If I have multiple parallel regions, would same thread be used for given thread index (returned by omp_get_thread_num()) in those parallel regions?
It is undefined by the OpenMP standard (version 5.1), except for the primary thread (master thread) encountering the two sections (assuming they are encountered by the same thread in your application):
The thread that encountered the
parallelconstruct becomes the primary thread of the new team, with a thread number of zero for the duration of the newparallelregion.
Note that the this could not be true if you use untied tasks in your parallel section:
The thread number may change during the execution of an untied task. The value returned by
omp_get_thread_numis not generally useful during the execution of such a task region.