- The extent to which suggestions made by using the
registerstorage-class specifier are effective (C90 6.5.1, C99 6.7.1).
registerspecifier affects code generation only in these ways:
- When used as part of the register variable extension, see Explicit Reg Vars.
- When -O0 is in use, the compiler allocates distinct stack
memory for all variables that do not have the
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.
- On some rare x86 targets,
setjmpdoesn't save the registers in all circumstances. In those cases, GCC doesn't allocate any variables in registers unless they are marked
- The extent to which suggestions made by using the inline function
specifier are effective (C99 6.7.4).
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.