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

Actually, Java should use string builder in your second example.

The basic problem with trying to optimize these examples away is that doing so would require theorem proving. Which means that the compiler would need to construct a mathematical proof of what you're code will actually do. And that's no small task at all. In fact, being able to prove that all code really does have an effect is equivalent to the halting problem.

Sure, you can come up with trivial examples, but the number of trivial examples is unlimited. You could always think of something else, so there is no way to catch them all.

Of course, it is possible for some code to be proven not to have any effect, as in your examples. What you would want to do is have the compiler optimize away every problem that can be proven unused in P time.

But anyway, that's a ton of work and it doesn't get you all that much. People spend a lot of time trying to figure out ways to prevent programs from having bugs in them, and type systems like those in Java and Scala are attempts to prevent bugs, but right now no one is using type systems to make statements about execution time, as far as I know.

You might want to look into Haskel, which I think has the most advanced theory proving stuff, although I'm not sure on that. I don't know it myself.