You saw it yourself: unreachable generates no code and suppresses the function epilog (duh - it's unreachable) but null deref generated a trap instruction in GCC. If you wanted a trap instruction, maybe this compiler reliably generates one from a hardcoded null deref.
> "the definition of std::unreachable is: invokes undefined behaviour" does not mean that all undefined behaviour is equivalent to calling std::unreachable.
Well, obviously not. The definition of UB is that anything can happen, so by transitive property it means that std::unreachable can trigger any behavior.
That's the whole point of UB, and the point of my post: It's UB! You cannot reason about what will happen! The whole point of the post is "No you CANNOT reason about what will happen". So of course I am making no claims about what will happen, just three interesting observations about what did in fact happen when I happened to try it, to prove this point.
> If you wanted a trap instruction, maybe this compiler reliably generates one from a hardcoded null deref.
No, even in the examples in my post I show that it's not reliable. It's UB!
Hell, we get very different output just between -O1 (https://godbolt.org/z/33e1nGE38) and -O2 (https://godbolt.org/z/3coTqP31W).
When you say "this UB does ... (like generates a trap instruction)" you are being hyper specific for a given compiler version and platform and optimization level and any other knob. It may not stay the same after you update the compiler. And what happens a lot is that newer, better optimizations (such as more opportunity to autovectorization), which attract people to newer compiler versions, may exploit UB in more creative ways.
For example, to exploit autovectorization, compilers may want to push some test out of a loop. If this test contains UB, this optimization will change where the UB is triggered. So that trap instruction may happen at a different place in the code, depending on optimizations.