected we only need to run the pass once
     if len(cmap_components) == 1:
+        if dag.num_qubits() > cmap_components[0].size():
+            raise TranspilerError(
+                "A connected component of the DAGCircuit is too large for any of the connected "
+                "components in the coupling map."
+            )
         return [run_func(dag, cmap_components[0])]
     dag_components = separate_dag(dag)
     mapped_components = map_components(dag_components, cmap_components)
@@ -127,11 +134,15 @@ def combine_barriers(dag: DAGCircuit, retain_uuid: bool = True):
                 node.op.label = None
 
 
-def check_layout_isolated_to_component(dag: DAGCircuit, coupling_map: CouplingMap) -> bool:
+def check_layout_isolated_to_component(
+    dag: DAGCircuit, coupling_map: CouplingMap, target: Optional[Target] = None
+) -> bool:
     """Check that the layout of the dag does not require connectivity across connected components
     in the CouplingMap"""
     qubit_indices = {bit: index for index, bit in enumerate(dag.qubits)}
-    component_sets = [set(x.graph.nodes()) for x in coupling_map.connected_components()]
+    component_sets = [
+        set(x.graph.nodes()) for x in coupling_map.connected_components(target=target)
+    ]
     for inst in dag.two_qubit_ops():
         component_index = None
         for i in range(len(component_sets)):

qiskit/transpiler/passes/layout/sabre_layout.py

@@ -218,7 +218,10 @@ def run(self, dag):
             return dag
         # Combined
         layout_components = disjoint_utils.run_pass_over_connected_components(
-            dag, self.coupling_map, self._inner_run
+            dag,
+            self.coupling_map,
+            self._inner_run,
+            target=self.target,
         )
         initial_layout_dict = {}
         final_layout_dict = {}

qiskit/transpiler/passes/layout/vf2_layout.py

@@ -141,6 +141,16 @@ def run(self, dag):
         cm_graph, cm_nodes = vf2_utils.shuffle_coupling_graph(
             self.coupling_map, self.seed, self.strict_direction
         )
+        # Filter qubits without any supported operations. If they don't support any operations
+        # They're not valid for layout selection
+        if self.target is not None:
+            for qubit, graph_index in enumerate(cm_nodes):
+                for qargs in self.target.qargs:
+                    if qubit in qargs:
+                        break
+                else:
+                    cm_graph.remove_node(graph_index)
+
         # To avoid trying to over optimize the result by default limit the number
         # of trials based on the size of the graphs. For circuits with simple layouts
         # like an all 1q circuit we don't want to sit forever trying every possible
@@ -239,6 +249,10 @@ def mapping_to_layout(layout_mapping):
                 reverse_im_graph_node_map,
                 self.avg_error_map,
             )
+            # No free qubits for free qubit mapping
+            if chosen_layout is None:
+                self.property_set["VF2Layout_stop_reason"] = VF2LayoutStopReason.NO_SOLUTION_FOUND
+                return
             self.property_set["layout"] = chosen_layout
             for reg in dag.qregs.values():
                 self.property_set["layout"].add_register(reg)

qiskit/transpiler/passes/layout/vf2_post_layout.py

@@ -211,6 +211,15 @@ def run(self, dag):
                         ops.update(global_ops[2])
                     cm_graph.add_edge(qargs[0], qargs[1], ops)
             cm_nodes = list(cm_graph.node_indexes())
+            # Filter qubits without any supported operations. If they
+            # don't support any operations They're not valid for layout selection.
+            # This is only needed in the undirected case because in strict direction
+            # mode the node matcher will not match since none of the circuit ops
+            # will match the cmap ops.
+            if not self.strict_direction:
+                for node_index in cm_graph.node_indices():
+                    if not cm_graph[node_index]:
+                        cm_graph.remove_node(node_index)
         else:
             cm_graph, cm_nodes = vf2_utils.shuffle_coupling_graph(
                 self.coupling_map, self.seed, self.strict_direction

qiskit/transpiler/passes/layout/vf2_utils.py

@@ -233,6 +233,8 @@ def map_free_qubits(
             set(range(num_physical_qubits)) - partial_layout.get_physical_bits().keys()
         )
     for im_index in sorted(free_nodes, key=lambda x: sum(free_nodes[x].values())):
+        if not free_qubits:
+            return None
         selected_qubit = free_qubits.pop(0)
         partial_layout.add(reverse_bit_map[im_index], selected_qubit)
     return partial_layout

qiskit/transpiler/passes/routing/basic_swap.py

@@ -72,7 +72,9 @@ def run(self, dag):
 
         if len(dag.qubits) > len(self.coupling_map.physical_qubits):
             raise TranspilerError("The layout does not match the amount of qubits in the DAG")
-        disjoint_utils.check_layout_isolated_to_component(dag, self.coupling_map)
+        disjoint_utils.check_layout_isolated_to_component(
+            dag, self.coupling_map, target=self.target
+        )
 
         canonical_register = dag.qregs["q"]
         trivial_layout = Layout.generate_trivial_layout(canonical_register)

qiskit/transpiler/passes/routing/bip_mapping.py

@@ -167,7 +167,9 @@ def run(self, dag):
                 "BIPMapping requires the number of virtual and physical qubits to be the same. "
                 "Supply 'qubit_subset' to specify physical qubits to use."
             )
-        disjoint_utils.check_layout_isolated_to_component(dag, self.coupling_map)
+        disjoint_utils.check_layout_isolated_to_component(
+            dag, self.coupling_map, target=self.target
+        )
 
