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[The extent to which suggestions made by using the register storage-class specifier are effective (6.7.1).]
The register specifier affects code generation only in these ways:
When used as part of the register variable extension, see Section 6.40 Variables in Specified Registers.
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 (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.