Add conversion transpiler pass from c_if to IfElseOp (#8764)

* Add conversion transpiler pass from c_if to IfElseOp

This is intended to be used to ease the transition from the old-style
conditions to the new-style form.  Backends may use it in their custom
pipelines, so they can guarantee they only need to deal with one form.

A further classical optimisation may be to group `IfElseOp` blocks that
share the same condition into a single operation, but this would likely
be a separate pass, if added, or potentially be the domain of a separate
part of the circuit-description/compilation/running pipeline.

* Remove nondeterministic bit ordering

* Add test of no-op behaviour

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/transpiler/passes/__init__.py

@@ -170,6 +170,7 @@
    FixedPoint
    ContainsInstruction
    GatesInBasis
+   ConvertConditionsToIfOps
 """
 
 # layout selection (placement)
@@ -278,3 +279,4 @@
 from .utils import RemoveBarriers
 from .utils import ContainsInstruction
 from .utils import GatesInBasis
+from .utils import ConvertConditionsToIfOps

qiskit/transpiler/passes/utils/__init__.py

@@ -26,6 +26,7 @@
 from .remove_barriers import RemoveBarriers
 from .contains_instruction import ContainsInstruction
 from .gates_basis import GatesInBasis
+from .convert_conditions_to_if_ops import ConvertConditionsToIfOps
 
 # Utility functions
 from . import control_flow

qiskit/transpiler/passes/utils/convert_conditions_to_if_ops.py

@@ -0,0 +1,86 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2022.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Replace conditional instructions with equivalent :class:`.IfElseOp` objects."""
+
+from qiskit.converters import dag_to_circuit, circuit_to_dag
+from qiskit.circuit import (
+    CircuitInstruction,
+    ClassicalRegister,
+    Clbit,
+    ControlFlowOp,
+    IfElseOp,
+    QuantumCircuit,
+)
+from qiskit.dagcircuit import DAGCircuit
+from qiskit.transpiler import TransformationPass
+
+
+class ConvertConditionsToIfOps(TransformationPass):
+    """Convert instructions whose ``condition`` attribute is set to a non-``None`` value into the
+    equivalent single-statement :class:`.IfElseBlock`.
+
+    This is a simple pass aimed at easing the conversion from the old style of using
+    :meth:`.InstructionSet.c_if` into the new style of using more complex conditional logic."""
+
+    def _run_inner(self, dag):
+        """Run the pass on one :class:`.DAGCircuit`, mutating it.  Returns ``True`` if the circuit
+        was modified and ``False`` if not."""
+        modified = False
+        for node in dag.op_nodes():
+            if isinstance(node.op, ControlFlowOp):
+                modified_blocks = False
+                new_dags = []
+                for block in node.op.blocks:
+                    new_dag = circuit_to_dag(block)
+                    modified_blocks |= self._run_inner(new_dag)
+                    new_dags.append(new_dag)
+                if not modified_blocks:
+                    continue
+                dag.substitute_node(
+                    node,
+                    node.op.replace_blocks(dag_to_circuit(block) for block in new_dags),
+                    inplace=True,
+                )
+            elif getattr(node.op, "condition", None) is None:
+                continue
+            else:
+                target, value = node.op.condition
+                clbits = list(node.cargs)
+                condition_clbits = [target] if isinstance(target, Clbit) else list(target)
+                clbits_set = set(clbits)
+                clbits += [bit for bit in condition_clbits if bit not in clbits_set]
+                block_body = QuantumCircuit(list(node.qargs) + clbits)
+                if isinstance(target, ClassicalRegister):
+                    block_body.add_register(target)
+                new_op = node.op.copy()
+                new_op.condition = None
+                block_body._append(CircuitInstruction(new_op, node.qargs, node.cargs))
+                # Despite only being a node-for-node replacement, control-flow ops contain the
+                # condition bits in their cargs, which requires slightly different handling in the
+                # DAGCircuit methods right now.
+                replacement = DAGCircuit()
+                replacement.add_qubits(block_body.qubits)
+                replacement.add_clbits(block_body.clbits)
+                if isinstance(target, ClassicalRegister):
+                    replacement.add_creg(target)
+                replacement.apply_operation_back(
+                    IfElseOp((target, value), block_body), block_body.qubits, block_body.clbits
+                )
+                wire_map = {bit: bit for bit in block_body.qubits + block_body.clbits}
+                dag.substitute_node_with_dag(node, replacement, wire_map, propagate_condition=False)
+            modified = True
+        return modified
+
+    def run(self, dag):
+        self._run_inner(dag)
+        return dag

releasenotes/notes/c_if-to-if_else-converter-2d48046de31814a8.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    A new transpiler pass, :class:`.ConvertConditionsToIfOps` was added, which
+    can be manually run to convert old-style :meth:`.Instruction.c_if`-conditioned
+    instructions into :class:`.IfElseOp` objects.  This is to help ease the transition
+    from the old type to the new type for backends.  For most users, there is no
+    need to add this to your pass managers, and it is not included in any preset
+    pass managers.

