Why are compilers so stupid?

Question

I always wonder why compilers can\'t figure out simple things that are obvious to the human eye. They do lots of simple optimizations, but never something even a little bit

Answer 1

As others have addressed the first part of your question adequately, I'll try to tackle the second part, i.e. "automatically uses StringBuilder instead".

There are several good reasons for not doing what you're suggesting, but the biggest factor in practice is likely that the optimizer runs long after the actual source code has been digested & forgotten about. Optimizers generally operate either on the generated byte code (or assembly, three address code, machine code, etc.), or on the abstract syntax trees that result from parsing the code. Optimizers generally know nothing of the runtime libraries (or any libraries at all), and instead operate at the instruction level (that is, low level control flow and register allocation).

Second, as libraries evolve (esp. in Java) much faster than languages, keeping up with them and knowing what deprecates what and what other library component might be better suited to the task would be a herculean task. Also likely an impossible one, as this proposed optimizer would have to precisely understand both your intent and the intent of each available library component, and somehow find a mapping between them.

Finally, as others have said (I think), the compiler/optimizer writer can reasonably assume that the programmer writing the input code is not brain-dead. It would be a waste of time to devote significant effort to asinine special cases like these when other, more general optimizations abound. Also, as others have also mentioned, seemingly brain-dead code can have an actual purpose (a spin lock, busy wait prior to a system-level yield, etc.), and the compiler has to honor what the programmer asks for (if it's syntactically and semantically valid).