Honor initial_layout when coupling map is None

qiskit/transpiler/layout.py

@@ -129,6 +129,11 @@ def __delitem__(self, key):
     def __len__(self):
         return len(self._p2v)
 
+    def __eq__(self, other):
+        if isinstance(other, Layout):
+            return self._p2v == other._p2v and self._v2p == other._v2p
+        return False
+
     def copy(self):
         """Returns a copy of a Layout instance."""
         layout_copy = type(self)()

qiskit/transpiler/passes/layout/full_ancilla_allocation.py

@@ -65,10 +65,17 @@ def run(self, dag):
         if layout is None:
             raise TranspilerError('FullAncillaAllocation pass requires property_set["layout"].')
 
-        layout_physical_qubits = layout.get_physical_bits().keys()
-        coupling_physical_qubits = self.coupling_map.physical_qubits
-        idle_physical_qubits = [q for q in coupling_physical_qubits
-                                if q not in layout_physical_qubits]
+        if layout:
+            layout_physical_qubits = list(range(max(layout.get_physical_bits()) + 1))
+        else:
+            layout_physical_qubits = []
+
+        idle_physical_qubits = [q for q in layout_physical_qubits
+                                if q not in layout.get_physical_bits()]
+
+        if self.coupling_map:
+            idle_physical_qubits = [q for q in self.coupling_map.physical_qubits
+                                    if q not in layout.get_physical_bits()]
 
         if idle_physical_qubits:
             if self.ancilla_name in dag.qregs:

qiskit/transpiler/passes/utils/check_map.py

@@ -43,6 +43,9 @@ def run(self, dag):
         """
         self.property_set['is_swap_mapped'] = True
 
+        if self.coupling_map is None:
+            return
+
         for gate in dag.two_qubit_ops():
             physical_q0 = gate.qargs[0].index
             physical_q1 = gate.qargs[1].index

qiskit/transpiler/preset_passmanagers/level0.py

@@ -160,7 +160,7 @@ def _direction_condition(property_set):
 
     # Build pass manager
     pm0 = PassManager()
-    if coupling_map:
+    if coupling_map or initial_layout:
         pm0.append(_given_layout)
         pm0.append(_choose_layout, condition=_choose_layout_condition)
         pm0.append(_embed)

qiskit/transpiler/preset_passmanagers/level1.py

@@ -193,7 +193,7 @@ def _opt_control(property_set):
 
     # Build pass manager
     pm1 = PassManager()
-    if coupling_map:
+    if coupling_map or initial_layout:
         pm1.append(_given_layout)
         pm1.append(_choose_layout_and_score, condition=_choose_layout_condition)
         pm1.append(_improve_layout, condition=_not_perfect_yet)

qiskit/transpiler/preset_passmanagers/level2.py

@@ -188,7 +188,7 @@ def _opt_control(property_set):
 
     # Build pass manager
     pm2 = PassManager()
-    if coupling_map:
+    if coupling_map or initial_layout:
         pm2.append(_given_layout)
         pm2.append(_choose_layout_1, condition=_choose_layout_condition)
         pm2.append(_choose_layout_2, condition=_choose_layout_condition)

qiskit/transpiler/preset_passmanagers/level3.py

@@ -207,7 +207,7 @@ def _opt_control(property_set):
     pm3 = PassManager()
     pm3.append(_unroll3q)
     pm3.append(_reset + _meas)
-    if coupling_map:
+    if coupling_map or initial_layout:
         pm3.append(_given_layout)
         pm3.append(_choose_layout_1, condition=_choose_layout_condition)
         pm3.append(_choose_layout_2, condition=_choose_layout_condition)

