Port collect 2q blocks function from qiskit-terra

[mtreinish]

What is the expected enhancement?

In qiskit-terra there is a transpiler pass collect2q blocks which primarily consists of a graph traversal. Doing the actual graph traversal and search will be much faster in retworkx, so we should add a function that can be used in place of terra's python version. The terra function code is:

https://github.com/Qiskit/qiskit-terra/blob/b058856e2aee9c5819120dc9c95f1e2478c95f4e/qiskit/transpiler/passes/optimization/collect_2q_blocks.py#L55-L246

This is similar to collect_runs which already has a native retworkx version:

https://github.com/Qiskit/retworkx/blob/b1f973630cb352950fbdeabd4b4c60191a62a6e7/src/lib.rs#L1121-L1173

and will operate in a similar manner with a filter function argument except it will collect 2q blocks instead of 1q runs.

[nahumsa]

I'll take a look on this issue.

[IvanIsCoding]

I'm working on it

[IvanIsCoding]

Giving an update on this after profiling the code and trying to convert it to Rust.

I am a bit concerned about porting all the Collect2qBlocks.run function to retworkx because it is very specific and it calls Python code very frequently. It won't generalize as collect_runs, because it has multiple conditions for handling cases and most of them mix checking DAG properties and quantum properties. So porting it might make it much harder to maintain the transpiler pass in qiskit-terra and the function in retworkx.

Nevertheless, I still think we can improve the performance by porting some core parts to Rust. Mainly about porting quantum_successors, quantum_predecessors, and some Python code that checks for elements in successors and predecessors.

I profiled the code with:

from random import randint
from qiskit import QuantumRegister, QuantumCircuit
from qiskit.dagcircuit import DAGCircuit
from qiskit.converters import circuit_to_dag
from qiskit.transpiler.passes.optimization import Collect2qBlocks

q = QuantumRegister(1000, 'q')
qc = QuantumCircuit(q)
for i in range(100000):
    a = randint(0, 999)
    b = randint(0, 999)
    if a == b:
        qc.h(a)
    else:
        if i % 2 == 0:
            qc.cz(a, b)
        else:
            qc.cx(a, b)

dag = circuit_to_dag(qc)

c2q = Collect2qBlocks()

for i in range(100):
    c2q.run(dag)

The results showed we spend around 10% of the computing time in quantum_predecessors, 7% of the time in quantum_successors, 7% in successors and 6% in predecessors.

I think the culprits are this and this check in predecessors/successors.

To address them, instead of rewriting the whole function I think we can:

• add a filtered_sucessors/filtered_predecessors function in PyDiGraph to avoid having to convert all nodes into Python to then run a filter function
• add a is_predecessors/is_sucessor function in PyDiGraph to avoid having to convert all nodes into Python to do a simple check

I will make a Draft PR with the additions and rewrite the function in qiskit-terra using those new functions. Hopefully that helps boost speed and make the code remain maintainable.