1. Invitation to Computer Science 5th Edition
Chapter 11
Compilers and Language Translation

2. 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
Invitation to Computer Science, 5th Edition

3. 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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4. 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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5. Figure 11.1 General Structure of a Compiler
Invitation to Computer Science, 5th Edition

6. Figure 11.2 Overall Execution Sequence of a High-level Language Program
Invitation to Computer Science, 5th Edition

7. 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
Invitation to Computer Science, 5th Edition

8. Figure 11.3 Typical Token Classifications
Invitation to Computer Science, 5th Edition

9. Phase II: Parsing During the parsing phase: Parsing
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
Invitation to Computer Science, 5th Edition

10. 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
Invitation to Computer Science, 5th Edition

11. 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: <, >, and ::=
Invitation to Computer Science, 5th Edition

12. 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
Invitation to Computer Science, 5th Edition

13. Figure 11.4 First Attempt at a Grammar for a Simplified Assignment Statement
Invitation to Computer Science, 5th Edition

14. Figure 11.5 Parse Tree Produced by the Parser
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15. Figure 11.6 Second Attempt at a Grammar for Assignment Statements
Invitation to Computer Science, 5th Edition

16. Figure 11.7 Two Parse Trees for the Statement x = x + y + z
Invitation to Computer Science, 5th Edition

17. Figure 11.8 Third Attempt at a Grammar for Assignment Statements
Invitation to Computer Science, 5th Edition

18. Figure 11.9 Grammar for a Simplified Version of an if-else Statement
Invitation to Computer Science, 5th Edition

19. Figure 11.10 Parse Tree for the Statement if (x55y)x5z; else x5y;
Invitation to Computer Science, 5th Edition

20. 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
Invitation to Computer Science, 5th Edition

21. 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
Invitation to Computer Science, 5th Edition

22. Figure 11.11 Code Generation for the Assignment Statement x = x + y + z
Invitation to Computer Science, 5th Edition

23. 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 programmer’s task and increase productivity
Integrated development environment
Compiler is embedded within a collection of supporting software development routines
Invitation to Computer Science, 5th Edition

24. Phase IV: Code Optimization (continued)
Two types of optimization
Local optimization and global optimization
Possible local optimizations
Constant evaluation
Strength reduction
Eliminating unnecessary operations
Invitation to Computer Science, 5th Edition

25. Figure 11.12 Optimized Code for the Assignment Statement x = x + y + z
Invitation to Computer Science, 5th Edition

26. Summary Compiler Goals of a compiler Source program
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
Invitation to Computer Science, 5th Edition

27. Summary (continued) Object program Phases of the compilation process
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
Invitation to Computer Science, 5th Edition