procedure and function calls, - clear.rice.edu · •at runtime, those sequences (prolog, epilog,...

ProcedureandFunctionCalls,— PartII—

Comp412

COMP412FALL2017

Copyright2017,KeithD.Cooper&LindaTorczon,allrightsreserved.

StudentsenrolledinComp412atRiceUniversityhaveexplicitpermissiontomakecopiesofthesematerialsfortheirpersonaluse.

Facultyfromothereducationalinstitutionsmayusethesematerialsfornonprofiteducationalpurposes,providedthiscopyrightnoticeispreserved.

Chapter6 inEaC2e

FrontEnd Optimizer BackEnd

IR IRsource

code

target

code

…

LinkageConvention:High-LevelView

Thelinkagemust:• Preserve& protectthecaller’senvironmentfromcallee (&itscallees)– Forexample,itmustpreservevaluesinregistersthatareliveacrossthecall

• Createanewenvironmentforthecallee (newnamespace)– Atruntime,itcreatesalocaldataareaforthecallee & tiesittothecontext

• Mapnames& resourcesfromcallerintothecallee,asneeded

Toaccomplishthesegoals:• Conventiondividesresponsibilitybetweencaller& callee• Caller& callee mustusethesamesetofconventions– Implieslimitedopportunitiesforcustomization& optimization

Weexpectcompiledcodetoworkevenwhencreatedwithdifferentcompilers,perhapsindifferentlanguages

COMP412,Fall2017 1

fee fie

foe

foemustoperateinmultipledistinctcallingcontexts.

Callers

Callee

LinkageConvention

Aprocedureorfunctionhastwodistinctrepresentations

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Codetoimplementtheprocedure• Codeforallthestatements• Codetoimplementthelinkageconvention• Labelsonentrypoints andreturnpoints♦ Usenamemanglingtocreateconsistentlabels

Datatorepresentaninvocation• Activationrecord(AR)• Localdata+savedcontextofcaller• Controlinformation• Storagetopreservecaller’senv’t♦ ChainARstogethertocreatecontext

• Staticdataarea(namemangling)

prolog

epilog

@foe_:

prolog

epilog

pre-call

post-return

@fee_:

@fee_r1:

ARP

fee’sAR foe’sAR

#fee_:#foe_:

The“Meta”Issue

Oneofthemostdifficultaspectsofunderstandinghowcompilersworkiskeepingtrackofwhathappensatcompiler-designtime,compiler-buildtime,compiletime,andruntime.

• Atcompiletime,thecompileremitscodefortheapplicationprogram,includingallofitsprocedures(andtheirprologs,epilogs,&sequences)– Thecompilerusessymboltablestokeeptrackofnames– Thosetablesarecompile-time datastructures

• Atruntime,thosesequences(prolog,epilog,pre-call,&post-return)create,populate,use,anddestroyARsforinvocationsofprocedures– Therunningcodestoresdata(bothprogramdataandcontroldata)intheARs– ThoseARsarerun-time datastructures

Toconfusemattersfurther,thecompilermaypreservethesymboltablessothattheruntimedebuggingenvironmentcanlocatevariablesandvalues,andlocatethevariousdataareasandtheARs

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ProcedureLinkages

StandardProcedureLinkage

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Eachprocedurehas• Astandardprolog• Astandardepilog

Eachcalluses• Apre-callsequence• Apost-returnsequence

Thecodeforthepre-callandpost-returnsequencesmustbecompletelypredictablefromthecallsite.Itdependsonthenumber&thetypesoftheactualparameters.

