Comment by Joky

LLVM instruction selection is slow, there is a "fast-path" which hasn't received much attention (it is only used for -O0 in clang). The new instruction selector work just started and will take a couple of years, considering the tradeoff between spending 3 months on it and waiting a few years for LLVM to be improved (without any guarantee of LLVM reaching the same speed as what they did). See also some thoughts from a LLVM developer on optimizing for high-level languages: http://lists.llvm.org/pipermail/llvm-dev/2016-February/09546...