This might be a bit too late to reply. It’s undefined in C, but it’s well defined at a processor level (on x86). IIRC, there was one particular bit of code that

This might be a bit too late to reply. It’s undefined in C, but it’s well defined at a processor level (on x86). IIRC, there was one particular bit of code that’d crash on AArch64 due to unaligned pointers, but would run a bit faster than aligned code on x86-64. On the other hand, this trick wouldn’t work in multithreaded code even if the semantics were well defined, due to false sharing of cache lines.