Note: Thegapbetweenpre-call&post-returncodeisartisticlicense.

functionsqrt

prolog

epilog

proceduremainprolog

epilog

…z =a*a+b*bx=sqrt(y,z)

pre-call

post-returnL:

…

caller

callee

ActivationRecordBasics

Inmostsystems,ActivationRecordshaveasimilarlayout

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spaceforparameters

registersavearea

slot forreturnvalue

slotforreturnaddress

slot foraddressability

slotforcaller’sARP

spaceforlocalvariables

ARP

OneARforeachinvocationofaprocedureOneregisterdedicatedtoholdthecurrentARP

ARP ≅ ActivationRecordPointer

Spaceforparameterstothecurrentprocedure

Contentsofsavedregisters

Iffunction,spaceforreturnvalue

Addresstoresumecaller

Helpwithnon-localaccess

Torestorecaller’sAR onreturn

Spaceforlocalvariables,temporaries,&spills

n

…

10

IfARsarestackallocated,thestackgrowsinthisdirection

k

…

0

01

…

m

ProcedureLinkages

Pre-callSequence• Setsupcallee’sbasicAR• Helpspreserveitsownenvironment

TheDetails• Allocatesspaceforthecallee’sAR– exceptspaceforlocalvariables

• Evaluateseachparameter&storesvalueoraddress• Savesreturnaddress& caller’sARP intocallee’sAR• Addressabilitymaintenance,ifnecessary• Savesanycaller-saveregistersthatareliveacrossthecall– Saveintospaceincaller’sAR

• Jumpstoaddressofcallee’sprologcode

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Wheredoparametervaluesreside?Conventionalwisdomsays:• Inregisters (First3or4 )• Incallee’sAR (therest )

prolog

epilog

pre-call

post-return

prolog

epilog

Pre-callsequenceisalotofwork

ProcedureLinkages

Post-returnSequence• Finishesrestoringcaller’senvironment• Placesanyvaluebackwhereitbelongs– e.g.,anyreferenceparameterorglobal

thatiskeptinaregisterinthecallee

TheDetails• CopiesreturnvaluefromAR intoaregister,ifneeded• Freesthecallee’sAR– Ifaccesslinksareused,thisactiondiscardscallee’slink

• Restoresanycaller-saveregisters• Restoresanycall-by-referenceparameterstoregisters,ifneeded– Alsocopybackcall-by-value/resultparameters

• Continuesexecutionafterthecall

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prolog

epilog

pre-call

post-return

prolog

epilog

ProcedureLinkages

PrologCode• Finishessettingupcallee’senvironment• Preservespartsofcaller’senvironmentthatwillbedisturbedbythecallee

TheDetails• Preservesanycallee-saveregisters• Addressabilitymaintenance,ifnecessary• Allocatesspaceforlocaldata– EasiestscenarioistoextendtheAR

• Findsanystaticdataareasreferencedinthecallee• Handlesanylocalvariableinitializations– Notanycheapertosay“int x=0;”

WithheapallocatedAR,mayneedaseparateheapobjectforlocalvariables.(Mustknowsizeoflocaldataarea)COMP412,Fall2017 8

prolog

epilog

pre-call

post-return

prolog

epilog

ProcedureLinkages

EpilogCode• Windsupthebusinessofthecallee• Startsrestoringthecaller’senvironment

TheDetails• Storesreturnvalue?– Placeit,orapointer,inthereturnvalue

slotofthecaller’sAR 1

– Otherschemesareequallyfeasible

• Restorecallee-saveregisters,asneeded• Addressabilitymaintenance,ifnecessary• Freespaceforlocaldata,ifnecessary(ontheheap)• Restorereturnaddress&caller’sARP fromAR• Jumptothereturnaddress

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prolog

epilog

pre-call

post-return

prolog

epilog

Takeawaypoint:procedurecallshavemanycyclesofoverhead.

1Returnvalueneedsstoragethatsurvivesthecall

IfARsarestackallocated,deallocationisimplicit(&cheap)whenstacktopisresettoitspre-callvalue.

WhereDoSavedRegistersGo?

