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HotSpot Internals

This wiki is written by and for programmers working on the HotSpot JVM system. This includes programmers of systems which generate JVM bytecodes (i.e., language implementors).

Contents

• Building
  • IncludeDB
• Compiler
  • C2 Register Allocator Notes
  • EscapeAnalysis
  • Graph Coloring Register Allocation Papers
  • HotSpot Tools
    • IdealGraphVisualizer
    • LogCompilation tool
      • PrintCompilation
  • LogCompilation overview
  • Overview of Ideal, C2's high level intermediate representation
• PerformanceTechniques — What code shapes does the JVM optimize best? Here is a list.
  • Inlining — Methods are often inlined, widening the compiler's "horizon" of optimization.
  • LogCompilation
  • MicroBenchmarks — Here are some rules, in priority order, to know about when you write micro-benchmarks for Hotspot.
  • PerformanceTacticIndex
  • PrintAssembly — Puzzled by a performance glitch? You might have to look at the generated code.
  • RangeCheckElimination — For loop-invariant arrays, range checks can usually be eliminated.
• Publications JKU
• Runtime
  • CallingSequences — A calling sequence is a contract between two blocks of code, a call site (within a caller) and and entry point (within a callee).
    • InterfaceCalls
    • StaticCalls
    • VirtualCalls
  • CompressedOops
  • JavaControlStack
  • Synchronization
• Serviceability
• StorageManagement
  • StoreBarriers — Changes to the heap's object graph need to be tracked by the GC; such tracking is done via store barriers.
• StyleGuide — How will my new code best fit in with the Hotspot code base? Here are some guidelines.

Intentions and non-intentions

This wiki contains:

• An architectural guide to the Runtime, Compiler, GC, Serviceability, etc.
• Details on performance techniques.
• A place to document likely future work (major refactorings or cleanups, new optimizations, etc.).
• Quick links to similar information in other places on the web.
  • http://java.sun.com/javase/technologies/hotspot ("Java SE HotSpot at a Glance")
  • http://openjdk.java.net/groups/hotspot/ (OpenJDK HotSpot Project home)
  • HotSpot-related research at the Johannes Kepler University Linz
• Information which, once written, is expected to remain accurate and relevant, as long as the related code remains stable.

Please look for the following types of information elsewhere:

• Not non-architectural or non-technical developer notes.
• Not an introduction or quick-start for working on the JVM. (TO DO: Put links here.)
• Not build instructions.
• Not a substitute for comments in the source code.
• Not a list of bugs or tasks.
• Not a blog, forum, or the comments section thereof.

Interesting tasks to do

• Extract links and info. from messages and blog postings, especially via hotspot-dev@openjdk.
• Extract links and info. from historical Sun mailing lists and wikis. (Sun engineers have to do this!)
• Write down understandings of major modules as they are (re-)acquired from trips through the code.

Who writes this wiki?

• We are using the Community Write pattern.
• There is no public write access, except for leaving page comments. You must be registered and logged in to leave comments.
• Please do add page comments when a page should answer your question but doesn't. (That means, potentially on any page.)
• The most effective page comments will include links to relevant email discussion threads, e.g., on the Hotspot compiler-dev list.
• Any member of the OpenJDK Hotspot Group or a related group should feel free to contribute.
• Actually, anybody with a (demonstrable) working knowledge of the Hotspot internals will be welcome to contribute content.
• Write permissions are not automatic until and unless the DBs get on speaking terms. If you think you should have write permission and don't contact the wiki administrator.