Retain all deprecated Bit properties in QPY roundtrip
#9525
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A major bugfix for QPY's handling of loose circuit bits in c0ac5fb (Qiskitgh-9095) caused the deprecated bit properties `index` and `register` to be lost after the QPY roundtrip. As far as Terra's data model is concerned, the two circuits would still compare equal after this loss; the output registers would still be equal to the inputs, and bit equality is not considered directly since it general bit instances are not expected to be equal between circuits. Losing this information caused some downstream issues for the IBM runtime, which was still relying on `Bit.index` working in some cases, despite it issuing a deprecating warning since Terra 0.17 (April 2021). While this can be corrected downstream, QPY can still do a better job of roundtripping the deprecated information while it is still present. The QPY format does not currently store enough information to _completely_ roundtrip this information in cases that some but not all owned bits from a register are present in the circuit. (This partial data is a decent part of the cause of the bugs that Qiskitgh-9095 fixed.) Since this is just in the support of deprecated functionality that Terra's data model does not even require for circuit equality (QPY's goal), it seems not worth it to produce a new QPY binary format version to store this, when the deprecated properties being removed would obsolete the format again immediately.
jakelishman
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mtreinish
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Feb 6, 2023
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At a high level this LGTM, we're restoring some information which was missing from the deserialized circuit. Just a couple comments inline.
TBH, I probably should have caught this in review for #9095, a big part of the old very complex/fragile code was to try and recreate registers with standalone bits vs old style registers which owned all their bits as close to the user input as possible. I relied a bit too heavily on the tests to assume we hadn't missed anything and the __eq__ not checking this got us.
mtreinish
reviewed
Feb 6, 2023
This allows complete round-tripping of all the deprecated register+index information in the `Bit` instances through QPY, restoring us to the _intended_ behaviour before Qiskitgh-9095. The behaviour in the dumper before that did not allow full reconstruction, because some of the information was lost for bits that were encountered in more than one register. This fixes the dumper to always output all the indexing information for all bits, not to use `-1` whenever a bit _is_ in the circuit but has previously been encountered. The `standalone` field on a register is sufficient to tell whether the bits contained in it should have their "owner" information set; it's not possible (in valid Qiskit code) to have a register that owns only _some_ of its bits. To accomodate this, the register reader now needs to be two-pass.
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From offline discussions: I was actually able to sort out the logic to completely reconstruct the deprecated register and index information for all bits, without breaking the QPY format. The format already had the spaces for the right information, it just wasn't being stored entirely accurately by the dumper. This PR is now the bugfix that #9095 should have been. |
jakelishman
changed the title
Bit properties in QPY roundtripBit properties in QPY roundtrip
mtreinish
reviewed
Feb 14, 2023
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Overall this LGTM, I think this all looks correct to me. I had a couple of small inline comments, mostly about adding some more comments on the deserialization side. The logic for this is pretty tricky and I think if we have more inline comments to describe the expected flow will be useful when/if we revisit this block in the future.
