fix(qasm3): record output only for results a measurement wrote - #294
fix(qasm3): record output only for results a measurement wrote#294ryanhill1 wants to merge 6 commits into
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Output recording iterated over qubits, so it emitted result_record_output
for results no mz ever produced. Two distinct symptoms, same cause:
A circuit with no measurement emitted zero mz instructions yet recorded
one result per qubit, giving a module that declares required_num_results=0
while reading back N results. That is spec-invalid QIR, and LLVM's verifier
does not catch it because the violation is at QIR profile level. qir-runner
reports whatever uninitialised state it finds as a measurement outcome, so
such programs returned confident wrong counts instead of failing: a Bell
state submitted from the circuit composer came back as {"01": 1000} and a
GHZ state as {"010": 1000}, both marked COMPLETED.
When a program declared more classical bits than it measured, recording
followed the qubit count and read the wrong registers. Given
qubit[4] q; bit[4] c; bit[4] c0;
h q; measure q -> c0;
mz writes results 4-7, but recording emitted 0-3 — reporting the untouched
`c` register and discarding the actual measurement. The complex_if
reference .ll files encoded that output, so they are regenerated here.
Track the result ids mz writes and record exactly those, in both the base
profile and the adaptive profile's fallback branch. Only the qasm3 frontend
routes through profiles/core.py; the qiskit and cirq visitors have their own
record_output and were already correct.
The custom-op validators in qir_utils asserted required_num_results=0 and
two result_record_output calls in the same test, pinning the invalid
combination; their fixtures declare no classical register, so the expected
recording is now empty.
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WalkthroughThe QASM3 visitor tracks result IDs written by measurements. Base and adaptive profiles record only those results. Regression tests and QIR fixtures cover circuits without measurements, measured registers, and custom operations without classical results. ChangesMeasurement Result Recording
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The base-path fix narrows output recording, but the adaptive path can still return incorrect counts when measurements target later classical registers or some registers are untouched. Conditional measurements may also emit results from branches that do not execute, so merge should wait for the adaptive-path correction and focused validation. Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@qbraid_qir/profiles/core.py`:
- Around line 213-217: The register-aware adaptive path around
result_record_output must restrict output generation to IDs in
visitor._measured_results, not every declared classical bit; preserve the
existing register mapping while skipping unmeasured registers and bits, and add
an adaptive regression case covering measurements into a later register and an
unmeasured register.
Apply the same fix in `@tests/qasm3_qir/converter/test_measurement.py` around
lines 81 - 105.
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📒 Files selected for processing (7)
CHANGELOG.mdqbraid_qir/profiles/core.pyqbraid_qir/qasm3/visitor.pytests/qasm3_qir/converter/test_measurement.pytests/qasm3_qir/fixtures/resources/complex_if_opaque.lltests/qasm3_qir/fixtures/resources/complex_if_typed.lltests/qir_utils.py
mypy rejected the profile methods reading visitor._measured_results, since the attribute was only set on the qasm3 subclass while profiles/core.py is typed against the QIRVisitor base. Declaring it alongside the other attributes the profiles reach for (_record_output, _clbit_labels, _global_creg_size_map) is where it belongs, and makes the subclass's own initialisation redundant.
The fallback ran only when no classical register was declared, and a measurement needs a classical target, so _measured_results was always empty there and the loop body was dead. Recording nothing is the correct behaviour for that case and is now stated in the docstring instead of expressed as an empty loop. Adds the adaptive-profile test that covers it. Also trims the changelog entry.
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Argus · 7/10 — Output recording still has a critical result-tracking defect
Verdict: Argus found 1 blocking finding. Fix the findings before you merge.
Findings: 1 blocking · 2 warning · 0 suggestion · 0 praise · 9 files reviewed
Findings summary
- Result tracking in
qbraid_qir/profiles/core.pystill causes a critical output-recording defect. - Measurement tests add warnings for incomplete coverage of output-recording behavior.
Findings outside the diff (1)
These findings refer to lines outside the diff.
Unverified: Argus did not compile or run this suggestion.
- 🟡
tests/qasm3_qir/converter/test_measurement.py:L106[warning] The regression tests do not cover repeated writes or genuinely sparse result IDsDuplicate or reordered output records can reach production while this suite remains green.
