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PyPy and The Art of Generating Virtual Machines Antonio Cuni DISI, Universit` a degli Studi di Genova PyCon Due 2008 - Firenze May 10, 2008 Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 1 / 47

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PyPy PyPy is both: a Python interpreter written in Python a framework for writing dynamic languages Today we will focus on the latter. Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 2 / 47

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Agenda quick overview & motivations the translation framework JIT generator (demo) Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 3 / 47

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PyPy status update (1) JVM backend completed, pypy-jvm some new GCs, much faster than ever ctypes for PyPy JIT refactoring, needed to make the JIT production-ready improved .NET integration for pypy-cli new blog: http://morepypy.blogspot.com Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 4 / 47

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PyPy status update (2) various performance improvements slighly slower than CPython on pystone (10-20%) but faster on richards (20-24%) less than 2x slower on other benchmarks Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 5 / 47

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Interpreters ...are good to implement dynamic languages: Easy to write Portable Flexible and easy to evolve, if written in high-level language (without low-level details) Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 6 / 47

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Folk Wisdom ...about interpreters for Dynamic Languages: There are unavoidable tradeoffs between flexibility, maintainability, and speed Fast, Maintainable, Flexible -- pick one Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 7 / 47

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What this means in Practice Current popular open source dynamic language implementations: are relatively slow are not very flexible are harder to maintain than we would like them to be Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 8 / 47

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Not very flexible Low-level decisions permeate the entire code base. Not ideal to experiment - cannot simply plug-in a new garbage collector, memory model, or threading model Early decisions come back to haunt you. Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 9 / 47

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Hard to maintain because they are traditionally written in low-level languages: the community generates experts in the dynamic language but requires experts in C or C++ for its own maintenance every time a new VM is needed, the language’s community forks (CPython - Jython - IronPython) Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 10 / 47

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The PyPy Project We built enough infrastructure such that: speed is regained features requiring low-level manipulations are (re-)added as aspects interpreters are kept simple and uncluttered Targets as different as C and the industry OO VMs (JVM, CLR) are supported. A special aspect: Generating JIT compilers Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 11 / 47

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PyPy as a project We operate both as an open source with production usage aspirations and research project. We focus on the whole system. We want the tool-chain itself to be as simple as possible (but not simpler). Some of what we do is relatively straight-forward, some is challenging (generating dynamic compilers!). Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 12 / 47

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PyPy Approach Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 13 / 47

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Going from interpreters to VMs In PyPy interpreters are written in RPython: A subset of Python amenable to static analysis Still fully garbage collected Rich built-in types RPython is still close to Python. Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 14 / 47

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Translation details (1) First, load and initialize RPython code inside a normal Python VM RPython translation starts from the resulting“live” bytecode Unified“intermediate code”representation: a forest of Control Flow Graphs Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 15 / 47

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Translation details (2) PyPy uses abstract interpretation extensively: to construct Flow Graphs for type inference to gather info for some optimisations for Partial Evaluation in the generated Dynamic Compilers... also uses Flow Graph transformation and rewriting. Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 16 / 47

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Type Systems (1) We model the different targets through different type systems: LL (low-level C-like targets): data and function pointers, structures, arrays... OO (object oriented targets): classes and instances with inheritance and dispatching Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 17 / 47

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Type Systems (2) Translation: starts from RPython Flow Graphs turns them into LL Flow Graphs or OO Flow Graphs the flowgraphs are transformed in various ways then they are sent to the backends. Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 18 / 47

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Translation aspects (1) The interpreters in RPython are free of low-level details (as required to target platforms as different as Posix/C and the JVM/.NET). Advanced features related to execution should not need wide-spread changes to the interpreters Instead, the interpreters should use support from the translation framework Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 19 / 47

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Translation aspects (2) Examples: GC and memory management memory layout stack inspection and manipulation unboxed integers as tagged pointers Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 20 / 47

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Implementation Translation aspects are implemented as transformation of low-level graphs Calls to library/helper code can be inserted too The helper code is also written in RPython and analyzed and translated Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 21 / 47

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GC Framework The LL Type System is extended with allocation and address manipulation primitives, used to express GC in RPython directly. GCs are linked by substituting memory allocation operations with calls into them Transformation inserts bookkeeping code, e.g. to keep track of roots Inline fast paths of allocation and barriers Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 22 / 47

