Virtual Support for Dynamic Join Points

C. Bockisch, M. Haupt, M. Mezini, K. Ostermann

Presented by Itai Sharon (itaish@cs.technion.ac.il)

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Agenda Classification of crosscuts Handling dynamic crosscuts Steamloom and support for aspects

on VM level Evaluation Related work

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Terms Join Pointwell defined place in the structure or execution where additional behavior can be attached

Pointcut Designator (PCD) Description of a set of join points and their values

Crosscutset of related join points defined by a PCD

Weaving the process of composing core functionality modules with aspects, thereby yielding a working system

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Crosscuts Code level crosscuts pointcuts that may directly be mapped to locations in the program code.e.g. “whenever method x is called”.

Dynamic corsscutsPointcuts that emerge depending on the dynamics of the program

e.g. “whenever exception y occurs”

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Dynamic Crosscuts

Statically bound dynamic crosscutspointcuts for which it is possible to statically determine a set of potentially affected code.e.g. “whenever method x is called in the control flow of y”.

Unbound dynamic crosscuts pointcuts whose correspondence to code locations cannot be restricted in a reasonable way before run time.

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Quiz Whenever foo() is called from bar()

Whenever foo() is called from cflow(bar())

Whenever x is calledwhere x is a method obtained from the user

Unbound dynamic crosscut

Code-level crosscut

Statically bound dynamic crosscut

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Dynamic Crosscuts Support

For statically bound dynamic crosscuts: instrument the set of potentially affected

code locations with dynamic checks

For unbound dynamic crosscuts: Same as for the statically bound dynamic

crosscuts...

Or Programmatic aspect deployment Fluid code, using just-in-time (JIT)

compilation

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Programmatic Aspect

Deployment

An aspect has to be deployed for its pointcuts and advice to take effect Fits unbound dynamic crosscuts better

than the (too) general approach used in languages like AspectJ.

First implemented in CAESAR:deploy(anAspectInstance) {aBlock} deploy public class AnAspect {

...

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The Fibonacci class

class TestApp {public void m1(int n)

{fibstart(n);}public void m2(int n)

{fibstart(n);}public void fibstart(int n)

{fib(n);}public int fib(int k) {

return (k>1) ? fib(k-1)+fib(k-2) : k;}

}

Fibstart

m1

m2

fib

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Fibstart

m1

m2

fib

Counting Calls to fib() via m2() (a la AspectJ)

public aspect FibonacciAspect {private int ctr = -1;pointcut m2cf() : cflow(call(void TestApp.m2(int)));before() :

execution(void TestApp.fibstart(int)) && m2cf(){ ctr = 0;}

after() : execution(void TestApp.fibstart(int)) && m2cf()

{System.out.println(ctr);}before () :

execution(void TestApp.fib (int)) && m2cf(){ ctr++;}

}

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Fibstart

m1

m2

fib

Counting Calls to fib() via m2() (a la CAESAR)

public class FibonacciAspect {private int ctr = -1;before() : execution(void TestApp.fibstart(int))

{ ctr = 0;}after() : execution(void TestApp.fibstart(int))

{System.out.println(ctr);}before () : execution(void TestApp.fib(int)) && m2cf()

{ ctr++;}}deploy public class DeploymentAspect {

around() : call(void TestApp.m2(int)) {deploy new (FibonacciAspect())

{proceed();}}

}

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Drawbacks of Pre-Runtime

invasive Weaving

Impedance Mismatch code loses its original modular structure

Pointcuts are implemented as dynamic checks in the set of potentially affected joinpoints

advices are regular Java classes (AspectJ)

Debugging and profiling are harder Separate compilation is impossible

Expensive both in terms of space and execution time

Cure: aspects support at VM level

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Just In Time Compilation

(JIT)

Get intermediate language code, compile it at runtime on the target machine to native machine code Code remains platform independent but

executes fast Per-machine/user optimizations are

possible Rather then optimizing the code, optimize

the compiler

Used in commercial compilers such as Sun’s HotSpot and Microsoft’s .Net environment First introduced as part of the Smalltalk

implementation

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Steamloom Java interpreter with built-in support for aspects Based on IBM’s Jikes Research Virtual

Machine (RVM) for Java Aspects, PCDs and advices are first-class

entities taken care of by the VM Built-in support for CAESAR deployment

approach Deployment scope can be either global,

thread-local or instance-local Pointcuts may include function calls only

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steam+loom a la google

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Based on Just-in-time compilation Three modes of operation

Baseline compiler Optimizing compiler Adaptive optimization system (AOS)

Classes and methods are kept as instances of VM_Class and VM_Method

Jikes RVM in a nutshell

TIB

An object

TIB status ...

