This is not supported by the C++ language. The "register" storage-class specifier can only be used at block scope or in parameters. Even if it was supported, it seems like a really bad idea to reserve a CPU register for an entire application based off of a single use of it.
You should not try to use "register" even in cases where it is allowed. This is deprecated by the C++ language and most compilers ignore it. The language lets them ignore it (it is just a hint) and it generally makes sense to do so. Let the compiler handle code generation.
thanks for a well written answer !
now, I need fast INPUT there for wanted to use a "global register int" variable
anyone knows how to achieve 100KHz INPUT with the GALILEO (pins 2,3 - with INPUT_FAST mode on)