Qiskit / qiskit / 13532095632

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Committed by web-flow

Commit Message

Add `ContractIdleWiresInControlFlow` optimisation pass (#13779)

* Add `ContractIdleWiresInControlFlow` optimisation pass

This transpiler pass removes data dependencies on idle qubits from
control-flow operations.  For example, given a circuit such as::

    from qiskit.circuit import QuantumCircuit

    qc = QuantumCircuit(1, 1)
    qc.x(0)
    with qc.if_test((qc.clbits[0], True)):
        qc.x(0)
        qc.x(0)
    qc.x(0)

the current optimisation passes will collapse the inner control-flow
block to the identity, but the qubit dependency will remain, preventing
the outer two X gates from being cancelled.  This pass removes the
now-spurious dependency, making it possible to detect and remove the two
X gates in a follow-up loop iteration.

As an accidental side-effect of their algorithms, the control-flow-aware
routing passes currently do this when they run.  This aims to move the
logic into a suitable place to run before routing (so the spurious
dependency never arises in routing in the first place) and in the
low-level optimisation stage.

The aim of this pass is also to centralise the logic, so when the
addition of the new `box` scope with different semantics around whether
a wire is truly idle in the box or not, the routers aren't accidentally
breaking them, and it's clearer when the modifications happen.

* Handle `None` return from `control_flow_op_nodes`

* Remove dead argument

Run Details

51 of 51 new or added lines in 3 files covered. (100.0%)

6 existing lines in 1 file now uncovered.

77560 of 88148 relevant lines covered (87.99%)

337643.83 hits per line