More conservative caching in the CommutationChecker (Qiskit#13600)

* conservative commutation check

* tests and reno

* reno in the right location

* more tests for custom gates

Author: Cryoris

crates/accelerate/src/commutation_checker.rs

@@ -28,24 +28,29 @@ use qiskit_circuit::circuit_instruction::{ExtraInstructionAttributes, OperationF
 use qiskit_circuit::dag_node::DAGOpNode;
 use qiskit_circuit::imports::QI_OPERATOR;
 use qiskit_circuit::operations::OperationRef::{Gate as PyGateType, Operation as PyOperationType};
-use qiskit_circuit::operations::{Operation, OperationRef, Param, StandardGate};
+use qiskit_circuit::operations::{
+    get_standard_gate_names, Operation, OperationRef, Param, StandardGate,
+};
 use qiskit_circuit::{BitType, Clbit, Qubit};
 
 use crate::unitary_compose;
 use crate::QiskitError;
 
+const TWOPI: f64 = 2.0 * std::f64::consts::PI;
+
+// These gates do not commute with other gates, we do not check them.
 static SKIPPED_NAMES: [&str; 4] = ["measure", "reset", "delay", "initialize"];
-static NO_CACHE_NAMES: [&str; 2] = ["annotated", "linear_function"];
+
+// We keep a hash-set of operations eligible for commutation checking. This is because checking
+// eligibility is not for free.
 static SUPPORTED_OP: Lazy<HashSet<&str>> = Lazy::new(|| {
     HashSet::from([
         "rxx", "ryy", "rzz", "rzx", "h", "x", "y", "z", "sx", "sxdg", "t", "tdg", "s", "sdg", "cx",
         "cy", "cz", "swap", "iswap", "ecr", "ccx", "cswap",
     ])
 });
 
-const TWOPI: f64 = 2.0 * std::f64::consts::PI;
-
-// map rotation gates to their generators, or to ``None`` if we cannot currently efficiently
+// Map rotation gates to their generators, or to ``None`` if we cannot currently efficiently
 // represent the generator in Rust and store the commutation relation in the commutation dictionary
 static SUPPORTED_ROTATIONS: Lazy<HashMap<&str, Option<OperationRef>>> = Lazy::new(|| {
     HashMap::from([
@@ -322,15 +327,17 @@ impl CommutationChecker {
             (qargs1, qargs2)
         };
 
-        let skip_cache: bool = NO_CACHE_NAMES.contains(&first_op.name()) ||
-            NO_CACHE_NAMES.contains(&second_op.name()) ||
-            // Skip params that do not evaluate to floats for caching and commutation library
-            first_params.iter().any(|p| !matches!(p, Param::Float(_))) ||
-            second_params.iter().any(|p| !matches!(p, Param::Float(_)))
-            && !SUPPORTED_OP.contains(op1.name())
-            && !SUPPORTED_OP.contains(op2.name());
-
-        if skip_cache {
+        // For our cache to work correctly, we require the gate's definition to only depend on the
+        // ``params`` attribute. This cannot be guaranteed for custom gates, so we only check
+        // the cache for our standard gates, which we know are defined by the ``params`` AND
+        // that the ``params`` are float-only at this point.
+        let whitelist = get_standard_gate_names();
+        let check_cache = whitelist.contains(&first_op.name())
+            && whitelist.contains(&second_op.name())
+            && first_params.iter().all(|p| matches!(p, Param::Float(_)))
+            && second_params.iter().all(|p| matches!(p, Param::Float(_)));
+
+        if !check_cache {
             return self.commute_matmul(
                 py,
                 first_op,
@@ -630,21 +637,24 @@ fn map_rotation<'a>(
 ) -> (&'a OperationRef<'a>, &'a [Param], bool) {
     let name = op.name();
     if let Some(generator) = SUPPORTED_ROTATIONS.get(name) {
-        // if the rotation angle is below the tolerance, the gate is assumed to
+        // If the rotation angle is below the tolerance, the gate is assumed to
         // commute with everything, and we simply return the operation with the flag that
-        // it commutes trivially
+        // it commutes trivially.
         if let Param::Float(angle) = params[0] {
             if (angle % TWOPI).abs() < tol {
                 return (op, params, true);
             };
         };
 
-        // otherwise, we check if a generator is given -- if not, we'll just return the operation
-        // itself (e.g. RXX does not have a generator and is just stored in the commutations
-        // dictionary)
+        // Otherwise we need to cover two cases -- either a generator is given, in which case
+        // we return it, or we don't have a generator yet, but we know we have the operation
+        // stored in the commutation library. For example, RXX does not have a generator in Rust
+        // yet (PauliGate is not in Rust currently), but it is stored in the library, so we
+        // can strip the parameters and just return the gate.
         if let Some(gate) = generator {
             return (gate, &[], false);
         };
+        return (op, &[], false);
     }
     (op, params, false)
 }

crates/circuit/src/operations.rs

@@ -431,6 +431,11 @@ static STANDARD_GATE_NAME: [&str; STANDARD_GATE_SIZE] = [
     "rcccx",        // 51 ("rc3x")
 ];
 
