CompilerDesignSpring2017

10Intermediaterepresentation(s)

Dr.Zoltán Majó

CompilerGroup– JavaHotSpot VirtualMachineOracleCorporation

Outline

§ DesignconcernsforIR

§ SSA:StaticSingleAssignment

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Simplecompiler:Tree-basedIR

§ Oneassignmentstatement:onebinary treeint A, B, C, D;

A = B + C * D;

§ Atreehasaroot§ Topnode§ Rightandleftsubtrees

§ Sequencesofstatements:forest oftrees

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x = a + b;

d = x + 1;

b = a + c;

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IRconcerns

§ Makedatadependencesexplicit§ Captureproducer-consumerrelationship

// 1 // x = a + b ;

// 2 // d = x + 1;

// 3 // b = a + c;

§ x producedby1,consumedby2

§ b in1mustbereadbeforeb iswrittenin3

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IRconcerns

§ Makedatadependencesexplicit§ Captureproducer-consumerrelationship

// 1 // x = a + b ;

// other basic blocks //

// 2 // d = x + 1;

// 3 // b = a + c;

§ x producedby1,consumedby2

§ b in1mustbereadbeforeb iswrittenin3

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§ Simplesolution:execute1,2,and3inorder

§ Whatmattersis1readstheold value(ofb)and3producesanew value(ofb)

§ Idea:renamevariableand/ormakecopies// 1 // x = a + b;

// 2 // d = x + 1;

// 3 // b = a + c;

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// 1 // x = a + foo;

// 2 // d = x + 1;

// 3 // bar = a + c;

§ Compilerintroducesanewnamewheneveranewvalueisproduced.

§ Coulduseanynamebutusuallyusesubscripts:b1,b2,…// 1 // x = a + b34;

// 2 // d = x + 1;

// 3 // b56 = a + c;

§ IRusedinoptimizingcompilers:staticsingleassignment(SSA)

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10.2StaticSingleAssignmentIR

§ Thereisoneassignmentstatementthatwritesavariable/field/memorylocation§ Assignmentforshort:statement,expression,…§ Static:inthesource/IR§ Single:eachstatementwritesadifferentvariable

§ SSAmakesdatadependencesexplicit

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Example

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x1 = a0 + b0d1 = x1 + 1

b1 = a0 + c0

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Example

§ Statement1producesavalueforstatement2

§ Onlytruedependencesrecorded§ Noconstraintsduetovariablenames

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x1 = a0 + b0d1 = x1 + 1

b1 = a0 + c0

SSAasIR

§ Disclaimer:§ Onlymethod-localscalarvariablesconsidered§ Arrays&fieldsignoredfornow§ Mustbeconservative:nopointer-basedassignmentsfornow

§ SSAformforstraightlinecode§ Howtoturna(JavaLi/C/Java/…)programintoSSAform

§ Conditionalstatements

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SSAforabasicblock

§ Assumption:Program(method)translatedintobasicblocks,forestofIRtreesforeachbasicblock§ E.g.,ASTorsimilarIR§ Allsourcesanddestinationsofoperationsvisible§ ProgramwrittenwithoutconsiderationofSSAformat

§ Goal:transformonebasicblockintoSSAformat

§ Approach:considerallstatements(IRtrees)insequence

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SSAconstructionforabasicblock

§ ForeachvariableX:CounterCX§ CX initializedto0atstartofmethod

§ CX indicatesthe“current”version

§ Givenastatementorexpression

D=S⊗ Tor S⊗ T

1. LookupCS andCT§ Yieldscurrentversion,saySn andTm

2. IncrementCD§ Yieldsnewversion(Dk)

3. ReplaceS,T,D:Dk =Sn ⊗ Tm or Sn ⊗ Tm22

Example

NoSSAx = a + b

y = x + 2

z = a + 1

x = c * 2

w = x + 1

SSAx1 = a0 + b0

y1 = x1 + 2

z1 = a0 + 1

x2 = c0 * 2

w1 = x2 + 1

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Example

PleaseturnthefollowingexampleintoSSAform(3min)

a = a + x

x = a + b

c = a + b

x = c + 2

a = x + b

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SSAconstructionforabasicblock

