anne bracy cs 3410 · 2016-10-18 · anne bracy cs 3410 computer science cornell university the...

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