At the risk of going off topic: That's for GCC. As far as I know, MSVC uses something like __declspec( thread ).
What are the options for generic C99 compilers, other than staying slow?

As a practical matter: does CPython even support "generic C99 compilers"?  AFAIK we support three specific compilers: GCC, Clang, and MSVC.

(Maybe we also support icc?  I think mostly because it supports GCC language extensions.)

/arry