tests/fio: regression coverage for ref_time_ticks() ns normalization

[borisbat]

Summary

PR #2685 normalized ref_time_ticks() to return nanoseconds on every platform (Windows used to return raw QPC ticks at the underlying counter's frequency — typically 10 MHz). The fix shipped without a unit test that would have caught a units regression.

This PR adds tests/fio/perf_time.das with four tests:

• monotonic — 1000 successive reads never go backwards. Catches signed/unsigned mixups and wrap-around bugs in the ns conversion arithmetic.
• sleep_roundtrip — sleep(100 ms) → delta must land in [80 ms, 500 ms]. The 80 ms lower bound is the load-bearing assertion: if Windows reverted to raw QPC ticks, a 100 ms sleep would surface as ~1 ms in the ns interpretation, tripping the test. Wide upper bound covers CI runner scheduler jitter.
• get_time_usec_agrees — get_time_usec(t0) and (ref_time_ticks() - t0) / 1000 agree within 5 ms. Two helpers reading the same underlying clock should not drift.
• units_are_nanoseconds — three back-to-back sleep(100 ms) deltas stay within 200 ms of each other.

tests/aot/CMakeLists.txt:224 already covers tests/fio/*.das via FILE(GLOB CONFIGURE_DEPENDS); cmake reconfigure picks the new file up automatically (no CMake edit needed).

Test plan

• daslang.exe dastest.das -- --test tests/fio/perf_time.das — interpreter mode, 4/4 PASS on Windows local
• daslang.exe dastest.das -- --test tests/fio — full fio dir, 175/175 PASS (interpreter)
• test_aot.exe -use-aot dastest.das -- --use-aot --test tests/fio/perf_time.das — AOT mode, 4/4 PASS on Windows local
• CI: confirm green on all OSes

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new test file tests/fio/perf_time.das with four regression tests that pin the post‑PR‑#2685 contract that ref_time_ticks() returns nanoseconds on every platform. The tests exercise monotonicity, a sleep(100ms) round‑trip with a load‑bearing 80 ms lower bound (would catch a regression to raw QPC ticks on Windows), agreement between get_time_usec and the raw delta, and inter‑sample uniformity across three back‑to‑back sleeps. The file is automatically picked up by the existing tests/fio/*.das glob in tests/aot/CMakeLists.txt, so no build changes are required.

Changes:

• New tests/fio/perf_time.das with 4 [test] functions covering monotonicity, sleep round‑trip bounds, helper/delta agreement, and ns unit sanity.
• Uses the standard dastest/testing_boost public + daslib/fio + math imports consistent with the rest of tests/.
• Tolerances chosen to absorb CI scheduler jitter (Windows up to ~200 ms) while still detecting a unit regression.

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