This patch is to address https://bugs.llvm.org/show_bug.cgi?id=47835.
A relevant discussion regarding pthread_self and TLS can be found here: http://lists.llvm.org/pipermail/llvm-dev/2020-November/146766.html.
A coroutine may suspend and resume on a different thread, and hence the address of a thread_local variable may change after coroutine suspension.
In the existing design, getting the address of a TLS variable is through a direct reference, like @tls_variable. Such kind of value can be
arbitrarily moved around/replaced in the IR within the same function. This will lead to incorrect caching of TLS variable address in coroutines across suspension points.
To fix it, we have to turn the TLS address access into an intrinsics call, so that it will not be simply CSE-ed.
After CoroSplit, we no longer have coroutines, and hence can safely lower the TLS intrinsics back into references.
The current placement of the LowerThreadLocalIntrinsicPass may not be ideal. I am not quite sure how to organize it. Suggestions welcome!
Testing isn't sufficient, and there may also be failing tests. I will add/fix more tests if this patch is along the right direction.