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Chapter 1: Introduction to Compiling

Chapter 1: Introduction to Compiling. Aggelos Kiayias Computer Science & Engineering Department The University of Connecticut 371 Fairfield Road, Box U-1155 Storrs, CT 06269-3155. aggelos@cse.uconn.edu http://www.cse.uconn.edu/~akiayias. Notes Credits: Steven A. Demurjian CSE, UCONN

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Chapter 1: Introduction to Compiling

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  1. Chapter 1: Introduction to Compiling Aggelos Kiayias Computer Science & Engineering Department The University of Connecticut 371 Fairfield Road, Box U-1155 Storrs, CT 06269-3155 aggelos@cse.uconn.edu http://www.cse.uconn.edu/~akiayias Notes Credits: Steven A. Demurjian CSE, UCONN Robert LaBarre United Technologies Research Center

  2. Introduction to Compilers Sourceprogram Target Program Compiler Error messages Diverse & Varied • As a Discipline, Involves Multiple CS&E Areas • Programming Languages and Algorithms • Theory of Computing & Software Engineering • Computer Architecture & Operating Systems • Has Deceivingly Simplistic Intent:

  3. Classifications of Compilers • Compilers Viewed from Many Perspectives • However, All utilize same basic tasks to accomplish their actions Single Pass Multiple Pass Load & Go Construction Debugging Optimizing Functional

  4. The Model • The TWO Fundamental Parts: • We Will Discuss Both in This Class, andFOCUS on analysis. Analysis: Decompose Source into an intermediate representation Synthesis: Target program generation from representation

  5. Important Notes • Today: There are many Software Tools for helping with the Analysis Part. This Wasn’t the Case in Early Days. (some) analysis is also important in: • Structure / Syntax directed editors: Force “syntactically” correct code to be entered • Pretty Printers: Standardized version for program structure (i.e., blank space, indenting, etc.) • Static Checkers: A “quick” compilation to detect rudimentary errors • Interpreters: “real” time execution of code a “line-at-a-time”

  6. Important Notes • Compilation Is Not Limited to Programming Language Applications • Text Formatters • LATEX & TROFF Are Languages Whose Commands Format Text • Silicon Compilers • Textual / Graphical: Take Input and Generate Circuit Design • Database Query Processors • Database Query Languages Are Also a Programming Language • Input is compiled Into a Set of Operations for Accessing the Database

  7. The Many Phases of a Compiler Source Program 5 1 2 6 Code Optimizer Lexical Analyzer Code Generator Syntax Analyzer 3 Semantic Analyzer Error Handler Symbol-table Manager 4 Intermediate Code Generator Target Program 1, 2, 3 : Analysis - Our Focus 4, 5, 6 : Synthesis

  8. Language-Processing System 5 1 2 Loader Link/Editor Pre-Processor Compiler 3 Assembler Library,relocatable object files 4 RelocatableMachine Code Source Program Executable

  9. The Analysis Task For Compilation • Three Phases: • Linear / Lexical Analysis: • L-to-r Scan to Identify Tokenstoken: sequence of chars having a collective meaning • Hierarchical Analysis: • Grouping of Tokens Into Meaningful Collection • Semantic Analysis: • Checking to ensure Correctness of Components

  10. Phase 1. Lexical Analysis All are tokens Easiest Analysis - Identify tokens which are the basic building blocks For Example: Position := initial + rate * 60 ; _______ __ _____ _ ___ _ __ _ Blanks, Line breaks, etc. are scanned out

  11. Phase 2. Hierarchical Analysisaka Parsing or Syntax Analysis assignment statement := identifier expression + position expression expression * identifier expression expression initial identifier number rate 60 For previous example, we would have Parse Tree: Nodes of tree are constructed using a grammar for the language

  12. What is a Grammar? • Grammar is a Set of Rules Which Govern the Interdependencies & Structure Among the Tokens statement is an assignment statement, or while statement, or if statement, or ... assignment statement is an identifier := expression ; expression is an (expression), or expression + expression, or expression * expression, or number, or identifier, or ...

  13. Why Have We Divided Analysis in This Manner? • Lexical Analysis - Scans Input, Its Linear Actions Are Not Recursive • Identify Only Individual “words” that are the the Tokens of the Language • Recursion Is Required to Identify Structure of an Expression, As Indicated in Parse Tree • Verify that the “words” are Correctly Assembled into “sentences” • What is Third Phase? • Determine Whether the Sentences have One and Only One Unambiguous Interpretation • … and do something about it! • e.g. “John Took Picture of Mary Out on the Patio”

  14. Phase 3. Semantic Analysis := := position + position + initial * initial * rate 60 rate inttoreal 60 • Find More Complicated Semantic Errors and Support Code Generation • Parse Tree Is Augmented With Semantic Actions Compressed Tree Conversion Action

  15. Phase 3. Semantic Analysis • Most Important Activity in This Phase: • Type Checking - Legality of Operands • Many Different Situations: Real := int + char ; A[int] := A[real] + int ; while char <> int do …. Etc.

