/GT (Support Fiber-Safe Thread-Local Storage)
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The /GT option supports fiber safety for data allocated using static thread-local storage, that is, data allocated with __declspec( thread ).
A fiber is a lightweight object that consists of a stack and a register context and can be scheduled on various threads. A fiber can run on any thread. Because a fiber may get swapped out and restarted later on a different thread, it must not be cached or optimized as a common subexpression across a function call (see the /Og option for details). The /GT switch prevents such optimizations.