anne bracy cs 3410 · • assembler: creates object files (= machine code) • linker: joins object...

See:P&HAppendixA1-2,A.3-4and2.12

AnneBracyCS3410

ComputerScienceCornellUniversity

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

• Compiler:createsassemblyfiles

• Assembler:createsobjectfiles(=machinecode)

• Linker: joinsobjectfilesintooneexecutable

• Loader: bringsexecutableintomemoryandstartsexecutingaprocess

✔

calc.c

math.c

io.s

calc.s

math.s

Compiler

Csourcefiles

assemblyfiles libc.o

libm.o

io.o

calc.o

math.o

Assembler

obj files

calc.exe

Linker

executableprogram

Executingin

Memory

loader

process

existsondisk

Howdowe(ashumansorcompiler)programontopofagivenISA?

Assemblylanguageà binary machinecodeInput=Program:• MIPSinstructions• Programdata(strings,variables,etc)

Output=objectfile:.ofileinUnix,.obj inWindows,containingMIPSinstructionsinexecutableform

addi r5, r0, 10muli r5, r5, 2addi r5, r5, 15

001000000000010100000000000010100000000000000101001010000100000000100000101001010000000000001111

Assemblylanguageisusedtospecifyprogramsatalow-level

WillIprograminassembly?A:Ido...• ForCS3410(andsomeCS4410/4411)• Forkernelhacking,devicedrivers,GPU,etc.• Forperformance(butcompilersaregettingbetter)• Forhighlytimecriticalsections• Forhardwarewithouthighlevellanguages• Fornew&advancedinstructions:rdtsc,debugregisters,performancecounters,synchronization,...

Input:• assemblyinstructions• psuedo-instructions• dataandlayoutdirectives

Output:ObjectFile

Slightlyhigherlevelthanplainassemblye.g:takescareofdelayslots

(willreorderinstructionsorinsertnops)

Arithmetic/Logical• ADD,ADDU,SUB,SUBU,AND,OR,XOR,NOR,SLT,SLTU• ADDI,ADDIU,ANDI,ORI,XORI,LUI,SLL,SRL,SLLV,SRLV,SRAV,SLTI,SLTIU

• MULT,DIV,MFLO,MTLO,MFHI,MTHIMemoryAccess• LW,LH,LB,LHU,LBU,LWL,LWR• SW,SH,SB,SWL,SWR

Controlflow• BEQ,BNE,BLEZ,BLTZ,BGEZ,BGTZ• J,JR,JAL,JALR,BEQL,BNEL,BLEZL,BGTZL

Special• LL,SC,SYSCALL,BREAK,SYNC,COPROC


Output:Objectfile



Pseudo-InstructionsNOP#donothing• SLLr0,r0,0

MOVEreg,reg #copybetweenregs• ADDr2,r0,r1#copiescontentsofr1tor2

LIreg,imm #loadimmediate(upto32bits)LAreg,label#loadaddress(32bits)Blabel#unconditionalbranchBLTreg,reg,label#branchlessthan• SLTr1,rA,rB #r1=1ifR[rA]<R[rB];o.w.r1=0• BNEr1,r0,label#gotoaddresslabelifr1!=r0;i.t.rA <rB


Output:Objectfile



Programsconsistofsegmentsusedfordifferentpurposes• Text:holdsinstructions• Data:holdsstaticallyallocated

programdatasuchasvariables,strings,etc.

addr1,r2,r3

ori r2,r4,3

...

