[EdSchouten]

Exactly. This is also highly problematic if you try to perform atomic operations against bare 64 bit integers on those systems, because the atomic instructions do require them to be 8 byte aligned. In C11 and later it’s not an issue, because _Atomic(int64_t)’s alignment may be stricter than int64_t.

This was also an issue in Go, because it tries to use the same alignment rules as C to make cgo work. There they also solved it by adding dedicated atomic types.

https://github.com/golang/go/discussions/47141