Which major compiler is mostly implemented by academics? Neither GCC nor LLVM, for sure.
Us academics do have a lot of "fun" figuring out a way to put what the compiler developers do on solid footing [1]. But this is a game of whack-a-mole that will never end: each time we find a way to formally approach some new crazy optimization they came up with, and help them weed out all the bugs that were caused by not carefully thinking through all the implications [2], the next crazy optimization comes up [3].
Us academics do have a lot of "fun" figuring out a way to put what the compiler developers do on solid footing [1]. But this is a game of whack-a-mole that will never end: each time we find a way to formally approach some new crazy optimization they came up with, and help them weed out all the bugs that were caused by not carefully thinking through all the implications [2], the next crazy optimization comes up [3].
[1]: https://people.mpi-sws.org/~jung/twinsem/twinsem.pdf
[2]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752, https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82282, https://bugs.llvm.org/show_bug.cgi?id=35229, https://bugs.llvm.org/show_bug.cgi?id=34548
[3]: https://bugs.llvm.org/show_bug.cgi?id=21725, https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57359