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@sjb @tedu
anyway, I think the overall pattern is:
1. C standard authors thought of some edge case with an unusual architecture that has a weird implementation of X
2. C standard authors opted to add some UB so that implementations don't need to worry about the edge case
3. the edge case doesn't apply to most architectures
4. but optimizing compilers are happy to use any UB you invoke as an excuse to delete your code