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  • The extent to which suggestions made by using the register storage-class specifier are effective (C90 6.5.1, C99 and C11 6.7.1).

    The register specifier 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 register storage-class specifier; if register is 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, setjmp doesn’t save the registers in all circumstances. In those cases, GCC doesn’t allocate any variables in registers unless they are marked register.
  • The extent to which suggestions made by using the inline function specifier are effective (C99 and C11 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.