Fix T2C thread synchronization

[jserv]

This addresses reliability issue in JIT parallel mode by fixing race conditions in the T2C.

• Add portable ATOMIC_* macros in common.h with GNU __atomic builtins as primary, C11 stdatomic fallback, and single-threaded fallback
• Replace busy-wait polling with proper pthread_cond_wait in t2c_runloop
• Change queue_entry_t from block pointer to cache key (PC|SATP) to prevent use-after-free when blocks are evicted during compilation
• Add defensive check in t2c_compile to skip already-compiled blocks
• Use atomic operations for shared block fields
• Proper shutdown sequence: signal under lock, join, cleanup queue

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Summary by cubic

Fixes T2C JIT race conditions to make background compilation reliable and thread-safe. Replaces busy-waiting with condition variables, adds portable atomics, and prevents use-after-free in the compile queue.

• Bug Fixes

  • Replace busy-wait polling with pthread_cond_wait in t2c_runloop; signal on enqueue and during shutdown.
  • Add portable ATOMIC_* macros and use them for shared fields (hot2, compiled, n_invoke, is_interrupted) with acquire/release ordering.
  • Queue cache keys (PC | SATP) instead of block pointers to avoid use-after-free; look up and compile under the cache lock only if the block still exists.
  • Defensive checks to skip already-compiled blocks and atomic increment of n_invoke when T2C is enabled.

• Dependencies

  • Build with gnu11 instead of gnu99 to support the atomics fallback path.

^{Written for commit c2c6847. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

[jserv]

Benchmarks

Details

Benchmark suite	Current: c2c6847	Previous: 7dad667	Ratio
Dhrystone	1657.333 DMIPS	1657 DMIPS	1.00
CoreMark	1025.5 iterations/sec	1017.224 iterations/sec	0.99

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