[ori_b]

There are rules and requirements documented in the spec, and there are de-facto rules and requirements that programs expect. Not only that, but when they do exploit these rules, often the code generated is obviously incorrect, and could have been flagged at compile time.

Right now, it seems like compiler vendors are playing a game of chicken with their users.

[saagarjha]

I think the issue is that many of these "obviously incorrect" things are not obvious at the level that the optimizations are taking place. Perhaps it would be worth considering adding higher-level passes in the compiler that can detect these kinds of surprising changes and warn about them.

[a1369209993]

Well, no, the issue is that the compiler writers refuse to acknowledge the these obviously incorrect things are incorrect in the first place and tend to blame users for tripping over compiler bugs. If it were just that they didn't know how to fix said bugs, that would be a qualitatively different and much less severe problem.

[mpweiher]

> not obvious at the level that the optimizations are taking place

Hmm...then it's up to the optimisers to up their game.

Optimisation is supposed to be behaviour-preserving. Arguing that almost all real-world programs invoke UB and therefore don't have well-defined behaviour (by the standard as currently interpreted) is a bit of a cop-out.