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spaceforparameters

registersavearea

slot forreturnvalue





ARP

OneARforeachinvocationofaprocedureOneregisterdedicatedtoholdthecurrentARP








p savesitsownregistersinitsownRSA

psavesitsspillsinitsownlocaldataarea.

Ifp alwayssavesregistersintoitsownAR,thenthecompilerknowshowmuchspacetoreserveintheARforthosepurposes— p’sowncallersavesregisters(inprolog),p’sowncallee savesregisters(inpre-callwhenpcallsq),andp’sownspillsintheregisterallocatorrunonp. (prologpatchedafterallocation)

WhereDoParametersGo?

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spaceforparameters

registersavearea

slot forreturnvalue





ARP








Parametersfromcalleegohere

spaceforparameters

Parameterspassedtoanothercallgohere

Callee’s activationrecord

CreatingandDestroyingActivationRecords

Theactivationrecordembodiestheruntimestateoftheprocedure

• HowareARscreatedanddestroyed?– Procedurecallmustallocate&initialize (preservecaller’sworld)

– Returnmustdismantleenvironment (andrestorecaller’sworld)

• Caller&callee mustcollaborateontheproblems– Calleraloneknowssomeofthenecessarystate➝ Returnaddress,parametervalues,accesstootherscopes

– Calleealoneknowstherest➝ Sizeoflocaldataarea(withspills),registersitwilluse

Theircollaborationtakestheformofalinkageconvention

Assume,forthemoment,anAlgol-60-likeenvironmentwheretheARisdeadafterthereturn.

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1. Linkageconventionisdefined atcompilerdesigntime2. Linkagecodeisemitted atcompiletime3. Linkagecodeexecutes atruntime,whenaprocedureiscalled.

StoringActivationRecords

Ifactivationrecordsarestoredonthestack• Easytoextend— simplybumptopofstackpointer• Caller&callee shareresponsibility– Callercanpushparameters,spaceforregisters,returnvalueslot,return

address,addressabilityinfo,&itsownARP– Calleecanpushspaceforlocalvariables(fixed&variablesize)

Algol-60rules

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Whatisthisstack?

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AddressSpaceLayout

Mostlanguageruntimeslayouttheaddressspaceinasimilarway

• Pieces(stack,heap,code,&globals)maymove,butallwillbethere• Stackandheapgrowtowardeachother(ifheapgrows)

Heap

Stacks

Grow

thsp

ace

forstacks

Glob

als

Code

JavaMemoryLayout

LIFORuntimeStackAR

forfeeAR

forfieAR

forfoeAR

forfieAR

forfumAR

forfoe

NewARs

Lecture21,StorageLayout(slide9)




Ifactivationrecordsarestoredontheheap• Hardtoextend• Severaloptions– Callerpasseseverythinginregisters;callee allocates&fillsAR– Storeparameters,returnaddress,etc.,incaller’sAR!– Storecallee’sAR sizeinadefinedstaticconstant

Algol-60rules

MLrules

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Namemangling,again

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AddressSpaceLayout



Stacks

Grow

thsp

ace

forstacks

Glob

als

Code

JavaMemoryLayout

ARforfee

ARforfie

ARforfie

ARforfie

ARforfum

ARforfoe




Ifactivationrecordsarestoredontheheap• Hardtoextend• Severaloptions– Callerpasseseverythinginregisters;callee allocates&fillsAR– Storeparameters,returnaddress,etc.,incaller’sAR!– Storecallee’sAR sizeinadefinedstaticconstant

Withoutrecursion,activationrecordscanbestatic

Algol-60rules

MLrules

Fortran66&77

COMP412,Fall2017 17

Namemangling,again

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AddressSpaceLayout



StaticallyallocatedARsgointothe“Global”dataarea—whichwewouldneedtoredrawinasalargerarea…

HEAP

Stacks

Grow

thsp

ace

forstacks

Glob

als

Code

JavaMemoryLayout

WhereDoVariablesLive?

