Building a stage3 compiler as a(n optional?) compiler perf test in CI
I think a stage3 build would make for a marvellous ghc/alloc test case. We could compare the ghc/alloc metric between the stage1 (or the stage2 compiler of a previous an ancestor commit) and the stage2 compiler.
Although we'd have to be careful to build stage1 and stage2 with the same flags throughout. So probably better just compare to the stage2 metric of the ancestor commit.
It's also a pretty good way to assess whether a new optimisation should go in -O1 or -O2.
Maybe we could make one of the full-build jobs build the stage3 compiler?