Add check for the size during transpiler optimization

qiskit/transpiler/preset_passmanagers/level1.py

@@ -40,6 +40,7 @@
 from qiskit.transpiler.passes import EnlargeWithAncilla
 from qiskit.transpiler.passes import FixedPoint
 from qiskit.transpiler.passes import Depth
+from qiskit.transpiler.passes import Size
 from qiskit.transpiler.passes import RemoveResetInZeroState
 from qiskit.transpiler.passes import Optimize1qGatesDecomposition
 from qiskit.transpiler.passes import ApplyLayout
@@ -234,10 +235,12 @@ def _direction_condition(property_set):
     _reset = RemoveResetInZeroState()
 
     # 9. Merge 1q rotations and cancel CNOT gates iteratively until no more change in depth
+    # or size of circuit
     _depth_check = [Depth(), FixedPoint("depth")]
+    _size_check = [Size(), FixedPoint("size")]
 
     def _opt_control(property_set):
-        return not property_set["depth_fixed_point"]
+        return (not property_set["depth_fixed_point"]) or (not property_set["size_fixed_point"])
 
     _opt = [Optimize1qGatesDecomposition(basis_gates), CXCancellation()]
 
@@ -291,7 +294,7 @@ def _contains_delay(property_set):
         pm1.append(_direction_check)
         pm1.append(_direction, condition=_direction_condition)
     pm1.append(_reset)
-    pm1.append(_depth_check + _opt + _unroll, do_while=_opt_control)
+    pm1.append(_depth_check + _size_check + _opt + _unroll, do_while=_opt_control)
     if inst_map and inst_map.has_custom_gate():
         pm1.append(PulseGates(inst_map=inst_map))
     if scheduling_method:

qiskit/transpiler/preset_passmanagers/level2.py

@@ -41,6 +41,7 @@
 from qiskit.transpiler.passes import EnlargeWithAncilla
 from qiskit.transpiler.passes import FixedPoint
 from qiskit.transpiler.passes import Depth
+from qiskit.transpiler.passes import Size
 from qiskit.transpiler.passes import RemoveResetInZeroState
 from qiskit.transpiler.passes import Optimize1qGatesDecomposition
 from qiskit.transpiler.passes import CommutativeCancellation
@@ -268,10 +269,12 @@ def _direction_condition(property_set):
     _reset = RemoveResetInZeroState()
 
     # 8. 1q rotation merge and commutative cancellation iteratively until no more change in depth
+    # and size
     _depth_check = [Depth(), FixedPoint("depth")]
+    _size_check = [Size(), FixedPoint("size")]
 
     def _opt_control(property_set):
-        return not property_set["depth_fixed_point"]
+        return (not property_set["depth_fixed_point"]) or (not property_set["size_fixed_point"])
 
     _opt = [
         Optimize1qGatesDecomposition(basis_gates),
@@ -329,7 +332,9 @@ def _contains_delay(property_set):
         pm2.append(_direction_check)
         pm2.append(_direction, condition=_direction_condition)
     pm2.append(_reset)
-    pm2.append(_depth_check + _opt + _unroll, do_while=_opt_control)
+
+    pm2.append(_depth_check + _size_check + _opt + _unroll, do_while=_opt_control)
+
     if inst_map and inst_map.has_custom_gate():
         pm2.append(PulseGates(inst_map=inst_map))
     if scheduling_method:

qiskit/transpiler/preset_passmanagers/level3.py

@@ -42,6 +42,7 @@
 from qiskit.transpiler.passes import EnlargeWithAncilla
 from qiskit.transpiler.passes import FixedPoint
 from qiskit.transpiler.passes import Depth
+from qiskit.transpiler.passes import Size
 from qiskit.transpiler.passes import RemoveResetInZeroState
 from qiskit.transpiler.passes import Optimize1qGatesDecomposition
 from qiskit.transpiler.passes import CommutativeCancellation
@@ -266,9 +267,10 @@ def _direction_condition(property_set):
     # 8. Optimize iteratively until no more change in depth. Removes useless gates
     # after reset and before measure, commutes gates and optimizes contiguous blocks.
     _depth_check = [Depth(), FixedPoint("depth")]
+    _size_check = [Size(), FixedPoint("size")]
 
     def _opt_control(property_set):
-        return not property_set["depth_fixed_point"]
+        return (not property_set["depth_fixed_point"]) or (not property_set["size_fixed_point"])
 
     _reset = [RemoveResetInZeroState()]
 
@@ -346,12 +348,14 @@ def _contains_delay(property_set):
         # inserted by UnitarySynthesis which is direction aware but only via
         # the coupling map which with a target doesn't give a full picture
         if target is not None:
-            pm3.append(_depth_check + _opt + _unroll + _direction, do_while=_opt_control)
+            pm3.append(
+                _depth_check + _size_check + _opt + _unroll + _direction, do_while=_opt_control
+            )
         else:
-            pm3.append(_depth_check + _opt + _unroll, do_while=_opt_control)
+            pm3.append(_depth_check + _size_check + _opt + _unroll, do_while=_opt_control)
     else:
         pm3.append(_reset)
-        pm3.append(_depth_check + _opt + _unroll, do_while=_opt_control)
+        pm3.append(_depth_check + _size_check + _opt + _unroll, do_while=_opt_control)
     if inst_map and inst_map.has_custom_gate():
         pm3.append(PulseGates(inst_map=inst_map))
     if scheduling_method:

test/python/transpiler/test_preset_passmanagers.py

@@ -919,3 +919,48 @@ def test_input_dag_copy(self):
         qc.cx(1, 0)
         circ = transpile(qc, basis_gates=["u3", "cz"])
         self.assertIsInstance(circ, QuantumCircuit)
+
+
+@ddt
+class TestOptimizationOnSize(QiskitTestCase):
+    """Test the optimization levels for optimization based on
+    both size and depth of the circuit.
+    See https://github.com/Qiskit/qiskit-terra/pull/7542
+    """
+
+    @data(2, 3)
+    def test_size_optimization(self, level):
+        """Test the levels for optimization based on size of circuit"""
+        qc = QuantumCircuit(8)
+        qc.cx(1, 2)
+        qc.cx(2, 3)
+        qc.cx(5, 4)
+        qc.cx(6, 5)
+        qc.cx(4, 5)
+        qc.cx(3, 4)
+        qc.cx(5, 6)
+        qc.cx(5, 4)
+        qc.cx(3, 4)
+        qc.cx(2, 3)
+        qc.cx(1, 2)
+        qc.cx(6, 7)
+        qc.cx(6, 5)
+        qc.cx(5, 4)
+        qc.cx(7, 6)
+        qc.cx(6, 7)
+
+        circ = transpile(qc, optimization_level=level).decompose()
+
+        circ_data = circ.data
+        free_qubits = set([0, 1, 2, 3])
+
+        # ensure no gates are using qubits - [0,1,2,3]
+        for gate in circ_data:
+            qubits = gate[1]
+            indices = {circ.find_bit(qubit).index for qubit in qubits}
+            common = indices.intersection(free_qubits)
+            for common_qubit in common:
+                self.assertTrue(common_qubit not in free_qubits)
+
+        self.assertLess(circ.size(), qc.size())
+        self.assertLessEqual(circ.depth(), qc.depth())