I updated the post with the point on visibility; in my tests inspired by https://lobste.rs/s/b5dnjh/0_0_0_c_thread_local_storage_performance#c_dkzbw3 https://lo

I updated the post with the point on visibility; in my tests inspired by https://lobste.rs/s/b5dnjh/0_0_0_c_thread_local_storage_performance#c_dkzbw3 https://lobste.rs/s/b5dnjh/0_0_0_c_thread_local_storage_perf... I see that clang improves codegen thanks to hidden visibility but g++ does not, and that comment says clang doesn't do it on all platforms. I stick to the broad conclusion that thread_locals without constructors linked into an executable rather than a shared library are the fastest and most performance-portable by far, but the visibility point is very worth mentioning.