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Cross-Repo PR Coverage
- ✅ qBraid/qbraid-simulators#216 — feat(qbraid-qir): port qir-runner v0.9.6 and the result-ordering fix — compatible
Updated at 15:37 UTC on 2026-08-19
The base profile was changed to record only results a measurement wrote. The adaptive profile was not, so the bug this PR names survived on the path that declares a classical register. `_clbit_labels` is populated when a register is *declared*, so a bit that was never measured still has a label and the grouping loop recorded it anyway. A program declaring `bit[3] c` and measuring `c[0]` emitted three result records under `array_record_output(i64 3)`: two uninitialised slots the runtime reports as outcomes, and an array whose length promises elements that never follow. Declaring a register and measuring nothing recorded the whole register, where the base profile correctly records nothing. Filtering on `_measured_results` brings adaptive in line with base -- 1 of 3 measured now records 1, none measured records 0, all measured is unchanged. Registers with no measured bits are skipped rather than emitted as empty arrays, and the array length counts what is actually recorded. Two regression tests cover the partially-measured register and the unused-register case; both fail against the previous implementation. The existing no-measurement test also now asserts required_num_results and the absence of read_result calls, rather than only the absence of record_output. Reported by Argus Eye on #294.
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Both Argus Eye findings were valid. Fixed in e455b95. P0 — confirmed, and it was this PR's own bug left half-doneThe base profile got the Reproduced before changing anything, measuring
The "none measured" row is the case this PR's title describes: with a register declared and nothing measured, adaptive recorded the whole register while base correctly recorded nothing. I took the suggested shape — collect the measured ids first, skip registers with none, size the array to what is actually recorded. Sizing to the declared width would promise elements that never follow, and recording an unwritten result is what hands the runtime an uninitialised slot. P1 — valid, and its assertions check out
VerificationTwo regression tests added, for the partially-measured register and the unused register. Both fail against the previous implementation and pass now — I checked by reverting the fix and re-running. Full suite 330 passed / 8 skipped; no One note on scope: running isort reformatted |
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Summary of changes
Problem
Output recording iterated over qubits, emitting
result_record_outputfor results nomzever wrote. Two symptoms, one cause:1. A circuit with no measurement recorded one result per qubit while the entry point declared
required_num_results=0. LLVM's verifier does not catch this — the violation is at QIR profile level, andModule.verify()returns clean.qir-runnerreports whatever uninitialised state it finds as a measurement outcome, so these programs returned confident wrong numbers instead of failing. In production, a Bell state submitted from the circuit composer came back as{"01": 1000}and a GHZ state as{"010": 1000}, both marked COMPLETED, against correct answers of ~50/50 over00/11and000/111.2. A program declaring more classical bits than it measured recorded the wrong registers:
mzwrites results 4-7; recording emitted 0-3 — reporting the untouchedcregister and discarding the actual measurement.Recording now follows the result ids an
mzwrote, in both the base profile and the adaptive profile's fallback branch.The
complex_ifreference files are regenerated, not adjustedcomplex_if_{typed,opaque}.llencoded symptom 2 above: they expected results 0-3 for a program that measures into 4-7. I confirmed against themztargets in the generated module before changing them. This is the part worth checking hardest — if you disagree that the old expectation was wrong, the rest of the fix is wrong too.Scope
Only the qasm3 frontend routes through
profiles/core.py. The qiskit and cirq visitors have their ownrecord_outputand already recorded correctly; I verified both emit self-consistent modules for an unmeasured circuit.Why the custom-op expectations shrank
The three validators in
qir_utilsassertedrequired_num_results=0and tworesult_record_outputcalls in the same test, pinning exactly the invalid combination. Their fixtures declare no classical register, so the expected recording is now empty.Caveats
An unmeasured circuit now returns empty counts rather than fabricated ones. That is honest but unhelpful on its own — the user gets no explanation. A companion guard in qbraid-simulators fails such jobs with a message instead; it is stacked on qBraid/qbraid-simulators#216 and can land independently.
Not exercised against a real
qir-runnerbinary. The evidence here is the generated modules plus the production jobs above.Summary by CodeRabbit
Bug Fixes
Tests
Also in this PR:
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