[Panzer04]

“Decided that it’s fine to exploit it for optimisation tricks” is a poor characterisation. The reality is, if you define particular behaviours you will harm performance in some cases. If you define how something in particular should happen, then all architectures will need to implement that, regardless of their underlying semantics.

eg. C leaves the case of exceeding the size of an int undefined. In most cases it has a predictable effect on modern, mostly similar architectures but that is by no means guaranteed, and forcing an architecture to calculate overflow a particular way seems like a negative.

That being said, everyone has a pet example of a compiler doing some really odd and deep optimisations - I suspect that’s mostly due to successive layers and optimisers adding up to have unexpected effects, rather than a deliberate effort by compiler writers - but I’m no expert on the matter.