WehaveseenARs,staticdataareas,globaldataareas,…

Howdoesthecompilerdecidewheretoplaceeachvariable• Acombinationofvisibilityandlifetimedeterminesthatplacement

Variablelengthitems?• Putadescriptorinthenaturalplace&allocatespaceinAR orintheheap– Requiresonelevelofindirection(a“pointer”)– Allowsuniformaddressability– InAR ifAR isextendible– OnheapifAR isnotextendible

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Lifetime Scope Location

automatic local declaringprocedure’sAR

static any named staticdataareaforitslifetime

dynamic any heap

Automatic: LifetimematchesprocedureactivationStatic: Lifetimemaybeaslongasentire

executionDynamic: Lifetimeisunderprogramcontrol&not

known atcompiletime

HowDoWeAddressVariables?

Localvariables• NeedamechanismtolocatethelocaldataareainappropriateAR• Representthevariableasastaticcoordinate:< level,offset>– Level isthelexicalnestingleveloftheprocedurethatdeclaresthevariable– Offset istheoffsetwithintheAR’slocaldataareaforthatprocedure

• Mechanismtakesstaticcoordinatetorun-timeaddress

Staticvariables(includingglobal)• Needabase-address,offsetpair (manglednameforbaseaddress)– Generatealoadofthebaseaddress– Addtheoffsetandloadthevalue (loadAI ifoffsetissmallenough)

• Mustberecognizableasnon-localcase (representdifferently)

Dynamicvariables• Programmermanagesaccessthroughpointers,names,…

COMP412,Fall2017 20

HowDoWeAddressVariables?

Localvariableofcurrentprocedure• Converttostaticcoordinate• Addoffset torARP andload

Localvariableinsurroundinglexicalscope• Converttostaticcoordinate• Convertlevel toARP forprocedureatthatlevel• Addoffset tothatARP andload

Localvariablesofsurroundingscopes,staticvariablesatvariousscopes,andglobalvariablesareoftencalled“freevariables”inthePL literature

COMP412,Fall2017 21

MustfindthecorrectAR

FindingtheCorrectAR

EachAR hasafieldforaddressability

COMP412,Fall2017 22

parameters

registersreturnvaluereturnaddresscaller’s ARP

addressability

localvariables

ARP

Onecommonschemeusesaccess links• EachAR hasalinktotheAR ofitslexicalancestor(surroundingscope)• Lexicalancestorneednotbetheprocedurethatcalledit• CompilercanusestaticcoordinatetogeneratecodethatchasesthechainofaccesslinkstofindthecorrectAR♦ Referenceto<p,16>runsupchaintofindlevelp’sARP,thenadds16

♦ Numberofderefences is(c– p),wherecisthecurrentlexicallevel

• Costofaccessisproportionaltolexicaldistancebetweenc andprARP isaphysicalregister,

dedicatedtoholdingtheARP

rARP

FindingtheCorrectAR

COMP412,Fall2017 23

parameters


addressability

localvariables

AR for foe

ARP

parameters


addressability

localvariables

AR for fie

parameters


addressability

localvariables

AR for fee

procedure fee {procedure fie {

procedure foe {}call foe()

}call fie()

}

Static Coord GeneratedCode

<3,8> loadAI rARP,8 => r10

<2,12> loadAI rARP,4 => r10loadAI rARP,12 => r10

<1,16> loadAI rARP,4 => r10loadAI rARP,4 => r10loadAI rARP,12 => r10

UsingAccessLinks

FindingtheCorrectAR

COMP412,Fall2017 24

parameters


addressability

localvariables

ARP

Ateachcall:• Inpre-callsequence,callermustfindtheappropriateARP• Assumecallerisatlevelk♦ Calleeatlevelk+1→ UsecurrentARP aslink

♦ Calleeatlevelj,0<j ≤k→ Walkupaccesslinkchaintolevelj -1→ UsethatARP aslink

♦ Calleeisglobal(e.g.,atlevel0)→ UseNULL aslink→ Caller&calleesharenolexicalcontext

