[damon_dam]

> How do I allocate memory for addressing it with FS/GS?

Allocate the variable normally, then compute the offset in one thread (e.g. offset = uintptr_t(&variable) - get_fs()), then access it by adding the offset to FS in any thread (e.g. (vartype *) (offset + get_fs())). The only difference from how it normally works is that you can manually force it to be inlined, sidestepping the codegen problems you described in your post. But if you can avoid those problems by not using constructors instead, that's definitely better.

I used "FS/GS" because GS is used instead of FS on some systems for the same purpose.

The shared library-specific issues are one of the reasons I was suggesting maybe looking into hashing, e.g. perhaps as a fallback solution when the TLS approach fails.