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SCOPE, FUNCTION CALLS AND STORAGE MANAGEMENT

Topics

  Block-structuredlanguagesandstackstorage  In-lineBlockso  ac5va5onrecordso  storageforlocal,globalvariables  First-orderfunc5onso  parameterpassingo  tailrecursionanditera5on  Higher-orderfunc5onso  devia5onsfromstackdisciplineo  languageexpressiveness=>implementa5oncomplexity

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Block-StructuredLanguages

  Nestedblocks,localvariableso  Example{intx=2; {inty=3;x=y+2;

}}o  Storagemanagement

ü Enterblock:allocatespaceforvariablesü Exitsblock:someorallspacemaybedeallocated

new variables declared in nested blocks

inner block

outer block local variable

global variable

Examples

  Blocksincommonlanguageso  C,{…}o  Algolbegin…endo  MLlet…in…end  Twoformsofblockso  In-lineblockso  Blocksassociatedwithfunc5onsorprocedures

  Topic:block-basedmemorymanagement,accesstolocalvariables,parameters,globalvariables

* JavaScript functions provide blocks

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SimplifiedMachineModelRegisters

Environment Pointer

Program Counter

Data Code

Heap

Stack

InterestedinMemoryMgmtOnly

  Registers,Codesegment,Programcountero  Ignoreregisterso  Detailsofinstruc5onsetwillnotmaWer

  DataSegmento  Stackcontainsdatarelatedtoblockentry/exito  Heapcontainsdataofvaryinglife5meo  Environmentpointerpointstocurrentstackposi5on

ü Blockentry:addnewac5va5onrecordtostackü Blockexit:removemostrecentac5va5onrecord

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Somebasicconcepts

  Scopeo  Regionofprogramtextwheredeclara5onisvisible  Life5meo  Periodof5mewhenloca5onisallocatedtoprogram

•  Inner declaration of x hides outer one. •  Called “hole in scope” •  Lifetime of outer x includes time when

inner block is executed •  Lifetime ≠ scope •  Lines indicate “contour model” of scope.

{ int x = … ; { int y = … ; { int x = … ; …. }; }; };

In-lineBlocks

  Ac5va5onrecordo  Datastructurestoredonrun-5mestacko  Containsspaceforlocalvariables  Example

May need space for variables and intermediate results like (x+y), (x-y)

{ int x=0;

int y=x+1;

{ int z=(x+y)*(x-y);

};

};

Push record with space for x, y Set values of x, y Push record for inner block Set value of z Pop record for inner block Pop record for outer block

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Ac5va5onrecordforin-lineblock

  Controllinko  pointertopreviousrecordonstack

  Pushrecordonstack:o  Setnewcontrollinktopointtooldenvptr

o  Setenvptrtonewrecord  Poprecordoffstacko  Followcontrollinkofcurrentrecordtoresetenvironmentpointer

Control link

Local variables

Intermediate results

Control link

Local variables

Intermediate results

Environment Pointer

Can be optimized away, but assume not for purpose of discussion.

Example{ int x=0;

int y=x+1;

{ int z=(x+y)*(x-y);

};

}; Push record with space for x, y Set values of x, y Push record for inner block Set value of z Pop record for inner block Pop record for outer block

Control link x y

0 1

x+y x-y

Environment Pointer

1 -1

Control link z -1

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Scopingrules

  Globalandlocalvariables{ int x=0;

int y=x+1;

{ int z=(x+y)*(x-y);

};

};

•  x, y are local to outer block •  z is local to inner bock •  x, y are global to inner block

◆ Static scope •  global refers to declaration in closest enclosing block

◆ Dynamic scope •  global refers to most recent activation record

These are same until we consider function calls.

Func5onsandprocedures

  Syntaxofprocedures(Algol)andfunc5ons(C)procedureP(<pars>)<type>func5onf(<pars>)begin{<localvars><localvars><procbody><func5onbody>end;}  Ac5va5onrecordmustincludespacefor•  parameters •  return address •  local variables,

intermediate results

•  return value (an intermediate result)

•  location to put return value on function exit

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Ac5va5onrecordforfunc5on

  Returnaddresso  Loca5onofcodetoexecuteonfunc5onreturn

  Return-resultaddresso  Addressinac5va5onrecordofcallingblocktoreceivereturnaddress

  Parameterso  Loca5onstocontaindatafromcallingblock

Control link

Local variables

Intermediate results

Environment Pointer

Parameters

Return address

Return-result addr

Example

  Func5onfact(n)=ifn<=1then1elsen*fact(n-1)o  Returnresultaddresso  loca5ontoputfact(n)  Parametero  settovalueofnbycallingsequence

  Intermediateresulto  loca5onstocontainvalueoffact(n-1)

Control link

Local variables

Intermediate results

Environment Pointer

Parameters

Return address

Return result addr

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Control link

fact(n-1) n

Return-result addr 3

fact(3)

