wai-quantum: the branching test pins its reference run and forces the state vector's split paths #393

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dcharlot merged 1 commit from wai/quantum-branching-threads-pinned into main 2026-10-21 12:11:03 -04:00

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2026-10-21

Transaction Science

wai-quantum: the branching test pins its reference run and forces the state vector's split paths ...

branching_runs_depend_on_neither_threads_nor_the_state_limit failed intermittently on
the linux-arm64 runner on 2026-10-19 (5 runs in 20 that morning, in debug), and has not
failed since. In the failing run the reference run and a variant both moved, by a few
parts in 10^5 and differently, with the counts and histories unchanged; so the test
could not say which run was wrong.

- The reference run is now pinned by a digest of its bits (estimates, counts, shots,
  histories), the same on every target. A failure of the pin says the reference
  itself moved; a failure of a comparison alone, a variant.
- From 16 qubits a run is one tree on the state vector's own threads, which since
  0.6.8 split a pass only from 2^18 amplitudes a thread. So this test's 16-qubit case
  no longer took the split paths it ran on that morning. It now forces them, with the
  state vector on 2 and 4 threads (not on wasm32, which has no threads).

What was checked before this, on today's main, found no defect:
- the whole library suite (716 tests) under AddressSanitizer: no error;
- the threaded tests under ThreadSanitizer, with the split paths forced: no race;
- the whole suite 8 times in the arm64 runner's own image: all passed;
- 240 forced-thread runs of the 16-qubit case in debug: all the same bits;
- the last 23 CI runs of the workflow on arm64: all passed.

The cause stays unproven. These tests now catch it if it recurs, and say which run moved.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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