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JIT motivation Flexibility vs. Performance: Interpreters are easy to write and evolve For high performance, dynamic compilation is required Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 23 / 47

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Traditional JIT compilers Huge resource investment The richer the semantics, the harder to write Poor encoding of language semantics Hard to evolve Need for novel approaches! Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 24 / 47

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PyPy Architecture Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 25 / 47

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Basics Use partial evaluation techniques to generate a dynamic compiler from an interpreter Inspiration: Psyco Our translation tool-chain was designed for trying this Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 26 / 47

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Futamura Partial evalution of computation process - an approach to a compiler-compiler, 1971 Generating compilers from interpreters with automatic specialization Relatively little practical impact so far Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 27 / 47

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General idea Partial evaluation (PE): Assume the Python bytecode to be constant, and constant-propagate it into the Python interpreter. Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 28 / 47

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PE for dummies Example def f(x, y): x2 = x * x y2 = y * y return x2 + y2 case x=3 def f_3(y): y2 = y * y return 9 + y2 case x=10 def f_10(y): y2 = y * y return 100 + y2 Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 29 / 47

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PE for dummies Example def f(x, y): x2 = x * x y2 = y * y return x2 + y2 case x=3 def f_3(y): y2 = y * y return 9 + y2 case x=10 def f_10(y): y2 = y * y return 100 + y2 Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 29 / 47

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PE for dummies Example def f(x, y): x2 = x * x y2 = y * y return x2 + y2 case x=3 def f_3(y): y2 = y * y return 9 + y2 case x=10 def f_10(y): y2 = y * y return 100 + y2 Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 29 / 47

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Challenges A shortcoming of PE is that in many cases not much can be really assumed constant at compile-time: poor results Effective dynamic compilation requires feedback of runtime information into compile-time For a dynamic language: types are a primary example Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 30 / 47

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Solution: Promotion Enhance PE with the ability to“promote”run-time values to compile-time Leverage the dynamic setting Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 31 / 47

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Overall ingredients The pieces to enable effective dynamic compiler generation in PyPy: a few hints in the Python interpreter to guide the JIT generator promotion lazy allocation of objects (only on escape) use CPU stack and registers for the contents of the Python frame Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 32 / 47

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Language-agnostic The dynamic generation process and primitives are language-agnostic. The language implementations should be able to evolve up to maintaining the hints. By construction all interpreter/language features are supported Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 33 / 47

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pypy-c-jit PyPy 1.0 contains both the dynamic compiler generator and the start of its application to PyPy’s Python intepreter. JIT refactoring in-progress. included are backends for IA32 and PPC experimental/incomplete CLI backend integer arithmetic operations are optimized for these, we are in the speed range of gcc -O0 Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 34 / 47

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EXTRA MATERIAL More about the JIT Generation: The Rainbow interpreter Virtuals and Promotion Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 35 / 47

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Execution steps Translation time pypy-c-jit is translated into an executable the JIT compiler is automatically generated Compile-time: the JIT compiler runs Runtime: the JIT compiled code runs Compile-time and runtime are intermixed Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 36 / 47

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The Rainbow interpreter A special interpreter whose goal is to produce executable code Written in RPython Guided by a binding time analysis ( “color”of the graphs) Green operations: executed at compile-time Red operations: produce code that executes the operation at runtime Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 37 / 47

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Rainbow architecture Translation time Low-level flowgraphs are produced The hint-annotator colors the variables The rainbow codewriter translates flowgraphs into rainbow bytecode Compile-time The rainbow interpreter executes the bytecode As a result, it procude executable code Runtime The produced code is executed Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 38 / 47

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Rainbow architecture Translation time Low-level flowgraphs are produced The hint-annotator colors the variables The rainbow codewriter translates flowgraphs into rainbow bytecode Compile-time The rainbow interpreter executes the bytecode As a result, it procude executable code Runtime The produced code is executed Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 38 / 47

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Rainbow architecture Translation time Low-level flowgraphs are produced The hint-annotator colors the variables The rainbow codewriter translates flowgraphs into rainbow bytecode Compile-time The rainbow interpreter executes the bytecode As a result, it procude executable code Runtime The produced code is executed Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 38 / 47

