alt Hacker NewsIn practice, GCC -O2 will happily erase entire swathes of code and turn perfectly logical source into nonsense assembly whenever it gets as much as a sniff of UB anywhere in the code path. Nobody would be talking about UB if GCC wasn't so aggressive in abusing the freedom UB gives.
To paraphrase your earlier comment - you lose low-friction polymorphism (unpredictable compiler output causes a lot of friction). You lose control of memory allocations (because they may have been elided) and lose control of lifetimes (because free can be moved before last use causing a crash, or removed entirely causing a leak). You lose control of (almost) anything that happens in the system. And it has everything to do with not following the standard.
You do retain control of the memory layout of data structures, though.
Then I'm almost ashamed to admit that I'm not sure I've ever witnessed any surprising form of UB in the wild. For example, I will reliably get segfaults on NULL dereference in practice. Typical manifestations of UB are entirely predictable and obvious. Of course I'm also running most code without most optimizations, most of the time, while developing.
On the other hand, what I've observed with my own eyes is interesting phenomenons like performance drops, e.g. memory bandwidth dropping from gigabytes/sec to 300 KB/sec due to false sharing on an ARM SOC for example.