...

m()

...

lazy compilation stub

m()’s compiled code

m()’s optimized code

TIB

An object

TIB status ...

...

m()

...

lazy compilation stub

m()’s compiled code

m()’s optimized code

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Adaptive Optimization

System (AOS)

Methods are initially compiled by the regular compiler and optimized whenever the system sees fit

Has three modules: Run-time measurements

subsystem – gathers profiling data and stores it in the AOS database

Controller – decides on re-compilation of methods

Recompilation unit – called by the controller whenever recompilation of a method is required

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Adding Weaving Support

Upon weaving of an aspect, modify and recompile relevant methods.

Concern: Methods may be inlined.

Advice: For each method m(), keep the set of

methods in which m() was inlined Upon recompilation of m(), recompile all

affected methods

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Input: m0, method to be re-compiledOutput: REC, the set of affected methods

REC = {m0}M = REC;do

M’ = ; foreach mM do

M’ inline_locations(m);M = M’\ M;REC M;

until M =

Setting the order of

recompilation

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TIB

An object

TIB status ...

...

m()

...

lazy compilation stublazy compilation stub

m()’s compiled codem()’s compiled code

m()’s optimized code with advice invocation code

Class-Wide Aspect

Weaving

Modify m()’s bytecode Change m()’s entry in the TIB

m() was not optimized: to the lazy compilation stub

m() was optimized: Recompile m() and all affected methods

m()’s compiled code with advice invocation code

m()’s optimized code with advice invocation code

m()’s compiled code with advice invocation code

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Instance-Local Aspect

Weaving

Clone original TIB and method Treat m() as in the class-wide case

Methods of classes for which an instance local aspect exists cannot be inlined!

TIB

o1 TIB status ...

...

m()

...

o2 TIB status ...

m()’s original code

m() with aspects

TIB

o1 TIB status ...

...

m()

...

o2 TIB status ...

TIB...

m()

...

m()’s original code

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Thread-Local Aspect

Weaving

A brief snippet of code is inserted before every thread-local join point: Check thread identity Skip advice invocation in case the aspect

need not to be active in current thread

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Performance Evaluation

Steamloom has been compared against AspectJ, executed on Sun’s HotSpot

(AspectJ/HS) AspectJ, executed on Jikes RVM (AspectJ/RVM)

Effect on code with no aspects: AspectJ/HS is 2.5 times faster than Steamloom, AspectJ/RVM is a little faster than Steamloom

Effect on code with static aspects: similar to the no-aspects case

Effect on code with dynamic aspects: Steamloom is 2-4 times faster than AspectJ/HS, Steamloom is 17 times faster than AspectJ/RVM

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Related Work Pre-run-time instrumentation EAOP, JAC, Jboss, AOP, PROSE 2

Run-time event monitoring PROSE 1

Run-time weaving Wool, AspectS

Continuous weaving

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Conclusions and Future

Work

Support on the VM level is required for efficient implementation of dynamic pointcuts

Recompilation at runtime is required for such support

Many features are still missing, in particular the around() advice

Implementation of Steamloom’s features on HotSpot

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References Virtual Machine Support for Dynamic Join points:http://www.st.informatik.tu-darmstadt

.de/database/publications/data/Steamloom.pdf?id=80

Steamloom’s homepage:http://www.st.informatik.tu-darmstadt

.de/static/pages/projects/AORTA/Steamloom.jsp

Just-in-time compilers:http://en.wikipedia.org/wiki/Just_In_Time_compilation.html

Jikes RVM webbpagehttp://www-124.ibm.com/developerworks/oss/jikesrvm/

History of steam loomshttp://www.fordham.edu/halsall/mod/1823cotton.html