[nn3]

>Secondly, if you think any compiler is meaningfully doing anything optimal >>("whole program analysis") on a TU scale greater than say ~50kloc (ie ~10 files) >relative to compiling individually you're dreaming.

That's wrong. gcc generates summaries of function properties and propagate those up and down the call tree, which for LTO is then build in a distributed way. It does much more than mere inlining, but even advanced analysis like points to analysis.

https://gcc.gnu.org/onlinedocs/gccint/IPA.html https://gcc.gnu.org/onlinedocs/gccint/IPA-passes.html

It scales to millions of lines of code because it's partioned.