  16. Supporting Phases/ Activities for Analysis • Symbol Table Creation / Maintenance • Contains Info (storage, type, scope, args) on Each “Meaningful” Token, Typically Identifiers • Data Structure Created / Initialized During Lexical Analysis • Utilized / Updated During Later Analysis & Synthesis • Error Handling • Detection of Different Errors Which Correspond to All Phases • What Kinds of Errors Are Found During the Analysis Phase? • What Happens When an Error Is Found?

  17. The Many Phases of a Compiler Source Program 5 1 2 6 Code Optimizer Lexical Analyzer Code Generator Syntax Analyzer 3 Semantic Analyzer Error Handler Symbol-table Manager 4 Intermediate Code Generator Target Program 1, 2, 3 : Analysis - Our Focus 4, 5, 6 : Synthesis

  18. The Synthesis Task For Compilation • Intermediate Code Generation • Abstract Machine Version of Code - Independent of Architecture • Easy to Produce and Do Final, Machine Dependent Code Generation • Code Optimization • Find More Efficient Ways to Execute Code • Replace Code With More Optimal Statements • 2-approaches: High-level Language & “Peephole” Optimization • Final Code Generation • Generate Relocatable Machine Dependent Code

  19. Reviewing the Entire Process intermediate code generator lexical analyzer syntax analyzer semantic analyzer := + id1 id2l * id3 60 := + id1 id2l * id3 inttoreal 60 position := initial + rate * 60 id1 := id2 + id3 * 60 Symbol Table Errors position .... initial …. rate….

  20. Reviewing the Entire Process intermediate code generator code optimizer final code generator Errors Symbol Table position .... initial …. rate…. temp1 := inttoreal(60) temp2 := id3 * temp1 temp3 := id2 + temp2 id1 := temp3 3 address code temp1 := id3 * 60.0 id1 := id2 + temp1 MOVF id3, R2 MULF #60.0, R2 MOVF id2, R1 ADDF R1, R2 MOVF R1, id1

  21. Assemblers • Assembly code: names are used for instructions, and names are used for memory addresses. • Two-pass Assembly: • First Pass: all identifiers are assigned to memory addresses (0-offset)e.g. substitute 0 for a, and 4 for b • Second Pass: produce relocatable machine code: MOV a, R1 ADD #2, R1 MOV R1, b 0001 01 00 00000000 * 0011 01 10 00000010 0010 01 00 00000100 * relocation bit

  22. Loaders and Link-Editors • Loader: taking relocatable machine code, altering the addresses and placing the altered instructionsinto memory. • Link-editor: taking many (relocatable) machine code programs (with cross-references) and produce a single file. • Need to keep track of correspondence between variable names and corresponding addresses in each piece of code.

  23. Compiler Cousins:PreprocessorsProvide Input to Compilers 1. Macro Processing #define in C: does text substitution before compiling #define X 3 #define Y A*B+C #define Z getchar()

  24. 2. File Inclusion defs.h main.c ////// ////// ////// #include “defs.h” …---…---…--- …---…---…--- …---…---…--- ////// ////// ////// …---…---…--- …---…---…--- …---…---…--- #include in C - bring in another file before compiling

  25. 3. Rational Preprocessors • Augment “Old” Languages With Modern Constructs • Add Macros for If - Then, While, Etc. • #Define Can Make C Code More Pascal-like #define begin { #define end } #define then

  26. 4. Language Extensions for a Database System EQUEL - Database query language embedded in C ## Retrieve (DN=Department.Dnum) where ## Department.Dname = ‘Research’ is Preprocessed into: ingres_system(“Retr…..Research’”,____,____); a procedure call in a programming language.

  27. The Grouping of Phases Front End : Analysis + Intermediate Code Generation vs. Back End : Code Generation + Optimization Number of Passes: A pass: requires r/w intermediate files Fewer passes: more efficiency. However: fewer passes require more sophisticated memory management and compiler phase interaction. Tradeoffs ……..

  28. Compiler Construction Tools Parser Generators : Produce Syntax Analyzers Scanner Generators : Produce Lexical Analyzers Syntax-directed Translation Engines : Generate Intermediate Code Automatic Code Generators : Generate Actual Code Data-Flow Engines : Support Optimization

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