1. Advanced Compiler Techniques LIU Xianhua School of EECS, Peking University Course Introduction

2. “Advanced Compiler Techniques” Outline  Course Overview Course Topics Course Requirements Grading  Preparation Materials Compiler Review 2

3. “Advanced Compiler Techniques” Course Overview  Graduate level compiler course Focusing on advanced materials on program analysis and optimization. Assuming that you have basic knowledge & techniques on compiler construction. Gain hands - on experience through a programming project to implement a specific program analysis or optimization technique.  Course website : http://mprc.pku.edu.cn/~liuxianhua/ACT13 3

4. Administrivia  Time : 10-12 (6:40 pm -) every Thursday  Location : 2-420  TA : WANG Wei, DONG Yin Email: act13 [at] mprc.pku.edu.cnact13 [at] mprc.pku.edu.cn  Office Hour : 4-5:30 pm Tuesdays or by appointment via email  Contact : Phone : 62765828-809, 62759129 Room 1818, 1 st Science Building Email: lxh [at] mprc.pku.edu.cnlxh [at] mprc.pku.edu.cn  Include [ ACT 13] in the subject “Advanced Compiler Techniques” 4

5. Course Materials  Dragon Book Aho, Lam, Sethi, Ullman, “ Compilers : Principles, Techniques, and Tools ”, 2 nd Edition, Addison 2007  Related Papers Class website 5 “Advanced Compiler Techniques”

6. Requirements  Basic Requirements Read materials before / after class. Work on your homework individually.  Discussions are encouraged but don ’ t copy others ’ work. Get you hands dirty !  Experiment with ideas presented in class and gain first - hand knowledge ! Come to class and DON ’ T hesitate to speak if you have any questions / comments / sugge stions ! Student participation is important ! 6

7. “Advanced Compiler Techniques” Grading  Grading based on Homework : 20%  ~ 5 homework assignments Midterm : 30%  Week 11 or 12 ( Nov 21 st or 28 th ) Final Project : 40% Class participation : 10% 7

8. “Advanced Compiler Techniques” Final Project  Groups of 2-3 students Pair Programming recommended !  Topic Problem of your choice ( recommend project list will be provided ) Should be an interesting enough ( non - trivial ) problem  Suggested environment LLVM ( UIUC ), SUIF ( Stanford ), gcc ( GNU ), Soot ( McGill Univ.), JoeQ, Jikes ( IBM ), OpenJDK, Dalvik, V 8 … 8

9. “Advanced Compiler Techniques” Project Req.  Week 5: Introduction  Week 7: Proposal due  Week 8: Proposal Presentation  Week 12: Progress Report due  Week 16: Final Presentation  Week 17: Final Report due 9

10. “Advanced Compiler Techniques” Course Topics  Basic analysis & optimizations Data flow analysis & implementation Control flow analysis SSA form & its application Loops / Instruction scheduling Pointer analysis Localization & Parallelization optimization  Selected topics ( TBD ) Architecture - based optimization Program slicing, program testing Power - aware compilation 10

11. Advanced Compiler Techniques LIU Xianhua School of EECS, Peking University Compiler Review

12. “Advanced Compiler Techniques” What Is a Compiler?  A program that translates a program in one language to another language The essential interface between applications & architectures  Typically lowers the level of abstraction analyzes and reasons about the program & architecture  We expect the program to be optimized, i. e., better than the original ideally exploiting architectural strengths and hiding weaknesses 12

13. Why Study Compilers? (1)  Become a better programmer (!) Insight into interaction between languages, compilers, and hardware Understanding of implementation techniques What is all that stuff in the debugger anyway? Better intuition about what your code does

14. Why Study Compilers? (2)  Compiler techniques are everywhere Parsing ( little languages, interpreters, XML ) Software tools ( verifiers, checkers, … ) Database engines, query languages AI, etc. : domain - specific languages Text processing  Tex / LaTex -> dvi -> Postscript -> PDF Hardware : VHDL ; model - checking tools Mathematics ( Mathematica, Matlab )

15. Why Study Compilers? (3)  Fascinating blend of theory and engineering Direct applications of theory to practice  Parsing, scanning, static analysis Some very difficult problems ( NPH or worse )  Resource allocation, “ optimization ”, etc.  Need to come up with good - enough approximations / heurist ics “Advanced Compiler Techniques” 15

