Is it possible to share a register between threads?

Question

I know that when the OS/Hardware switch between the execution of different threads it manage the store/restore the context of each thread, however I do not know many of the deta

Answer 1

If you are writing all the code that will go in a system, you may dedicate registers to whatever purpose you want, subject to the limitation that any register which is dedicated to a particular function will be unusable for any other purpose. There are some very specialized situations where this may be worth doing; these generally entail, bizarre as it may seem, programs that are very simple but need to run very fast. Some compilers like gcc can facilitate such usage by allowing a programmer to specify particular registers that the code it generates should not use for any purpose unless explicitly requested. In general, because the efficiency of compiled code will be reduced by restricting the number of registers the compiler can use, it will be more efficient to simply use statically-defined memory locations to exchange information between threads. While memory locations cannot be accessed as quickly as registers, one can reserve many of them for various purposes without affecting the compiler's ability to optimize register usage.

The one situation I've seen on the ARM where using a dedicated register was helpful was a situation where a significant plurality of methods needed to share a common static data structure. Specifying that a certain register should always be assumed to hold a pointer to that data structure, and that code must never modify it, eliminates the need for code to load the address of that structure before accessing items therein. If you want to share information among threads, that might be a useful approach, since accessing an arbitrary static location generally requires a PC-relative load to fetch the address followed by a load of the actual data; having a dedicated register would eliminate one of the loads.