“cornell cs”

13

25data

text

Assemblyfilesconsistofamixof• Instructions• pseudo-instructions• assembler(data/layout)directives on

howtolayoutvaluesinmemory

AssembledtoanObjectFile• Header• TextSegment• DataSegment• RelocationInformation• SymbolTable• DebuggingInformation

.text

.ent mainmain: la $4, Larray

li $5, 15...li $4, 0jal exit

.end main

.dataLarray:

.long 51, 491, 3991

Assemblyisalow-leveltask• Needtoassembleassemblylanguageintomachinecodebinary.Requires– Assemblylanguageinstructions– pseudo-instructions– AndSpecifylayoutanddatausingassemblerdirectives

• Modern(Harvard VonNeumann)processorsstorebothdataandinstructionsinmemory…butkeptinseparatesegments…andhasseparatecaches

Putitalltogether:Anexampleofcompilingaprogramfromsourcetoassemblytomachineobjectcode.

add100.c add100.s

Compiler

Csourcefiles

assemblyfiles

add100.o

Assembler

obj filesadd100

Linkerexecutableprogram

Executingin

Memory

loader

process

existsondisk

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

int i;intm=n;int sum=0;

for(i =1;i <=m;i++)sum+=i;

printf ("Sum1to%dis%d\n",n,sum);}#Compile[csug03] mipsel-linux-gcc –S add1To100.cexportPATH=${PATH}:/courses/cs3410/mipsel-linux/bin:/courses/cs3410/mips-sim/binorsetenv PATH${PATH}:/courses/cs3410/mipsel-linux/bin:/courses/cs3410/mips-sim/bin

$L2: lw $2,24($fp)lw $3,28($fp)slt $2,$3,$2bne $2,$0,$L3lw $3,32($fp)lw $2,24($fp)addu $2,$3,$2sw $2,32($fp)lw $2,24($fp)addiu $2,$2,1sw $2,24($fp)b $L2

$L3: la $4,$str0lw $5,28($fp)lw $6,32($fp)jal printfmove $sp,$fplw $31,44($sp)lw $fp,40($sp)addiu $sp,$sp,48j $31

.data

.globl n

.align 2 n: .word 100

.rdata

.align 2$str0: .asciiz

"Sum 1 to %d is %d\n".text.align 2.globl main

main: addiu $sp,$sp,-48sw $31,44($sp)sw $fp,40($sp)move $fp,$spsw $4,48($fp)sw $5,52($fp)la $2,nlw $2,0($2)sw $2,28($fp)sw $0,32($fp)li $2,1sw $2,24($fp)

Example: Add 1 to 100

prologue

epilogue

printf

$L2: lw $2,24($fp)lw $3,28($fp)slt $2,$3,$2bne $2,$0,$L3lw $3,32($fp)lw $2,24($fp)addu $2,$3,$2sw $2,32($fp)lw $2,24($fp)addiu $2,$2,1sw $2,24($fp)b $L2

$L3: la $4,$str0lw $5,28($fp)lw $6,32($fp)jal printfmove $sp,$fplw $31,44($sp)lw $fp,40($sp)addiu $sp,$sp,48j $31

.data

.globl n

.align 2 n: .word 100

.rdata

.align 2$str0: .asciiz

"Sum 1 to %d is %d\n".text.align 2.globl main

main: addiu $sp,$sp,-48sw $31,44($sp)sw $fp,40($sp)move $fp,$spsw $4,48($fp)sw $5,52($fp)la $2,nlw $2,0($2)sw $2,28($fp)sw $0,32($fp)li $2,1sw $2,24($fp)

Example: Add 1 to 100

prologue

epilogue

printf

$v0

$v0$v1

$v0=100m=100sum=0

i=1

i=1m=100

if(m<i)100<1

v0=1(i)v1=0(sum)

v0=1(0+1)

i=1sum=1

i=2(1+1)i=2

$a0$a1$a2

strm=100sum

$a0$a1

# Assemble[csug01] mipsel-linux-gcc –c add1To100.s

# Link[csug01] mipsel-linux-gcc –o add1To100 add1To100.o ${LINKFLAGS}# -nostartfiles –nodefaultlibs# -static -mno-xgot -mno-embedded-pic -mno-abicalls -G 0 -DMIPS -Wall

# Load[csug01] simulate add1To100Sum 1 to 100 is 5050MIPS program exits with status 0 (approx. 2007 instructions in 143000 nsec at 14.14034 MHz)

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 ("Sum1to%dis%d\n",n,sum);

