Invitation to Computer Science 5th Edition

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Invitation to Computer Science 5th Edition

• Chapter 11
• Compilers and Language Translation

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Objectives

• In this chapter, you will learn about
• The compilation process
• Phase I Lexical analysis
• Phase II Parsing
• Phase III Semantics and code generation
• Phase IV Code optimization

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Introduction

• Compiler
• Translates high-level language into machine
  language prior to execution
• Assembly language and machine language are
  related one to one
• One to many
• Relationship between a high-level language and
  machine language
• Compiler goals
• Correctness efficient and concise

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The Compilation Process

• Four phases of compilation
• Phase I Lexical analysis
• Phase II Parsing
• Phase III Semantic analysis and code generation
• Phase IV Code optimization

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Figure 11.1 General Structure of a Compiler
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Figure 11.2 Overall Execution Sequence of a
High-level Language Program
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Phase I Lexical Analysis

• Lexical analyzer
• Groups input characters into units called tokens
• Scanner
• Discards nonessential characters, such as blanks
  and tabs
• Groups remaining characters into high-level
  syntactical units such as symbols, numbers, and
  operators

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Figure 11.3 Typical Token Classifications
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Phase II Parsing

• During the parsing phase
• Compiler determines whether the tokens recognized
  by the scanner during phase I fit together in a
  grammatically meaningful way
• Parsing
• Process of diagramming a high-level language
  statement
• Done by a program called a parser

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Grammars, Languages, and BNF

• Backus-Naur Form
• Named after its designers John Backus and Peter
  Naur
• Syntax of a language is specified as a set of
  rules, also called productions
• Entire collection of rules is called a grammar
• Lefthand side of a BNF rule is the name of a
  single grammatical category
• Operator means is defined as, and
  definition, also called righthand side

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Grammars, Languages, and BNF (continued)

• Backus-Naur Form
• Nonterminal intermediate grammatical category
  used to help explain and organize the language
• Goal symbol final nonterminal
• Language collection of all statements that can
  be successfully parsed
• Metasymbols lt, gt, and

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Parsing Concepts and Techniques

• Parser
• Receives as input the BNF description of a
  high-level language and a sequence of tokens
  recognized by the scanner
• Look-ahead parsing algorithms
• Look down the road a few tokens to see what
  would happen if a certain choice is made
• Ambiguous
• Grammar that allows the construction of two or
  more distinct parse trees for the same statement

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Figure 11.4 First Attempt at a Grammar for a
Simplified Assignment Statement
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Figure 11.5 Parse Tree Produced by the Parser
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Figure 11.6 Second Attempt at a Grammar for
Assignment Statements
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Figure 11.7 Two Parse Trees for the Statement x
x y z
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Figure 11.8 Third Attempt at a Grammar for
Assignment Statements
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Figure 11.9 Grammar for a Simplified Version of
an if-else Statement
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Figure 11.10 Parse Tree for the Statement if
(x55y)x5z else x5y
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Phase III Semantics and Code Generation

• Semantic record
• Data structure that stores information about a
  nonterminal
• First part of code generation
• Involves a pass over the parse tree to determine
  whether all branches of the tree are semantically
  valid
• Code generation
• Compiler must determine how transformation of
  grammatical objects can be accomplished in
  machine language

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Phase III Semantics and Code Generation
(continued)

• Optimization
• Compiler polishes and fine-tunes the translation
  so that it runs a little faster or occupies a
  little less memory

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Figure 11.11 Code Generation for the Assignment
Statement x x y z
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Phase IV Code Optimization

• Efficiency
• Ability to write highly optimized programs that
  contained no wasted microseconds or unnecessary
  memory cells
• Goal in compiler design today
• Provide a wide array of compiler tools to
  simplify the programmers task and increase
  productivity
• Integrated development environment
• Compiler is embedded within a collection of
  supporting software development routines

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Phase IV Code Optimization (continued)

• Two types of optimization
• Local optimization and global optimization
• Possible local optimizations
• Constant evaluation
• Strength reduction
• Eliminating unnecessary operations

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Figure 11.12 Optimized Code for the Assignment
Statement x x y z
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Summary

• Compiler
• Piece of system software that translates
  high-level languages into machine language
• Goals of a compiler
• Correctness and the production of efficient and
  concise code
• Source program
• High-level language program

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Summary (continued)

• Object program
• The machine language translation of the source
  program
• Phases of the compilation process
• Phase I Lexical analysis
• Phase II Parsing
• Phase III Semantic analysis and code generation
• Phase IV Code optimization