16. Why Study Compilers? (4)  Ideas from many parts of CSE AI : Greedy algorithms, heuristic search Algorithms : graph algorithms, union - find, dynamic programming, approximation algorithms Theory : Grammars, DFAs and PDAs, pattern matching, fixed - point algorithms, lattice theory for analysis Systems : Allocation & naming, synchronization, locality Architecture : pipelines, instruction set use, memory hierarchy management “Advanced Compiler Techniques” 16

17. Why Study Advanced Compilers?  An opportunity to explore compiler techniques in both breadth and depth Parallelization? Functional? Optimizations with more details  Compiler optimizations rely on both program analysis and transformation, which are useful in many related areas Software engineering : program understanding / reverse engineering / debugging Run - time support and improvement  Open problems Engineering effort : limits and issues Motivate research topics “Advanced Compiler Techniques” 17

18. “Advanced Compiler Techniques” Compiler vs. Interpreter  Compilers : Translate a source ( human - writable ) program to an executable ( machine - readable ) program  Interpreters : Convert a source program and execute it at the same time. 18

19. “Advanced Compiler Techniques” Compiler vs. Interpreter Ideal concept: Compiler Executable Source codeExecutable Input dataOutput data Interpreter Source code Input data Output data 19

20. “Advanced Compiler Techniques” Compiler vs. Interpreter  Most languages are usually thought of as using either one or the other : Compilers : FORTRAN, C, C ++, Pascal, COBOL, PL /1 Interpreters : Lisp, Scheme, BASIC, APL, Perl, Python, Smalltalk, Javascript, RUBY, Shellscripts / awk / sed …  BUT : not always implemented this way Virtual Machines ( think Java ) Linking of executables at runtime JIT ( Just - in - time ) compiling 20

21. “Advanced Compiler Techniques” Compiler vs. Interpreter  Actually, no sharp boundary between them  General situation is a combo : Translator Virtual machine Source code Intermed. code Input Data Output 21

22. Hybrid Approaches  Classic example : Java Compile Java source to byte codes – Java Virtual Machine language (. class files ) Execution  Interpret byte codes directly, or  Compile some or all byte codes to native code Just - In - Time compiler ( JIT ) – detect hot spots & compile on the fly to native code – standard these days  Variations used for. NET ( compile always ) & in high - performance compilers for dynamic languages, e. g., JavaScript 22 “Advanced Compiler Techniques”

23. Compiler vs. Interpreter 23 “Advanced Compiler Techniques” Compiler  Pros Less space Fast execution  Cons Slow processing  Partly Solved ( Separate compilation ) Debugging  Improved thru IDEs Interpreter Pros –Easy debugging –Fast Development –Interaction Cons –Not for large projects Exceptions : Perl, Python –Requires more space –Slower execution Interpreter in memory all the time TRADE OFF

24. Traditional Compiler intermediate representation ( IR ) front end maps legal code into IR back end maps IR onto target machine simplify retargeting allows multiple front ends multiple passes  better code 24 Scanner (lexical analysis) Parser (syntax analysis) Code Optimizer Semantic Analysis (IR generator) Code Generator Symbol Table tokens Syntactic structure IR Source program Target program IR

25. Fallacy  Front - end, IR and back - end must encode knowledge needed for all n  m combinations ! 25 “Advanced Compiler Techniques”

26. Optimizer (Middle End)  Modern optimizers are usually built as a set of passes constant propagation and folding code motion reduction of operator strength common sub - expression elimination redundant store elimination dead code elimination 26

27. “Advanced Compiler Techniques” Phase of Compilations

28. “Advanced Compiler Techniques” Scanning/Lexical Analysis  Break program down into its smallest meaningful symbols ( tokens, atoms )  Tools for this include lex, flex  Tokens include e. g.: Reserved words : do if float while Special characters : ( {, + - = ! / Names & numbers : myValue 3.07 e 02  Start symbol table with new symbols found 28 index := start + step * 20 Input: index := start + step * 20 After scanning: identifier operatornumber

29. “Advanced Compiler Techniques” Parsing  Construct a parse tree from symbols  A pattern - matching problem Language grammar defined by set of rules that identify legal ( meaningful ) combinations of symbols Each application of a rule results in a node in the parse tree Parser applies these rules repeatedly to the program until leaves of parse tree are “ atoms ”  If no pattern matches, it ’ s a syntax error  yacc, bison are tools for this ( generate c code that parses specified language ) 29

30. “Advanced Compiler Techniques” Parse Tree  Output of parsing  Top - down description of program syntax Root node is entire program  Constructed by repeated application of rules in Context Free Grammar ( CFG )  Leaves are tokens that were identified during lexical analysis 30

