@dabrahams I think that what @Joe_Groff is saying is that when library evolution is disabled, passing -cross-module-optimization to the compiler will provide you with some of what you want, today (ignoring the heuristic).
The main thing here is that we don't want to just do it blindly, we are worried about other implications like code-size for instance. So the idea has been, ok use a heuristic so that we can tune this thing to not have too much of a code-size impact/vs the perf win and carefully expand the cases that we support over time. So any test cases that you have would be very interesting.
That being said, my memory may be incorrect: +CC @Erik_Eckstein who is in this area.
One last thing. I do think that it would be useful to have a mode that does /all/ the things and then measuring the code-size/perf difference to help inform the decision. But I don't know if that is possible (I don't know how the heuristic is implemented, but Erik will).