Listoflectures
TDDA69DataandProgramStructure
VirtualMachinesandBytecode
CyrilleBerger
1IntroductionandFunctionalProgramming
2ImperativeProgrammingandDataStructures
3Parsing
4Evaluation
5ObjectOrientedProgramming
6Macrosanddecorators
7VirtualMachinesandBytecode
8GarbageCollectionandNativeCode
9DistributedComputing
10DeclarativeProgramming
11LogicProgramming
12Summary
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Howisaprograminterpreted?
Sourcecode
Parser
Lecturecontent
VirtualMachines
Typesofvirtual/hardwaremachines
Parser
Bytecode
AbstractSyntaxTree
Treevisitor
Generator
Sourcecode
Bytecode
VirtualMachine
Assembler
Assembly
Simpleinstructionset
FromASTtoBytecode
BytecodeInterpreter
...
Meta-circularevaluator
OperatingSystem
CPU
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WhatisaVirtualMachine?
AVirtualMachineisahardwareor
softwareemulationofarealor
hypotheticalcomputersystem
Asystemvirtualmachineemulatesa
completesystemandisintentedto
executeacompleteoperatingsystem
VirtualMachines
Examples:VirtualBox,VMWare,Parallels...
Aprocess/languagevirtualmachineruns
asingleprograminasingleprocess
Examples:JVM,CPython,V8,Dalvik...
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NativeEnvironmentvsEmulatedEnvironment
LanguageVirtualMachine
OperatingSystem
SourceCode
Input/Output
NativeProgram
Compiler
Hardware
Libraries
ByteCode
VirtualMachine
VirtualMachine
VirtualMachine
Windows
Linux
Mac
EmulatedProgram
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VirtualMachine
OperatingSystem
Emulated
Input/Output
Input/Output
Emulated
HardwareInterface
Hardware
Bindings
Libraries
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BenefitsofVirtualMachines
Portability:VirtualMachinesare
compatiblewithvarioushardware
platformsandOperatingSystems
Isolation:VirtualMachinesare
isolatedfromeachother
Typesofvirtual/hardwaremachines
Forrunningincompatibleapplicationsconcurrently
Encapsulation:computationis
seperatedfromtheoperatingsystem
Beneficialforsecurity
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Typesofvirtual/hardwaremachines
RegisterMachine:Formaldefinition
A(in)finitesetofregisters,whichholdsa
singlenon-negativeinteger
Aninstructionset,whichdefinesthe
operationonregisters
RegisterMachine
StackMachine
Arithmetic,control,input/output...
Astateregister,whichholdsthecurrent
instructionanditsindex
Sequentiallistoflabeledinstructions
whichdefinestheprogramtobeexecuted
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StackMachine:Formaldefinition
An(almost)infinitestack,whichholds
integers
Aninstructionset,whichdefinesthe
operationonthestack
StackMachinesvsRegisterMachines
StackMachinesneedmorecompactcode
Arithmeticinstructionaresmaller
StackMachineshavesimplercompiler,
interpretersandaminimalprocessorstate
StackMachineshaveaperformance
disadvantages
Arithmeticoperationsarealwaysappliedonthetoptwo
elementsandtheresultsisstoredinthetop
Astateregister,whichholdsthecurrent
instructionanditsindex
Sequentiallistoflabeledinstructions
whichdefinestheprogramtobeexecuted
Morememoryreferences,lesscachingoftemporaries
Highercostoffactoringoutcommonsubexpressions
Ithastobestoredasatemporaryvariable
Mostcommonhardwareareregistermachines
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VirtualMachinesforDynamicTyping
Remember:
Withstatictyping,typesarecheckedduring
compilation
Withdyamictyping,typesarecheckedduring
execution
Bytecode
Implication
Stack/Registerscontainspointertoobjects
Functioncallconvention
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Bytecode
Foravirtualstackmachine
Instructionset
Generatethebytecode
Interpretingthebytecode
Simpleinstructionset
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Stackmanagment
Arithmeticoperators
PUSH[constant_value]
ADD,MUL...
Pushtheconstantonthestack
Poptwoargumentsonthestack
Pushtheresult
POP[number]
Popacertainnumbersofvariablesfromthe
stack
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Variables
Jumps
LOAD[varname]
JMP[idx]
Pushthevalueofvariable
Jumptoexecuteinstructionatthegivenindex
DCL[varname]
IFJMP[idx]
Declarethevariable
Popthevalueandiftruejumpto[idx]
STORE[varname]
Getthevalue,storetheresultandpushthe
value
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Functions
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Objects
CALL[arguments]
LOAD_MEMBER[varname]
Popthefunctionobjectandcallitwiththe
givennumberofarguments
Poptheobjectandpushthevalueofvariable
STORE_MEMBER[varname]
RET
Poptheobjectandvalueandstoreitandpush
thevalue
Return
Tosimplifythebytecodethiscan
beimplementedasfunctioncall
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FromASTtoBytecode
Withatreevisitor...