}

Variables Visibility Lifetime Location

Function-Local

Global

Dynamic

int *trouble(){ int a; …return &a;

}char *evil() { char s[20];gets(s); return s;

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

}

//“addr of”somethingonthestack!//invalidafterreturn

//bufferoverflow

//freed(i.e.adangling)pointer

calc.c

math.c

io.s

calc.s

math.s

Compiler

Csourcefiles


libm.o

io.o

calc.o

math.o

Assembler

obj files

calc.exe

Linker

executableprogram

Executingin

Memory

loader

process

existsondisk

vector* v = malloc(8);v->x = prompt(“enter x”);v->y = prompt(“enter y”);int c = pi + tnorm(v);print(“result %d”, c);

calc.c

int tnorm(vector* v) {return abs(v->x)+abs(v->y);}

math.c

global variable: pientry point: promptentry point: printentry point: malloc

lib3410.o

systemreserved

stack

systemreserved

code(text)

staticdata

dynamicdata(heap)

Compiller producesassemblyfiles• (containMIPSassembly,pseudo-instructions,directives,etc.)

Assemblerproducesobjectfiles• (containMIPSmachinecode,missingsymbols,somelayoutinformation,etc.)

Linkerproducesexecutablefile• (containsMIPSmachinecode,nomissingsymbols,somelayoutinformation)

Loaderputsprogramintomemoryandjumpstofirstinstruction• (machinecode)

Compiler outputisassemblyfiles

Assembler outputisobj files• Howdoestheassemblerresolvereferences/labels?• Howdoestheassemblerresolveexternalreferences?

Linker joinsobjectfilesintooneexecutable• Howdoesthelinkercombineseparatelycompiledfiles?• Howdoeslinkerresolveunresolvedreferences?• Howdoeslinkerrelocatedataandcodesegments

Loader bringsitintomemoryandstartsexecution• Howdoestheloaderstartexecutingaprogram?• Howdoestheloaderhandlesharedlibraries?

calc.c

math.c

io.s

calc.s

math.s

Compiler

Csourcefiles

assemblyfiles

io.o

calc.o

math.o

Assembler

obj files

.o=Linux

.objWindows

Outputofassembler:objectfiles• Binarymachinecode,butnotexecutable

Eachfileassembledseparately• Howdoesassemblerhandleforwardreferences?

Howdoestheassemblerhandlelocalreferences?

Two-pass assembly• Firstpassthroughwholeprogram:allocateinstructions,layoutdata,determineaddresses

• Secondpass:emitinstructionsanddata,usinglabeloffsetsfrom1st pass

One-pass (orbackpatch)assembly• Onepassthroughwholeprogram:emittinstructions,emit0forjumpstolabelsnotyetdetermined(keeptrackofthese)

• Backpatch,fillin0offsetsaslabelsaredefined

Example:bne $1,$2,Lsll $0,$0,0

L:addiu $2,$3,0x2

Theassemblerwillchangethistobne $1,$2,+1sll $0,$0,0addiu $2,$3,$0x2

Finalmachinecode0X14220001 #bne0x00000000#sll0x24620002#addiu

000101000010001000000000000000010000000000000000000000000000000000100100011000100000000000000010

Outputofassembler:objectfiles• Binarymachinecode,notexecutable• Howdoesassemblerhandleforwardreferences?• Mayrefertoexternalsymbols• Eachobjectfilehasillusionofitsownaddressspace

– Addresseswillneedtobefixedlater

math.c math.o .o=Linux.objWindows

e.g..text(code)startsataddr 0x00000000.datastarts@addr 0x10000000

needa“symboltable”

math.s

Howdoestheassemblerhandleexternalreferences?

Globallabels: Externallyvisible“exported”symbols• Canbereferencedfromotherobjectfiles• Exportedfunctions,globalvariables

Locallabels: Internalvisibleonlysymbols• Onlyusedwithinthisobjectfile• staticfunctions,staticvariables,looplabels,…

e.g.pi(fromacoupleofslides ago)

e.g.staticfoostaticbarstaticbaz

e.g.$str$L0$L2

Header• Sizeandpositionofpiecesoffile

TextSegment• instructions

DataSegment• staticdata(local/globalvars,strings,constants)

DebuggingInformation• linenumberà codeaddressmap,etc.

