A register variable is a type of local variable. The register keyword tells the compiler “Make accesses to this variable as fast as possible.” Increasing the access speed is implementation dependent, but, as the name suggests, it is often done by placing the variable in a register. There is no guarantee that the variable will be placed in a register or even that the access speed will increase. It is a hint to the compiler.

There are restrictions to the use of register variables. You cannot take or compute the address of a register variable. A register variable can be declared only within a block (you cannot have global or static register variables). You can, however, use a register variable as a formal argument in a function (i.e., in the argument list).

In general, you shouldn’t try to second-guess the compiler’s optimizer, since it will probably do a better job than you can. Thus, the register keyword is best avoided.