The register specifier shall be applied only to names of variables declared in a block or to function parameters. It specifies that the named
variable has automatic storage duration. A variable declared without a storage-class-specifier at block scope or declared as a function parameter
has automatic storage duration by default.
A register specifier is a hint to the implementation that the variable so declared will be heavily used.[ Note: The hint can be ignored and in
most implementations it will be ignored if the address of the variable is taken. This use is deprecated - end note ]
In upcoming versions of C/C++ we can expect this deprecated specifier to not be supported anymore.