For more QCon posts, see my live blog table of contents. This presentation is about the compiler and also the runtime.

Major pieces

• Execution engine
  • Heap management/garbage collection
  • JIT compilation
• Runtime
  • VM Class loading
  • Interpretter
  • Byte code verification,etc

Runtime goal – convert from bytecode to native code and do optimizations along the way

Compilation Techniques and Notes

• Pre-compiled/ahead of time
• profile guided – based on critical hotspots
• Adaptive optimization (Java uses Profile guided and Adaptive optimization)
• Identify root of compilation
• replace method or on stack – depends on number of times through loop
• Server compiler has a higher threshold than client compiler for the threshold at which you need optimizations
• Tiered compilation – tier 1 is client compiler with no profiling info, tier 2 and 3 are client compiler with profiling info. Then comes server compiler
• CodeCache order of magnitude larger when tiered compilation is enabled. If need more can use -XX:ReservedCodeCacheSize
• Inlining – many different parameters when figuring out when to inline
• Vectorization – SIMD (SIngle Instruction Multiple Data). Can generate stubs and benefit from caching size chunks. For SuperWord Level Parallelism, you need to unroll the loop, do analysis/pre-optimization, etc. Still in infancy with Hotspot.
• Escape analysis – Want to see if object only is used in a compile method. Need entire graph to confirm not in a static field/returned from method/passed as parameter/etc. If really local, can optimize by storing in registers.
• Objects are 8 byte aligned by default. Fields are aligned by type.
• OOP (ordinary object pointer) is a managed pointer. The size can be changed to optimize.
• Compressed Class Pointers – part of the Metaspace. Class data is outside of heap.

Deoptimization

1. dependency issues
2. class unloading/redefinition
3. uncommon path
4. profiled info isn’t useful for path [like with databases when db assumes something different than you want]

If curious about details

To get information about what compiler thinks/did:

• PrintCompilation – ex: what level instructions were compiled at
• PrintInlining – use -XX:+UnlockDiagonsticVMOptions