FunctionsandtheMIPSCallingConvention 2

CS64:ComputerOrganizationandDesignLogicLecture#11

ZiadMatni

Dept.ofComputerScience,UCSB

Administrative•  Lab5dueendofdaytomorrow

•  Midterm:toreview,seetheT.A.s–  LastnamesAthruN:gotoJinjin–  LastnamesOthruZ: gotoBay-Yuan

•  Mid-quarterevaluations–  LinksonthelastslideandwillputuponPiazzatoo– Optionaltodo,butveryappreciatedbyus!–  There’sstilltheESCIevalsattheendofthequarter…

2/22/18 Matni,CS64,Wi18 2

LectureOutline

•  RecappingMIPSCallingConvention

– Functioncallingfunctionexample

– Recursivefunctionexample

2/22/18 Matni,CS64,Wi18 3

POP!QUIZ!ConsiderthisC/C++code:voidthird(){}voidsecond(){third();}voidfirst(){second();exit()}AndconsiderthissupposedlyequivalentMIPScodeèèèèa)  Arethereanyerrorsinit?

(i.e.willitrun?)b)  DoesitfollowtheMIPSC.C.?

EXPLAINYOURANSWER2/22/18 Matni,CS64,Wi18 4

third:jr$ra

second:move$t0,$rajalthirdjr$t0

first:jalsecond

li$v0,10syscall

TheMIPSConventionInItsEssence•  Remember:PreservedvsUnpreservedRegs•  Preserved: $s0-$s7,and$sp•  Unpreserved: $t0-$t9,$a0-$a3,and$v0-$v1

•  ValuesheldinPreservedRegsimmediatelybeforeafunctioncallMUSTbethesameimmediatelyafterthefunctionreturns.

•  ValuesheldinUnpreservedRegsmustalwaysbeassumedtochangeafterafunctioncallisperformed.–  $a0-$a3areforpassingargumentsintoafunction–  $v0-$v1areforpassingvaluesfromafunction

2/22/18 Matni,CS64,Wi18 5

Matni,CS64,Wi18 6

intsubTwo(inta,intb){intsub=a-b;returnsub;}intdoSomething(intx,inty){inta=subTwo(x,y);intb=subTwo(y,x);returna+b;}

AnIllustrativeExamplesubTwodoesn’tcallanythingWhatshouldImapaandbto?

$a0and$a1CanImapsubto$t0?

Yes,b/cIdon’tcareabout$t*Eventually,Ihavetohavesubbe$v0

doSomethingDOEScallafunctionWhatshouldImapxandyto?

SincewewanttopreservethemacrossthecalltosubTwo,weshouldmapthemto$s0and$s1WhatshouldImapaandbto?

“a+b”hastoeventuallybe$v0.Ishouldmakeatleastabeapreservedreg($s2).SinceIgetbbackfromacallandthere’snoothercallafterit,Icanlikelygetawaywithnotusingapreservedregforb.

2/22/18

Matni,CS64,Wi18 72/22/18

subTwo:sub$t0,$a0,$a1move$v0,$t0jr$radoSomething:addiu$sp,$sp,-16sw$s0,0($sp)sw$s1,4($sp)sw$s2,8($sp)sw$ra,12($sp)move$s0,$a0move$s1,$a1jalsubTwo

move$s2,$v0move$a0,$s1move$a1,$s0jalsubTwoadd$v0,$v0,$s2lw$ra,12($sp)lw$s2,8($sp)lw$s1,4($sp)lw$s0,0($sp)addiu$sp,$sp,16jr$ra

intsubTwo(inta,intb){intsub=a-b;returnsub;}intdoSomething(intx,inty){inta=subTwo(x,y);intb=subTwo(y,x);returna+b;}

intsubTwo(inta,intb){intsub=a-b;returnsub;}intdoSomething(intx,inty){inta=subTwo(x,y);intb=subTwo(y,x);…returna+b;}

subTwo:sub$t0,$a0,$a1move$v0,$t0jr$radoSomething:addiu$sp,$sp,-16sw$s0,0($sp)sw$s1,4($sp)sw$s2,8($sp)sw$ra,12($sp)move$s0,$a0move$s1,$a1jalsubTwo

