They do a good job but the scheduling aspects are really really fuzzy. LLVM and GCC both have models of out of order pipelines but other than making sure they

They do a good job but the scheduling aspects are really really fuzzy. LLVM and GCC both have models of out of order pipelines but other than making sure they don't stall the instruction decoder it's really hazy whether they actually do anything. The processors themselves are designed around the code the compilers emit and vice versa.