§ Eachbasicblockcanbehandledthatway…

§ Onlythevariablenamesarechanged§ Otherwiseusetreesasbefore§ CoulduseanyotherIR

§ Sometimesvariableaj iscalledaversionofa

§ NeedrightvalueforcountersCX atthestartofabasicblock

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Conditionalstatements

§ Simpleexamplea = 1;

if (b ≠ 0) {

a = 0;

}

x = a ;

§ CountersCa =

Cb =

Cx =

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BB0

BB1

BB2

Findingthecurrentversion

§ Simpleexamplea = 1;

if (b ≠ 0) {

a = 0;

}

x = a ;

§ CountersCa =

Cb =

Cx =

a1 = 1;

if (b0 ≠ 0) {

a2 = 0;

}

x1 = a???;

§ Can’tusea1§ Can’tusea2

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BB0

BB1

BB2

Solution:f function

§ Introducea“magic”functionf § Insertanodewiththef function(f node)

§ f functiondeliversthecorrectversion§ (intheexample)§ If(b0 =0):returnsa1§ If(b0 ≠0):returnsa2

§ ThefunctionpicksthecorrectversiondependingonthepathtakentoreachBB2§ Moreprecisely:thepointwhereweneedtouseeithera1 ora2

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a1 = 1;if (b0 ≠ 0) {

a2 = 0;}x1 = a???;

Paths

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a1 = 1;

if (b0 ≠ 0) {

a2 = 0;

}

a3 = f (a1, a2)

x1 = a3 ;

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f function

§ Result(returnvalue)dependsonpathtaken§ Resultvalueassignedtoanewversionofvariable

§ Argumentsarepossiblereturnvalues§ Differentversionsofthesame(conceptually)variable

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a1 = 1;

if (b0 ≠ 0) {

a2 = 0;

}

a3 = f (a1, a2)

x1 = a3 ;

f function– Notes

§ f functionappearsonlyontherighthandsideofanassignment

§ f functionplacedatbeginningofbasicblock§ Notmandatorybutsimplifiesreadingexamples§ Simplifiesoptimizations/transformations

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f function– Notes

§ f functionreadsargumentsinpredecessorbasicblock§ (Ifpathwastaken)

if (b0 ≠ 0) {

a2 = 0;

}

else {

a1 = 1;

}

a3 = f (a2 , a1)

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BB0BB1

BB2

BB2

ConvertingtoSSA

§ GivenaCFGwithENTRYnode,forestofIRtreesorAST

§ StartwithENTRYnode§ Converttheoperationsinthisbasicblock

§ Insertf functionsasneeded§ MoreonthisinAdvancedCompilerDesign

§ Processnextbasicblockuntilallblockshavebeenprocessed

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Implementingf functions

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BB0

BB1 BB2

BB2

if (b0 ≠ 0)

a2 = 0 a1 = 1

a3 = f (a2 , a1)

Implementingf functions

§ Option1:Assigna1 anda2 tothesameregister§ Botha1 anda2 areallocatedtoaregisterand theygetthesame

register§ Usefulifa3 isreadbeforeitisspilled

§ Option2:Createatemporaryt (storedinmemory)andinsertcopystatements

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BB1 BB2

BB2

a2 = 0t = a2

a1 = 1t = a1

a3 = t

SSAinpractice

§ Usedinmanycompilers§ InGCC3.0(2001)§ HotSpot JVMC1andC2

§ Consequence:Optimizations(transformations)simplified§ Def-usechainsexplicitintheIR

§ Interestingconsequencesforregisterallocation§ Interferencegraphchordalifvariableversionsdefinenodes

§ Edgesindicatethatthereisaconflict

§ Butthereisnofreelunch§ Interferencegraphbasedonvariableversionshasalargernumberof

nodesthaninterferencegraphbasedonliveranges

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CompilerDesignSpring2017

10Intermediaterepresentation(s)

Dr.Zoltán Majó

CompilerGroup– JavaHotSpot VirtualMachineOracleCorporation

IRssofar

§ JavaLi:Tree-based§ Globalprogramanalysis:Three-addresscode§ SSA

§ OtherIRs§ C1:CFGwith“instructions”

§ High-levelIR:usesvirtualregisters§ Low-levelIR:moremachinedetails,physicalregisters

§ C2:“Seaofnodes”§ NoCFG,nobasicblocks§ Dataandcontroldependencesexplicit

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