> you can use an AI to generate vaguely realistic PGO data (profile guidance is where the real speed comes from)

That's not a bad idea, but PGO is pretty limited. I'm thinking about things like rewriting a loop to use vector instructions; something which compilers are absolutely terrible at but ChatGPT is actually pretty good at.

PGO is limited if you are chasing throughput on really tight programs that spend most of their time in a small number of loops (preferably with textbooks written about them), but it's really useful if you have a bunch of branchy code that mainly does one thing but has to cater for other things too (compilers come to mind).

Enabling PGO when building the D compiler made it almost 50% faster on some parts of the test suite (and fairly reasonable big files at that) - speedup elsewhere was not as dramatic but still a tidy win.

50% is probably a pretty extreme case for PGO, but when rewriting code to use vector instructions instead of scalar often has a speedup between 2x and 4x. Both approaches are interesting is all I'm saying.