An extension to this infact is the following - https://github.com/llvm-mirror/llvm/blob/f36485f7ac2a8d72ad0e0f2134c17fd365272285/lib/Transforms/Scalar/LoopIdiom

An extension to this infact is the following - https://github.com/llvm-mirror/llvm/blob/f36485f7ac2a8d72ad0e0f2134c17fd365272285/lib/Transforms/Scalar/LoopIdiomRecognize.cpp#L960 https://github.com/llvm-mirror/llvm/blob/f36485f7ac2a8d72ad0... So the original user's comment might infact get detected by this and get optimised down to using popcnt. Will need to try it out. :-)