+/// Get a slice of all standard gate names.
+pub fn get_standard_gate_names() -> &'static [&'static str] {
+    &STANDARD_GATE_NAME
+}
+
 impl StandardGate {
     pub fn create_py_op(
         &self,

releasenotes/notes/conservative-commutation-checking-b728e7b6e1645615.yaml

@@ -0,0 +1,18 @@
+---
+fixes:
+  - |
+    Commutation relations of :class:`~.circuit.Instruction`\ s with float-only ``params``
+    were eagerly cached by the :class:`.CommutationChecker`, using the ``params`` as key to
+    query the relation. This could lead to faulty results, if the instruction's definition
+    depended on additional information that just the :attr:`~.circuit.Instruction.params`
+    attribute, such as e.g. the case for :class:`.PauliEvolutionGate`.
+    This behavior is now fixed, and the commutation checker only conservatively caches
+    commutations for Qiskit-native standard gates. This can incur a performance cost if you were
+    relying on your custom gates being cached, however, we cannot guarantee safe caching for
+    custom gates, as they might rely on information beyond :attr:`~.circuit.Instruction.params`.
+  - |
+    Fixed a bug in the :class:`.CommmutationChecker`, where checking commutation of instruction
+    with non-numeric values in the :attr:`~.circuit.Instruction.params` attribute (such as the
+    :class:`.PauliGate`) could raise an error.
+    Fixed `#13570 <https://github.com/Qiskit/qiskit/issues/13570>`__.
+

test/python/circuit/test_commutation_checker.py

@@ -27,6 +27,7 @@
     Parameter,
     QuantumRegister,
     Qubit,
+    QuantumCircuit,
 )
 from qiskit.circuit.commutation_library import SessionCommutationChecker as scc
 from qiskit.circuit.library import (
@@ -37,9 +38,11 @@
     CRYGate,
     CRZGate,
     CXGate,
+    CUGate,
     LinearFunction,
     MCXGate,
     Measure,
+    PauliGate,
     PhaseGate,
     Reset,
     RXGate,
@@ -82,6 +85,22 @@ def to_matrix(self):
         return np.array([[1, 0, 0, 0], [0, 0, 0, 1], [0, 0, 1, 0], [0, 1, 0, 0]], dtype=complex)
 
 
+class MyEvilRXGate(Gate):
+    """A RX gate designed to annoy the caching mechanism (but a realistic gate nevertheless)."""
+
+    def __init__(self, evil_input_not_in_param: float):
+        """
+        Args:
+            evil_input_not_in_param: The RX rotation angle.
+        """
+        self.value = evil_input_not_in_param
+        super().__init__("<evil laugh here>", 1, [])
+
+    def _define(self):
+        self.definition = QuantumCircuit(1)
+        self.definition.rx(self.value, 0)
+
+
 @ddt
 class TestCommutationChecker(QiskitTestCase):
     """Test CommutationChecker class."""
@@ -137,7 +156,7 @@ def test_standard_gates_commutations(self):
     def test_caching_positive_results(self):
         """Check that hashing positive results in commutativity checker works as expected."""
         scc.clear_cached_commutations()
-        self.assertTrue(scc.commute(ZGate(), [0], [], NewGateCX(), [0, 1], []))
+        self.assertTrue(scc.commute(ZGate(), [0], [], CUGate(1, 2, 3, 0), [0, 1], []))
         self.assertGreater(scc.num_cached_entries(), 0)
 
     def test_caching_lookup_with_non_overlapping_qubits(self):
@@ -150,27 +169,29 @@ def test_caching_lookup_with_non_overlapping_qubits(self):
     def test_caching_store_and_lookup_with_non_overlapping_qubits(self):
         """Check that commutations storing and lookup with non-overlapping qubits works as expected."""
         scc_lenm = scc.num_cached_entries()
-        self.assertTrue(scc.commute(NewGateCX(), [0, 2], [], CXGate(), [0, 1], []))
-        self.assertFalse(scc.commute(NewGateCX(), [0, 1], [], CXGate(), [1, 2], []))
-        self.assertTrue(scc.commute(NewGateCX(), [1, 4], [], CXGate(), [1, 6], []))
-        self.assertFalse(scc.commute(NewGateCX(), [5, 3], [], CXGate(), [3, 1], []))
+        cx_like = CUGate(np.pi, 0, np.pi, 0)
+        self.assertTrue(scc.commute(cx_like, [0, 2], [], CXGate(), [0, 1], []))
+        self.assertFalse(scc.commute(cx_like, [0, 1], [], CXGate(), [1, 2], []))
+        self.assertTrue(scc.commute(cx_like, [1, 4], [], CXGate(), [1, 6], []))
+        self.assertFalse(scc.commute(cx_like, [5, 3], [], CXGate(), [3, 1], []))
         self.assertEqual(scc.num_cached_entries(), scc_lenm + 2)
 
