c++ - Why is a = i + i++ undefined and not unspecified behaviour

Question

I read through several very good answers about undefined behaviour and sequence points (e.g. Undefined behavior and sequence points) and I understand, that

   int i = 1;
   a = i + i++; //this is undefined behaviour

is undefined code, according to the C++ standard. But what is the deeper reasoning behind it being undefined behaviour? Wouldn't it be enough to make it unspecified behaviour? The normal argument is, that by having few sequence points, C++ compilers can optimize better for different architectures, but wouldn't leaving it unspecified allow those optimizations as well? In

   a = foo(bar(1), bar(2)); //this is unspecified behaviour

the compiler can also optimize, and it is not undefined behaviour. In the first example it seems clear, that a is either 2 or 3, so the semantics seems to be clear to me. I hope there is a reasoning, why some things are unspecified, and others are undefined.

Answer 1

Not all of those optimizations. Itanium, for example, could perform both the add and increment in parallel, and it might cough up, say, a hardware exception for trying to do something like this.

But this is a completely micro-optimization, writing the compiler to take advantage of that was extremely difficult, and it's an extremely rare architecture that can do it (none existed at the time, IIRC, it was mostly hypothetical). So the reality is that as of 2012, there is no reason for it not to be well-defined behaviour, and indeed, C++11 made more of these situations well-defined.