31. “Advanced Compiler Techniques” Example: Parsing Rules for Pascal These are like the following :  program ----> PROGRAM identifier ( identifier more_identifiers ) ; block  more_identifiers ---->, identifier more_identifiers | ε  block ----> variables BEGIN statement more_statements END  statement ----> do_statement | if_statement | assignment | …  if_statement ----> IF logical_expression THEN statement ELSE … 31

32. “Advanced Compiler Techniques” Pascal Code Example program gcd ( input, output ) var i, j : integer begin read ( i, j ) while i <> j do if i > j then i := i – j ; else j := j – i ; writeln ( i ); end. 32

33. “Advanced Compiler Techniques” Example: Parse Tree 33

34. “Advanced Compiler Techniques” Semantic Analysis  Discovery of meaning in a program using the symbol table Do static semantics check Simplify the structure of the parse tree ( from parse tree to abstract syntax tree ( AST ) )  Static semantics check Making sure identifiers are declared before use Type checking for assignments and operators Checking types and number of parameters to subroutines Making sure functions contain return statements … 34

35. “Advanced Compiler Techniques” Example: AST 35

36. “Advanced Compiler Techniques” (Intermediate) Code Generation  Go through the parse tree from bottom up, turning rules into code.  e. g. A sum expression results in the code that computes the sum and saves the result  Result : inefficient code in a machine - independent language 36

37. “Advanced Compiler Techniques” Target Code Generation  Convert intermediate code to machine instructions on intended target machine  Determine storage addresses for entries in symbol table 37

38. Types of Code Optimizations  Machine - independent optimizations Eliminate redundant computation Eliminate dead code Perform computation at compile time if possible Execute code less frequently  Machine - dependent optimizations Hide latency Parallelize computation Replace expensive computations with cheaper ones Improve memory performance 38 “Advanced Compiler Techniques”

39. Scopes of Code Optimizations  Peephole optimizations Consider a small window of instructions  Local optimizations Consider instruction sequences within a basic block  Intra - procedural ( global ) optimizations Consider multiple basic blocks within a procedure Need support from control flow analysis  Branches, loops, merging of flows  Inter - procedural optimizations Consider the whole program w / multiple procedures Need to analyze calls / returns 39 “Advanced Compiler Techniques”

40. Sample Optimizations  Redundant loads and stores elimination MOV R 0, a  MOV R 0, a MOV a, R 0  Unreachable code elimination GOTO L 2 x := x + 1  unreachable  Algebraic identities x := x + 0  can eliminate x := x * 1  Reduction in strength x := x * 2  x := x + x  Constant folding p = 2 * 3.14  p = 6.2.8  Constant propagation p = 6.28 p =6.28 x = x * p  x = x * 6.28 40 “Advanced Compiler Techniques”

41. Sample Optimizations  Common sub - expression elimination Local m = 2 * y * z t = 2 * y o = 2 * y - z  m = t * z o = t - z Global Global partial a:=d+eb:=d+e c:=d+e t:=d+e a:=t t:=d+e b:=t c:=t a:=d+e c:=d+e t:=d+e a:=t t:=d+e c:=t 41 “Advanced Compiler Techniques”

42. Sample Optimizations  Loop optimizations Code motion while ( i <= limit - 2) { … }  t = limit - 2 while ( i <= t ) { … } Loop unrolling do i =1 to n by 4 a ( i ) = a ( i )+ b ( i ) a ( i +1) = a ( i +1)+ b ( i +1) a ( i +2) = a ( i +2)+ b ( i +2) a ( i +3) = a ( i +3)+ b ( i +3) end … // process tail part do i =1 to n by 1 a ( i ) = a ( i )+ b ( i )  end 42 “Advanced Compiler Techniques”

43. When Should We Compile?  Ahead - of - time : before you run the program  Offline profiling : compile several times compile / run / profile.... then run again  Just - in - time : while you run the program required for dynamic class loading, i. e., Java, Python, etc. 43 “Advanced Compiler Techniques”

44. Aren’t Compilers a Solved Problem? “ Optimization for scalar machines is a problem that was solved ten years ago.” -- David Kuck, Fall 1990  Architectures keep changing New features pose new problems Changing costs lead to different concerns Old solutions need re - engineering  Languages keep changing  Applications keep changing - SPEC CPU?  When to compile keeps changing And compiling options / parameters?  Desired target properties keep changing Code size, running time, power consumption, security  Design flow also may change 44 “Advanced Compiler Techniques”

45. Role of compilers  Bridge complexity and evolution in architecture, languages, & applications  Help programs with correctness, reliability, program understanding  Compiler optimizations can significantly improve performance 1 to 10 x on conventional processors  Performance stability : one line change can dramatically alter performance unfortunate, but true 45

