Use TrivialLayout in optimization level 1 (#2975)

This reverts #2971

* Use TrivialLayout in optimization level 1

* lint

* typo

CHANGELOG.md

@@ -100,8 +100,7 @@ The format is based on [Keep a Changelog].
     using the option ``with_layout=False`` in the method
     ``QuantumCircuit.draw``. (\#2739)
 -   Q-sphere visualization is enhanced and corrected (\#2932)
--   The default layout selector for transpile optimization level 1 is ``DenseLayout``
+
-    instead of ``TrivialLayout`` (\#2845).
 
 ### Removed
 -   The ability to set the `Timeslot`s for a pulse `Instruction` at initialization.

qiskit/transpiler/preset_passmanagers/level1.py

@@ -27,7 +27,7 @@ from qiskit.transpiler.passes import Decompose
 from qiskit.transpiler.passes import CheckMap
 from qiskit.transpiler.passes import CXDirection
 from qiskit.transpiler.passes import SetLayout
-from qiskit.transpiler.passes import DenseLayout
+from qiskit.transpiler.passes import TrivialLayout
 from qiskit.transpiler.passes import BarrierBeforeFinalMeasurements
 from qiskit.transpiler.passes import StochasticSwap
 from qiskit.transpiler.passes import FullAncillaAllocation
@@ -65,13 +65,13 @@ def level_1_pass_manager(transpile_config):
     initial_layout = transpile_config.initial_layout
     seed_transpiler = transpile_config.seed_transpiler
 
-    # 1. Use dense layout if no layout given
+    # 1. Use trivial layout if no layout given
     _given_layout = SetLayout(initial_layout)
 
     def _choose_layout_condition(property_set):
         return not property_set['layout']
 
-    _choose_layout = DenseLayout(coupling_map)
+    _choose_layout = TrivialLayout(coupling_map)
 
     # 2. Use a better layout on densely connected qubits, if circuit needs swaps
     _layout_check = CheckMap(coupling_map)

test/python/compiler/test_transpiler.py

@@ -422,11 +422,12 @@ class TestTranspile(QiskitTestCase):
         theta = Parameter('theta')
         qc.rz(theta, qr[0])
 
-        transpiled_qc = transpile(qc, backend=FakeMelbourne())
+        transpiled_qc = transpile(qc, backend=FakeMelbourne(),
+                                  initial_layout=Layout.generate_trivial_layout(qr))
 
         qr = QuantumRegister(14, 'q')
         expected_qc = QuantumCircuit(qr)
-        expected_qc.u1(theta, qr[1])
+        expected_qc.u1(theta, qr[0])
 
         self.assertEqual(expected_qc, transpiled_qc)
 
@@ -457,11 +458,12 @@ class TestTranspile(QiskitTestCase):
         square = theta * theta
         qc.rz(square, qr[0])
 
-        transpiled_qc = transpile(qc, backend=FakeMelbourne())
+        transpiled_qc = transpile(qc, backend=FakeMelbourne(),
+                                  initial_layout=Layout.generate_trivial_layout(qr))
 
         qr = QuantumRegister(14, 'q')
         expected_qc = QuantumCircuit(qr)
-        expected_qc.u1(square, qr[1])
+        expected_qc.u1(square, qr[0])
 
         self.assertEqual(expected_qc, transpiled_qc)
 

test/python/transpiler/test_preset_passmanagers.py

@@ -257,23 +257,19 @@ class TestFinalLayouts(QiskitTestCase):
         qc.cx(qr2[0], qr2[1])
 
         ancilla = QuantumRegister(15, 'ancilla')
-
+        trivial_layout = {0: qr1[0], 1: qr1[1], 2: qr1[2], 3: qr2[0], 4: qr2[1],
+                          5: ancilla[0], 6: ancilla[1], 7: ancilla[2], 8: ancilla[3],
+                          9: ancilla[4], 10: ancilla[5], 11: ancilla[6], 12: ancilla[7],
+                          13: ancilla[8], 14: ancilla[9], 15: ancilla[10], 16: ancilla[11],
+                          17: ancilla[12], 18: ancilla[13], 19: ancilla[14]}
         # TrivialLayout
-        expected_layout_level0 = {0: qr1[0], 1: qr1[1], 2: qr1[2], 3: qr2[0], 4: qr2[1],
+        expected_layout_level0 = trivial_layout
-                                  5: ancilla[0], 6: ancilla[1], 7: ancilla[2], 8: ancilla[3],
+        expected_layout_level1 = trivial_layout
-                                  9: ancilla[4], 10: ancilla[5], 11: ancilla[6], 12: ancilla[7],
-                                  13: ancilla[8], 14: ancilla[9], 15: ancilla[10], 16: ancilla[11],
-                                  17: ancilla[12], 18: ancilla[13], 19: ancilla[14]}
-        # DenseLayout
-        expected_layout_level1 = {0: qr2[1], 1: ancilla[0], 2: ancilla[1], 3: ancilla[2],
-                                  4: ancilla[3], 5: qr2[0], 6: qr1[2], 7: ancilla[4], 8: ancilla[5],
-                                  9: ancilla[6], 10: qr1[1], 11: qr1[0], 12: ancilla[7],
-                                  13: ancilla[8], 14: ancilla[9], 15: ancilla[10], 16: ancilla[11],
-                                  17: ancilla[12], 18: ancilla[13], 19: ancilla[14]}
 
         # NoiseAdaptiveLayout (in FakeTokyo, no errors. Therefore, TrivialLayout)
-        expected_layout_level2 = expected_layout_level0
+        # TODO See https://github.com/Qiskit/qiskit-terra/issues/2970
-        expected_layout_level3 = expected_layout_level0
+        expected_layout_level2 = trivial_layout
+        expected_layout_level3 = trivial_layout
         expected_layouts = [expected_layout_level0,
                             expected_layout_level1,
                             expected_layout_level2,