[ritter2a]

There will probably always be particular code transformations (algorithmic or target-machine-specific) that speed up your code and that your general purpose compilers will not find. (And that's good, because otherwise the compile time would be horrible!)

The good thing about compiler optimizations is not that they produce actual "optimal" binaries, but that they allow you to not care too much about performance when writing code (e.g. using abstractions that will be inlined, etc.) and still get a decent binary. If you then require more performance (which will probably not be the case for most applications) you can still search for bottlenecks and start optimizing by hand there.