AnneBracyCS3410

ComputerScienceCornellUniversity

The slides are the product of many rounds of teaching CS 3410 by Professors Weatherspoon, Bala, Bracy, McKee, and Sirer.

Write-BackMemory

InstructionFetch Execute

InstructionDecode

extend

registerfile

control

alu

memory

din dout

addrPC

programmemory

newpc

inst

IF/ID ID/EX EX/MEM MEM/WB

imm

BA

ctrl

ctrl

ctrl

BD D

M

computejump/branch

targets

+4

forwardunitdetect

hazard

2

0xfffffffc

0x00000000

top

bottom

0x7ffffffc0x80000000

0x10000000

0x00400000

systemreserved

stack

systemreserved

code(text)

staticdata

dynamicdata(heap)

“DataMemory”

“ProgramMemory”

3

Stackcontainsstackframes(aka“activationrecords”)• 1stackframeperdynamicfunction• Existsonlyforthedurationoffunction• Growsdown,“top”ofstackis$sp,r29• Example:lw $r1,0($sp)putswordattopofstackinto$r1Eachstackframecontains:• Localvariables,returnaddress(later),register

backups(later) myfn stackframe

myfn stackframesystemreserved

stack

systemreserved

codestaticdata

heap

main stackframe

int main(…) {...

myfn(x);}int myfn(int n) {

...

myfn();}

$spà

4

Heapholdsdynamicallyallocatedmemory• Programmustmaintainpointerstoanythingallocated

• Example:if$r3holdsx• lw $r1,0($r3)getsfirstwordxpointsto

• Dataexistsfrommalloc()tofree()

2000bytes

1000bytes

systemreserved

stackxyz

systemreserved

codestaticdata

heap

3000bytes

void some_function() {int *x = malloc(1000);int *y = malloc(2000);free(y);int *z = malloc(3000);

}

5

Datasegmentcontainsglobalvariables• Existforalltime,accessibletoallroutines• Accessedw/globalpointer

• $gp,r28,pointstomiddleofsegment• Example:lw $r1,0($gp)getsmiddle-mostword

(here,max_players)

systemreserved

stack

systemreserved

codestaticdata

heap

int max_players = 4;

int main(...) {...

}

gpà 4

6

int n = 100;int main (int argc, char* argv[ ]) {

int i, m = n, sum = 0; int* A = malloc(4*m + 4);for (i = 1; i <= m; i++) {

sum += i; A[i] = sum; }printf ("Sum 1 to %d is %d\n", n, sum);

}

Variables Visibility Lifetime Location

Function-Local

Global

Dynamic

7

Variables Visibility Lifetime Location

Function-Local

Global

Dynamic

i,m,sum,A

n,str

w/infunction functioninvocation stack

wholeprogram programexecution .data

b/wmallocandfree heap

Anywherethathasapointer*A

int n = 100;int main (int argc, char* argv[ ]) {

int i, m = n, sum = 0; int* A = malloc(4*m + 4);for (i = 1; i <= m; i++) {

sum += i; A[i] = sum; }printf ("Sum 1 to %d is %d\n", n, sum);

}8

Don’teverwritecodelikethis!

void some_function() {int *x = malloc(1000);int *y = malloc(2000);free(y);int *z = malloc(3000);y[20] = 7;

}

void f1() {int *x = f2();int y = *x + 2;

}int *f2() {

int a = 3;return &a;

}

Danglingpointersintofreedheapmem

Danglingpointersintooldstackframes

9

Whichofthefollowingistrouble-freecode?

10

int *bubble(){ int a; …return &a;

}

int *toil() { s = malloc(20); … return s;

}

A char *rubble() { char s[20];gets(s); return s;

}

int *trouble() { s = malloc(20); … free(s); … return s;

}

B

C D

int main (int argc, char* argv[ ]) {int n = 9;int result = myfn(n);

}

int myfn(int n) {int f = 1;int i = 1;int j = n – 1;while(j >= 0) {

f *= i;i++;j = n - i;

}return f;

}11

TransferControl• Callerà Routine• Routineà Caller

PassArgumentstoandfromtheroutine• fixedlength,variablelength,recursively• Getreturnvaluebacktothecaller

ManageRegisters• Alloweachroutinetouseregisters• Preventroutinesfromclobberingeachothers’data

WhatisaConvention?Warning: ThereisnoonetrueMIPScallingconvention.

lecture!=book!=gcc !=spim !=web 12

main: j myfn

after1:add $1,$2,$3

j myfnafter2:

sub $3,$4,$5

myfn: …

…j after1

JumpstothecalleeJumpsbackWhataboutmultiplesites?