SymbolTable• External(exported)references• Unresolved(imported)references

ObjectFile

int pi=3;int e=2;staticint randomval =7;

externchar*username;externint printf(char*str,…);

int square(int x){…}staticint is_prime(int x){…}int pick_prime(){…}int pick_random(){

returnrandomval;}

math.cgcc -S …math.cgcc -c …math.sobjdump --disassemblemath.oobjdump --syms math.o

CompilerAssemblerglobal

local(tocurrentfile)

external(defined inanother file)

globallocal

csug01 ~$ mipsel-linux-objdump --disassemble math.omath.o: file format elf32-tradlittlemipsDisassembly of section .text:

00000000<pick_random>:0: 27bdfff8 addiu sp,sp,-84: afbe0000 sw s8,0(sp)8: 03a0f021 move s8,spc: 3c020000 lui v0,0x010: 8c420008 lw v0,8(v0)14: 03c0e821 move sp,s818: 8fbe0000 lw s8,0(sp)1c: 27bd0008 addiu sp,sp,820: 03e00008 jr ra24: 00000000 nop

00000028<square>:28: 27bdfff8 addiu sp,sp,-82c: afbe0000 sw s8,0(sp)30: 03a0f021 move s8,sp34: afc40008 sw a0,8(s8)…

csug01 ~$ mipsel-linux-objdump --disassemble math.omath.o: file format elf32-tradlittlemipsDisassembly of section .text:

00000000<pick_random>:0: 27bdfff8 addiu sp,sp,-84: afbe0000 sw s8,0(sp)8: 03a0f021 move s8,spc: 3c020000 lui v0,0x010: 8c420008 lw v0,8(v0)14: 03c0e821 move sp,s818: 8fbe0000 lw s8,0(sp)1c: 27bd0008 addiu sp,sp,820: 03e00008 jr ra24: 00000000 nop

00000028<square>:28: 27bdfff8 addiu sp,sp,-82c: afbe0000 sw s8,0(sp)30: 03a0f021 move s8,sp34: afc40008 sw a0,8(s8)…

Address instruction Mem[8]=instruction0x03a0f021(moves8,sp)

prologue

body

epilogue

symbol

resolved(fixed)later

csug01 ~$ mipsel-linux-objdump --syms math.omath.o: file format elf32-tradlittlemips

SYMBOL TABLE:00000000 l df *ABS* 00000000 math.c00000000 l d .text 00000000 .text00000000 l d .data 00000000 .data00000000 l d .bss 00000000 .bss00000000 l d .mdebug.abi32 00000000 .mdebug.abi3200000008 l O .data 00000004 randomval00000060 l F .text 00000028 is_prime00000000 l d .rodata 00000000 .rodata00000000 l d .comment 00000000 .comment00000000 g O .data 00000004 pi00000004 g O .data 00000004 e00000000 g F .text 00000028 pick_random00000028 g F .text 00000038 square00000088 g F .text 0000004c pick_prime00000000 *UND* 00000000 username00000000 *UND* 00000000 printf

csug01 ~$ mipsel-linux-objdump --syms math.omath.o: file format elf32-tradlittlemips

SYMBOL TABLE:00000000 l df *ABS* 00000000 math.c00000000 l d .text 00000000 .text00000000 l d .data 00000000 .data00000000 l d .bss 00000000 .bss00000000 l d .mdebug.abi32 00000000 .mdebug.abi3200000008 l O .data 00000004 randomval00000060 l F .text 00000028 is_prime00000000 l d .rodata 00000000 .rodata00000000 l d .comment 00000000 .comment00000000 g O .data 00000004 pi00000004 g O .data 00000004 e00000000 g F .text 00000028 pick_random00000028 g F .text 00000038 square00000088 g F .text 0000004c pick_prime00000000 *UND* 00000000 username00000000 *UND* 00000000 printf

