Comment by charleslmunger

This is sort of true, except that behavior can be undefined in the standard but defined in your implementation. One of the best examples is -fwrapv, which makes signed integer overflow not UB. And Fil-C has no undefined behavior that I'm aware of. But most implementations target the standard and "define" what the standard doesn't to be "whatever I feel like, in service of implementation simplicity and performance". What matters is what your implementation guarantees, which should be a superset of what the standard does.