Leave code without obvious side effects alone (this is different from dead code)

What would "leave alone" even be? There's no "default" state of performance of Java code; it would be ridiculously stupid for there to be something saying that, say, "a+b" for int type values has to take at least 1 nanosecond or something. And you can't use big O complexity here either - the int type has a maximum of 2 billion, and thus a loop over it is trivially O(1), just with a potentially-big constant factor. (or, alternatively, the loop was sped up by a constant factor of 2 billion, and optimizing compilers should extremely obviously be allowed to optimize code by a constant factor)

But this is code obviously without side effect.

There are side effects, but in real world, not abstract

So non-obvious side effects?

You can say that about any code at all, so no optimising would ever be possible. The program running faster is a side effect after all.

No, that code has no side effects. The implementation is free to produce whatever side effects it wants or needs as part of execution, but that is absolutely none of the compiler's business.