### 4.8 Hints

- The extent to which suggestions made by using the `register` storage-class specifier are effective (C90 6.5.1, C99 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 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.