> The DAG-ness means it's always available when compiling the call site, even if it's a cross-package call. It means you don't have to do it at link time.
Why is it any harder to do at link time?
(I've implemented this in a production compiler, and choosing whether to do it at compile time or link time was a trivial decision.)
traditionally, this required a linker that understands there is ir in the files.
in practice, i don't believe this has been a problem for many years now (and again, was only a problem in the open source world, so saying it's related to the language is kind of strange.).
Every good production C++ compiler has had some form of link time optimization for many years.
IBM's, for example, has been happily cross-optimizing between C++, java, fortran, PL/IX, etc without any issues, going on at least 15, maybe 25+ years now (I know it's 15 for sure, i suspect it's closer to 25).
Why is it any harder to do at link time?
(I've implemented this in a production compiler, and choosing whether to do it at compile time or link time was a trivial decision.)