releasenotes/notes/initial_layout_cp_none-8753d058743eeb79.yaml

@@ -0,0 +1,48 @@
+---
+fixes:
+  - |
+    When the argument `coupling_map=None` (explicitly, implicitly as default value,  or via `backend`),
+    the transpiling process was not "embedding" the circuit.
+    That is, even when an `initial_layout` was specified, the virtual qubits were not assigned to physical
+    qubits. Now, the function :func:`qiskit.compiler.transpile` honors the `initial_layout` argument by embedding the circuit::
+
+    .. code-block::
+
+        from qiskit import QuantumCircuit, QuantumRegister
+        from qiskit.compiler import transpile
+
+        qr = QuantumRegister(2, name='qr')
+        circ = QuantumCircuit(qr)
+        circ.h(qr[0])
+        circ.cx(qr[0], qr[1])
+
+        transpile(circ, initial_layout=[1, 0]).draw()
+
+
+    .. code-block::
+
+                       ┌───┐
+        qr_1 -> 0 ─────┤ X ├
+                  ┌───┐└─┬─┘
+        qr_0 -> 1 ┤ H ├──■──
+                  └───┘  
+
+    If the `initial_layout` refers to more qubits than in the circuit, the transpiling process will extended the circuit with
+    ancillas.
+
+    .. code-block::
+
+        transpile(circ, initial_layout=[4, 2], coupling_map=None).draw()
+
+    .. code-block::
+
+        ancilla_0 -> 0 ──────────
+
+        ancilla_1 -> 1 ──────────
+                            ┌───┐
+             qr_1 -> 2 ─────┤ X ├
+                            └─┬─┘
+        ancilla_2 -> 3 ───────┼──
+                       ┌───┐  │  
+             qr_0 -> 4 ┤ H ├──■──
+                       └───┘     

test/python/transpiler/test_preset_passmanagers.py

@@ -17,7 +17,7 @@
 from qiskit import QuantumCircuit, ClassicalRegister, QuantumRegister
 from qiskit.circuit import Qubit
 from qiskit.compiler import transpile, assemble
-from qiskit.transpiler import CouplingMap
+from qiskit.transpiler import CouplingMap, Layout
 from qiskit.circuit.library import U2Gate, U3Gate
 from qiskit.test import QiskitTestCase
 from qiskit.test.mock import (FakeTenerife, FakeMelbourne, FakeJohannesburg,
@@ -167,6 +167,42 @@ def test_symmetric_coupling_map(self, level):
         self.assertIn('ApplyLayout', self.passes)
         self.assertNotIn('CheckCXDirection', self.passes)
 
+    @data(0, 1, 2, 3)
+    def test_inital_layout_fully_connected_cm(self, level):
+        """Honor initial_layout when coupling_map=None
+        See: https://github.com/Qiskit/qiskit-terra/issues/5345
+        """
+        qr = QuantumRegister(2, 'q')
+        qc = QuantumCircuit(qr)
+        qc.h(qr[0])
+        qc.cx(qr[0], qr[1])
+
+        transpiled = transpile(qc, initial_layout=[0, 1], optimization_level=level,
+                               callback=self.callback)
+
+        self.assertIn('SetLayout', self.passes)
+        self.assertIn('ApplyLayout', self.passes)
+        self.assertEqual(transpiled._layout, Layout.from_qubit_list([qr[0], qr[1]]))
+
+    @data(0, 1, 2, 3)
+    def test_partial_layout_fully_connected_cm(self, level):
+        """Honor initial_layout (partially defined) when coupling_map=None
+        See: https://github.com/Qiskit/qiskit-terra/issues/5345
+        """
+        qr = QuantumRegister(2, 'q')
+        qc = QuantumCircuit(qr)
+        qc.h(qr[0])
+        qc.cx(qr[0], qr[1])
+
+        transpiled = transpile(qc, initial_layout=[4, 2], optimization_level=level,
+                               callback=self.callback)
+
+        self.assertIn('SetLayout', self.passes)
+        self.assertIn('ApplyLayout', self.passes)
+        ancilla = QuantumRegister(3, 'ancilla')
+        self.assertEqual(transpiled._layout, Layout.from_qubit_list([ancilla[0], ancilla[1], qr[1],
+                                                                     ancilla[2], qr[0]]))
+
 
 @ddt
 class TestInitialLayouts(QiskitTestCase):