Understand JIT Compilation in the JVM: Interpret -> C1 -> C2

Tiered JIT in HotSpot

The JVM starts in the interpreter, then promotes hot methods through C1 (fast, lightly optimized) to C2 (aggressive optimizing). Tiered compilation balances startup time against peak throughput.

The Pipeline

Typical progression:

1. Interpreter: zero compile cost, high per-bytecode overhead
2. C1 (client): quick native code, some inlining and profiling hooks
3. C2 (server): full optimization after counters prove a method is hot

Method invocation counters and back-edge counters trigger compilation. OSR (on-stack replacement) compiles loops inside long-running methods without waiting for the next entry.

For example, a server loop may run interpreted for thousands of iterations, hit C1, then C2 with inlined callees and escape analysis.

Flags like \`-XX:+TieredCompilation\` control the policy on modern JDKs.

Design Notes

Compile thresholds exist because compiling everything at startup would slow launch unbearably. Server workloads benefit from higher thresholds; interactive apps sometimes lower them for snappier first interactions.

For This Exercise

This exercise asks you to document the interpret-to-C1-to-C2 pipeline. You will explain what triggers each tier and why cold code stays interpreted while hot paths get native code.

You will use the same mental model here when reading production interpreter source later in the track. Sketch one concrete input on paper, predict the outcome, then confirm with code. That discipline catches logic errors early and makes debugging far faster when you extend the implementation in follow-on tasks.