Compiler Design Spring 2017 7.0 Code generation Dr. Zoltán Majó Compiler Group – Java HotSpot Virtual Machine Oracle Corporation 46
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CompilerDesignSpring2017
7.0Codegeneration
Dr.Zoltán Majó
CompilerGroup– JavaHotSpot VirtualMachineOracleCorporation
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Outline
§ 7.1Accesstooperands
§ 7.2Assignmentstatement
§ 7.3Conditionalstatement
§ 7.4Loops
§ 7.5Methodinvocation
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A B
B
Extendingthecodegenerator
§ NewIRnodes
§ ARRAY§ (Index inframework)§ Specifiesarrayelement§ Requiresnameofarray…§ ...andtreeforindexexpression
§ FIELD
§ Couldincludesubtree tocomputeaddress§ Asdiscussedearlier
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Otherrestrictions
§ Didnothandlestatementsthatrequiredmoreregistersthanavailable§ Virtualregisterspostponedealingwiththisaspect
§ CouldbreakatreethatrequiresmorethanRregistersintoseparatetrees§ Introducetemporaryvariablestoholdintermediateresults
§ Recommendedoption:Handleregistershortageonthefly§ Needaregister?Freearegister§ Saveregistercontentsontostack§ Exampleonnextslide
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Outline
§ 7.1Accesstooperands
§ 7.2Assignmentstatement
§ 7.3Conditionalstatement
§ 7.4Loops
§ 7.5Methodinvocation
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7.3Conditionalstatement
JavaLi allowstwokindsofconditionalstatements
1. “if-then”if (expression) { <body> }
2. “if-then-else”if (expression) { <body> }
else { <body> }
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1. “if-then”if (expression) { <then-part> }InvokecodegeneratoronexpressionCodetodecideifthen-partisexecutedInvokecodegeneratoronthen-part
2. “if-then-else”if (expression) { <then-part> }
else { <else-part> }InvokecodegeneratoronexpressionCodetodecideifthen-partisexecutedInvokecodegeneratoronthen-partCodetoskiparoundelse-partInvokecodegeneratoronelse-part
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Codeoutline
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Eval expression
Test
Branch
dThen-part
…continue
Eval expression
Test
Branch
Then-part
…continue
Else-part
d
d
Uncond.branch
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Outline
§ 7.1Accesstooperands
§ 7.2Assignmentstatement
§ 7.3Conditionalstatement
§ 7.4Loops
§ 7.5Methodinvocation
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7.4Loops
§ JavaLi hasonlyoneloopingconstructwhile (expression) {
<loop-body>}
§ Cantranslateallotherkindsofloopingconstructsintowhile-loops
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§ Translationsimilartoconditionalstatement§ Samerulesforexpression
while-loopwhile (expression) { <loop-body> }
InvokecodegeneratoronexpressionCodetodecideifloop-bodyisexecutedInvokecodegeneratoronloop-bodyCodetojumpbacktostartofloop
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Codeoutline
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Eval expression
Test
Branch
Body
…continue
d
Uncond.branch
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§ Expressionmustbeevaluatedforeveryiteration
§ Simplescheme
§ Straightforwardtoimplement§ Closetoconditionalstatement
§ Oneunconditionalbranchforeachiteration
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Codeoutline2
§ Idea:eliminateunconditionalbranchattheendofeachloopbody
§ Moveloopcontinuationtesttotheendoftheloop
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Codeoutline2
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Eval expression
Test
Branch
Body
…continue
d
Uncond.branch
§ Possibleperformanceimprovement§ Reducednumberofunconditionalbranches§ Basedontheguessthattheloopbodyisexecuted§ Processorimplementationmayprefetch /pre-executeunconditional
branches
§ Otheroptionsexist
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Otherloopconstructs
Takeasheetofpaperandsketchhowyoucouldtranslatea“for”loop
for (int i = low; i < high; i++) {<body>
}
Showafigurethatoutlinesthecode(similartoslidesshownduringthelecture)
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Codeoutline
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Eval expression
Test
Branch
Body
…continue
d
Uncond.branch
Init loopcounter
Updateloopcounter
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Outline
§ 7.1Accesstooperands
§ 7.2Assignmentstatement
§ 7.3Conditionalstatement
§ 7.4Loops
§ 7.5Methodinvocation
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7.5Methodinvocation
§ Findingthecorrectbodyofcode§ Coveredearlier
§ Storageformethod-localvariables
§ Passingofparameters(arguments)
§ Handlingthereturnvalue
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Terminology
Considerasimpleclasswithonemethod:class B {
int bar(int x1, int x2, ..., int xn) { … }}// somewhere in some method fooB bref; int val = bref.bar(y1,y2, …, yn);
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callee
target(instance)receiver(y0– firstactual)
argumentsactualparametersactuals
callsite
caller
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Callingconvention
controlflow
infoo
continue
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controlflow
inbar
return
Callingconvention
§ Thecallingconventiondecideswhichsteps/actionsofmethodinvocation/returnare§ Donebythecaller§ Donebythecallee
§ …andhowanydatatransferisimplemented§ Wherethedataarestored§ Dataincludesreturnaddressandreturnvalue§ Mayhavetoincludetypecasts
§ Usuallydefinedforallcompilers/softwaredevelopersonagivenoperatingsystem§ Ensuresinteroperability
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Callingconvention
§ Stackprincipaldatastructure§ “Controlstack”
§ Setupbyrun-timesystem§ Supportedbyinstructionsand/orresources(registers)onsome
systems
§ Activationrecord keepsinformationonmethodinvocationsthatarecurrently“active”(or“live”)§ Alsoknownasframe
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Actions
Considerthismethodinvocation.// in some method fooB bref; int val = bref.bar(y1,y2, …, yn);
Listallthestepsthatarenecessarytoimplementthecallanditsreturn(onasheetofpaper).
