Optim level 2 and 3 modifiy circuits that match topology

[nonhermitian]

Information

• Qiskit Terra version: master
• Python version:
• Operating system:

What is the current behavior?

The following circuit fits Vigo-like topologies exactly:

But optimization level 3 gives:

Where as the default gives the expected original circuit (save for H -> U2):

Steps to reproduce the problem

qc = QuantumCircuit(5)
qc.h(3)
qc.cx([3, 3, 1, 1], [1, 4, 2, 0])
qc.measure_all()
new_qc = transpile(qc, backend, optimization_level=3)
new_qc.draw(output='mpl')

What is the expected behavior?

Suggested solutions

[1ucian0]

It seems like TrivialLayout was not an attempt before DenseLayout. #3324 could be a solution to this. #3043 is also an option. @ajavadia ?

It's important to notice that neither TrivialLayout nor CSPLayout are noise aware (yet).

[1ucian0]

In PR #3391 I implemented the second approach, using CSPLayout (which resorts to the trivial layout if that's the only possible map).