Unsigned integer overflow is well defined. And because of this, you see worse performance when working with unsigned integers in many cases, because the compiler needs to stick instructions in there to catch overflow at the appropriate width. It cannot just use hardware flags because that 32 bit unsigned integer needs to wrap correctly on both 32 bit and 64 bit machines.

Thus the word "efficiently" in my comment. By requiring weird and almost certainly buggy behavior to be defined, you require the program to behave more like an interpreted program, with associated performance costs.