This keywords is only a hint, was used in C in 1970s, but is useless
nowadays. Some reports indicate, that GCC never used it to actually
treat variable as a register.
It was deprecated from C++ in 2009:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4193.html#809
It was removed from language in C++17, and will likely see new meaning
in some future standard, similar to how 'auto' got replaced in C++11.
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