Introduction to JVM JIT Optimizations

Diego Parra | @dpsoft | Core Team @ Kamon

Agenda

  • What is JIT?
  • Tiered Compilation
  • Optimizations
  • Conclusion

JIT

  • Just-In-Time compilation
  • Compilation work happens during application execution
  • JIT'ing requires Profiling for more efficient optimization
    • ​Because you don't want JIT everything
    • Aggressively optimize  based on profile information
    • Tiered compilation
    • Runtime Overhead?

 

JIT Lifecycle

JIT Compilers

C1 Client

C2 Server

VS

  • Fast compiler
  • Invocation count > 1500
  • Produces Compilations quickly
  • Generated code runs Slow
  • Smart compiler
  • Invocation count > 10000
  • Produces Compilations slowly
  • Generated code runs Fast

 Profiled Guided

Speculative

 

10x faster than Interpreter

2x faster than C1

Tiered Compilation

  • Available in Java 7
  • Default in Java 8
  • Client Startup
  • Server Performance

Best of Both Worlds - C1 + C2

Common Transitions

Common Transitions Patterns : https://www.slideshare.net/maddocig/tiered

JIT Optimizations

Compiler tactics

  • delayed compilation
  • tiered compilation
  • on-stack replacement
  • delayed reoptimization
  • program dependence graph representation
  • static single assignment representation

Speculative (profile-based)

  • optimistic nullness assertions
  • optimistic type assertions
  • optimistic type strengthening
  • optimistic array length strengthening
  • untaken branch pruning
  • optimistic N-morphic inlining
  • branch frequency prediction
  • call frequency prediction

Proof-based techniques

  • exact type inference
  • memory value inference
  • memory value tracking
  • constant folding
  • reassociation
  • operator strength reduction
  • null check elimination
  • type test strength reduction
  • type test elimination
  • algebraic simplification
  • common subexpression elimination
  • integer range typing

Flow-sensitive rewrites

  • conditional constant propagation
  • dominating test detection
  • flow-carried type narrowing
  • dead code elimination

Text

and much more..

Null check Elimination

In equivalent machine code

Null check Elimination

Into the rabbit hole

-XX:+PrintCompilation

Compilation Level

Deoptimizations

Null check Elimination

Into the rabbit hole

  • -XX:+UnlockDiagnosticVMOptions
  • -XX:+PrintAssembly
  • -XX:CompileCommand=print,NullCheck::isPowerOfTwo

Someone said Assembly?

 

Null check Elimination

Into the rabbit hole

implicit exception: dispatches to 0x00007fdff5318274

Null check Elimination

  • JVM uses SIGSEGV as control flow

Into the rabbit hole

  • Speculative Optimization

  • Faster IF not null are encountered (Uncommon Trap)

Implicit Null Check?

  • Field and Array access is null checked

Method Inlining

-XX:+PrintInlining

Method Inlining

  • Inlining is the most important optimization
  • Combine caller and callee into one unit
  • Expands the scope for other optimizations
  • Bytecode size < 35 bytes
  • Bytecode size < 325 bytes for inlining hot methods

What about virtual call inlining?

Monomorphic

Bimorphic

Megamorphic

  • Class Hierarchy Analysis
  • Devirtualization

On-Stack Replacement

-XX:+PrintCompilation

-XX:+PrintInlining

 Compilation is OSR

Dead Code Elimination

Result is not used and the entire method is optimized away!

Escape Analysis

  • No synchronization lock when calling getNoEscapeSum method
  • No allocate a Sum object at all, just keep track of the individual fields of the object

Deoptimization

  • The compiler can back to previous versions of compiled code
  • Effect in performance when code is deoptimized
  • Deoptimize when previous optimizations are no longer valid

There are two cases of deoptimization

  • not entrant -> don't let new calls enter
  • zombi -> on this way to deadness

Loop Unrolling

Unrolled!

Intrinsics

  • Special code build-into the VM for a particular method
  • Often use special hardware capabilities
  • Not writen in Java

JITWatch 

Conclusion

  • Please use JMH, Don't lie to yourself
  • Code will be Optimized and De-optimized
  • JVM require Warmup
  • JVM is Smart
  • Keep It Simple, Stupid!, is perfect for JIT to make his job
  • Learn Assembly

Resources

Questions?

JIT Optimizations

By Diego Parra

JIT Optimizations

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