???Changetargetonthefly???

j after2

1

2

3

4

13

JALsavesthePCinregister$31Subroutinereturnsbyjumpingto$31

r31

main: jal myfn

after1:add $1,$2,$3

jal myfnafter2:

sub $3,$4,$5

myfn: …

…jr $31

after11

2

14

Firstcall

JALsavesthePCinregister$31Subroutinereturnsbyjumpingto$31Whathappensforrecursiveinvocations?

r31

main: jal myfn

after1:add $1,$2,$3

jal myfnafter2:

sub $3,$4,$5

myfn: …

…jr $31

after21

2

4

3

15

Secondcall

Problemswithrecursion:• overwritescontentsof$31

r31

main: jal myfn

after1:add $1,$2,$3

myfn:if (test)jal myfn

after2:

jr $31

after11

16

Firstcall

Problemswithrecursion:• overwritescontentsof$31

r31

main: jal myfn

after1:add $1,$2,$3

myfn:if (test)jal myfn

after2:

jr $31

after21

2

17

RecursiveCall

Problemswithrecursion:• overwritescontentsof$31

r31

main: jal myfn

after1:add $1,$2,$3

myfn:if (test)jal myfn

after2:

jr $31

12

3

18

ReturnfromRecursiveCall after2

Problemswithrecursion:• overwritescontentsof$31

r31

main: jal myfn

after1:add $1,$2,$3

myfn:if (test)jal myfn

after2:

jr $31

12

34 Stuck!

19

ReturnfromOriginalCall??? after2

Problemswithrecursion:• overwritescontentsof$31• Cometothinkofit…overwritesall theregisters!

r31

main: jal myfn

after1:add $1,$2,$3

myfn:

jal myfnafter2:

jr $31

after1

addi r1, r0, 5

addi r1, r1, 5

r1 5

12

…

20

1st timethroughmyfn

Problemswithrecursion:• overwritescontentsof$31• Cometothinkofit…overwritesall theregisters!

r31

main: jal myfn

after1:add $1,$2,$3

myfn:

jal myfnafter2:

jr $31

after2

addi r1, r0, 5

addi r1, r1, 5

r1 10

12

…

21

2nd timethroughmyfn

x2000x1FD0

StackManipulatedbypush/pop operationsContext:after2nd JALtomyfn (frommyfn)PUSH: ADDIU$sp,$sp,-20//movespdown

SW$31,16($sp)//storeretn PC1st

Context:2nd myfn isdone(r31==???)POP: LW$31,16($sp) //restoreretn PCàr31

ADDIU$sp,$sp,20//movespupJR$31 //return

myfn stackframe

mainstackframe

myfn stackframe

after2r31r29

x2000

Fornow:Assumeeachframe=x20bytes(justtomakethisexampleconcrete)

x1FD0

after2

XXXX22

WhydoweneedaJALinstructionforprocedurecalls?

A. TheonlywaytochangethePCofyourprogramiswithaJALinstruction.

B. Thesystemwon’tletyoujumptoaprocedurewithjustaJMPinstruction.

C. IfyouJMPtoafunction,itdoesn’tknowwheretoreturntouponcompletion.

D. Actually,JALonlyworksforthefirstfunctioncall.Withmultipleactivefunctions,JALisnottherightinstructiontouse.

23

TransferControl• Callerà Routine• Routineà Caller

PassArgumentstoandfromtheroutine• fixedlength,variablelength,recursively• Getreturnvaluebacktothecaller

ManageRegisters• Alloweachroutinetouseregisters• Preventroutinesfromclobberingeachothers’data

24

Firstfourarguments:passedinregisters$4-$7

• aka$a0,$a1,$a2,$a3Returnedresult:passedbackinaregister

• Specifically,$2,aka$v0main:

li $a0, 6li $a1, 7jal myfnaddi $r1, $v0, 2

main() {int x = myfn(6, 7);x = x + 2;

}

Note:Thisisnottheentirestoryfor1-4arguments.PleaseseetheFullStoryslides.

