Exactly. Last time I tried clang I specified -flto aswell as -O4 which supposedly turns on lto but there was no changes to the binaries. I later read that due to linker dependencies it was (as of that time) only available for the OSX platform. Hopefully that has changed with newer versions of clang/llvm. Also does anyone know the status of PGO (profile guided optimization) in clang//llvm?