I'm speculating heavily here, but it seems TLS seems to be userland thing, but with OS support (e.g. CLONE_SETTLS flags, *_thread_area()). Maybe it was thought

I'm speculating heavily here, but it seems TLS seems to be userland thing, but with OS support (e.g. CLONE_SETTLS flags, *_thread_area()). Maybe it was thought more like a thread-level thing, and not runtime thing. E.g. if we had some competing runtimes in one process space (e.g. go, jvm, libc), then they can all obtain TLS pointer via some unified method (via register or syscall). Otherwise there would be need for some chosen runtime, which would keep the pointer and distribute it around - which would create another set of problems. Putting it on the top of initial process stack would work I guess, though.