[sstangl]

They are optimizing only for code size, so they don't need to run the code: they only count bytes emitted.

This works by changing around the order / interleaving of various LLVM optimization phases, so the learning process does not require knowledge of program timing or correctness.

[liuliu]

I heard when they started to do this, LLVM folks were freaked out because they worried changing orders may affect correctness. But it seems to work so far :)