reference from AVR LibC
http://www.nongnu.org/avr-libc/user-manual/group__avr__interrupts.html#define sei ( )
Enables interrupts by setting the global interrupt mask. This function actually compiles into a single line of assembly, so there is no function call overhead. However, the macro also implies a memory barrier which can cause additional loss of optimization.
In order to implement atomic access to multi-byte objects, consider using the macros from <util/atomic.h>, rather than implementing them manually with cli() and sei().
