On Tue, Feb 06, 2007 at 04:14:30PM -0800, Ian Lance Taylor wrote: > I also think it would be good to have one option affecting it: turn > __builtin_unreachable() into an abort(), or turn it into a "cannot be > reached" marker. I think the former should be the default at -O0, the > latter at -O1 and above. > > Note that a "cannot be reached" marker permits further optimization to > delete code which leads up to it. Actually implementing that further > optimization may call for adding appropriate warnings, probably > controlled by -Wunreachable-code.
Ouch. In the case that motivated this discussion, the instruction that would immediately go before the __builtin_unreachable() is a trap instruction, but the compiler does not know that it traps. It would be very bad if the compiler eliminated the trap, since it is the presence of the trap that keeps the function from returning.