46. “Advanced Compiler Techniques” Performance Anxiety  But does performance really matter? Computers are really fast Moore ’ s law ( roughly ): hardware performance doubles every 18 months  Real bottlenecks lie elsewhere : Memory & storage access Communications in bus and network Human ! ( think interactive apps )  Human typing avg. 8 cps ( max 25 cps )  Waste time “ thinking ” 46

47. “Advanced Compiler Techniques” Compilers Don’t Help Much  Do compilers improve performance anyway? Proebsting ’ s law ( Todd Proebsting, Microsoft Research ):  Difference between optimizing and non - optimizing compiler ~ 4 x  Assume compiler technology represents 36 years of progress ( actually more )  Compilers double program performance every 18 years !  Not quite Moore ’ s Law… 47

48. “Advanced Compiler Techniques” A Big BUT  Why use high - level languages anyway? Easier to write & maintain Safer ( think Java ) More convenient ( think libraries, GC… )  But : people will not accept massive performance hit for these gains Compile with optimization ! Still use C and C ++!! Hand - optimize their code !!! Even write assembler code ( gasp )!!!!  Apparently performance does matter… 48

49. “Advanced Compiler Techniques” Why Compilers Matter  Key part of compiler ’ s job : make the costs of abstraction reasonable Remove performance penalty for :  Using objects  Safety checks ( e. g., array - bounds )  Writing clean code ( e. g., recursion )  Use program analysis to transform code primary topic of this course 49

50. “Advanced Compiler Techniques” Program Analysis  Source code analysis is the process of extracting information about a program from its source code or artifacts ( e. g., from Java byte code or execution traces ) generated from the source code using automatic tools. Source code is any static, textual, human readable, fully executable description of a computer program that can be compiled automatically into an executable form. To support dynamic analysis the description can include documents needed to execute or compile the program, such as program inputs. Source : Dave Binkely -” Source Code Analysis – A Roadmap ”, FOSE ’07 50

51. “Advanced Compiler Techniques” Anatomy of an Analysis 1. Parser parses the source code into one or more internal representations. 2. Internal representation CFG, call graph, AST, SSA, VDG, FSA Most common : Graphs 3. Actual Analysis 51

52. “Advanced Compiler Techniques” Analysis Properties  Static vs. Dynamic  Sound vs. unsound Safe vs. Unsafe  Flow sensitive vs. Flow insensitive  Context sensitive vs. Context insensitive  Precision - Cost trade - off 52

53. “Advanced Compiler Techniques” Levels of Analysis ( in order of increasing detail & complexity )  Local ( single - block ) [1960’ s ] Straight - line code Simple to analyze ; limited impact  Global ( Intraprocedural ) [1970’ s – today ] Whole procedure Dataflow & dependence analysis  Interprocedural [ late 1970’ s – today ] Whole - program analysis Tricky :  Very time and space intensive  Hard for some PL ’ s ( e. g., Java ) 53

54. “Advanced Compiler Techniques” Optimization = Analysis + Transformation  Key analysis : Control - flow  if - statements, branches, loops, procedure calls Data - flow  definitions and uses of variables  Representations : Control - flow graph Control - dependence graph Def / use, use / def chains SSA ( Static Single Assignment ) 54

55. “Advanced Compiler Techniques” Applications  architecture recovery  clone detection  program comprehension  debugging  fault location  model checking in formal analysis  model - driven development  optimization techniques in software engineering  reverse engineering  software maintenance  visualizations of analysis results  etc. 55

56. “Advanced Compiler Techniques” Current Challenges  Pointer Analysis  Concurrent Program Analysis  Dynamic Analysis  Information Retrieval  Data Mining  Multi - Language Analysis  Non - functional Properties  Self - Healing Systems  Real - Time Analysis 56

57. Exciting times New and changing architectures  Hitting the microprocessor wall  Multicore / manycore  Tiled architectures, tiled memory systems Object - oriented languages becoming dominant paradigm  Java and C # coming to your OS soon - Jnode, Singularity  Security and reliability, ease of programming Key challenges and approaches  Latency & parallelism still key to performance  Language & runtime implementation efficiency  Software / hardware cooperation is another key issue 57 Compiler ProgrammerRuntime Code Specification Future behavior Feedback Feedback H/S Profiling “Advanced Compiler Techniques”

58. Next Time  Control - Flow Analysis  Local Optimizations  Read Dragon book : § 8.4-8.5, 8.7-8.8