Comment by pcwalton

I notice that the paper doesn't claim to eliminate all reasoning about undefined behavior for optimizations. For example:

    int f() {
        int arr[3], i = 0;
        arr[3] = 5;
        return i;
    }

Optimizing this to "return 0" is relying on UB, because it's assuming that i wasn't laid out directly after arr in the stack frame. I believe this is what the paper calls "non-guardable UB".

I don't agree with the claim in the paper that their semantics offers a "flat memory model". A flat memory model would rule out the optimization above. Rather, the memory model still has the notion of object bounds; it's just simplified in some ways.