Address l:localg:global

segmentsize

segment

Staticlocalfunc@addr=0x60size=0x28bytes

f:funcO:obj

externalreference


• Assembler: createsobjectfiles(=machinecode)



✔

✔

Howdowelinktogetherseparatelycompiledandassembledmachineobjectfiles?

calc.c

math.c

io.s

calc.s

math.s

Compiler

Csourcefiles


libm.o

io.o

calc.o

math.o

Assembler

obj files

calc.exe

Linker

executableprogram

Executingin

Memory

loader

process

existsondisk

Linker combinesobjectfilesintoanexecutablefile• Relocateeachobject’stextanddatasegments• Resolveas-yet-unresolvedsymbols• Recordtop-levelentrypointinexecutablefile

Endresult:aprogramondisk,readytoexecute• E.g. ./calc Linux

./calc.exe Windowssimulatecalc ClassMIPSsimulator

.

main.o...

0C000000210350001b80050C8C040000210470020C000000

...00 T main00 D uname*UND* printf*UND* pi40,JAL, printf4C,LW/gp, pi50,JAL, square

math.o...

210320400C0000001b3014023C04000034040000

...20 T square00 D pi*UND* printf*UND* uname28,JAL, printf30,LUI, uname34,LA, uname

printf.o...

3C T printf

.text

Symbo

ltbl

Relocatio

ninfo

Externalreferencesneedtoberesolved(fixed)

Steps1) FindUNDsymbolsin

symboltable2) Relocatesegmentsthat

collide

e.g.uname@0x00pi@0x00square@0x00main@0x00

main.o...

0C000000210350001b80050C8C040000210470020C000000

...00 T main00 D uname*UND* printf*UND* pi40,JAL, printf4C,LW/gp, pi50,JAL, square

math.o...

210320400C0000001b3014023C04000034040000

...20 T square00 D pi*UND* printf*UND* uname28,JAL, printf30,LUI, uname34,LA, uname

printf.o...

3C T printf

...210320400C40023C1b3014023C04100034040004

...0C40023C210350001b80050c8C048004210470020C400020

...102010002104033022500102

...

Entry:0040 0100text:0040 0000data:1000 0000

calc.exe

000000030077616B

2 1

BA

3

1

2

3

00400000

00400100

00400200

1000000010000004

LUI1000ORI0004

unamepi

math

main

printf

.text

Symbo

ltbl

Relocatio

ninfo

LW$4,-32764($gp)$4=pi

JALsquare

JALprintfLAuname

Header• locationofmainentrypoint(ifany)

TextSegment• instructions

DataSegment• staticdata(local/globalvars,strings,constants)

RelocationInformation• Instructionsanddatathatdependonactualaddresses• Linkerpatchesthesebitsafterrelocatingsegments

SymbolTable• Exportedandimportedreferences

DebuggingInformation

ObjectFile

Unix• a.out• COFF:CommonObjectFileFormat• ELF:ExecutableandLinkingFormat• …

Windows• PE:PortableExecutable

Allsupportbothexecutableandobjectfiles


• Assembler: createsobjectfiles(=machinecode)



✔

✔

✔

calc.c

math.c

io.s

calc.s

math.s

Compiler

Csourcefiles


libm.o

io.o

calc.o

math.o

Assembler

obj files

calc.exe

Linker

executableprogram

Executingin

Memory

loader

process

existsondisk

Loader readsexecutablefromdiskintomemory• Initializesregisters,stack,argumentstofirstfunction• Jumpstoentry-point

PartoftheOperatingSystem(OS)

StaticLibrary:Collectionofobjectfiles(think:likeaziparchive)

Q:Buteveryprogramcontainsentirelibrary!A:Linkerpicksonlyobjectfilesneededtoresolveundefinedreferencesatlinktime

e.g.libc.a containsmanyobjects:• printf.o,fprintf.o,vprintf.o,sprintf.o,snprintf.o,…• read.o,write.o,open.o,close.o,mkdir.o,readdir.o,…• rand.o,exit.o,sleep.o,time.o,….

