Typically when you compile a Java program, it is first compiled into bytecode by a Java compiler. However, this bytecode is not optimized yet. In HotSpot (OpenJDK’s JVM) this instead happens at runtime, and is done by a JIT, a Just-In-Time compiler. This way of doing things allows the JIT to take maximum advantage of the conditions that the code is running in, such as hardware, and even to a certain degree the specific data that is fed into the program.

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This post provides techniques for debugging HotSpot's JIT compilation, including setting up a test project, printing assembly code, printing inlining traces, using compile commands, tracking down escaping objects, and debugging compilation using a native debugger.

Notes on debugging HotSpot's JIT compilation

2. Getting the assembly of a compiled method

6. Debugging compilation using a native debugger