But 100% code coverage isn't necessarily a useful thing in this context, as different code (and different options) mean different interactions between passes.

But 100% code coverage isn't necessarily a useful thing in this context, as different code (and different options) mean different interactions between passes. And you're never going to cover all the different paths through the compiler. Back when I worked on a commercial compiler, we had a smoke test which we ran before committing. It would take ~30 minutes to run. We had a longer, roughly eight hour, set of tests which we ran daily. And we had machines running tests 24/7, which managed to clock up roughly 20 million distinct test cases (combinations of code and options) over the course of a year. Every single compiler bug we'd ever seen was present (in a reduced form) in the conformance tests we'd run before release. That took about a week.