alt Hacker News> No - if something is UB in the spec, it's UB.
A compiler is still free to ignore the spec and declare that something is not UB. However, this is very much compiler based, not platform based. Windows might guarantee that aligned DWORD-sized memory accesses are atomic, but that doesn't mean Clang when compiling for Windows would respect this - but MSVC might.
No, a compiler obviously cannot do this. nothing is undefined behaviour under a known compiler, version, and settings. UB means you can't know what the code does in general not that you can't know what it does in a very specific case.