Fail safe in the QASM 3 exporter #7336
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There were previously parts of the QASM 3 exporter where it attempted to do its best guess at what should happen, even when this always produced entirely incorrect results. These were subroutine definitions with parameters, opaque gates (`defcal`) and non-included gates with parameters. We now choose to fail loudly with a message that we do not currently support certain features, rather than outputting a meaningless programme. For the parameterised gates, we now force the exporter to output a separate definition for every instance of a gate if it cannot be sure that it has the most general form to start with. The code it outputs looks silly (the gates take parameters, but do not use them in their definitions, and each gate may be defined many times), but the semantics of the programme will actually be correct. See Qiskitgh-7335 for the parameterisation problem.
jakelishman
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priority: high
mod: qasm2
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levbishop
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Dec 1, 2021
jakelishman
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mod: qasm3
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Summary
There were previously parts of the QASM 3 exporter where it attempted to
do its best guess at what should happen, even when this always produced
entirely incorrect results. These were subroutine definitions with
parameters, opaque gates (
defcal) and non-included gates withparameters.
We now choose to fail loudly with a message that we do not currently
support certain features, rather than outputting a meaningless
programme. For the parameterised gates, we now force the exporter to
output a separate definition for every instance of a gate if it cannot
be sure that it has the most general form to start with. The code it
outputs looks silly (the gates take parameters, but do not use them in
their definitions, and each gate may be defined many times), but the
semantics of the programme will actually be correct.
See gh-7335 for the parameterisation problem.
Details and comments
This very very lightly papers over #7335, but is not a proper solution. The full solution will need to come after release, and may well require a proper refactor of the expectations of
Instruction.params.