11. Notable Bugs and Fixes

11.1. Barrier Sense Mismatch Deadlock

Symptom: Program hangs when calling GOMP_parallel from a forkIO thread at -N4. No output at all. Works at -N1.

Root cause: Workers' local barrier sense variables (start_sense, end_sense) persisted from previous parallel regions (value 1), but spin_barrier_init() reset the barrier's global sense to 0. On the next region:

• Workers flipped 1→0, saw sense(0) == local_sense(0), passed through immediately
• Master (on a new OS thread from forkIO) had fresh sense=0, flipped to 1, but couldn't complete the barrier

Fix: Reset all local sense variables to 0 at the start of each parallel region, matching the freshly initialized barriers.

11.2. False Parallel-For Regression

Symptom: At 4 threads, parallel for appeared 1.65x slower than native libgomp (6.7ms vs 4.1ms).

Root cause: Single-sample measurement on a laptop with powersave CPU governor (i7-10750H at 46% clock). CPU boost state varied between process invocations.

Fix: Changed to best-of-10 within each process. Controlled interleaved testing confirmed parity (3.85ms vs 3.91ms).

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