Which assembler did Scala / Clojure use?

I am doing some research regarding compilers (to run the JVM). And I have successfully found two collectors that target this.

However, I found that one of them is Jasmin, which is not well supported (last update in 2010). And the other, Krakatoa, is pretty poorly documented.

I might notice that there are many mature JVM languages ​​out there. And I'm just wondering what he was using assembler? Or did they just make full use of raw buttock codes? As far as I understand, there is a lot more to it than simple bytecode instructions. And with assembler it will be much easier.

Are they any mature (well preserved and well documented) JVM builders? Or am I just starting from scratch, learning the basics of the JVM?