Remove deepcopy usage from dag/circuit converters

In Qiskit#1816 as part of the larger restructure the dag_to_circuit() and
circuit_to_dag() converter were modified to use a copy.deepcopy()
instead of native Instruction/Gate constructors. This caused a large
performance regressions (around 3x in the worst case) in certain
transpile calls. To avoid this performance penalty this commit
reintroduces a private function to call the class constructor for each
input gate to create a new instance of the object to append to the
output. This removes the deepcopy usage and should fix the performance
regression.

Fixes Qiskit#2131

qiskit/converters/circuit_to_dag.py

@@ -7,8 +7,8 @@
 
 """Helper function for converting a circuit to a dag"""
 
-import copy
-
+from qiskit.circuit import Gate
+from qiskit.circuit import Instruction
 from qiskit.dagcircuit.dagcircuit import DAGCircuit
 
 
@@ -35,7 +35,33 @@ def circuit_to_dag(circuit):
         else:
             control = (instruction.control[0], instruction.control[1])
 
-        instruction = copy.deepcopy(instruction)
-        dagcircuit.apply_operation_back(instruction, qargs, cargs, control)
+        def duplicate_instruction(inst):
+            """Create a fresh instruction from an input instruction."""
+            if issubclass(inst.__class__,
+                          Instruction) and inst.__class__ not in [
+                                Instruction, Gate]:
+                if inst.name == 'barrier':
+                    new_inst = inst.__class__(inst.num_qubits)
+                elif inst.name == 'initialize':
+                    params = getattr(inst, 'params', [])
+                    new_inst = inst.__class__(params)
+                elif inst.name == 'snapshot':
+                    label = inst.params[0]
+                    snap_type = inst.params[1]
+                    new_inst = inst.__class__(inst.num_qubits, inst.num_clbits,
+                                              label, snap_type)
+                else:
+                    params = getattr(inst, 'params', [])
+                    new_inst = inst.__class__(*params)
+            else:
+                new_inst = Instruction(name=inst.name,
+                                       num_qubits=inst.num_qubits,
+                                       num_clbits=inst.num_clbits,
+                                       params=inst.params)
+                new_inst.definition = inst.definition
+            return new_inst
+
+        dagcircuit.apply_operation_back(duplicate_instruction(instruction),
+                                        qargs, cargs, control)
 
     return dagcircuit

qiskit/converters/dag_to_circuit.py

@@ -6,11 +6,12 @@
 # the LICENSE.txt file in the root directory of this source tree.
 
 """Helper function for converting a dag to a circuit"""
-import copy
 import collections
 
-from qiskit.circuit import QuantumCircuit
 from qiskit.circuit import ClassicalRegister
+from qiskit.circuit import Instruction
+from qiskit.circuit import Gate
+from qiskit.circuit import QuantumCircuit
 from qiskit.circuit import QuantumRegister
 
 
@@ -51,7 +52,34 @@ def dag_to_circuit(dag):
             else:
                 control = (node.condition[0], node.condition[1])
 
-            inst = copy.deepcopy(node.op)
+            def duplicate_instruction(inst):
+                """Create a fresh instruction from an input instruction."""
+                if issubclass(inst.__class__,
+                              Instruction) and inst.__class__ not in [
+                                      Instruction, Gate]:
+                    if inst.name == 'barrier':
+                        new_inst = inst.__class__(inst.num_qubits)
+                    elif inst.name == 'initialize':
+                        params = getattr(inst, 'params', [])
+                        new_inst = inst.__class__(params)
+                    elif inst.name == 'snapshot':
+                        label = inst.params[0]
+                        snap_type = inst.params[1]
+                        new_inst = inst.__class__(inst.num_qubits,
+                                                  inst.num_clbits,
+                                                  label, snap_type)
+                    else:
+                        params = getattr(inst, 'params', [])
+                        new_inst = inst.__class__(*params)
+                else:
+                    new_inst = Instruction(name=inst.name,
+                                           num_qubits=inst.num_qubits,
+                                           num_clbits=inst.num_clbits,
+                                           params=inst.params)
+                    new_inst.definition = inst.definition
+                return new_inst
+
+            inst = duplicate_instruction(node.op)
             inst.control = control
             circuit.append(inst, qubits, clbits)