Preserve i->i initial layout when coupling_map satisfied #527
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qiskit/_compiler.py
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| graphs are equal to backend qubits coupling graph. | ||
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| Parameters: | ||
| instructions (List): List of circuit instructions. Noy all instructions |
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| logger = logging.getLogger(__name__) | ||
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| def compile(circuits, backend, | ||
| config=None, basis_gates=None, coupling_map=None, initial_layout=None, | ||
| shots=1024, max_credits=10, seed=None, qobj_id=None, hpc=None, | ||
| skip_translation=False): |
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Do we really want to remove this kwarg? For backward compatibility, should it not implicitly set skip_transpiler and raise a depreciation warning saying that it will be removed in the future?
| """ | ||
| for instruction in instructions: | ||
| if isinstance(instruction, Gate) and instruction.is_multi_qubit(): | ||
| if instruction.get_qubit_coupling() not in coupling_map: |
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For a two qubit gate between qubits i and j, I thought the check would be if j is in coupling_map[i] or if i in coupling_map[j]. Is that what is going on here, or (more likely) am I missing something?
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This checks for exact match, meaning if there is a CX(q[i], q[j]) in the circuit, it checks that the device also has CX(q[i], q[j]) in it. The reason is that I want to keep already-matching circuits untouched, but pass any other ones through the mapper. A good mapper may actually find a more compact circuit, but usually a user that writes an already-matching circuit wants to use those exact qubits.
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Yeah, I found this confusing at first, because I thought that the coupling_map was an adjacency list, but it's not. It more a list of coupled pairs: coupling_map = [[1, 0], [2, 0], [2, 1], [3, 2], [3, 4], [4, 2]].
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Yes, your right. This is what we want.
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* Preserve i->i initial layout when already satisfied. * skip_translation deprecated in favor of skip_transpiler.
When a circuit already satisfies the coupling_map on the backend, the swap_mapper should not insert any swaps. This means passing an
initial_layoutofq[i]->q[i].PR #478 optimizes the initial layout, but that creates unexpected behavior when the coupling map is already satisfied. Here, I only apply the optimization of #478 if swaps are indeed needed.
The other thing that this PR does is a slight name change:
skip_translation->skip_transpiler, to get ready for the transpiler PR.Motivation and Context
Issue was raised here:
https://quantumexperience.ng.bluemix.net/qx/community/question?questionId=5b0d17f943fb6e0038a623bf
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