It's not that rare for optimizers to be able to find more optimizations in a subsequent pass. For example, it's often true (for at least small gains) in assembl

It's not that rare for optimizers to be able to find more optimizations in a subsequent pass. For example, it's often true (for at least small gains) in assembly peephole optimizers. Once you've applied one round of peephole optimizations, which are basically just simple pattern-matching replacing instruction sequences with better ones, the resulting, improved instruction sequence may now have new sequences that the peephole optimizer could improve again if run a second time. You could try running this iteratively until some kind of fixed point if compilation time were no issue... although, it might also have cycles! That is often the nature of these kinds of pattern-matching, heuristic optimization passes. That's especially likely if you mixed-and-matched from different compilers, like running optimization passes from gcc, clang, and icc interleaved with each other. Though that example would be difficult to do in practice, because they aren't source-to-source optimizers, and have totally different IRs, so can't easily be chained in the way these CSS-to-CSS optimizers can be chained.