ConsolidateBlocks drops classical conditional

[kdk]

From https://travis-ci.com/Qiskit/qiskit-terra/jobs/242965523#L6482 :

Information

• Qiskit Terra version: master
• Python version: 3.5
• Operating system: Mac

>>> import qiskit as qk
>>> from qiskit.test.mock import FakeMelbourne
>>> qc = qk.QuantumCircuit.from_qasm_str('''
... OPENQASM 2.0;
... include "qelib1.inc";
... qreg q6476[8];
... qreg q6477[2];
... creg c1144[1];
... crz(0.0) q6476[7],q6476[0];
... if(c1144==1) u2(0.0,0.0) q6476[0];
... measure q6476[0] -> c1144[0];
... ''')
/Users/kevin.krsulichibm.com/q/qiskit-terra/qiskit/circuit/instruction.py:356: UserWarning: The instruction attribute, "control", will be renamed to "condition". The "control" method will later be used to create quantum controlled gates
  warnings.warn('The instruction attribute, "control", will be renamed '
>>> print(qc)
            ┌───────┐┌─────────┐┌─┐
q6476_0: |0>┤ Rz(0) ├┤ U2(0,0) ├┤M├
            └───┬───┘└────┬────┘└╥┘
q6476_1: |0>────┼─────────┼──────╫─
                │         │      ║
q6476_2: |0>────┼─────────┼──────╫─
                │         │      ║
q6476_3: |0>────┼─────────┼──────╫─
                │         │      ║
q6476_4: |0>────┼─────────┼──────╫─
                │         │      ║
q6476_5: |0>────┼─────────┼──────╫─
                │         │      ║
q6476_6: |0>────┼─────────┼──────╫─
                │         │      ║
q6476_7: |0>────■─────────┼──────╫─
                          │      ║
q6477_0: |0>──────────────┼──────╫─
                          │      ║
q6477_1: |0>──────────────┼──────╫─
                       ┌──┴──┐   ║
 c1144_0: 0 ═══════════╡ = 1 ╞═══╩═
                       └─────┘
>>> qk.execute(qc, qk.Aer.get_backend('qasm_simulator')).result().get_counts()
{'0': 1024}
>>> qk.execute(qc, qk.Aer.get_backend('qasm_simulator'), optimization_level=3).result().get_counts()
{'0': 506, '1': 518}
>>> print(qk.transpile(qc, qk.Aer.get_backend('qasm_simulator'), optimization_level=3))
            ┌──────────┐┌─┐
q6476_0: |0>┤0         ├┤M├
            │          │└╥┘
q6476_1: |0>┤          ├─╫─
            │          │ ║
q6476_2: |0>┤          ├─╫─
            │          │ ║
q6476_3: |0>┤          ├─╫─
            │  unitary │ ║
q6476_4: |0>┤          ├─╫─
            │          │ ║
q6476_5: |0>┤          ├─╫─
            │          │ ║
q6476_6: |0>┤          ├─╫─
            │          │ ║
q6476_7: |0>┤1         ├─╫─
            └──────────┘ ║
q6477_0: |0>─────────────╫─
                         ║
q6477_1: |0>─────────────╫─
                         ║
 c1144_0: 0 ═════════════╩═

Attaching a callback to print the pass name and output dag, it looks like the conditional is lost in ConsolidateBlocks.

RemoveResetInZeroState
            ┌───────┐┌───┐┌───────┐┌───┐┌─────────┐┌─┐
q6476_0: |0>┤ U1(0) ├┤ X ├┤ U1(0) ├┤ X ├┤ U2(0,0) ├┤M├
            └───────┘└─┬─┘└───────┘└─┬─┘└────┬────┘└╥┘
q6476_1: |0>───────────┼─────────────┼───────┼──────╫─
                       │             │       │      ║
q6476_2: |0>───────────┼─────────────┼───────┼──────╫─
                       │             │       │      ║
q6476_3: |0>───────────┼─────────────┼───────┼──────╫─
                       │             │       │      ║
q6476_4: |0>───────────┼─────────────┼───────┼──────╫─
                       │             │       │      ║
q6476_5: |0>───────────┼─────────────┼───────┼──────╫─
                       │             │       │      ║
q6476_6: |0>───────────┼─────────────┼───────┼──────╫─
                       │             │       │      ║
q6476_7: |0>───────────■─────────────■───────┼──────╫─
                                             │      ║
q6477_0: |0>─────────────────────────────────┼──────╫─
                                             │      ║
q6477_1: |0>─────────────────────────────────┼──────╫─
                                          ┌──┴──┐   ║
 c1144_0: 0 ══════════════════════════════╡ = 1 ╞═══╩═
                                          └─────┘
ConsolidateBlocks
            ┌──────────┐┌─┐
q6476_0: |0>┤0         ├┤M├
            │          │└╥┘
q6476_1: |0>┤          ├─╫─
            │          │ ║
q6476_2: |0>┤          ├─╫─
            │          │ ║
q6476_3: |0>┤          ├─╫─
            │  unitary │ ║
q6476_4: |0>┤          ├─╫─
            │          │ ║
q6476_5: |0>┤          ├─╫─
            │          │ ║
q6476_6: |0>┤          ├─╫─
            │          │ ║
q6476_7: |0>┤1         ├─╫─
            └──────────┘ ║
q6477_0: |0>─────────────╫─
                         ║
q6477_1: |0>─────────────╫─
                         ║
 c1144_0: 0 ═════════════╩═

[maddy-tod]

Collect2qBlocks includes multiple gates with conditionals into the same block. For example, the circuit

            ┌─────────┐┌───────┐┌───┐┌───────┐┌───┐┌─────────┐┌─┐
q6476_0: |0>┤ U2(0,0) ├┤ U1(0) ├┤ X ├┤ U1(0) ├┤ X ├┤ U2(0,0) ├┤M├
            └────┬────┘└───────┘└─┬─┘└───────┘└─┬─┘└────┬────┘└╥┘
q6476_1: |0>─────┼────────────────┼─────────────┼───────┼──────╫─
                 │                │             │       │      ║ 
q6476_2: |0>─────┼────────────────┼─────────────┼───────┼──────╫─
                 │                │             │       │      ║ 
q6476_3: |0>─────┼────────────────┼─────────────┼───────┼──────╫─
                 │                │             │       │      ║ 
q6476_4: |0>─────┼────────────────┼─────────────┼───────┼──────╫─
                 │                │             │       │      ║ 
q6476_5: |0>─────┼────────────────┼─────────────┼───────┼──────╫─
                 │                │             │       │      ║ 
q6476_6: |0>─────┼────────────────┼─────────────┼───────┼──────╫─
                 │                │             │       │      ║ 
q6476_7: |0>─────┼────────────────■─────────────■───────┼──────╫─
                 │                                      │      ║ 
q6477_0: |0>─────┼──────────────────────────────────────┼──────╫─
                 │                                      │      ║ 
q6477_1: |0>─────┼──────────────────────────────────────┼──────╫─
              ┌──┴──┐                                ┌──┴──┐   ║ 
 c1144_0: 0 ══╡ = 0 ╞════════════════════════════════╡ = 1 ╞═══╩═
              └─────┘                                └─────┘     

is collected into the blocks[['u2', 'u1', 'cx', 'u1', 'cx', 'u2'], ['measure']] which ConsolidateBlocks then joins together.

I don't think that this grouping is correct as I cannot see a logical value that the condition on the unitary could hold in this instance. Therefore, I think that the update that is needed is to refine how Collect2QBlocks deals with gates which contain conditions.