Can branches with undefined behavior be assumed unreachable and optimized as dead code?

Question

Consider the following statement:

*((char*)NULL) = 0; //undefined behavior

It clearly invokes undefined behavior. Does the existence of suc

Answer 1

If the program reaches a statement that invokes undefined behavior, no requirements are placed on any of the program's output/behavior whatsoever; it doesn't matter whether they would take place "before" or "after" undefined behavior is invoked.

Your reasoning about all three code snippets is correct. In particular, a compiler may treat any statement which unconditionally invokes undefined behavior the way GCC treats __builtin_unreachable(): as an optimization hint that the statement is unreachable (and thereby, that all code paths leading unconditionally to it are also unreachable). Other similar optimizations are of course possible.