move$s2,$v0move$a0,$s1move$a1,$s0jalsubTwoadd$v0,$v0,$s2lw$ra,12($sp)lw$s2,8($sp)lw$s1,4($sp)lw$s0,0($sp)addiu$sp,$sp,16jr$ra

a-b

inta intb

inta intb

$t0

$a0 $a1

$s0 $s1

Orig.$ra

Orig.$s2

Orig.$s1

stackOrig.$s0

Arguments

Preserved

Unpreserved

a–b$v0

ResultValue

$ra a–b$s2

subTwo:sub$t0,$a0,$a1move$v0,$t0jr$radoSomething:addiu$sp,$sp,-16sw$s0,0($sp)sw$s1,4($sp)sw$s2,8($sp)sw$ra,12($sp)move$s0,$a0move$s1,$a1jalsubTwo

move$s2,$v0move$a0,$s1move$a1,$s0jalsubTwoadd$v0,$v0,$s2lw$ra,12($sp)lw$s2,8($sp)lw$s1,4($sp)lw$s0,0($sp)addiu$sp,$sp,16jr$ra

intsubTwo(inta,intb){intsub=a-b;returnsub;}intdoSomething(intx,inty){inta=subTwo(x,y);intb=subTwo(y,x);…returna+b;}

b–a

intb inta

inta intb

$t0

$a0 $a1

$s0 $s1

Orig.$ra

Orig.$s2

Orig.$s1

stackOrig.$s0

Arguments

Preserved

Unpreserved

b–a$v0

ResultValue

$ra a–b$s2

0

subTwo:sub$t0,$a0,$a1move$v0,$t0jr$radoSomething:addiu$sp,$sp,-16sw$s0,0($sp)sw$s1,4($sp)sw$s2,8($sp)sw$ra,12($sp)move$s0,$a0move$s1,$a1jalsubTwo

move$s2,$v0move$a0,$s1move$a1,$s0jalsubTwoadd$v0,$v0,$s2lw$ra,12($sp)lw$s2,8($sp)lw$s1,4($sp)lw$s0,0($sp)addiu$sp,$sp,16jr$ra

intsubTwo(inta,intb){intsub=a-b;returnsub;}intdoSomething(intx,inty){inta=subTwo(x,y);intb=subTwo(y,x);…returna+b;}

b–a

intb inta

orig. orig.

$t0

$a0 $a1

$s0 $s1

Orig.$ra

Orig.$s2

Orig.$s1

stackOrig.$s0

Arguments

Preserved

Unpreserved

0$v0

ResultValue

$ra orig.$s2

Originalcaller$ra

b–a

intb inta

orig. orig.

$t0

$a0 $a1

$s0 $s1

Arguments

Preserved

Unpreserved

0$v0

ResultValue

orig.$s2

LessonsLearned•  Wepassedargumentsintothefunctionsusing$a*

•  Weused$s*toworkoutcalculationsinregistersthatwewantedtopreserve,sowemadesuretosavetheminthecallstack–  ThesevarvaluesDOneedtolivebeyondacall–  Intheend,theoriginalvalueswerereturnedback

•  Weused$t*toworkoutcalcs.inregsthatwedidnotneedtopreserve–  ThesevaluesDONOTneedtolivebeyond

afunctioncall

•  Weused$v*asregs.toreturnthevalueofthefunction

2/22/18 Matni,CS64,Wi18

AnotherExampleUsingRecursion

2/22/18 Matni,CS64,Wi18 12

RecursiveFunctions

•  Thissamesetuphandlesnestedfunctioncallsandrecursion–  i.e.Bysaving$ramethodicallyonthestack

•  Example:recursive_fibonacci.asm

2/22/18 Matni,CS64,Wi18 13

recursive_fibonacci.asmRecalltheFibonacciSeries:0,1,1,2,3,5,8,13,etc…

fib(n)=fib(n–1)+fib(n–2)

InC/C++,wemightwritetherecursivefunctionas:intfib(intn){

if(n==0) return(0);else if(n==1) return(1); else return(fib(n-1)+fib(n-2));

