functions and the stack - swarthmore college · “a” stack • a stack is a basic data structure...

FunctionsandtheStack10/4/16

Overview

• Stackdatastructure,appliedtomemory

• Behavioroffunctioncalls

• Storageoffunctiondata,atIA32level

“A”Stack• Astackisabasicdatastructure• Lastin,firstoutbehavior(LIFO)• Twooperations

• Push(additemtotopofstack)• Pop(removeitemfromtopofstack)

Oldestdata

Newestdata

Push(adddataitem)

Pop(removeandreturnitem)

• Applystackdatastructuretomemory• Storelocal(automatic)variables• Maintainstateforfunctions(e.g.,wheretoreturn)

• Organizedintounitscalledframes• Oneframerepresentsalloftheinformationforonefunction.• Sometimescalledactivationrecords

“The”Stack

MemoryModel

• Stackstartsatthehighestmemoryaddresses,growsintoloweraddresses.

0x0

0xFFFFFFFF

Operatingsystem

Stack

TextData

Heap

StackFrames

• Asfunctionsgetcalled,newframesaddedtostack.

• Example:Lab4• maincallsget_values()• get_values callsread_float()• read_float callsI/Olibrary

main

0xFFFFFFFF

get_values

read_float

(I/Olibrary)

StackFrames

• Asfunctionsreturn,framesremovedfromstack.

• Example:Lab4• I/Olibraryreturnstoread_float• read_float returnstoget_values• get_values returnstomain

main

0xFFFFFFFF

get_values

read_float

(I/Olibrary)

Allofthisstackgrowing/shrinkinghappensautomatically(fromtheprogrammer’sperspective).

Whatisresponsibleforcreatingandremovingstackframes?A. Theuser

B. Thecompiler

C. Clibrarycode

D. Theoperatingsystem

E. Something/someoneelse

Insight:EVERYfunctionneedsastackframe.Creating/destroyingastackframeisa(mostly)genericprocedure.

StackFrameContents• Whatneedstobestoredinastackframe?• Alternatively:Whatmust afunctionknow/access?

• Hint:Atleast5things

main

0xFFFFFFFF

get_values

read_float

StackFrameContents

• Whatneedstobestoredinastackframe?• Alternatively:Whatmust afunctionknow?

• Localvariables• Previousstackframebaseaddress• Functionarguments• Returnvalue• Returnaddress

• Savedregisters• Spilledtemporaries

main

0xFFFFFFFF

function1

function2

LocalVariables

Iftheprogrammersays:int x = 0;

Whereshouldxbestored?(Recallbasicstackdatastructure)

Whichmemoryaddressisthat?

main

0xFFFFFFFF

function1

function2

Xgoeshere

0x????????

Howshouldwedeterminetheaddresstouseforstoringanewlocalvariable?

A. Theprogrammerspecifiesthevariablelocation.

B. TheCPUstoresthelocationofthecurrentstackframe.

C. Theoperatingsystemkeepstrackofthetopofthestack.

D. Thecompilerknows/determineswherethelocaldataforeachfunctionwillbeasitgeneratescode.

E. Theaddressisdeterminedsomeotherway.

• Compiletime(static)• Informationthatisknownbyanalyzingyourprogram• Independentofthemachineandinputs

• Runtime(dynamic)• Informationthatisn’tknownuntilprogramisrunning• Dependsonmachinecharacteristicsanduserinput

TheCompilerCan…

• Determinehowmuchspaceyouneedonthestacktostorelocalvariables.

• InsertIA32instructionsforyoutosetupthestackforfunctioncalls.• Createstackframesonfunctioncall• Restorestacktopreviousstateonfunctionreturn

• Performtypechecking,etc.

CurrentStackFrame

LocalVariables

• CompilercanallocateNbytesonthestackbysubtractingNfromthe“stackpointer”:%esp

CurrentStackFrame

esp

esp - N

Nbytes

TheCompilerCan’t…• Predictuserinput.

int main() {

int x = get_user_input();

if (x > 5) {

funcA(x);

} else {

funcB();

}

}

main

0xFFFFFFFF

funcBfuncA ???

TheCompilerCan’t…• Predictuserinput.

• Assumeafunctionwillalwaysbeatacertainaddressonthestack.

