The value is per thread, so if you're caching it to main memory (including in the stack) then you need one copy per thread and you need to somehow know which on

The value is per thread, so if you're caching it to main memory (including in the stack) then you need one copy per thread and you need to somehow know which one to use based on what thread is running. You'd need to store that index somewhere so there'd be an infinite recursion. Even if it's stored at the base of the current stack you don't want to have to walk all the stack frames to find it. You can store it in a GPR instead (that's what some modern architectures do) but then it needs to be part of the ABI. Otherwise at the beginning of a function you can't assume any register contains the correct value.