Even things like instruction selection could be pretty interesting. People think compilers are basically sentient but they really aren't, even ignoring the tuning parameters, they really do need help when you get the edges of what the ISA designers incorporate.

Similarly, try and work out how a compiler schedules and lays out code for a modern superscalar processor. Both things do matter (potentially a lot) but it's not like the old days where you have a very fixed model of the pipeline to evaluate your schedule with (or a simple one at least)