test/python/transpiler/test_convert_conditions_to_if_ops.py

@@ -0,0 +1,139 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2022.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+# pylint: disable=missing-class-docstring,missing-module-docstring
+
+from qiskit.circuit import QuantumCircuit, QuantumRegister, ClassicalRegister, Qubit, Clbit
+from qiskit.test import QiskitTestCase
+from qiskit.test._canonical import canonicalize_control_flow
+from qiskit.transpiler import PassManager
+from qiskit.transpiler.passes import ConvertConditionsToIfOps
+
+
+class TestConvertConditionsToIfOps(QiskitTestCase):
+    def test_simple_loose_bits(self):
+        """Test that basic conversions work when operating on loose classical bits."""
+        bits = [Qubit(), Qubit(), Clbit(), Clbit()]
+
+        base = QuantumCircuit(bits)
+        base.h(0)
+        base.x(0).c_if(0, 1)
+        base.z(1).c_if(1, 0)
+        base.measure(0, 0)
+        base.measure(1, 1)
+        base.h(0)
+        base.x(0).c_if(0, 1)
+        base.cx(0, 1).c_if(1, 0)
+
+        expected = QuantumCircuit(bits)
+        expected.h(0)
+        with expected.if_test((expected.clbits[0], True)):
+            expected.x(0)
+        with expected.if_test((expected.clbits[1], False)):
+            expected.z(1)
+        expected.measure(0, 0)
+        expected.measure(1, 1)
+        expected.h(0)
+        with expected.if_test((expected.clbits[0], True)):
+            expected.x(0)
+        with expected.if_test((expected.clbits[1], False)):
+            expected.cx(0, 1)
+        expected = canonicalize_control_flow(expected)
+
+        output = PassManager([ConvertConditionsToIfOps()]).run(base)
+        self.assertEqual(output, expected)
+
+    def test_simple_registers(self):
+        """Test that basic conversions work when operating on conditions over registers."""
+        registers = [QuantumRegister(2), ClassicalRegister(2), ClassicalRegister(1)]
+
+        base = QuantumCircuit(*registers)
+        base.h(0)
+        base.x(0).c_if(base.cregs[0], 1)
+        base.z(1).c_if(base.cregs[1], 0)
+        base.measure(0, 0)
+        base.measure(1, 2)
+        base.h(0)
+        base.x(0).c_if(base.cregs[0], 1)
+        base.cx(0, 1).c_if(base.cregs[1], 0)
+
+        expected = QuantumCircuit(*registers)
+        expected.h(0)
+        with expected.if_test((expected.cregs[0], 1)):
+            expected.x(0)
+        with expected.if_test((expected.cregs[1], 0)):
+            expected.z(1)
+        expected.measure(0, 0)
+        expected.measure(1, 2)
+        expected.h(0)
+        with expected.if_test((expected.cregs[0], 1)):
+            expected.x(0)
+        with expected.if_test((expected.cregs[1], 0)):
+            expected.cx(0, 1)
+        expected = canonicalize_control_flow(expected)
+
+        output = PassManager([ConvertConditionsToIfOps()]).run(base)
+        self.assertEqual(output, expected)
+
+    def test_nested_control_flow(self):
+        """Test that the pass successfully converts instructions nested within control-flow
+        blocks."""
+        bits = [Clbit()]
+        registers = [QuantumRegister(3), ClassicalRegister(2)]
+
+        base = QuantumCircuit(*registers, bits)
+        base.x(0).c_if(bits[0], False)
+        with base.if_test((base.cregs[0], 0)) as else_:
+            base.z(1).c_if(bits[0], False)
+        with else_:
+            base.z(1).c_if(base.cregs[0], 1)
+        with base.for_loop(range(2)):
+            with base.while_loop((base.cregs[0], 1)):
+                base.cx(1, 2).c_if(base.cregs[0], 1)
+        base = canonicalize_control_flow(base)
+
+        expected = QuantumCircuit(*registers, bits)
+        with expected.if_test((bits[0], False)):
+            expected.x(0)
+        with expected.if_test((expected.cregs[0], 0)) as else_:
+            with expected.if_test((bits[0], False)):
+                expected.z(1)
+        with else_:
+            with expected.if_test((expected.cregs[0], 1)):
+                expected.z(1)
+        with expected.for_loop(range(2)):
+            with expected.while_loop((expected.cregs[0], 1)):
+                with expected.if_test((expected.cregs[0], 1)):
+                    expected.cx(1, 2)
+        expected = canonicalize_control_flow(expected)
+
+        output = PassManager([ConvertConditionsToIfOps()]).run(base)
+        self.assertEqual(output, expected)
+
+    def test_no_op(self):
+        """Test that the pass works when recursing into control-flow structures, but there's nothing
+        that actually needs replacing."""
+        bits = [Clbit()]
+        registers = [QuantumRegister(3), ClassicalRegister(2)]
+
+        base = QuantumCircuit(*registers, bits)
+        base.x(0)
+        with base.if_test((base.cregs[0], 0)) as else_:
+            base.z(1)
+        with else_:
+            base.z(2)
+        with base.for_loop(range(2)):
+            with base.while_loop((base.cregs[0], 1)):
+                base.cx(1, 2)
+        base = canonicalize_control_flow(base)
+        output = PassManager([ConvertConditionsToIfOps()]).run(base)
+        self.assertEqual(output, base)