}

2/22/18 Matni,CS64,Wi18 14

Basecases

recursive_fibonacci.asm•  We’llneedatleast3registerstokeeptrackof:

–  The(single)inputtothecall,i.e.varn–  Theoutput(orpartialoutput)tothecall–  Thevalueof$ra(sincethisisarecursivefunction)

Ifwemake$s0=nand$s1=fib(n–1)•  Thenweneedtosave$s0,$s1and$raonthestack

–  Sothatwedonotcorrupt/losewhat’salreadyintheseregs•  We’lluse$s*registersb/cweneedtopreservethem

beyondthefunctioncall

2/22/18 Matni,CS64,Wi18 15

recursive_fibonacci.asm•  First:Checkforthebasecases–  Isn($a0)equalto0or1?

•  Next:3registerscontainingintegers,meansweneedtoplanfor3wordsinthestack–  Push3wordsin(i.e.12bytes)–  $sp–=12–  Theorderbywhichyouputthemindoes

notstrictlymatter,butitmakesmore“organized”senseto push$s0,$s1,then$ra

Matni,CS64,Wi18 16

Orig.$s0

Orig.$s1

Orig.$ra

n$a0

stack

$sp

Orig.ra$ra

Orig.s0 Orig.s1$s0 $s1

recursive_fibonacci.asm•  Next:calculatefib(n–1)

–  Callrecursively,copyoutputin$s1•  Next:calculatefib(n–2)

Matni,CS64,Wi18 17

n fib(n-1)$s0 $s1

Orig.$s0

Orig.$s1

Orig.$ra

n$a0

stack

fib(n-1)$v0

$sp

Orig.ra$ra

Newra

recursive_fibonacci.asm•  Next:calculatefib(n–1)

–  Callrecursively,copyoutputin$s1•  Next:calculatefib(n–2)

–  Callrecursively,addoutputto$s1

Matni,CS64,Wi18 18

n$s0 $s1

Orig.$s0

Orig.$s1

Orig.$ra

n$a0

stack

fib(n-1)$v0

fib(n-2)fib(n-1)+f(n-2)

Orig.s1fib(n-1)

$sp

Newra$ra

recursive_fibonacci.asm•  Next:calculatefib(n–1)

–  Callrecursively,copyoutputin$s1•  Next:calculatefib(n–2)

–  Callrecursively,addoutputto$s1•  Next:restoreregisters

–  Popthe3wordsbackto$s0,$s1,and$ra•  Next:returntocaller

–  Issueajr$rainstruction

•  Notehowwhenweleavethefunctionandgobacktothe“callee”,wedidnotdisturbwhatwasintheregisterspreviously

•  Andnowwehaveouroutputwhereitshouldbe,in$v0 Matni,CS64,Wi18 19

n fib(n-1)$s0 $s1

Orig.$s0

Orig.$s1

Orig.$ra

n$a0

stack

$v0fib(n-1)+f(n-2)Orig.s1Orig.s0

$sp

$raNewraOrig.ra

TailRecursion•  Checkoutthedemofiletail_recursive_factorial.asmathome•  What’sspecialaboutthetailrecursivefunctions(seeexample)?

–  Wheretherecursivecallistheverylastthinginthefunction.–  Withtherightoptimization,itcanuseaconstantstackspace

(noneedtokeepsaving$raoverandover)

intTRFac(intn,intaccum){

if(n==0) returnaccum;else returnTRFac(n–1,n*accum);

}

2/22/18 Matni,CS64,Wi18 20

Forexample,ifyousaid:TRFac(4,1)Thentheprogramwouldreturn:TRFac(3,4),thenreturnTRFac(2,12),thenreturnTRFac(1,24),thenreturnTRFac(0,24),then,sincen=0,24

YOURTO-DOs

•  FinishLab#5byendofdayFriday

•  Taketheonlinemid-termevaluationsbyMONDAY!J–  TAs: https://goo.gl/forms/fK0VxzIQt09NG6iT2–  Prof: https://goo.gl/forms/HJXe9b7f6ekfOSSR2

•  Nextweek:DigitalLogic!

2/22/18 Matni,CS64,Wi18 21

2/22/18 Matni,CS64,Wi18 22