tracing jits are slightly harder to grasp than usual ones. The technique comes from real CPUs so the mindset of people behind the original idea is very different from the software world.

Metatracing ones are kind of an interesting twist on the original idea.

> So it takes normal interpreted code and jits it somehow?

Anyway, they use a patched LLVM to JIT-compile not just interpreted code but the main loop of the bytecode interpreter. Like, the C implementation itself.

> But you have to modify the source code of your program in some way?

Generally speaking, this is not normally the goal. All JIT-s try to support as much of the target language as possible. Some JIT-s do limit the subset of features supported.