25

Firstfourarguments:passedin$4-$7

• aka$a0-$a3Subsequentarguments:”spill”ontothestack

main: li $a0, 0li $a1, 1li $a2, 2li $a3, 3addiu $sp,$sp,-8li $8, 4sw $8, 0($sp)li $8, 5sw $8, 4($sp)jal myfn

main() {myfn(0,1,2,3,4,5);…

}

Note:Thisisnottheentirestoryfor5+arguments.PleaseseetheFullStoryslides.

spà

54

spà

26

Arguments1-4:passedin$4-$7roomonstackArguments5+:placedonstack

main: li $a0, 0li $a1, 1li $a2, 2li $a3, 3addiu $sp,$sp,-24li $8, 4sw $8, 16($sp)li $8, 5sw $8, 20($sp)jal myfn

main() {myfn(0,1,2,3,4,5);…

}

spà

spà

4spacefora3spacefora2spacefora1spacefora0

5

0($sp)

4($sp)

8($sp)

12($sp)

16($sp)

20($sp)

Stackdecrimented bymax(16,#args x4)Here:max(16,24)=24

27

• Consistentwayofpassingargumentstoandfromsubroutines

• Createssinglelocationforallarguments• Callermakesroomfor$a0-$a3onstack• Callee mustcopyvaluesfrom$a0-$a3tostack

à callee maytreatallargs asanarrayinmemory• Particularlyhelpfulforfunctionsw/variablelength

inputs:printf(“Scores: %d %d %d\n”, 1, 2, 3);

• Aside:notabadplacetostoreinputsifcalleeneedstocallafunction(yourinputcannotstayin$a0ifyouneedtocallanotherfunction!)

28

Callowspassingwholestructs• int dist(struct Point p1, struct Point p2);

• Treatedascollectionofconsecutive32-bitarguments– Registersforfirst4words,stackforrest

• Better:int dist(struct Point *p1, struct Point *p2);

Wherearetheargumentsto:void sub(int a, int b, int c, int d, int e);void isalpha(char c);void treesort(struct Tree *root);

Wherearethereturnvaluesfrom:struct Node *createNode();struct Node mynode();

Manycombinationsofchar,short,int,void*,struct,etc.• MIPStreatschar,short,int andvoid*identically 29

Whichisatruestatementabouttheargumentstothefunctionvoid sub(int a, int b, int c, int d, int e);

A. Argumentsa-e areallpassedinregisters.B. Argumentsa-e areallstoredonthestack.C. Onlyeisstoredonthestack,butspaceis

allocatedforall5arguments.D. Onlya-darestoredonthestack,butspaceis

allocatedforall5arguments.30

4spacefora3spacefora2spacefora1spacefora0

5

pink’sRetAddr

blue() {pink(0,1,2,3,4,5);

}pink(int a, int b, int c, int d, int e, int f) {

…}

blue’sRetAddr

pink’sstackframe

spà

spà

blue’sstackframe

Notice• Pink’sargumentsareonblue’s stack• spchangesasfunctionscallother

functions, complicatesaccessesà Convenienttokeeppointertobottomofstack==framepointer

$30,aka$fpcanbeusedtorestore$sponexitß fp

31

TransferControl• Callerà Routine• Routineà Caller

PassArgumentstoandfromtheroutine• fixedlength,variablelength,recursively• Getreturnvaluebacktothecaller

ManageRegisters• Alloweachroutinetouseregisters• Preventroutinesfromclobberingeachothers’data

32

Functions:• Arecompiledinisolation• Makeuseofgeneralpurposeregisters• Callotherfunctionsinthemiddleoftheirexecution

• Thesefunctionsalsousegeneralpurposeregisters!• Nowaytocoordinatebetweencaller&callee

à Needaconventionforregistermanagement

33

Registersthatthecallercaresabout:$t0…$t9Abouttocallafunction?• Needvalueinat-registerafterfunctionreturns?

à saveittothestackbeforefn callà restoreitfromthestackafterfn returns

• Don’tneedvalue?à donothing

Functions• Canfreelyusetheseregisters• Mustassumethattheircontentsaredestroyedbyotherfunctions

void myfn(int a) {int x = 10;int y = max(x, a);int z = some_fn(y);return (z + y);

}

Suppose:$t0holdsx$t1holdsy$t2holdsz

Wheredowesaveandrestore?

