Sun Studio 12: C User's Guide

E.1.8 Registers (G.3.8)

E.1.8.1 (6.5.1) The extent to which objects can actually be placed in registers by use of the register storage-class specifier:

The number of effective register declarations depends on patterns of use and definition within each function and is bounded by the number of registers available for allocation. Neither the compiler nor the optimizer is required to honor register declarations.