They could.

but.... running PGO is just too much pain.

We can't do it "incrementally", can we? How about combining with LTO?

edit: I was thinking profiling individual module on a test driver and link them after PGO

I don't know if an optimization is allowed to "invent" a write, but I would be surprised if an optimizer goes that far because I have to believe that the number of cases where more writes improve performance are pretty slim.