mtreinish
approved these changes
Feb 16, 2023
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* Retain more deprecated Bit properties in QPY roundtrip A major bugfix for QPY's handling of loose circuit bits in c0ac5fb (gh-9095) caused the deprecated bit properties `index` and `register` to be lost after the QPY roundtrip. As far as Terra's data model is concerned, the two circuits would still compare equal after this loss; the output registers would still be equal to the inputs, and bit equality is not considered directly since it general bit instances are not expected to be equal between circuits. Losing this information caused some downstream issues for the IBM runtime, which was still relying on `Bit.index` working in some cases, despite it issuing a deprecating warning since Terra 0.17 (April 2021). While this can be corrected downstream, QPY can still do a better job of roundtripping the deprecated information while it is still present. The QPY format does not currently store enough information to _completely_ roundtrip this information in cases that some but not all owned bits from a register are present in the circuit. (This partial data is a decent part of the cause of the bugs that gh-9095 fixed.) Since this is just in the support of deprecated functionality that Terra's data model does not even require for circuit equality (QPY's goal), it seems not worth it to produce a new QPY binary format version to store this, when the deprecated properties being removed would obsolete the format again immediately. * Fix lint * Correct deprecated bit information in QPY This allows complete round-tripping of all the deprecated register+index information in the `Bit` instances through QPY, restoring us to the _intended_ behaviour before gh-9095. The behaviour in the dumper before that did not allow full reconstruction, because some of the information was lost for bits that were encountered in more than one register. This fixes the dumper to always output all the indexing information for all bits, not to use `-1` whenever a bit _is_ in the circuit but has previously been encountered. The `standalone` field on a register is sufficient to tell whether the bits contained in it should have their "owner" information set; it's not possible (in valid Qiskit code) to have a register that owns only _some_ of its bits. To accomodate this, the register reader now needs to be two-pass. * Add deprecated-bit checks to backwards compatibility tests * Rewrite release note * Improve internal documentation --------- Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com> (cherry picked from commit b6aba59)
mergify bot
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Feb 16, 2023
* Retain more deprecated Bit properties in QPY roundtrip A major bugfix for QPY's handling of loose circuit bits in c0ac5fb (gh-9095) caused the deprecated bit properties `index` and `register` to be lost after the QPY roundtrip. As far as Terra's data model is concerned, the two circuits would still compare equal after this loss; the output registers would still be equal to the inputs, and bit equality is not considered directly since it general bit instances are not expected to be equal between circuits. Losing this information caused some downstream issues for the IBM runtime, which was still relying on `Bit.index` working in some cases, despite it issuing a deprecating warning since Terra 0.17 (April 2021). While this can be corrected downstream, QPY can still do a better job of roundtripping the deprecated information while it is still present. The QPY format does not currently store enough information to _completely_ roundtrip this information in cases that some but not all owned bits from a register are present in the circuit. (This partial data is a decent part of the cause of the bugs that gh-9095 fixed.) Since this is just in the support of deprecated functionality that Terra's data model does not even require for circuit equality (QPY's goal), it seems not worth it to produce a new QPY binary format version to store this, when the deprecated properties being removed would obsolete the format again immediately. * Fix lint * Correct deprecated bit information in QPY This allows complete round-tripping of all the deprecated register+index information in the `Bit` instances through QPY, restoring us to the _intended_ behaviour before gh-9095. The behaviour in the dumper before that did not allow full reconstruction, because some of the information was lost for bits that were encountered in more than one register. This fixes the dumper to always output all the indexing information for all bits, not to use `-1` whenever a bit _is_ in the circuit but has previously been encountered. The `standalone` field on a register is sufficient to tell whether the bits contained in it should have their "owner" information set; it's not possible (in valid Qiskit code) to have a register that owns only _some_ of its bits. To accomodate this, the register reader now needs to be two-pass. * Add deprecated-bit checks to backwards compatibility tests * Rewrite release note * Improve internal documentation --------- Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com> (cherry picked from commit b6aba59) Co-authored-by: Jake Lishman <[email protected]>
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Summary
A major bugfix for QPY's handling of loose circuit bits in c0ac5fb (gh-9095) caused the deprecated bit properties
indexandregisterto be lost after the QPY roundtrip. As far as Terra's data model is concerned, the two circuits would still compare equal after this loss; the output registers would still be equal to the inputs, and bit equality is not considered directly since it general bit instances are not expected to be equal between circuits.Losing this information caused some downstream issues for the IBM runtime, which was still relying on
Bit.indexworking in some cases, despite it issuing a deprecating warning since Terra 0.17 (April 2021). While this can be corrected downstream, QPY can still do a better job of roundtripping the deprecated information while it is still present.(Following paragraph obsoleted by b27da19.) The QPY format does not currently store enough information to completely roundtrip this information in cases that some but not all owned bits from a register are present in the circuit. (This partial data is a decent part of the cause of the bugs that gh-9095 fixed.) Since this is just in the support of deprecated functionality that Terra's data model does not even require for circuit equality (QPY's goal), it seems not worth it to produce a new QPY binary format version to store this, when the deprecated properties being removed would obsolete the format again immediately.
Details and comments
The best mitigation for the IBM providers is to fix the use of deprecated functionality in their program runners (I've provided a fix internally). However, this is still something we should fix.