Alternative:createstackframesrelativetothecurrent(dynamic)stateofthestack.

main

0xFFFFFFFF

funcBfuncA ???

funcB

StackFrameLocation

• Whereinmemoryisthecurrentstackframe?

main

0xFFFFFFFF

function1

function2

Currenttopofstack

Currentbottomofstack

Recall:IA32Registers

%eip

Generalpurposeregisters

Currentstacktop

Currentstackframe

Instructionpointer(PC)

CF ZF SF OF Conditioncodes

%eax

%ecx

%edx

%ebx

%esi

%edi

%esp

%ebp

StackFrameLocation

• Whereinmemoryisthecurrentstackframe?

• Maintaininvariant:• Thecurrentfunction’sstackframeisalwaysbetweentheaddressesstoredin%esp and%ebp

• %esp:stackpointer• %ebp:framepointer(basepointer) main

0xFFFFFFFF

function1

function2

%esp

%ebp

StackFrameLocation

• Compilerensuresthatthisinvariantholds.• We’llseehowabitlater.

• Thisiswhyalllocalvariableswe’veseeninIA32arerelativeto%ebp or%esp!

main

0xFFFFFFFF

function1

function2

%esp

%ebp

HowwouldweimplementpushingxtothetopofthestackinIA32?A. Increment%esp

Storexat(%esp)

B. Storexat(%esp)Increment%esp

C. Decrement%espStorexat(%esp)

D. Storexat(%esp)Decrement%esp

E. Copy%esp to%ebpStorexat(%ebp) main

0xFFFFFFFF

function1

function2

Xgoeshere%esp

(Topofstack)

%ebp(Framestart)

Push&Pop

• IA32providesconvenientinstructions:• pushl src

• Movestackpointerupby4bytes subl $4, %esp• Copy‘src’tocurrenttopofstack movl src, (%esp)

• popl dst• Copycurrenttopofstackto‘dst’ movl (%esp), dst• Movestackpointerdown4bytes addl $4, %esp

• src anddst arethecontentsofanyregister

LocalVariables

• Moregenerally,wecanmakespaceonthestackforNbytesbysubtractingNfrom%esp

CurrentStackFrame

CurrentStackFrame

esp esp - NNbytes

Newvariable

LocalVariables

• Moregenerally,wecanmakespaceonthestackforNbytesbysubtractingNfrom%esp• Whenwe’redone,freethespacebyaddingNbackto%esp

CurrentStackFrame

CurrentStackFrame

esp

esp - N

Nbytes

Newvariable

StackFrameContents

• Whatneedstobestoredinastackframe?• Alternatively:Whatmust afunctionknow?

• Localvariables• Previousstackframebaseaddress• Functionarguments• Returnvalue• Returnaddress

• Savedregisters• Spilledtemporaries

main

0xFFFFFFFF

function1

function2

StackFrameRelationships

• Iffunction1callsfunction2:• function1isthecaller• function2isthecallee

• Withrespecttomain:• mainisthecaller• function1isthecallee

main

0xFFFFFFFF

function1(caller)

function2(callee)

Whereshouldwestoreallthisstuff?

A. InregistersB. OntheheapC. Inthecaller’sstackframeD. Inthecallee’s stackframeE. Somewhereelse

PreviousstackframebaseaddressFunctionargumentsReturnvalueReturnaddress

CallingConvention

• Youcouldstorethisstuffwhereveryouwant!• ThehardwaredoesNOTcare.• Whatmatters:everyoneagreesonwheretofindthenecessarydata.

• Callingconvention:agreeduponsystemforexchangingdatabetweencallerandcallee

IA32CallingConvention(gcc)

• Inregister%eax:• Thereturnvalue

• Inthecallee’s stackframe:• Thecaller’s%ebp value(previousframepointer)

• Inthecaller’sframe(sharedwithcallee):• Functionarguments• Returnaddress(savedPCvalue)

FramePointer

• Mustmaintaininvariant:• Thecurrentfunction’sstackframeisalwaysbetweentheaddressesstoredin%esp and%ebp

• Mustadjust%esp,%ebp oncall/return.

caller

%esp

%ebp …

callee

FramePointer


• Immediatelyuponcallingafunction:1. pushl %ebp

caller

%esp

…%ebp

caller’s%ebp value

callee

FramePointer


• Immediatelyuponcallingafunction:1. pushl %ebp2. Set%ebp =%esp

caller

%esp

…%ebp


callee

FramePointer


• Immediatelyuponcallingafunction:1. pushl %ebp2. Set%ebp =%esp3. SubtractNfrom%esp

caller

%esp

…%ebp


Callee cannowexecute.

