Add VF2PostLayout pass

qiskit/transpiler/passes/__init__.py

@@ -140,6 +140,14 @@
    LinearFunctionsSynthesis
    LinearFunctionsToPermutations
 
+Post Layout (Post transpile qubit selection)
+============================================
+
+.. autosummary::
+   :toctree: ../stubs/
+
+   VF2PostLayout
+
 Additional Passes
 =================
 
@@ -169,6 +177,7 @@
 from .layout import SabreLayout
 from .layout import CSPLayout
 from .layout import VF2Layout
+from .layout import VF2PostLayout
 from .layout import ApplyLayout
 from .layout import Layout2qDistance
 from .layout import EnlargeWithAncilla

qiskit/transpiler/passes/layout/__init__.py

@@ -19,6 +19,7 @@
 from .sabre_layout import SabreLayout
 from .csp_layout import CSPLayout
 from .vf2_layout import VF2Layout
+from .vf2_post_layout import VF2PostLayout
 from .apply_layout import ApplyLayout
 from .layout_2q_distance import Layout2qDistance
 from .enlarge_with_ancilla import EnlargeWithAncilla

qiskit/transpiler/passes/layout/apply_layout.py

@@ -16,6 +16,7 @@
 from qiskit.dagcircuit import DAGCircuit
 from qiskit.transpiler.basepasses import TransformationPass
 from qiskit.transpiler.exceptions import TranspilerError
+from qiskit.transpiler.layout import Layout
 
 
 class ApplyLayout(TransformationPass):
@@ -25,6 +26,13 @@ class ApplyLayout(TransformationPass):
     by applying the Layout given in `property_set`.
     Requires either of passes to set/select Layout, e.g. `SetLayout`, `TrivialLayout`.
     Assumes the Layout has full physical qubits.
+
+    If a post layout pass is run and sets the ``post_layout`` property set field with
+    a new layout to use after ``ApplyLayout`` has already run once this pass will
+    compact the layouts so that we apply
+    ``original_virtual`` -> ``existing_layout`` -> ``new_layout`` -> ``new_physical``
+    so that the output circuit and layout combination become:
+    ``original_virtual`` -> ``new_physical``
     """
 
     def run(self, dag):
@@ -47,8 +55,7 @@ def run(self, dag):
         if len(layout) != (1 + max(layout.get_physical_bits())):
             raise TranspilerError("The 'layout' must be full (with ancilla).")
 
-        for qreg in dag.qregs.values():
-            self.property_set["layout"].add_register(qreg)
+        post_layout = self.property_set["post_layout"]
 
         q = QuantumRegister(len(layout), "q")
 
@@ -58,10 +65,33 @@ def run(self, dag):
         new_dag.add_clbits(dag.clbits)
         for creg in dag.cregs.values():
             new_dag.add_creg(creg)
-        virtual_phsyical_map = layout.get_virtual_bits()
-        for node in dag.topological_op_nodes():
-            qargs = [q[virtual_phsyical_map[qarg]] for qarg in node.qargs]
-            new_dag.apply_operation_back(node.op, qargs, node.cargs)
+        if post_layout is None:
+            for qreg in dag.qregs.values():
+                self.property_set["layout"].add_register(qreg)
+            virtual_phsyical_map = layout.get_virtual_bits()
+            for node in dag.topological_op_nodes():
+                qargs = [q[virtual_phsyical_map[qarg]] for qarg in node.qargs]
+                new_dag.apply_operation_back(node.op, qargs, node.cargs)
+        else:
+            # First build a new layout object going from:
+            # old virtual -> old phsyical -> new virtual -> new physical
+            # to:
+            # old virtual -> new physical
+            full_layout = Layout()
+            old_phys_to_virtual = layout.get_physical_bits()
+            new_virtual_to_physical = post_layout.get_virtual_bits()
+            qubit_index_map = {bit: index for index, bit in enumerate(dag.qubits)}
+            for new_virt, new_phys in new_virtual_to_physical.items():
+                old_phys = qubit_index_map[new_virt]
+                old_virt = old_phys_to_virtual[old_phys]
+                full_layout.add(old_virt, new_phys)
+            for reg in layout.get_registers():
+                full_layout.add_register(reg)
+            # Apply new layout to the circuit
+            for node in dag.topological_op_nodes():
+                qargs = [q[new_virtual_to_physical[qarg]] for qarg in node.qargs]
+                new_dag.apply_operation_back(node.op, qargs, node.cargs)
+            self.property_set["layout"] = full_layout
         new_dag._global_phase = dag._global_phase
 
         return new_dag