34

Registersafunctionintendstouse:$s0…$s9Abouttouseans-register?YouMUST:• Savethecurrentvalueonthestackbeforeusing• Restoretheoldvaluefromthestackbeforefn returns

Functions• Mustsavetheseregistersbeforeusingthem

• Mayassumethattheircontentsarepreservedevenacrossfn calls

void myfn(int a) {int x = 10;int y = max(x, a);int z = some_fn(y);return (z + y);

}

Suppose:$t0holdsx$s1holdsy$s2holdsz

Wheredowesaveandrestore?

35

Assumetheregistersarefreeforthetaking,usewithnooverhead

Sincesubroutineswilldothesame,mustprotectvaluesneededlater:

Savebeforefn callRestoreafterfn call

Notice:Goodregisterstouseifyoudon’tcalltoomanyfunctionsorifthevaluesdon’tmatterlateronanyway.

main:…[use$t0&$t1]…addiu $sp,$sp,-8sw $t1,4($sp)sw $t0,0($sp)jal multlw $t1,4($sp)lw $t0,0($sp)addiu $sp,$sp,8…[use$t0&$t1]

36

AssumecallerisusingtheregistersSaveonentryRestoreonexit

Notice:Goodregisterstouseifyoumakealotoffunctioncallsandneedvaluesthatarepreservedacrossallofthem.Also,goodifcallerisactuallyusingtheregisters,otherwisethesaveandrestoresarewasted.Buthardtoknowthis.

main:addiu $sp,$sp,-32sw $ra,28($sp)sw $fp,24($sp)sw $s1,20($sp)sw $s0,16($sp)addiu $fp,$sp,28

…[use$s0and$s1]…

lw $ra,28($sp)lw $fp,24($sp)lw $s1,20$sp)lw $s0,16($sp)addiu $sp,$sp,32jr $ra 37

• firstfourarg wordspassedin$a0-$a3• remainingargs passedinparent’sstackframe• returnvalue(ifany)in$v0,$v1• stackframe($fpto$sp)contains:

• $ra (clobberedonJALs)• localvariables• spacefor4argumentstoCallees• arguments5+toCallees

• callee saveregs:preserved• callersaveregs:notpreserved• globaldataaccessedvia$gp

savedrasavedfpsavedregs($s0...$s7)

locals

outgoingargs

$fpà

$spà 38

r0 $zero zeror1 $at assemblertempr2 $v0 function

returnvaluesr3 $v1r4 $a0

functionarguments

r5 $a1r6 $a2r7 $a3r8 $t0

temps(callersave)

r9 $t1r10 $t2r11 $t3r12 $t4r13 $t5r14 $t6r15 $t7

r16 $s0

saved(callee save)

r17 $s1r18 $s2r19 $s3r20 $s4r21 $s5r22 $s6r23 $s7r24 $t8 moretemps

(caller save)r25 $t9r26 $k0 reservedfor

kernelr27 $k1r28 $gp globaldatapointerr29 $sp stackpointerr30 $fp framepointerr31 $ra returnaddress 39

Assumeafunctionusestwocallee-saveregisters.Howdoweallocateastackframe?Howlargeisthestackframe?Whatshouldbestoredinthestackframe?Whereshouldeverythingbestored?

savedrasavedfpsavedregs($s0...$s7)

locals

outgoingargs

fpà

spà

40

ADDIU$sp,$sp,-32 #allocateframeSW$ra,28($sp) #save$raSW$fp,24($sp) #saveold$fpSW$s1,20($sp) #save...SW$s0,16($sp) #save...ADDIU$fp,$sp,28 #setnewframeptr… ...BODY… ...LW$s0,16($sp) #restore…LW$s1,20($sp) #restore…LW$fp,24($sp) #restoreold$fpLW$ra,28($sp) #restore$raADDIU$sp,$sp,32 #dealloc frameJR$ra

savedrasavedfpsavedregs($s0...$s7)

locals

outgoingargs

fpà

spà

41

pink’sra

blue() {pink(0,1,2,3,4,5);

}pink(int a, int b, int c, int d, int e, int f) {

int x;orange(10,11,12,13,14);

}orange(int a, int b, int c, int, d, int e) {

char buf[100];gets(buf); // no bounds check!

