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

Many standards for many kinds of things expend a lot of effort on describing things which implementations SHOULD or SHOULD NOT do, using nomenclature similar to that defined in IETF RFC 2119 (though not necessarily citing the definitions in that document). In many cases, descriptions of things that implementations should do except in cases where they would be useless or impractical are more important than the requirements to which all conforming implementations must conform.

Unfortunately, C and C++ Standards tend to eschew descriptions of things which, while not 100% required, should nonetheless be expected of quality implementations which don't document contrary behavior. A suggestion that implementations should do something might be seen as implying that those which don't are inferior, and in cases where it would generally be obvious which behaviors would be useful or practical, versus impractical and useless, on a given implementation, there was little perceived need for the Standard to interfere with such judgments.

A clever compiler could conform to the Standard while eliminating any code that would have no effect except when code receives inputs that would inevitably cause Undefined Behavior, but "clever" and "dumb" are not antonyms. The fact that the authors of the Standard decided that there might be some kinds of implementations where behaving usefully in a given situation would be useless and impractical does not imply any judgment as to whether such behaviors should be considered practical and useful on others. If an implementation could uphold a behavioral guarantee for no cost beyond the loss of a "dead-branch" pruning opportunity, almost any value user code could receive from that guarantee would exceed the cost of providing it. Dead-branch elimination may be fine in cases where it wouldn't require giving up anything, but if in a given situation user code could have handled almost any possible behavior other than dead-branch elimination, any effort user code would have to expend to avoid UB would likely exceed the value achieved from DBE.