     def test_caching_negative_results(self):
         """Check that hashing negative results in commutativity checker works as expected."""
         scc.clear_cached_commutations()
-        self.assertFalse(scc.commute(XGate(), [0], [], NewGateCX(), [0, 1], []))
+        self.assertFalse(scc.commute(XGate(), [0], [], CUGate(1, 2, 3, 0), [0, 1], []))
         self.assertGreater(scc.num_cached_entries(), 0)
 
     def test_caching_different_qubit_sets(self):
         """Check that hashing same commutativity results over different qubit sets works as expected."""
         scc.clear_cached_commutations()
         # All the following should be cached in the same way
         # though each relation gets cached twice: (A, B) and (B, A)
-        scc.commute(XGate(), [0], [], NewGateCX(), [0, 1], [])
-        scc.commute(XGate(), [10], [], NewGateCX(), [10, 20], [])
-        scc.commute(XGate(), [10], [], NewGateCX(), [10, 5], [])
-        scc.commute(XGate(), [5], [], NewGateCX(), [5, 7], [])
+        cx_like = CUGate(np.pi, 0, np.pi, 0)
+        scc.commute(XGate(), [0], [], cx_like, [0, 1], [])
+        scc.commute(XGate(), [10], [], cx_like, [10, 20], [])
+        scc.commute(XGate(), [10], [], cx_like, [10, 5], [])
+        scc.commute(XGate(), [5], [], cx_like, [5, 7], [])
         self.assertEqual(scc.num_cached_entries(), 1)
 
     def test_zero_rotations(self):
@@ -377,12 +398,14 @@ def test_serialization(self):
         """Test that the commutation checker is correctly serialized"""
         import pickle
 
+        cx_like = CUGate(np.pi, 0, np.pi, 0)
+
         scc.clear_cached_commutations()
-        self.assertTrue(scc.commute(ZGate(), [0], [], NewGateCX(), [0, 1], []))
+        self.assertTrue(scc.commute(ZGate(), [0], [], cx_like, [0, 1], []))
         cc2 = pickle.loads(pickle.dumps(scc))
         self.assertEqual(cc2.num_cached_entries(), 1)
         dop1 = DAGOpNode(ZGate(), qargs=[0], cargs=[])
-        dop2 = DAGOpNode(NewGateCX(), qargs=[0, 1], cargs=[])
+        dop2 = DAGOpNode(cx_like, qargs=[0, 1], cargs=[])
         cc2.commute_nodes(dop1, dop2)
         dop1 = DAGOpNode(ZGate(), qargs=[0], cargs=[])
         dop2 = DAGOpNode(CXGate(), qargs=[0, 1], cargs=[])
@@ -430,6 +453,36 @@ def test_rotation_mod_2pi(self, gate_cls):
                     scc.commute(generic_gate, [0], [], gate, list(range(gate.num_qubits)), [])
                 )
 
+    def test_custom_gate(self):
+        """Test a custom gate."""
+        my_cx = NewGateCX()
+
+        self.assertTrue(scc.commute(my_cx, [0, 1], [], XGate(), [1], []))
+        self.assertFalse(scc.commute(my_cx, [0, 1], [], XGate(), [0], []))
+        self.assertTrue(scc.commute(my_cx, [0, 1], [], ZGate(), [0], []))
+
+        self.assertFalse(scc.commute(my_cx, [0, 1], [], my_cx, [1, 0], []))
+        self.assertTrue(scc.commute(my_cx, [0, 1], [], my_cx, [0, 1], []))
+
+    def test_custom_gate_caching(self):
+        """Test a custom gate is correctly handled on consecutive runs."""
+
+        all_commuter = MyEvilRXGate(0)  # this will commute with anything
+        some_rx = MyEvilRXGate(1.6192)  # this should not commute with H
+
+        # the order here is important: we're testing whether the gate that commutes with
+        # everything is used after the first commutation check, regardless of the internal
+        # gate parameters
+        self.assertTrue(scc.commute(all_commuter, [0], [], HGate(), [0], []))
+        self.assertFalse(scc.commute(some_rx, [0], [], HGate(), [0], []))
+
+    def test_nonfloat_param(self):
+        """Test commutation-checking on a gate that has non-float ``params``."""
+        pauli_gate = PauliGate("XX")
+        rx_gate_theta = RXGate(Parameter("Theta"))
+        self.assertTrue(scc.commute(pauli_gate, [0, 1], [], rx_gate_theta, [0], []))
+        self.assertTrue(scc.commute(rx_gate_theta, [0], [], pauli_gate, [0, 1], []))
+
 
 if __name__ == "__main__":
     unittest.main()