Transpiler with target unrolls even if gate is natively supported

[burgholzer]

Environment

• Qiskit Terra version: 0.25.0
• Python version: 3.11
• Operating system: Ubuntu 22.04

What is happening?

Calling transpile with a custom target on a circuit causes transpilation even though all gates of a circuit are in the target's list of supported operations. See the below MRE that demonstrates the issue.
The underlying issue is much broader than that. Given a target that contains the U3Gate and any other single-qubit gate, the transpiled circuit will always contain a u3 gate instead of the original gate.

How can we reproduce the issue?

import qiskit.circuit.library as qcl
from qiskit import QuantumCircuit, transpile
from qiskit.circuit import Parameter
from qiskit.transpiler import Target

target = Target(num_qubits=1)
target.add_instruction(qcl.XGate())
target.add_instruction(qcl.U3Gate(Parameter("theta"), Parameter("phi"), Parameter("lam")))

qc = QuantumCircuit(1)
qc.x(0)
qc_transpiled = transpile(qc, target=target)
print(qc_transpiled.data[0][0])

yields

u3

What should happen?

The X gate is in the list of supported instructions. As such, I would have expected no transpilation to take place and that qc == qc_transpiled.

Interestingly, using

qc_transpiled = transpile(qc, basis_gates=["x", "u3"])

produces the desired result.

Any suggestions?

If the behavior above is not desired, it is at least unexpected and some clarification would be great.
If this is really a bug, I suspect that some early stopping criterion is missing somewhere.

[mtreinish]

I came across the same thing too, I had opened: #10568 to track it. I'm going to close this as a duplicate of it. The specific issue is basically this function: https://github.com/Qiskit/qiskit/blob/main/qiskit/transpiler/passes/optimization/optimize_1q_decomposition.py#L152-L180 isn't taking into account that the default should be to not translate the gate if the all have the same error rates (which in a simulator will be None).

[burgholzer]

I came across the same thing too, I had opened: #10568 to track it. I'm going to close this as a duplicate of it. The specific issue is basically this function: https://github.com/Qiskit/qiskit/blob/main/qiskit/transpiler/passes/optimization/optimize_1q_decomposition.py#L152-L180 isn't taking into account that the default should be to not translate the gate if the all have the same error rates (which in a simulator will be None).

Ah thanks for bringing that up and sorry for the noise. Is that something you have on the radar internally and are working on a fix or would you appreciate a community contribution there?

[mtreinish]

I haven't started to work on it besides identifying the root cause a few weeks ago. If you have the bandwidth to work on it that would be greatly appreciated. If you leave a comment on #10568 I can assign the issue to you.