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Actions (relatedtomethodinvocation)
§ Identifytarget
§ Identifystartingaddressofcallee
§ Handleparameters§ Evaluateyi
§ Castasnecessary§ Putintolocationwhereitcanbefound
§ Pushontostack§ Placeintoregister§ Leavesomewhereelse
§ Saveregisters
§ Findspacefortemporaries
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Actions (relatedtomethodinvocation)
§ Determineandstorereturnaddress
§ Findspaceforreturnvalue
§ Setupactivationrecordforcallee§ Linktoactivationrecordofcaller(“controllink”)
§ Transfercontroltocallee
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Callling convention
§ Someofthesesteps§ Mustbedonebycaller§ Mustbedonebycallee
§ Methodthatwasinvoked§ Canbedonebyeitherone
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Actions (relatedtomethodreturn)
§ Transfercontroltocaller/returnaddress
§ Deliverreturnvalue
§ Removeactivationrecordforcallee§ Reclaimanytemporarystorage§ Switchtoactivationrecordofcaller
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Simpleactivationrecord
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Temp k
...
Temp0
Local m
…
Local0
oldSP
oldFP
Returnaddress
target(parametery0)
parametery1
…
parameteryn
Returnvalue
framepointer
stackpointer
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Comments
§ SufficientforJavaLi§ Willnotworkfor“varargs”
§ Methodswithavariablenumberofarguments(actuals)
§ Parametersarefoundonthestack§ Actuals:positiveoffsetfromframepointer§ Offsetrecordedinsymboltable
§ Localsarefoundonthestack§ Negativeoffsetfromframepointer§ Offsetrecordedinsymboltable
§ Temporaryregioncangroww/olimit§ Temporariescanbeaccessedviaframepointer
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Symboltable
§ Mustcompletesymboltablepriortocodegeneration
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local0 - 4
local1 - 8
parameter1 +16
parameter 2 +20
return value +24
this (implicitparameter) + 12
Parameterpassing
§ (Recall)JavaLi passesallargumentsbyvalue
§ Nottheaddressofavariableispassedbutitsvalue
§ Thevalueofareferencecanbeusedtoread/writetheobjectthereferencerefersto§ Butthereferencecannotbechanged
§ Atcallsite(ofbref.bar(y1,y2, …, yn))§ Evaluateyn,pushontostack
§ Evaluateyn-1,yn-2,…y1,y0§ Couldbeanexpression,i.e.bref.bar(y1, a × b, …, yn)
§ Mayrequirecast
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Actual-formalcorrespondence
§ Otherlanguagesarebasedon(orinclude)differentmodels§ Languagespecshouldspecifymodel
§ Herewelookatthecompiler’sside7.5.1Call-by-value7.5.2Call-by-reference7.5.3Call-by-result7.5.4Call-by-name
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7.5.1Call-by-value
§ TheonlyoptioninJava(andJavaLi)
§ Easytounderstand§ Lookatmethodinisolation§ Withreferencesriskofaliasing
§ Easytoimplement§ Callerknowshowtoevaluateactuals§ Callee usesaparameterlikealocal– initializedbycaller
§ Potentiallyinefficient§ Passanarray/anobjectbyvalue?§ Mayrequirecopy(timeconsuming)§ Languagerestrictscall-by-valueto“word-size”entities(ints,references)
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Adminissues
§ Norecitationsessiontoday
§ Easterbreakstartssoon§ Usefreetime(ifany)toworkonHomework3(dueonApril27)§ Andonoptionalparsing-relatedhomework(HWPRS)
§ Outtoday
§ Nextrecitation:April27at10:15inCABG61§ DiscussionofHomework4(completecodegenerator)
§ Willsendoutreminder
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Wherearewenow?
§ Pastfirsthalfofthelecture§ Covered8weeksofmaterial
§ Lexing§ Parsing§ Semanticanalysis§ Codegeneration§ SomeVM/runtime-relatedissues
§ Intheremaining6weekswe’llcover§ Programanalysis§ Registerallocation§ OtherIRs/JITcompilation(iftimepermits)
§ Nextlecture:May4at10:15inCABG61
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