C has the register keyword, originally designed as a hint to the compiler that a variable should be placed in a register rather than on the stack. However this is generally considered an unneeded feature in this day and age:
Most modern compilers do that automatically, and are better at picking them than us humans.
Intuitively, a variable that is used a lot in a lifetime would be a register, at least to a human programmer such as myself. But if modern compilers are better at picking them than us humans as the linked answer suggests, what heuristics/tests do they use to determine what should go into a register, besides having a short lifetime and/or being accessed very frequently?