}

Whathappensifmorethan100bytesiswrittentobuf?

savedregsargs forpink

savedfp

savedfp

savedregs

blue’sra

pink’sstackframe

fpà

blue’sstackframe

xargs fororange

spà

pink’srasavedfpsavedregs

orangestackframe

buf[100]

42

Leaffunctiondoesnotinvokeanyotherfunctionsint f(int x, int y) {

return (x+y); }

Optimizations?Nosavedregs (orlocals)NooutgoingargsDon’tpush$raNoframeatall?Possibly…

savedrasavedfpsavedregs($s0...$s7)

locals

outgoingargs

$fpà

$spà43

int test(int a, int b) {int tmp = (a&b)+(a|b);int s = sum(tmp,1,2,3,4,5);int u = sum(s,tmp,b,a,b,a);return u + a + b;

}

CorrectOrder:1. BodyFirst2. Determinestackframesize3. CompletePrologue/Epilogue

44

int test(int a, int b) {int tmp = (a&b)+(a|b);int s = sum(tmp,1,2,3,4,5);int u = sum(s,tmp,b,a,b,a);return u + a + b;

}

test:

MOVE$s0,$a0MOVE$s1,$a1AND$t0,$a0,$a1OR$t1,$a0,$a1ADD$t0,$t0,$t1MOVE$a0,$t0LI$a1,1LI$a2,2LI$a3,3LI$t1,4SW$t116($sp)LI$t1,5SW$t1,20($sp)SW$t0,24($sp)JALsumNOPLW$t0,24($sp)

Prologue

We’llassumetheyellowinordertoforceyourhandontherest.$s0for$a0/a$s1for$a1/b$t0fortmp

CanwegetridoftheNOP?Wewanttodothelw… 45

int test(int a, int b) {int tmp = (a&b)+(a|b);int s = sum(tmp,1,2,3,4,5);int u = sum(s,tmp,b,a,b,a);return u + a + b;

}

MOVE$a0,$v0#sMOVE$a1,$t0#tmpMOVE$a2,$s1#bMOVE$a3,$s0#aSW$s1,16($sp)#bSW$s0,20($sp)#aJALsumNOP

ADD$v0,$v0,$s0#u+aADD$v0,$v0,$s1#+b

Epilogue

46

int test(int a, int b) {int tmp = (a&b)+(a|b);int s = sum(tmp,1,2,3,4,5);int u = sum(s,tmp,b,a,b,a);return u + a + b;

}

How many bytes dowe need to allocate for the stack frame?a) 24b) 36c) 44d) 48e) 52Minimumstacksizeforastandardfunction?

savedrasavedfpsavedregs

($s0and$s1)

locals($t0)

outgoingargsspacefora0- a3and5th and6th arg 47

int test(int a, int b) {int tmp = (a&b)+(a|b);int s = sum(tmp,1,2,3,4,5);int u = sum(s,tmp,b,a,b,a);return u + a + b;

}savedrasavedfpsavedregs

($s0and$s1)

locals($t0)

outgoingargsspacefora0- a3and5th and6th arg

savedrasavedfp

fpà

spà

savedreg $s1savedreg $s0local$t0

outgoing6th argoutgoing5th argspacefor$a3spacefor$a2spacefor$a1spacefor$a0

48

1216

0

202428323640

How many bytes dowe need to allocate for the stack frame?

44Minimumstacksizeforastandardfunction?

$ra +$fp+4args =6x4bytes=24bytes

48

#allocateframe#save$ra#saveold$fp#callee save...#callee save...#setnewframeptr

...

...

#restore…#restore…#restoreold$fp#restore$ra#dealloc frame

49

#allocateframe#save$ra#saveold$fp#callee save...#callee save...#setnewframeptr

...

...

#restore…#restore…#restoreold$fp#restore$ra#dealloc frame

savedrasavedfp

savedreg $s1savedreg $s0local$t0

outgoing6th argoutgoing5th argspacefor$a3spacefor$a2spacefor$a1spacefor$a0

48

1216

0

202428323640fpà

spà

ADDIU$sp,$sp,-44SW$ra,40($sp)SW $fp,36($sp)SW$s1,32($sp)SW$s0,28($sp)ADDIU$fp,$sp,40

LW$s0,28($sp)LW $s1,32($sp)LW$fp,36($sp)LW$ra,40($sp)ADDIU$sp,$sp,44JR$raNOP

Body(previousslide,Activity#1)

50