• Aslongasthecodeiscorrect♦ Caller’schainwillbelongenough♦ Calleecannotwalkoffendofchain

MaintainingAccessLinks

FindingtheCorrectAREachAR hasafieldforaddressability

COMP412,Fall2017 25

parameters


addressability

localvariables

ARP

TheLinkageCanAlsoUseADisplay• GlobalvectorofpointerstotheARsofsurroundingprocedures• AnynameableAR canbereachedinconstanttime♦ CurrentAR isreachedthroughARP♦ Surroundingscopesarereachedbytreatingdisplayasavector

• Referenceto<p,16>,forp <k,looksupthelevelp ARP inthedisplayandadds16toit• Costofaccessisconstant

FindingtheCorrectARUsingADisplay

COMP412,Fall2017 26

parameters


addressability

localvariables

AR for foe

ARP

parameters


addressability

localvariables

AR for fie

parameters


addressability

localvariables

AR for fee

procedure fee {procedure fie {

procedure foe {}call foe()

}call fie()

}

Static Coord GeneratedCode

<3,8> loadAI rARP,8 => r10

<2,12> loadAI _display_ => r10loadAI r10,4 => r10loadAI r10,12 => r10

<1,16> loadI _display_ => r10load r10 => r10loadAI rARP,12 => r10

level1

level2

level 3

_display_:

FindingtheCorrectAR

COMP412,Fall2017 27

parameters


addressability

localvariables

ARP

MaintainingADisplay• Displaymaintenanceissimple,aslongasthecompilerwriteriswillingto“waste”onedisplayslot• Inprologofeachleveljprocedure♦ Savethelevelj displayentryintotheprocedure’sownAR (addressabilityslot)

♦ StorecurrentARP intolevelj displayslot

• Inepilogofeachlevelj procedure♦ Restoreoldlevelj displayentryfromthecurrentAR’saddressabilityslot

Simple,clean,andconstanttime

AccessLinksversusDisplay

AccessLinks• Maintenanceaddsoverheadtoeachcall– Overheaddependsonnesting

• Addsoverheadoneachreferencetoalocalinsurroundingscope– Overheaddependsonnesting

• IfARsoutliveprocedure,accesslinksstillwork– AccesslinksbetweenARsensures

thattheyarelive(&uncollected)

COMP412,Fall2017 28

Display• Maintenanceaddsoverheadtoeachcall&return♦ Overheadisconstant

• Addsoverheadoneachreferencetoalocalinsurroundingscope♦ Overheadisconstant

• IfARsoutliveprocedure,displaylinksarenotpreserved♦ NeedtopreservetheminawaythatensuresARsarelive(collectorissue)

Yourmileagewillvary• Dependsonratioofnon-localaccessestocalls• Dependsondemandforregisters (availableregistertohold_display_?)

AccessLinksandDisplays

TheMetaIssue• Decisiontouseoneortheotherismadewhenthelinkage is designed– Maybebeyondthecontrolofthecompilerwriter

• Lexicallevelsforproceduresandvariablesaredeterminedinparsing– Requirestheuseofalexicallyscopedsymboltable

• Codetomaintainaccesslinkordisplayisemittedatcompiletime– Whencompilergeneratesaprocedureoracall,itemitsthecode– Accesslinks: Lexicallevelsareknownatcompiletime,socompilercan

generatethecodetofindalevelj display(knowshowfartorunupthechain)– Display:Lexicallevelsareknownatcompiletime,socompilercangenerate

thesave(inprolog)andrestore(inepilog)

• Eitheranaccesslinkoradisplayisaruntimeobject– Existsatruntime,– PointstoARs,whichareruntimeobjects

COMP412,Fall2017 29

procedure and function calls, - clear.rice.edu · •at runtime, those sequences (prolog, epilog,...

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