         original_dag = dag
 

qiskit/transpiler/passes/routing/lookahead_swap.py

@@ -130,7 +130,9 @@ def run(self, dag):
                 f"The number of DAG qubits ({len(dag.qubits)}) is greater than the number of "
                 f"available device qubits ({number_of_available_qubits})."
             )
-        disjoint_utils.check_layout_isolated_to_component(dag, self.coupling_map)
+        disjoint_utils.check_layout_isolated_to_component(
+            dag, self.coupling_map, target=self.target
+        )
 
         register = dag.qregs["q"]
         current_state = _SystemState(

qiskit/transpiler/passes/routing/sabre_swap.py

@@ -213,7 +213,9 @@ def run(self, dag):
             heuristic = Heuristic.Decay
         else:
             raise TranspilerError("Heuristic %s not recognized." % self.heuristic)
-        disjoint_utils.check_layout_isolated_to_component(dag, self.coupling_map)
+        disjoint_utils.check_layout_isolated_to_component(
+            dag, self.coupling_map, target=self.target
+        )
 
         self.dist_matrix = self.coupling_map.distance_matrix
 

qiskit/transpiler/passes/routing/stochastic_swap.py

@@ -105,7 +105,9 @@ def run(self, dag):
         if len(dag.qubits) > len(self.coupling_map.physical_qubits):
             raise TranspilerError("The layout does not match the amount of qubits in the DAG")
 
-        disjoint_utils.check_layout_isolated_to_component(dag, self.coupling_map)
+        disjoint_utils.check_layout_isolated_to_component(
+            dag, self.coupling_map, target=self.target
+        )
 
         self.rng = np.random.default_rng(self.seed)
 

test/python/compiler/test_transpiler.py

@@ -45,6 +45,7 @@
     CZGate,
     XGate,
     SXGate,
+    HGate,
 )
 from qiskit.circuit import IfElseOp, WhileLoopOp, ForLoopOp, ControlFlowOp
 from qiskit.circuit.measure import Measure
@@ -2731,3 +2732,58 @@ def test_six_component_circuit_dense_layout(self, routing_method):
             if op_name == "barrier":
                 continue
             self.assertIn(qubits, self.backend.target[op_name])
+
+    @data(0, 1, 2, 3)
+    def test_transpile_target_with_qubits_without_ops(self, opt_level):
+        """Test qubits without operations aren't ever used."""
+        target = Target(num_qubits=5)
+        target.add_instruction(XGate(), {(i,): InstructionProperties(error=0.5) for i in range(3)})
+        target.add_instruction(HGate(), {(i,): InstructionProperties(error=0.5) for i in range(3)})
+        target.add_instruction(
+            CXGate(), {edge: InstructionProperties(error=0.5) for edge in [(0, 1), (1, 2), (2, 0)]}
+        )
+        qc = QuantumCircuit(3)
+        qc.x(0)
+        qc.cx(0, 1)
+        qc.cx(0, 2)
+        tqc = transpile(qc, target=target, optimization_level=opt_level)
+        invalid_qubits = {3, 4, 5}
+        for inst in tqc.data:
+            for bit in inst.qubits:
+                self.assertNotIn(tqc.find_bit(bit).index, invalid_qubits)
+
+    @data(0, 1, 2, 3)
+    def test_transpile_target_with_qubits_without_ops_circuit_too_large(self, opt_level):
+        """Test qubits without operations aren't ever used and error if circuit needs them."""
+        target = Target(num_qubits=5)
+        target.add_instruction(XGate(), {(i,): InstructionProperties(error=0.5) for i in range(3)})
+        target.add_instruction(HGate(), {(i,): InstructionProperties(error=0.5) for i in range(3)})
+        target.add_instruction(
+            CXGate(), {edge: InstructionProperties(error=0.5) for edge in [(0, 1), (1, 2), (2, 0)]}
+        )
+        qc = QuantumCircuit(4)
+        qc.x(0)
+        qc.cx(0, 1)
+        qc.cx(0, 2)
+        qc.cx(0, 3)
+        with self.assertRaises(TranspilerError):
+            transpile(qc, target=target, optimization_level=opt_level)
+
+    @data(1, 2, 3)
+    def test_transpile_target_with_qubits_without_ops_circuit_too_large_disconnected(
+        self, opt_level
+    ):
+        """Test qubits without operations aren't ever used if a disconnected circuit needs them."""
+        target = Target(num_qubits=5)
+        target.add_instruction(XGate(), {(i,): InstructionProperties(error=0.5) for i in range(3)})
+        target.add_instruction(HGate(), {(i,): InstructionProperties(error=0.5) for i in range(3)})
+        target.add_instruction(
+            CXGate(), {edge: InstructionProperties(error=0.5) for edge in [(0, 1), (1, 2), (2, 0)]}
+        )
+        qc = QuantumCircuit(5)
+        qc.x(0)
+        qc.x(1)
+        qc.x(3)
+        qc.x(4)
+        with self.assertRaises(TranspilerError):
+            transpile(qc, target=target, optimization_level=opt_level)