Q:Buteveryprogramstillcontainspartoflibrary!A:sharedlibraries• executablefilesallpointtosinglesharedlibrary ondisk

• finallinking(andrelocations)donebytheloader

Optimizations:• Librarycompiledatfixednon-zeroaddress

• Jumptableineachprograminsteadofrelocations• Canevenpatchjumpson-the-fly

Directcall:00400010 <main>:

...jal 0x00400330...jal 0x00400620...jal 0x00400330...

00400330 <printf>:...

00400620 <gets>:...

Drawbacks:Linkerorloadermustediteveryuseofasymbol(callsite,globalvar use,…)

Idea:Putallsymbolsinasingle“globaloffsettable”

Codedoeslookupasneeded

00400010 <main>:...jal 0x00400330...jal 0x00400620...jal 0x00400330...

00400330 <printf>:...

00400620 <gets>:...

GOT:globaloffsettable

0x00400330#printf0x00400620#gets

0x00400010#main

Indirectcall:


00400330 <printf>:...

00400620 <gets>:...

GOT:globaloffsettable

0x00400330#printf0x00400620#gets

0x00400010#main

Indirectcall: # data segment

# global offset table# to be loaded# at -32712($gp)# printf = 4+(-32712)+$gp# gets = 8+(-32712)+$gp

048

lw $t9,-32708($gp)jalr $t9

lw $t9,-32704($gp)jalr $t9

lw $t9,-32708($gp)jalr $t9


00400330 <printf>:...

00400620 <gets>:...

.got

0x00400330#printf0x00400620#gets

0x00400010#main

Indirectcall: # data segment

# global offset table# to be loaded# at -32712($gp)# printf = 4+(-32712)+$gp# gets = 8+(-32712)+$gp

.word

.word.word

lw $t9,-32708($gp)jalr $t9

lw $t9,-32704($gp)jalr $t9

lw $t9,-32708($gp)jalr $t9

Indirectcallwithon-demanddynamiclinking:00400010 <main>:

...# load address of prints# from .got[1]lw t9, -32708(gp)

# now call itjalr t9...

.got .word 00400888 # open.word 00400888 # prints.word 00400888 # gets.word 00400888 # foo

Indirectcallwithon-demanddynamiclinking:00400010 <main>:

...# load address of prints# from .got[1]lw t9, -32708(gp)# also load the index 1li t8, 1# now call itjalr t9...

.got .word 00400888 # open.word 00400888 # prints.word 00400888 # gets.word 00400888 # foo

...00400888 <dlresolve>:

# t9 = 0x400888# t8 = index of func that# needs to be loaded

# load that func... # t7 = loadfromdisk(t8)

# save func’s address so# so next call goes direct... # got[t8] = t7

# also jump to funcjr t7# it will return directly # to main, not here

Windows:dynamicallyloadedlibrary(DLL)• PEformat

Unix:dynamicsharedobject(DSO)• ELFformat

UnixalsosupportsPositionIndependentCode(PIC)– Programdeterminesitscurrentaddresswheneverneeded(noabsolutejumps!)

– Localdata:accessviaoffsetfromcurrentPC,etc.– Externaldata:indirectionthroughGlobalOffsetTable(GOT)

– …whichinturnisaccessedviaoffsetfromcurrentPC

Staticlinking• Bigexecutablefiles(all/mostofneededlibrariesinside)

• Don’tbenefitfromupdatestolibrary• Noload-timelinking

Dynamiclinking• Smallexecutablefiles(justpointtosharedlibrary)• Libraryupdatebenefitsallprogramsthatuseit• Load-timecosttodofinallinking

– Butdll codeisprobablyalreadyinmemory– Andcandothelinkingincrementally,on-demand

anne bracy cs 3410 · • assembler: creates object files (= machine code) • linker: joins object...

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