anne bracy cs 3410 · • assembler: creates object files (= machine code) • linker: joins object...
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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
Input:• assemblyinstructions• psuedo-instructions• dataandlayoutdirectives
Output:Objectfile
Slightlyhigherlevelthanplainassemblye.g:takescareofdelayslots
(willreorderinstructionsorinsertnops)
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
Input:• assemblyinstructions• psuedo-instructions• dataandlayoutdirectives
Output:Objectfile
Slightlyhigherlevelthanplainassemblye.g:takescareofdelayslots
(willreorderinstructionsorinsertnops)
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
assemblyfiles libc.o
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
• Compiler:createsassemblyfiles
• Assembler: createsobjectfiles(=machinecode)
• Linker: joinsobjectfilesintooneexecutable
• Loader: bringsexecutableintomemoryandstartsexecutingaprocess
✔
✔
Howdowelinktogetherseparatelycompiledandassembledmachineobjectfiles?
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
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
• 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
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:
00400010 <main>:...jal 0x00400330...jal 0x00400620...jal 0x00400330...
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 $t9
00400010 <main>:...jal 0x00400330...jal 0x00400620...jal 0x00400330...
00400330 <printf>:...
00400620 <gets>:...
.got
0x00400330#printf0x00400620#gets
0x00400010#main
Indirectcall: # data segment
# global offset table# to be loaded# at -32712($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
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