How can `a+b` be almost 4× faster than `b+a`?

Interesting.

So, how do you find out what optimization does the JIT? With compiled languages, you can look at the assembly to figure out which optimizations were applied.

What can you do with JIT? Are there JIT implementations that provide "asm-like" output or any information about the optimizations done? What about Java? Is it also opaque like in JS?