Why does a 4 billion-iteration Java loop take only 2 ms?

There are one of two possibilities going on here:

1. The compiler realized that the loop is redundant and doing nothing so it optimized it away.

2. The JIT (just-in-time compiler) realized that the loop is redundant and doing nothing, so it optimized it away.

Modern compilers are very intelligent; they can see when code is useless. Try putting an empty loop into GodBolt and look at the output, then turn on -O2 optimizations, you will see that the output is something along the lines of

main():
    xor eax, eax
    ret

I would like to clarify something, in Java most of the optimizations are done by the JIT. In some other languages (like C/C++) most of the optimizations are done by the first compiler.