Fix ignored errors/durations in generate_preset_pass_manager if dt is set (#14065)

* Fix dt oversight in generate_preset_pass_manager

* Update warning to mention explicitly the invalidation of custom durations and error rates.

* Do not overwrite original backend values inside transpile, iterate over target to reset internal instruction_durations value so that new dt can be taken into account

* One iteration is enough to reset the value of self._instruction_durations

* Estimate duration instead of hardcoding it

* Apply suggestion from Matt's code review

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Fix example

* Move cache invalidation to target dt setter.

* Add reno

---------

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

Author: ElePT

crates/accelerate/src/target_transpiler/mod.rs

@@ -150,7 +150,6 @@ pub(crate) struct Target {
     pub description: Option<String>,
     #[pyo3(get)]
     pub num_qubits: Option<usize>,
-    #[pyo3(get, set)]
     pub dt: Option<f64>,
     #[pyo3(get, set)]
     pub granularity: u32,
@@ -741,6 +740,17 @@ impl Target {
 
     // Instance attributes
 
+    /// The dt attribute.
+    #[getter(_dt)]
+    fn get_dt(&self) -> Option<f64> {
+        self.dt
+    }
+
+    #[setter(_dt)]
+    fn set_dt(&mut self, dt: Option<f64>) {
+        self.dt = dt
+    }
+
     /// The set of qargs in the target.
     #[getter]
     #[pyo3(name = "qargs")]

qiskit/transpiler/preset_passmanagers/generate_preset_pass_manager.py

@@ -13,7 +13,7 @@
 """
 Preset pass manager generation function
 """
-
+import copy
 import warnings
 
 from qiskit.circuit.controlflow import CONTROL_FLOW_OP_NAMES, get_control_flow_name_mapping
@@ -200,12 +200,14 @@ def generate_preset_pass_manager(
     # If there are no loose constraints => use backend target if available
     _no_loose_constraints = basis_gates is None and coupling_map is None and dt is None
 
+    # If the only loose constraint is dt => use backend target and modify dt
+    _adjust_dt = backend is not None and dt is not None
+
     # Warn about inconsistencies in backend + loose constraints path (dt shouldn't be a problem)
     if backend is not None and (coupling_map is not None or basis_gates is not None):
         warnings.warn(
             "Providing `coupling_map` and/or `basis_gates` along with `backend` is not "
-            "recommended. This may introduce inconsistencies in the transpilation target, "
-            "leading to potential errors.",
+            "recommended, as this will invalidate the backend's gate durations and error rates.",
             category=UserWarning,
             stacklevel=2,
         )
@@ -226,13 +228,17 @@ def generate_preset_pass_manager(
                 raise ValueError(
                     f"Gates with 3 or more qubits ({gate}) in `basis_gates` or `backend` are "
                     "incompatible with a custom `coupling_map`. To include 3-qubit or larger "
-                    " gates in the transpilation basis, use a custom `target` instance instead."
+                    " gates in the transpilation basis, provide a custom `target` instead."
                 )
 
     if target is None:
         if backend is not None and _no_loose_constraints:
             # If a backend is specified without loose constraints, use its target directly.
             target = backend.target
+        elif _adjust_dt:
+            # If a backend is specified with loose dt, use its target and adjust the dt value.
+            target = copy.deepcopy(backend.target)
+            target.dt = dt
         else:
             if basis_gates is not None:
                 # Build target from constraints.
@@ -255,7 +261,7 @@ def generate_preset_pass_manager(
                     dt=dt,
                 )
 
-    # update loose constraints to populate pm options
+    # Update loose constraints to populate pm options
     if coupling_map is None:
         coupling_map = target.build_coupling_map()
     if basis_gates is None and len(target.operation_names) > 0:

qiskit/transpiler/target.py

@@ -270,6 +270,17 @@ def __init__(
         self._instruction_durations = None
         self._instruction_schedule_map = None
 
+    @property
+    def dt(self):
+        """Return dt."""
+        return self._dt
+
+    @dt.setter
+    def dt(self, dt):
+        """Set dt and invalidate instruction duration cache"""
+        self._dt = dt
+        self._instruction_durations = None
+
     def add_instruction(self, instruction, properties=None, name=None):
         """Add a new instruction to the :class:`~qiskit.transpiler.Target`
 

releasenotes/notes/fix-dt-oversight-be7e9b00f9c7b7a8.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed an oversight in the :class:`.Target` class where setting a new value for the ``dt`` attribute
+    and subsequently calling ``target.durations()`` would not show the updated ``dt`` value in the returned
+    :class:`.InstructionDurations` object. This is now fixed through an invalidation of the internal target
+    instruction durations cache in the ``dt`` setter.

test/python/compiler/test_transpiler.py

@@ -1677,12 +1677,13 @@ def test_scheduling_dt_constraints(self):
         """Test that scheduling-related loose transpile constraints
         work with BackendV2."""
 
-        backend_v2 = GenericBackendV2(num_qubits=2, seed=1)
+        original_dt = 2.2222222222222221e-10
+        backend_v2 = GenericBackendV2(num_qubits=2, dt=original_dt, seed=3)
         qc = QuantumCircuit(1, 1)
         qc.x(0)
         qc.measure(0, 0)
-        original_dt = 2.2222222222222221e-10
-        original_duration = 5059
+        scheduled = transpile(qc, backend=backend_v2, scheduling_method="asap")
+        original_duration = scheduled.duration
 
         # halve dt in sec = double duration in dt
         scheduled = transpile(qc, backend=backend_v2, scheduling_method="asap", dt=original_dt / 2)
@@ -2272,6 +2273,36 @@ def test_no_contraction_of_wires_in_routed_box(self, level):
         active_indices = {qubit_map[bit] for instruction in body for bit in instruction.qubits}
         self.assertNotIn(8, active_indices)
 
+    def test_custom_dt_preserves_properties(self):
+        """Test that setting the `dt` parameter with a `backend` doesn't affect the target properties
+        and vf2 runs as expected.
+        """
+
+        coupling_map = [[0, 1], [1, 0], [1, 2], [1, 3], [2, 1], [3, 1], [3, 4], [4, 3]]
+        backend = GenericBackendV2(
+            num_qubits=5,
+            basis_gates=["id", "sx", "x", "cx", "rz"],
+            coupling_map=coupling_map,
+            seed=0,
+        )
+        qubits = 3
+        qc = QuantumCircuit(qubits)
+        for i in range(5):
+            qc.cx(i % qubits, int(i + qubits / 2) % qubits)
+
+        # transpile with no gate errors
+        tqc_no_error = transpile(qc, coupling_map=coupling_map, seed_transpiler=4242)
+        # transpile with gate errors
+        tqc_no_dt = transpile(qc, backend=backend, seed_transpiler=4242)
+        # confirm that the output layouts are different
+        self.assertNotEqual(
+            tqc_no_dt.layout.final_index_layout(), tqc_no_error.layout.final_index_layout()
+        )
+        # now modify dt with gate errors
+        tqc_dt = transpile(qc, backend=backend, seed_transpiler=4242, dt=backend.dt * 2)
+        # confirm that dt doesn't affect layout
+        self.assertEqual(tqc_no_dt.layout.final_index_layout(), tqc_dt.layout.final_index_layout())
+
 
 @ddt
 class TestPostTranspileIntegration(QiskitTestCase):