Keep guards that prove later code unreachable - #154
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Two TO_BOOL sites reading the same local can specialize on different types, so the JIT ends up guarding one value as both a list and None. The second guard can only fail, its output is Bottom, and the block is truncated to `guard; Unreachable`. Nothing consumed the list-ness, so GuardTypeRemoval dropped the GuardType<ListExact>; the None guard then succeeded and the compiled function hit its own ud2 (SIGILL). guardNeeded() now recomputes the output type of a Bottom-valued use with the relaxed operand type. If it is no longer Bottom, the guard is what makes the code unreachable, so keep it. Hit in SQLAlchemy's inspect_formatargspec under jit.auto(). Free-threaded builds only: on GIL builds emitGetLengthInt64 pins the list type with a UseType, which kept the guard alive by accident.
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Hit in SQLAlchemy's inspect_formatargspec under jit.auto(). Free-threaded builds only: on GIL builds emitGetLengthInt64 pins the list type with a UseType, which kept the guard alive by accident.
fixes #115