Func5oncall

ReturnaddressomiWed;wouldbeptrintocodesegment

Control link

fact(n-1) n

Return-result addr 2

fact(2)

fact(n) = if n<= 1 then 1 else n * fact(n-1)

Control link

fact(n-1) n

Return-result addr k

fact(k)

Environment Pointer

Control link

fact(n-1) n

Return-result addr 1

fact(1)

Function return next slide →

Func5onreturn

Control link

fact(n-1) n

Return result addr 3

fact(3)

Control link

fact(n-1) n

Return result addr

1 2

fact(2)

Control link

fact(n-1) n

Return result addr 1

fact(1)

fact(n) = if n<= 1 then 1 else n * fact(n-1)

Control link

fact(n-1) n

Return result addr

2 3

fact(3)

Control link

fact(n-1) n

Return result addr

1 2

fact(2)

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Topicsforfirst-orderfunc5ons  Parameterpassingo  pass-by-value:copyvaluetonewac5va5onrecordo  pass-by-reference:copyptrtonewac5va5onrecord  Accesstoglobalvariableso  globalvariablesarecontainedinanac5va5onrecordhigher“up”thestack

  Tailrecursiono  anop5miza5onforcertainrecursivefunc5ons

Seethisyourself:writefactorialandrununderdebugger

Parameterpassing

  Generalterminology:L-valuesandR-valueso  Assignmenty:=x+3

ü Iden5fieronlebreferstoloca5on,calleditsL-valueü Iden5fieronrightreferstocontents,calledR-value

  Pass-by-referenceo  PlaceL-value(address)inac5va5onrecordo  Func5oncanassigntovariablethatispassed  Pass-by-valueo  PlaceR-value(contents)inac5va5onrecordo  Func5oncannotchangevalueofcaller’svariableo  Reducesaliasing(alias:twonamesrefertosameloc)

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Example

function f (x) = { x = x+1; return x; } var y = 0; print (f(y)+y);

pseudo-code activation records

pass-by-re

f

pass-by-value

f(y) y 0

Control link

x Return result addr

•

f(y)

f(y) y 0

Control link

x Return result addr

0

f(y)

Accesstoglobalvariables

  Twopossiblescopingconven5onso  Sta5cscope:refertoclosestenclosingblocko  Dynamicscope:mostrecentac5va5onrecordonstack  Examplevar x=1; function g(z) { return x+z; } function f(y) { var x = y+1; return g(y*x); } f(3);

x 1

x 4 y 3

z 12

outer block

f(3)

g(12)

Which x is used for expression x+z ?

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Ac5va5onrecordforsta5cscope

  Controllinko  Linktoac5va5onrecordofprevious(calling)block

  Accesslinko  Linktoac5va5onrecordofclosestenclosingblockinprogramtext

  Differenceo  Controllinkdependsondynamicbehaviorofprog

o  Accesslinkdependsonsta5cformofprogramtext

Control link

Local variables

Intermediate results

Environment Pointer

Parameters

Return address

Return result addr

Access link

Complexnes5ngstructure

var x=1; function g(z) { return x+z; } function f(y) { var x = y+1; return g(y*x); } f(3);

function m(…) { var x=1; … function n( … ){ function g(z) { return x+z; } … { … function f(y) { var x = y+1; return g(y*x); } … f(3); … } … n( … ) …} … m(…)

Simplify to

Simplified code has same block nesting, if we follow convention that each declaration begins a new block.

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Sta5cscopewithaccesslinks

Useaccesslinktofindglobalvariable:ü Accesslinkisalwayssettoframeofclosestenclosinglexicalblock

ü Forfunc5onbody,thisisblockthatcontainsfunc5ondeclara5on

var x=1; function g(z) = { return x+z; } function f(y) = { var x = y+1; return g(y*x); } f(3);

x 1

x 4 y 3

z 12

outer block

f(3)

g(12) control link access link

g …

f …

control link access link

control link access link

access link control link

Higher-OrderFunc5ons

  Languagefeatureso  Func5onspassedasargumentso  Func5onsthatreturnfunc5onsfromnestedblockso  Needtomaintainenvironmentoffunc5on  Simplercaseo  Func5onpassedasargumento  Needpointertoac5va5onrecord“higherup”instack  Morecomplicatedsecondcaseo  Func5onreturnedasresultoffunc5oncallo  Needtokeepac5va5onrecordofreturningfunc5on

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Passfunc5onasargument

intx=4;funf(y)=x*y;fung(h)=letintx=7inh(3)+x;g(f);Therearetwodeclara5onsofxWhichoneisusedforeachoccurrenceofx?

{ var x = 4; { function f(y) {return x*y}; { function g(h) { var x = 7; return h(3) + x; }; g(f); } } }

Haskell Pseudo-JavaScript

Sta5cScopeforFunc5onArgument

intx=4;funf(y)=x*y;fung(h)=letintx=7inh(3)+x;g(f);

x 4

h

y 3

f

g

Code for f

Code for g g(f)

h(3)

x * y

x 7

follow access link local var

How is access link for h(3) set?

