> what does that mean 'maintained by the runtime'?
> void bar( ptr_scheduler_thread_local_var) {
>   printf("%p\n", ptr_scheduler_thread_local_var);
>   suspend_and_migrate_fiber();
> }
>
> void foo() {
>    while ( true) {
>       bar( return_ptr_of_scheduler_thread_local_var() );
>       ...
>    }
> }
> in this example b) accesses a) (TLS of scheduler) prints in to stdout
> suspends active fiber and migrates it to another thread
> after resumption the address of the TLS-scheduler is printed out ...
> how does your code look like after compiler optimization?!

a) is safe as all threads sharing the same scheduler store the same reference (fibers can be moved only inside that scheduler)
b) is safe as the runtime sets the reference to the fiber it is about to run in TLS and resets it right after the fiber returned.