GCC provides a couple of special builtin functions to aid in the writing of interrupt handlers in C.
__bic_SR_register_on_exit (int mask)This clears the indicated bits in the saved copy of the status register currently residing on the stack. This only works inside interrupt handlers and the changes to the status register will only take affect once the handler returns.
__bis_SR_register_on_exit (int mask)This sets the indicated bits in the saved copy of the status register currently residing on the stack. This only works inside interrupt handlers and the changes to the status register will only take affect once the handler returns.
__delay_cycles (long long cycles)This inserts an instruction sequence that takes exactly cycles cycles (between 0 and about 17E9) to complete. The inserted sequence may use jumps, loops, or no-ops, and does not interfere with any other instructions. Note that cycles must be a compile-time constant integer - that is, you must pass a number, not a variable that may be optimized to a constant later. The number of cycles delayed by this builtin is exact.