client compiler for the java hotspot virtual machine … · session 3198 client compiler for the...

S ess ion 3198

Client Compiler for the Java HotSpot™ Virtual Machine

Tom Rodriguez, Ken RussellSun Microsystems, Inc.

Technology and Application

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Overall Presentation Goal

L earn about compilation in the J ava HotS pot™ V irtual Machine, and the C lient C ompiler

Understand how the C lient C ompiler deals with specific J ava™ programming language features

G et to know tuning and trouble- shooting techniques

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Learning Objectives

• A s a result of this presentation, you will be able to:– Understand “J ava HotS pot compilation”– Write better code in the J ava programming

language– Improve the performance of your applications– S ee what’s new in 1.4 and what’s coming

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Speaker’s Qualifications

• T om R odriguez is the technical lead for the J ava HotS pot C lient C ompiler

• K en R ussell has worked on the J ava HotS pot runtime and compiler for over two years

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Presentation Agenda

• C ompilation in the J ava HotS pot™ V M

• S tructure of the C lient C ompiler

• Implications for C ode written in the J ava™ P rogramming L anguage

• Miscellaneous

• S ummary

• Q &A

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Compilation in theJava HotSpot™ VM

• V M C onfigurations

• C ompilation S teps

• O n-S tack R eplacement

• C lass Hierarchy A nalys is

• D eoptimization

• Q uick S ummary

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VM Configurations

O S L ibraries

O S

J ava

Hardware

libjvm.so/jvm.dll

Typ

ica

l Ja

va V

M S

oft

wa

re S

tack

?C ore V M

C lientC ompiler

S erver C ompiler

javajava-client

java- server

T opic of T his T alk

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Compilation Steps

• E very method is interpreted firs t

• Hot methods are scheduled for compilation– Method invocations– L oops

• C ompilation can be foreground/background– F oreground compilation default for C lient V M– Background compilation in parallel

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OnStack Replacement (1)

• C hoice between interpreted/compiled execution

• P roblem with long- running interpreted methods– L oops!

• Need to switch to compiled method in the middle of interpreted method execution– O n-S tack R eplacement (O S R )

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OnStack Replacement (2)

S tackbefore O S R

void m1() { ... while (i < n-1) { // OSR here ... } ...}

dead m1frame

m0 frame

S tackafter O S R

m1compiled

frame

m1interpreted

frame

m0 frame

O S R

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Class Hierarchy Analysis (1)

• D ynamic pruning of receiver class set– S tatic calls instead of virtual calls– Inlining across virtual calls– F aster type tests

• C lass Hierarchy A nalys is (C HA )– A nalys is of loaded classes– C an change over time– E ffective

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Class Hierarchy Analysis (2)

Avirtual m

Bvirtual m

Dvirtual n

Cvirtual m

Evirtual n F

extends

loaded

unloaded

a.m A .m, B .mb.m B.md.m A .md.n D .nf.m A .mf.n D .n

Gvirtual m

A a;B b;C c;D d;E e;F f;G g;

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Deoptimization (1)

• C ompile- time assumptions may become invalid over time– C lass loading

• D ebugging of program des ired– S ingle- s tepping

• A ctive compiled methods become invalid

• Need to switch to interpreted method in the middle of compiled method execution– D eoptimization

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Deoptimization (2)

T3 m3(...) { ... // Deopt. here}

T2 m2(...) { m3(...);}

T1 m1() { ... m2(...); ...}

S tack

m0 frame

m1interpreted

m2interpreted

m3 interpr.

m0 frame

comp. m1w/ inlinedm2, m3frame

D eopt.

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Quick Summary

• C lient V M differs from S erver V M in compiler

• Hotspots trigger compilation– C ompiled method invocation– O S R

• C lass loading, debugging changes compile- time assumptions– D eoptimization

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What’s New in 1.4

• L ow- level Intermediate R epresentation

• D eoptimization

• Inlining

• Improved D ebugging and P rofiling S upport

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Deoptimization and Inlining

• 1.3.x client compiler inlined s imple accessors

• 1.4 uses deoptimization and C HA– More inlining poss ibilities

• 1.4.0 client compiler does not inline:– Methods with exception handlers– S ynchronized methods

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Improved Debugging and Profiling

• F ull- speed debugging– C ompiler no longer disabled when us ing

debuggers (-X debug)– A pplication runs at full speed• Breakpoint setting causes deoptimization

– S erver compiler support coming in 1.4.1

• P rofiling– S ubstantial work on J V M P I robustness– E xisting tools work much better with 1.4

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Structure of the Client Compiler

• R epresentation of bytecodes

• F rontend

• B ackend

• Machine code generation

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Representation of Code

• High L evel Intermediate (HIR )– C losely mimics bytecode– D irected graph to enforce evaluation order

• L ow L evel Intermediate (L IR )– C lose to machine instructions– S ome high level instructions• V irtual calls• T ype checks• Intrins ics

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Frontend

• P arse bytecodes into HIR– E liminate loads (copy propagation)– L ocal value numbering– C onstant folding and identities– Inline– Use C HA to optimize calls

• G lobal O ptimizations– Block merging– E liminate null checks

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Intermediate Representation

basic block

instructions

successor(s)

CFG

Bytecodes

0 aload_1 1 bipush 46 3 invokevirtual #139 6 istore_2 7 iload_2 8 ifgt 1811 aload_012 getfield #215 invokevirtual #9318 aload_119 iconst_020 ...