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Coloring Green: compile-time value Red: runtime value The hints give constraints from which the colors of all values are derived We reuse the type inference framework to propagate colors Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 39 / 47

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Partial Evaluation with Colors Green operations: unchanged, executed at compile-time Red operations: converted into corresponding code emitting code Example def f(x, y): x2 = x * x y2 = y * y return x2 + y2 case x=10 def f_10(y): y2 = y * y return 100 + y2 Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 40 / 47

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Partial Evaluation with Colors Green operations: unchanged, executed at compile-time Red operations: converted into corresponding code emitting code Example def f(x, y): x2 = x * x y2 = y * y return x2 + y2 case x=10 def f_10(y): y2 = y * y return 100 + y2 Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 40 / 47

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Partial Evaluation with Colors Green operations: unchanged, executed at compile-time Red operations: converted into corresponding code emitting code Example def f(x, y): x2 = x * x y2 = y * y return x2 + y2 case x=10 def f_10(y): y2 = y * y return 100 + y2 Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 40 / 47

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Partial Evaluate Control Flow red split points: schedule multiple compilation states merge points: merge logic to reuse code for equivalent states Example if x: print "x is true" if y: print "y is true" case y != 0 if x: print "x is true" print "y is true" Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 41 / 47

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Partial Evaluate Control Flow red split points: schedule multiple compilation states merge points: merge logic to reuse code for equivalent states Example if x: print "x is true" if y: print "y is true" case y != 0 if x: print "x is true" print "y is true" Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 41 / 47

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Partial Evaluate Control Flow red split points: schedule multiple compilation states merge points: merge logic to reuse code for equivalent states Example if x: print "x is true" if y: print "y is true" case y != 0 if x: print "x is true" print "y is true" Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 41 / 47

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Promotion Promotion is implemented generating a switch that grows to cover the seen runtime values First compilation stops at a promotion point and generates a switch with only a default case. The default will call back into the compiler with runtime values. On callback the compiler adds one more case to the switch and generate more code assuming the received value. Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 42 / 47

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Promotion (example) Example def f(x, y): x1 = hint(x, promote=True) return x1*x1 + y*y original def f_(x, y): switch x: pass default: compile_more(x) augmented def f_(x, y): switch x: case 3: return 9 + y*y default: compile_more(x) Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 43 / 47

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Promotion (example) Example def f(x, y): x1 = hint(x, promote=True) return x1*x1 + y*y original def f_(x, y): switch x: pass default: compile_more(x) augmented def f_(x, y): switch x: case 3: return 9 + y*y default: compile_more(x) Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 43 / 47

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Promotion (example) Example def f(x, y): x1 = hint(x, promote=True) return x1*x1 + y*y original def f_(x, y): switch x: pass default: compile_more(x) augmented def f_(x, y): switch x: case 3: return 9 + y*y default: compile_more(x) Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 43 / 47

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Virtuals + Promotion Example from PyPy (simplified!) def add_python_objects(obj1, obj2): obj1cls = hint(obj1.__class__, promote=True) obj2cls = hint(obj2.__class__, promote=True) if obj1cls is IntObject and obj2cls is IntObject: x = obj1.intval y = obj2.intval z = x + y return IntObject(intval=z) Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 44 / 47

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Conclusion (JIT) Effective dynamic compiler generation flexibility and ease of evolution orthogonal to the performance question. Languages implemented as understandable interpreters. PyPy proves this a viable approach worth of further exploration. Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 45 / 47

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Open Issues inlining control promotion switch explosion fallbacks jit only the hot-spots more hints needed in PyPy’s Python JIT backends for CLI/JVM Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 46 / 47

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Virtualizable Frames frames need to live in the heap (tracebacks ...) and be introspectable jit code wants local variables to live in registers and on the stack => mark the frame class as“virtualizable” jit code uses lazy allocation and stores some contents (local variables...) in register and stack outside world access gets intercepted to be able to force lazy virtual data into the heap Antonio Cuni (PyCon Due 2008) PyPy and The Art of Generating VMs May 10, 2008 47 / 47