Behavioral Reflection
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The document discusses behavioral reflection and metaprogramming techniques in Smalltalk. It introduces sub-method structures like bytecode, the ByteSurgeon library for bytecode transformation, and Geppetto which enables partial behavioral reflection through hooksets and metaobjects. This allows profiling and caching to be selectively added to applications at runtime without restarting the system. Future work includes improving tool support and integrating with AST representations.
![ByteSurgeon Library for bytecode transformation in Smalltalk Full flexibility of Smalltalk: Runtime Provides high-level API For Squeak, but portable [Main Source] Marcus Denker, Stéphane Ducasse and Éric Tanter. Runtime Bytecode Transformation for Smalltalk. Journal of Computer Languages, Systems and Structures, vol. 32, no. 2-3.](https://image.slidesharecdn.com/meta0606behavioralreflection-090827202022-phpapp02/85/Behavioral-Reflection-14-320.jpg)
![Examples Counts the number of Bytecodes: Counts the number of Sends: Introspection: InstrCounter reset. Example instrument: [:instr | InstrCounter increase] InstrCounter reset. Example instrumentSend: [:instr|InstrCounter increase] (Example>>#aMethod) instrumentSend: [:send | Transcript show: send selector printString]](https://image.slidesharecdn.com/meta0606behavioralreflection-090827202022-phpapp02/85/Behavioral-Reflection-15-320.jpg)
![Transformations Modification: inlining of code insertBefore:, insertAfter:, replace: (Example>>#aMethod) instrumentSend: [ :send | send insertAfter: ’ [ I nstrCounter increase ]’ (Example>>#aMethod) instrumentSend: [:send | send insertAfter: ‘ T ranscript show: ’ , send selector printString].](https://image.slidesharecdn.com/meta0606behavioralreflection-090827202022-phpapp02/85/Behavioral-Reflection-16-320.jpg)
![Meta Variables Goal: extend a send with after logging Problem: How to access receiver and args? Solution: metavariables #receiver, #arguments, #argn, #result.... #value, #newvalue Example instrumentSend: [:s | s insertAfter: ‘ L ogger logSendTo: <meta: #receiver>‘]](https://image.slidesharecdn.com/meta0606behavioralreflection-090827202022-phpapp02/85/Behavioral-Reflection-17-320.jpg)
![Geppetto Partial Behavioral Reflection pioneered in Java Code transformation at load time Not unanticipated (it’s Java...) Geppetto: Partial Behavioral Reflection for Smalltalk For Squeak 3.9 with ByteSurgeon but portable to other dialects Let’s see an example! [Main Source] David R öthlisberger , Marcus Denker and Éric Tanter. Unanticipated Partial Behavioral Reflection: Adapting Applications at runtime . to appear](https://image.slidesharecdn.com/meta0606behavioralreflection-090827202022-phpapp02/85/Behavioral-Reflection-21-320.jpg)
Behavioral Reflection
- 1. Metaprogramming and Reflection Behavioral Reflection Universit ät Bern Marcus Denker Hasso-Plattner-Institut Potsdam Software Architecture Group Prof. Dr. Robert Hirschfeld http://www.swa.hpi.uni-potsdam.de WS 2006/2007
- 2. Topics Covered Overview Introduction Open implementations OMG meta object facility CLOS metaobject protocol Smalltalk/Squeak Behavioral reflection Refactoring browser AspectS and ContextS Traits and mirrors Metacircular interpreters Objects
- 3. Outline Introduction: Reflection in Squeak Sub-method Structure: Bytecode ByteSurgeon: Bytecode Transformation Partial Behavioral Reflection
- 4. Reflection Reflection: computation about computation Base level / meta level Causally connected Structural Reflection Reification of structure Behavioral Reflection Reification of execution
- 5. Reflection in Squeak Squeak has support for reflection Structural Reflection Classes / Methods are Objects Can be changed at runtime Behavioral Reflection Current execution reified (thisContext) #doesNotUnderstand / MethodWrappers
- 6. Can we do better? Structural Reflection stops at method level Bytecode in the CompiledMethod: Numbers Text: Just a String, needs to be compiled Behavior hard coded in the Virtual Machine Message Sending Variable Access Behavioral Reflection is limited in Squeak We should do better!