Itcantakeseveralpass:
FromASTtoBytecode
Findthevariables
Computethejumps
Generatethecode
Therealchallengeistomaphigh
levellanguagetoinstructions
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Arithmeticoperation
Functions
1+2
a-2
vara
a=1
vara=1
a+=2
a=b*c+2
a.b=c
func(1)
func(1,2)
console.log('Helloworld!')
functionfunc(a,b){returna+b;
}
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Controls
if(a){console.log('test')}
if(a){console.log('hello')}
else{console.log('world')}
vara=10;
while(a){a-=1;}
for(vara=0;a<10;a+=1)
{console.log(a);}
for(vara=0;a<10;a+=1)
{console.log(a);if(a+b){break;}}
BytecodeInterpreter
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Componentsofbytecodeinterpreter
Verifier
Verification
Verifiercheckscorrectnessof
bytecode
Bytecodemaycomefrombuggycompileror
malicioussource
Dynamiccheckingoftypes,arraybounds,
functionarguments...
Everyinstructionmusthaveavalidoperation
code
Everyinstructionmusthavevalidparameters
Everybranchinstructionmustbranchtothe
startofsomeotherinstruction,notmiddleof
instruction
Instructionexecuter
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BytecodeInterpreter
Interpreterloop
instruction_index=0
instructions=[...]
stack=[]
current_env=Environment()
whileinstruction_index<len(instructions):
next_instruction=instructions[instruction_index]
switch(next_instruction.opcode):
caseADD:
..
caseJMP:
...
Standardvirtualmachine
interpretsinstructions
Performrun-timecheckssuchasarraybounds
andtypesandfunctionarguments
Possibletocompilebytecodetonativecode
(JIT:Just-In-Time)
Callnativemethods
TypicallyfunctionswritteninC
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InterpreterExample
FunctionCall
Printtheabsolutevalueof4-1:
Recursivecall
1PUSH4
2PUSH1
3SUB
4DUP
5PUSH0
6SUP
7IFJMP9
8NEG
9LOAD'print'
10CALL
Forafunctioncall,instantiateanew
interpreterloop
Stack
Pushonthestacksomeinformationonhowto
restoretheinterpreterwhenreturning
Morecomplexcode,buthigherperformance
andflexibility
Allowinfiniterecursion
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HandlingExceptions(1/2)
FunctionCall-Stack
instruction_index=0
instructions=[...]
stack=[]
current_env=Environment()
whileinstruction_index<len(instructions):
next_instruction=instructions[instruction_index]
switch(next_instruction.opcode):
...
caseCALL:
env=Environment()
func=stack.pop()
forarginfunc.args:
value=stack.pop()
env.set(arg.name,value)
stack.push([next_instruction,instructions,current_env])
next_instruction=0
instructions=func.instructions
current_env=env
Usetheimplementationlanguage
exceptions
Addexceptioninformationtothe
stack
caseRET:
retval=stack.pop()
info=stack.pop()
stack.push(retval)
next_instruction=info[0]
instructions=info[1]
current_env=info[2]
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HandlingExceptions(2/2)
instruction_index=0
instructions=[...]
stack=[]
current_env=Environment()
whileinstruction_index<len(instructions):
next_instruction=instructions[instruction_index]
switch(next_instruction.opcode):
...
caseTRY_PUSH:
stack.push(Exception(next_instruction.rescue_index,instructions))
caseTRY_POP:
stack.pop()
caseTHROW:
whileTrue:
info=stack.pop()
if(infoisException):
instructions=info.instructions
next_instruction=info.rescue_index
Meta-circularevaluator
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Meta-circularevaluator
Anevaluatorthatiswritteninthe
samelanguagethatitevaluatesis
saidtobemetacircular(SICP4.1).
RPython
RPythonisasubsetofPython
Variablescontainsvalueofatmostonetype
Moduleglobalsareconstants
Restrictionsoncontrolflow,objects,
exceptions...
Easytoextendthelanguage
Makeiteasiertobuildsophisticated
debuggers
Moresuitedforbuildingnewlanguages
Meanttobeeasytointerpret
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RPythonandPyPy
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PyPyVirtualMachine
Usuallyinterpretersarewrittenina
targetplatformlanguagesuchasC
ButCisacomplexandunsafe
language,andinterpretertendto
getcomplicated
PyPyusesRPython:
SourceCode
Compiler
PyPyVM
ByteCode
toprovideasetoftoolsforimplementing
interpretersforanylanguage
apythonimplementationusingthosetools
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PyPyVM
PyPyVM
PyPyVM
Windows
Linux
Mac
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BenefitsofRPythonoverhostlanguage
Easiertodevelop
Conclusion
VirtualMachinesforinterpreting
programs
Stackmachinesareeasierto
implementbutslowerthan
registermachines
VirtualMachinesintroduce
compilationoverhead,butare
fastertoexecute
IfyouknowPython,youknowRPython
Easiertodebug
SinceyoucanrunRPythonprogramwitha
regularPythoninterpreter
Factortheporttodifferent
platform
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