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Sta5cScopeforFunc5onArgument

{varx=4;{func5onf(y){returnx*y};{func5ong(h){intx=7;returnh(3)+x;};g(f);}}}

x 4

h

y 3

f

g

Code for f

Code for g g(f)

h(3)

x * y

x 7

follow access link local var

How is access link for h(3) set?

Closures

  Func5onvalueispairclosure=〈env,code〉  Whenafunc5onrepresentedbyaclosureiscalled,o  Allocateac5va5onrecordforcall(asalways)o  Settheaccesslinkintheac5va5onrecordusingtheenvironmentpointerfromtheclosure

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Func5onArgumentandClosures

intx=4;funf(y)=x*y;fung(h)=letintx=7inh(3)+x;g(f);

x 4

access link set from closure

Code for f

f access

Run-time stack with access links

Code for g

h(3) y 3

access

g(f) h

access

x 7

g access

{ var x = 4;

{ function f(y){return x*y};

{ function g(h) {

int x=7;

return h(3)+x;

};

g(f);

}}}

Func5onArgumentandClosures

x 4

access link set from closure

Code for f

f access

Run-time stack with access links

Code for g

h(3) y 3

access

g(f) h

access

x 7

g access

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Summary:Func5onArguments

  Useclosuretomaintainapointertothesta5cenvironmentofafunc5onbody  Whencalled,setaccesslinkfromclosure  Allaccesslinkspoint“up”instacko  Mayjumppastac5vrecordstofindglobalvarso  S5lldeallocateac5vrecordsusingstack(lifo)order

ReturnFunc5onasResult  Languagefeatureo  Func5onsthatreturn“new”func5onso  Needtomaintainenvironmentoffunc5on  Examplefunc5oncompose(f,g){returnfunc5on(x){returng(f(x))}};  Func5on“created”dynamicallyo  expressionwithfreevariables

valuesaredeterminedatrun5meo  func5onvalueisclosure=〈env,code〉o  codenotcompileddynamically(inmostlanguages)

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Example:Returnfctnwithprivatestate

funmk_counter(init:int)=letvalcount=refinit

funcounter(inc:int)=(count:=!count+inc;!count)incounterend;

valc=mk_counter(1);c(2)+c(2);

•  Function to “make counter” returns a closure

• How is correct value of count determined in c(2) ?

ML

Example:Returnfctnwithprivatestate

func5onmk_counter(init){varcount=init;func5oncounter(inc){count=count+inc;returncount};returncounter};varc=mk_counter(1);c(2)+c(2);Func5onto“makecounter”returnsaclosureHowiscorrectvalueofcountdeterminedincallc(2)?

JS

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Func5onResultsandClosuresfunmk_counter(init:int)=letvalcount=refinit

funcounter(inc:int)=(count:=!count+inc;!count)incounterend

end;valc=mk_counter(1);c(2)+c(2);

c access

Code for counter

Code for mk_counter

c(2) access inc 2

1 mk_counter(1)

count init 1

access

counter

mk_c

Call changes cell value from 1 to 3

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ML

Func5onResultsandClosuresfunc5onmk_counter(init){varcount=init;func5oncounter(inc){count=count+inc;returncount};returncounter};varc=mk_counter(1);c(2)+c(2);

c access

Code for counter

Code for mk_counter

c(2) access inc 2

mk_counter(1)

count 1 init 1

access

counter

mk_c

JS

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ClosuresinWebprogramming

  UsefulforeventhandlersinWebprogramming:func5onAppendBuWon(container,name,message){varbtn=document.createElement('buWon');btn.innerHTML=name;btn.onclick=func5on(evt){alert(message);}container.appendChild(btn);}

  EnvironmentpointerletsthebuWon’sclickhandlerfindthemessagetodisplay

Summary:ReturnFunc5onResults

  Useclosuretomaintainsta5cenvironment  Mayneedtokeepac5va5onrecordsaberreturno  Stack(lifo)orderfails!

  Possible“stack”implementa5ono  Forgetaboutexplicitdealloca5ono  Putac5va5onrecordsonheapo  Invokegarbagecollectorasneededo  Notastotallycrazyasissounds

Mayonlyneedtosearchreachabledata

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Summaryofscopeissues

  Block-structuredlangusesstackofac5vrecordso  Ac5va5onrecordscontainparameters,localvars,…o  Alsopointerstoenclosingscope  Severaldifferentparameterpassingmechanisms  Tailcallsmaybeop5mized  Func5onparameters/resultsrequireclosureso  Closureenvironmentpointerusedonfunc5oncallo  Stackdealloca5onmayfailiffunc5onreturnedfromcallo  Closuresnotneedediffunc5onsnotinnestedblocks