> memcpy(0x1, 0x1, 0) is a well-defined no-op

Sure about that?

It really should be a no-op. The C and C++ standards are seriously broken in this respect, and compiler authors take advantage of this brokenness to get away with miscompiling programs that, to anyone with common sense, are fine.

Well, just before I just saw a "read" and a "write" to address 0.

Not as mechanism that can cause serious bugs, when the compiler can statically prove length argument to be 0 and you can have NULL arguments as input.

Calls to memcpy with zero length are definitely not uncommon. Macros are an obvious way, but it's worse. All you need is a data flow where compiler can statically prove that all paths lead to zero length argument.

I have to agree it's very scary that memcpy(NULL, NULL, 0) is undefined.

Passing a non-null pointer that points outside your address space is made UB by the exact same sentence as the one that makes passing null UB.