Really? http://www.complang.tuwien.ac.at/kps2015/proceedings/KPS_201... https://news.ycombinator.com/item?id=11219874
While GCC and LLVM do care about implementing the standard correctly, they show little mercy to the poor developer who failed to avoid one of the many sharp corners of undefined behaviour.
There was a long thread about this just a few days ago (https://news.ycombinator.com/item?id=11219874)
I already commented there, and since i didn't always have pleasant things to say, i'm not going to repeat them here.