The information rustc can hand to llvm should allow for great optimization, but llvm's optimizer has bugs that mean rustc can't pass much of it, or the optimize

The information rustc can hand to llvm should allow for great optimization, but llvm's optimizer has bugs that mean rustc can't pass much of it, or the optimizer won't make a valid binary (it will crash). Also, llvm's optimizer is way behind gcc's (way way behind intel's)for hpc type stuff. So clang is probably on par with rustc, but GCC or ICC (and certainly ifort) will outperform rustc in the hpc arena until llvm catches up.