[cesarb]

No. Nothing guarantees that the array is aligned within the stack frame, even if the stack frame is aligned. What if the compiler introduced a boolean flag (for instance, a drop flag) immediately before the array, in the same stack frame?

[mdip]

Good point, here. As is often said, when the documentation says "undefined behavior", it means the compiler can do whatever it wants, including "work just fine"; and sometimes it'll cause time travel[0]. Hence the "nasal demons" lore. Often, it'll cause optimizations to be applied that would have otherwise been avoided resulting in a bug that appears to occur somewhere else and a programmer to look at the result of execution and ... if it actually continues executing ... swear a lot. These are especially fun because the problem frequently won't appear in debug builds.

[0] https://blogs.msdn.microsoft.com/oldnewthing/20140627-00/?p=... - worth a read for some entertainment - basically what happens when the compiler assumes "undefined behavior" can't happen and optimizes accordingly.