registerstorage-class specifier are effective (6.7.1).
register specifier affects code generation only in these ways:
registerstorage-class specifier; if
registeris specified, the variable may have a shorter lifespan than the code would indicate and may never be placed in memory.
setjmpdoesn't save the registers in all circumstances. In those cases, GCC doesn't allocate any variables in registers unless they are marked
GCC will not inline any functions if the -fno-inline option is used or if -O0 is used. Otherwise, GCC may still be unable to inline a function for many reasons; the -Winline option may be used to determine if a function has not been inlined and why not.