Make BarrierBeforeFinalMeasurements deterministic
#9568
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The barrier-merging step at the end directly converted its inner (unordered) sets of bits into (ordered) lists, which is an operation dependent on `PYTHONHASHSEED`. This can have an impact on subsequent topological iteration orders, if there are any barriers that are not full-width in the circuit, which can in turn cause later stochastic transpiler passes to have different outputs for fixed random seeds, depending on `PYTHONHASHSEED`.
jakelishman
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Changelog: Bugfix
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mtreinish
reviewed
Feb 10, 2023
| @@ -57,6 +57,7 @@ class MergeAdjacentBarriers(TransformationPass): | |||
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| def run(self, dag): | |||
| """Run the MergeAdjacentBarriers pass on `dag`.""" | |||
| indices = {qubit: index for index, qubit in enumerate(dag.qubits)} | |||
releasenotes/notes/deterministic-barrier-before-final-measurements-04e817d995794067.yaml
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mtreinish
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Feb 10, 2023
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I'm nervous about backporting because of the RNG changes, but I think we're technically within our rights to if we wanted, because it is a bugfix. |
nbronn
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Feb 13, 2023
* Make `BarrierBeforeFinalMeasurements` deterministic The barrier-merging step at the end directly converted its inner (unordered) sets of bits into (ordered) lists, which is an operation dependent on `PYTHONHASHSEED`. This can have an impact on subsequent topological iteration orders, if there are any barriers that are not full-width in the circuit, which can in turn cause later stochastic transpiler passes to have different outputs for fixed random seeds, depending on `PYTHONHASHSEED`. * Add direct test of MergeAdjacentBarriers
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Summary
The barrier-merging step at the end directly converted its inner (unordered) sets of bits into (ordered) lists, which is an operation dependent on
PYTHONHASHSEED. This can have an impact on subsequent topological iteration orders, if there are any barriers that are not full-width in the circuit, which can in turn cause later stochastic transpiler passes to have different outputs for fixed random seeds, depending onPYTHONHASHSEED.Details and comments
This is what's causing the non-determinism in the test on #9560.