If the focus is on energy efficiency, I would want to see some
comparisons across various hardware platforms. Related to that. it would
presumably be instructive to see at least a histogram of hardware
instruction usage (since ultimately that is where the energy use is) and
a discussion of how that usage relates to the source language and
performance. I am having difficulty understanding the point of the
present paper, but perhaps I am missing something, lengthy IBM mainframe
experience may predispose me to a particular perspective.
On a broader issue, what seems an interesting question is whether
program performance can be analysed adequately by solely external means
(as contrasted with simply being time measured) or whether it requires
also some sort of internal instrumentation (as provided for instance by
some PL/I options).