callee

FramePointer


• Toreturn,reversethis:

caller

%esp

…%ebp


FramePointer


• Toreturn,reversethis:1. set%esp =%ebp

caller

%esp

…%ebp


FramePointer


• Toreturn,reversethis:1. set%esp =%ebp2. popl %ebp

caller

%esp

…%ebp


FramePointer


• Toreturn,reversethis:1. set%esp =%ebp2. popl %ebp

caller

%esp

…%ebpBacktowherewestarted.

IA32hasanotherconvenienceinstructionforthis:leave

Recall:AssemblyWhileLoop

some_function:pushl %ebpmovl %esp, %ebp

# Your code here

movl $10, %eaxleaveret

Setupthestackframeforthisfunction.

Storereturnvaluein%eax.

Restorecaller’s%esp,%ebp.

Lab4:swap.s

swap:

pushl %ebp

movl %esp, %ebp

subl $16, %esp

# Your code here

leave

ret

IA32CallingConvention(gcc)

• Inregister%eax:• Thereturnvalue

• Inthecallee’s stackframe:• Thecaller’s%ebp value(previousframepointer)

• Inthecaller’sframe(sharedwithcallee):• Functionarguments• Returnaddress(savedPCvalue)

FunctionArguments

• Argumentsarepushedontothestackbeforethecallinstructionjumpsintothecallee.

callee

caller

…

caller’s%ebp valueCallee arguments

InstructionsinMemory

0x0

0xFFFFFFFF

Operatingsystem

Stack

TextData

Heap

funcA:…call funcB…

funcB:pushl %ebpmovl %esp, %ebp…

FunctionA

FunctionB

…

ProgramCounter

ProgramCounter(PC)

funcA:addl $5, %ecxmovl %ecx, -4(%ebp)…call funcBaddl %eax, %ecx…

funcB:pushl %ebpmovl %esp, %ebp…movl $10, %eaxleaveret

TextMemoryRegionRecall:PCstorestheaddressofthenextinstruction.(Apointertothenextinstruction.)

Whatdowedonow?

FollowPC,fetchinstruction:

addl $5, %ecx

ProgramCounter

ProgramCounter(PC)



TextMemoryRegionRecall:PCstorestheaddressofthenextinstruction.(Apointertothenextinstruction.)

Whatdowedonow?


addl $5, %ecx

UpdatePCtonextinstruction.

Executetheaddl.

ProgramCounter

ProgramCounter(PC)



Recall:PCstorestheaddressofthenextinstruction.(Apointertothenextinstruction.)

Whatdowedonow?


movl $ecx, -4(%ebp)

TextMemoryRegion

ProgramCounter

ProgramCounter(PC)




Whatdowedonow?


movl $ecx, -4(%ebp)

UpdatePCtonextinstruction.

Executethemovl.

TextMemoryRegion

ProgramCounter

ProgramCounter(PC)




Whatdowedonow?

Keepexecutinginastraightlinedownwardslikethisuntil:

Wehitajumpinstruction.Wecallafunction.

TextMemoryRegion

ChangingthePC:Jump

• Onajump:• Checkconditioncodes• SetPCtoexecuteelsewhere(notnextinstruction)

• Doweeverneedtogobacktotheinstructionafterthejump?

Maybe(andifso,we’dhavealabeltojumpbackto),butusuallynot.

ChangingthePC:Functions

ProgramCounter(PC)



Whatwe’dlikethistodo:

TextMemoryRegion


ProgramCounter(PC)




SetupfunctionB’sstack.

TextMemoryRegion


ProgramCounter(PC)





ExecutethebodyofB,produceresult(storedin%eax).

TextMemoryRegion


ProgramCounter(PC)





ExecutethebodyofB,produceresult(storedin%eax).

RestorefunctionA’sstack.

TextMemoryRegion


ProgramCounter(PC)




Return:GobacktowhatweweredoingbeforefuncB started.

Unlikejumping,weintendtogoback!

TextMemoryRegion

functions and the stack - swarthmore college · “a” stack • a stack is a basic data structure...

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