- 7. Sub-Method Abstraction We need a model for method bodies Posibilities: Text Bytecode (CompiledMethod) AST (Abstract Syntax Tree) For now: Bytecode Bytecode is causally connected Quite fast to edit Later: AST (next Lecture)
- 8. Outline Introduction: Reflection in Squeak Sub-method Structure: Bytecode ByteSurgeon: Bytecode Transformation Partial Behavioral Reflection
- 9. Squeak VM Virtual machine provides a virtual processor Smalltalk (like Java): Stack machine easy to implement interpreters for different processors most hardware processors are register machines Bytecode: ‘Machine code’ of the VM
- 10. CompiledMethods CompiledMethod format: Number of temps, literals... Array of all Literal Objects Pointer to Source (Number methodDict at: #asInteger) inspect (Number>>#asInteger) inspect Trailer Bytecode Literals Header
- 11. Squeak Bytecode 256 Bytecodes, four groups: Stack Bytecodes Stack manipulation: push / pop / dup Send Bytecodes Invoke Methods Return Bytecodes Return to caller Jump Bytecodes Control flow inside a method
- 12. Editing Bytecode Why Bytecode? No source needed Performance Language independent (e.g. Ruby-on-Squeak) Problems of direct Bytecode editing Hard to edit directly (e.g jump offsets) Programmers don‘t know bytecode Easy to make errors
- 13. Outline Introduction: Reflection in Squeak Sub-method Structure: Bytecode ByteSurgeon: Bytecode Transformation Partial Behavioral Reflection
- 14. ByteSurgeon Library for bytecode transformation in Smalltalk Full flexibility of Smalltalk: Runtime Provides high-level API For Squeak, but portable [Main Source] Marcus Denker, Stéphane Ducasse and Éric Tanter. Runtime Bytecode Transformation for Smalltalk. Journal of Computer Languages, Systems and Structures, vol. 32, no. 2-3.
- 15. Examples Counts the number of Bytecodes: Counts the number of Sends: Introspection: InstrCounter reset. Example instrument: [:instr | InstrCounter increase] InstrCounter reset. Example instrumentSend: [:instr|InstrCounter increase] (Example>>#aMethod) instrumentSend: [:send | Transcript show: send selector printString]
- 16. Transformations Modification: inlining of code insertBefore:, insertAfter:, replace: (Example>>#aMethod) instrumentSend: [ :send | send insertAfter: ’ [ I nstrCounter increase ]’ (Example>>#aMethod) instrumentSend: [:send | send insertAfter: ‘ T ranscript show: ’ , send selector printString].
- 17. Meta Variables Goal: extend a send with after logging Problem: How to access receiver and args? Solution: metavariables #receiver, #arguments, #argn, #result.... #value, #newvalue Example instrumentSend: [:s | s insertAfter: ‘ L ogger logSendTo: <meta: #receiver>‘]
- 18. ByteSurgeon Provides simple model for submethod structure Too low level? Used in a number of projects Trace Debugger Test Coverage Next Step: Behavioral Reflection Bytesurgeon just concerned with inlining of bytecode We want to have reifications + meta objects
- 19. Outline Introduction: Reflection in Squeak Sub-method Structure: Bytecode ByteSurgeon: Bytecode Transformation Partial Behavioral Reflection
- 20. Partial Reflection: Reflex Hooksets: collection of operation occurrences Links Bind hooksets to metaobjects Define Protocol between base and meta Goals Highly selective reification Flexible metalevel engineering Protocol specification Cross-cutting hooksets
- 21. Geppetto Partial Behavioral Reflection pioneered in Java Code transformation at load time Not unanticipated (it’s Java...) Geppetto: Partial Behavioral Reflection for Smalltalk For Squeak 3.9 with ByteSurgeon but portable to other dialects Let’s see an example! [Main Source] David R öthlisberger , Marcus Denker and Éric Tanter. Unanticipated Partial Behavioral Reflection: Adapting Applications at runtime . to appear
- 22. Example Typical Web Applications (e.g. Wiki) Shows performance problem under high load Goals: Profile and fix the problem No restart / interruption of service
- 23. Towards a Solution Analyze the problem Install Profiler Analyze Retract Profiler Solve the Problem Introduce a caching mechanism Experiment with different solutions
- 24. Solution with Reflection Operation Method Evaluation Hookset All method executions in the wiki Metaobject A profiler tool
- 25. Profiler: Hookset + Link Hookset allExecs := Hookset new. allExecs inPackage: ’Wiki’; operation: MethodEval. profile := Link id: #profiler hookset: allExecs metaobject: Profiler new. profile control: Control around. Link
- 26. Profiler: Protocol profile callDescriptor: (CallDescriptor selector: #profileMethod:in:withArguments: parameters:{ Parameter selector. Parameter self. Parameter arguments.} passingMode: PassingMode plain). Protocol Install / Retract profile install. profile uninstall.
- 27. Solving the Problem: Caching Operation: Method Execution (Around) Hookset The one slow method ( #toughWork: ) Metaobject The cache
- 28. Cache: Hookset and Link toughWorks := Hookset new. toughWorks inClass: Worker; inMethod: #toughWork:; operation: MethodEval. Hookset: Link: cache := Link id: #cache hookset: toughWorks metaobject: Cache new. cache control: Control around. cache callDescriptor:(CallDescriptor selector: #cacheFor: parameters: {Parameter arg1} passingMode: PassingMode plain).
- 29. Geppetto Operations MethodEval MessageSend, InstVarAccess, TempAccess Control Before, After, Around, Replace Activation condition per Link
- 30. Future Work Pluggable Backends Bytecode AST VM Support? AST: integrate with annotation framework Better tool support
- 31. Conclusion Introduction: Reflection in Squeak Sub-method Structure: Bytecode ByteSurgeon: Bytecode Transformation Partial Behavioral Reflection Next Lecture: Refactoring Engine
- 32. Conclusion Introduction: Reflection in Squeak Sub-method Structure: Bytecode ByteSurgeon: Bytecode Transformation Partial Behavioral Reflection Next Lecture: Refactoring Engine Questions?