There's a lot of work going on in link time optimization at the moment, both in LLVM and GCC. It's not quite ready for prime time, it still takes more than a small change in your Makefile to deploy it (e.g. dealing with linkers etc).

With LLVM toolchain you can compile C code (or other high level code) into LLVM IR, link the IR files together and run that through the optimizer.

You will notice that modern optimizers will want to inline everything if possible and a lot of functions will be missing from the resulting binary. Boundaries of object files are perhaps the biggest obstacle in optimization today.

With the automake-based build system you'd pass "--enable-final" and the buildsystem would cat all the source files together and compile the whole damn thing at once (and really stress-test the kernel and gcc).

With KDE 4 I believe it is -DKDE4_ENABLE_FINAL=TRUE passed to cmake.

It was never quite 100%... sometimes you'd run into things like different source files in a modules declaring the same class name, insufficiently-namespaced header include guards, etc. But it was definitely interesting.