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Backend

• G enerate L IR by walking HIR– E xpress ion level register allocation

• O ptimize L IR– A ss ign locals to registers• L oop focused

– P eephole optimizer

• E mit machine code from L IR– G enerate object maps for use by G C– G enerate information for deoptimization

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Quick Summary

• F rontend does– P arse and analyze bytecodes– Build and optimize IR– Use C HA for very effective O O optimizations– R eorder C F G for code generation

• B ackend does– L IR– R egister allocation, low- level optimizations– C ode generation

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Implications for Code Written in the Java™ Programming Language

• A ccessors

• Usage of final

• O bject A llocation (new)

• E xception Handling

• O ther Issues

• Q uick S ummary

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Accessors

• Use accessors

– XType getX() { return _x; }

– void setX(XType x) { _x = x; }

• A bstracts from implementation

• E as ier to maintain

• No performance penalty– Inlining!

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Usage of final

• D on’t use final for performance tuning

• C HA will do the work

– Where C HA can’t do it, final doesn’t help either

• K eep software extens ible

• No performance penalty

– S tatic calls

– Inlining

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Object Allocation (new)

• O bject allocation (new) inlined

– Works in most cases

– E xtremely fast (~ 10–20 clock cycles )

• D o not manage memory yourself

– G C will s low down

– L arger memory footprint

• K eep software s imple

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Exception Handling

• E xception object creation is very expens ive

• E xception handling is not optimized– Use it for exceptional s ituations– D on’t use it as programming paradigm– D on’t use instead of regular return

• E xception handling costs only when used– S afe to declare exceptions

• If you must use exceptions for control flow:– P reallocate exception object

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Other Issues

• C lient C ompiler optimized for clean O O code– “Hand- tuning” often counterproductive– G enerated code can be problematic• O bfuscators

• D o not optimize prematurely– Use profiling information

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Quick Summary

• Write clean O O code– Use accessors– Use final by des ign only– Use new for object allocation– Use exception handling for exceptional cases

• K eep it s imple, keep it clean

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Miscellaneous

• B uilt- in P rofiler

• When to use the C lient C ompiler

• What’s coming in 1.4.1

• Q uick S ummary

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BuiltIn Profiler

• O ption: -X prof– E .g.: java -X prof - jar J ava2D emo.jar

• S tatis tical (sampling) flat profiler– Not hierarchical

• P er thread– O utput when thread terminates

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Sample Profiler Output

Flat profile of 27.38 secs (2574 total ticks): AWT-EventQueue-0

Interpreted + native Method 7.2% 0 + 90 sun.java2d.loops.Blit.Blit 0.7% 0 + 9 sun.awt.windows.Win32BlitLoops.Blit ... 19.8% 72 + 174 Total interpreted (including elided)

Compiled + native Method 9.2% 115 + 0 java.awt.GradientPaintContext.clip... ... 15.0% 179 + 8 Total compiled (including elided)

Thread-local ticks: 51.7% 1330 Blocked (of total) 0.2% 2 Class loader 0.3% 4 Interpreter 10.0% 124 Compilation 0.5% 6 Unknown: running frame 0.2% 2 Unknown: thread_state

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When to Use the Client Compiler

• C lient C ompiler characteris tics– F ast compilation• Q uick s tartup time

– S mall footprint

• Use for apps with same expectations

• R ecommendation– T ry client and server, choose best• java - client• java - server

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What’s New in 1.4.1

• B etter peephole optimization

• R educed footprint for compiled code

• New fast subtype check

• Inlining of js rs

• Improved exception handling– Better full- speed debugging

• R educed footprint on the S olaris™ O E

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Overall Summary

• C ompilation with the J ava HotS pot™ V M

• C lient C ompiler Internals

• P rogramming and T uning Hints

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References

• R obert G riesemer, S rdjan Mitrovic: “A C ompiler for the J ava HotS pot™ V irtual Machine”, in The School of Niklaus Wirth—The Art of Simplicity, Morgan K aufmann P ublishers , 2000, IS B N 1-55860-723-4

• J ava HotS pot™ T echnology D ocuments, http://java.sun.com/products/hotspot/2.0/docs .html

S ess ion 3198

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