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

Compilers in general are very smart.

What you must consider is that they must account for every possibly exception or situation where optimizing or re-factoring code could cause unwanted side-effects.

Things like, threaded programs, pointer aliasing, dynamically linked code and side effects (system calls/memory alloc) etc. make formally prooving refactoring very difficult.

Even though your example is simple, there still may be difficult situations to consider.

As for your StringBuilder argument, that is NOT a compilers job to choose which data structures to use for you.

If you want more powerful optimisations move to a more strongly typed language like fortran or haskell, where the compilers are given much more information to work with.

Most courses teaching compilers/optimisation (even acedemically) give a sense of appreciation about how making gerneral formally prooven optimisatons rather than hacking specific cases is a very difficult problem.