1. CS 432: Compiler Construction Lecture 1
Department of Computer Science Salisbury University Fall 2017 Instructor: Dr. Sophie Wang
11/8/2017

2. Goals of the Course
Understand the concepts of compilers and interpreters.
Parser, scanner, tokens
Symbol tables, intermediate code
Executors, code generators
Compiler-compilers
Learn important job skills that employers want.
Work as a member of a small programming team.
Understand Big Application.
Use modern software engineering practices to develop a complex application.
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3. Course Notes Class website
Latest news
Lecture notes and handouts
The textbook and Java source code from the textbook:
Writing Compilers and Interpreters, 3rd edition
Second reference textbook:
Generating Parsers with JavaCC, 2nd edition
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4. How to instruct a computer
In the beginning…
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5. How to instruct a computer
Programming bit-by-bit doesn’t scale
we want to instruct the computer at a higher level of abstraction
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6. High-level Abstract Description to Low-level Implementation Details
President
My poll ratings are low,
lets invade a small nation
General
Cross the river and take
defensive positions
Sergeant
Forward march, turn left
Stop!, Shoot
Foot Soldier
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7. How to instruct a computer
Programming bit-by-bit doesn’t scale
we want to instruct the computer at a higher level of abstraction
Natural Languages:
Powerful, but…
Ambiguous
Same expression describes many possible action
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8. The computer understands you as well as your mother understands you.
“At last, a computer that understands you like your mother” McDonnell-Douglas Ad
The computer understands you as well as your mother understands you.
The computer understands that you like your mother.
The computer understands you as well as it understands your mother.
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9. “ I know you believe you understood what you think I said, but I am not sure you realize that what you heard is not what I meant.”
S. I. Hayakawa ( ),
U.S. senator from California
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10. George Romney (1907-1995), U.S. presidential candidate
“I didn’t say that I didn’t say it. I said that I didn’t say I said it. I want to make that very clear.”
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11. Programming Languages
We want a high level of abstraction
We also want
precision (avoid ambiguity)
conciseness
expressiveness
modularity
The goal is to achieve:
Programmer Productivity
Also Efficiency and Performance
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12. The role of the compiler
Translate a program
from high-level description to low-level implementation
High-level of Abstraction
Low-level of Abstraction
Program written in a Programming Languages
Assembly
Language
Translation
Compiler
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13. What is a Compiler?
A software utility that is extremely important for developing applications …
… but is otherwise overlooked and taken for granted
UNLESS … you can’t get your program to compile! _
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14. A Compiler is a Translator
A compiler translates a program you’ve written
... in a high-level language
C, C++, Java, Pascal, etc.
... into a low-level language
assembly language or machine language
... that a computer can understand and eventually execute.
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15. More Definitions
source program: the program (application) you write in a high-level language that the compiler will translate
Usually stored in a source file.
source language: the high-level language in which you write your source program
Pascal
object language: the low-level language (AKA target language) into which the compiler translates the source program
Do not confuse object language with object-oriented language.
Jasmin assembly language
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16. Still More Definitions
object program: your program after it has been translated into the object language
target machine: the computer that will eventually execute the object program
Java Virtual Machine (JVM)
The JVM runs on your workstation or laptop (any computer that supports Java)
implementation language: the language that the compiler itself is written in
Java _
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17. Overview of the Compilation Process
Sort.pas
Sort.java
What’s in this box?
Pascal compiler
(you will write this in Java)
Java compiler
translation:
javac Sort.java ...
Sort.asm
Sort.class
assembly language
object program
Jasmin assembler
(provided for you)
translation:
Sort.class
Java linker & loader
(provided for you)
Java linker & loader
java Sort ...
Java Virtual Machine
(provided for you)
Java Virtual Machine
execution:
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18. What is Conceptual Design?
The conceptual design of a program is a high-level view of its software architecture.
It includes the primary components of the program, how they’re organized, and how they interact with each other.
It does not necessarily say how these components will be implemented.
Rather, it allows you to examine and understand the components first without worrying about how you’re eventually going to develop them.
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19. Conceptual Design (Version 1)
Parser
Controls the translation process.
Repeatedly asks the scanner for the next token.
Scanner
Repeatedly reads characters from the source to construct tokens for the parser.
Token
A source language element
identifier (name)
number
special symbol (+ - * / = etc.)
reserved word
Also reads from the source
Source
The source program
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20. Token A low-level element of the source language.
AKA lexeme
Pascal language tokens
Identifiers
names of variables, types, procedures, functions, enumeration values, etc.
Numbers
integer and real (floating-point)
Reserved words
BEGIN END IF THEN ELSE AND OR NOT etc.
Special symbols
+ - * / := < <= = >= > . , .. : ( ) [ ] { } ′
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21. Parser Controls the translation process.
Repeatedly asks the scanner for the next token.
Knows the syntax (“grammar”) of the source language’s statements and expressions.
Analyzes the sequence of tokens to determine what kind of statement or expression it is translating.
Verifies that what it’s seeing is syntactically correct.
Flags any syntax errors that it finds and attempts to recover from them.
What the parser does is called parsing – it parses the source program in order to translate it.
AKA syntax analyzer
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22. Scanner
Reads characters sequentially from the source in order to construct the next token whenever requested by the parser.
Knows the syntax of the source language’s tokens.
What the scanner does is called scanning – it scans the source program in order to extract tokens.
AKA lexical analyzer _
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23. Conceptual Design (Version 2)
We can architect a compiler with three major parts:
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24. Major Parts of a Compiler
Front end
Parser, Scanner, Source, Token
Intermediate tier
Intermediate code (icode)
“Predigested” form of the source code that the back end can process efficiently.
Example: parse trees
AKA intermediate representation (IR)
Symbol table (symtab)
Stores information about the symbols (such as the identifiers) contained in the source program.
Back end
Code generator
Processes the icode and the symtab in order to generate the object code.
Only the front end needs to be source language-specific.
The intermediate tier and the back end can be language-independent!
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25. What Else Can Compilers Do?
Compilers allow you to program in a high-level language and think about your algorithms, not about machine architecture.
Compilers provide language portability.
You can run your C++ and Java programs on different machines because their compilers enforce language standards.
Compilers can optimize and improve how well your programs execute.
Optimize the object code for speed.
Optimize the object code for size.
Optimize the object code for power consumption.
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26. What about Interpreters?
An interpreter executes a source program instead of generating object code.
It executes a source program using the intermediate code and the symbol table.
It shares many of the components of a compiler.
Instead of a code generator in the back end, it has an executor. _
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27. Conceptual Design (Version 3)
A compiler and an interpreter can both use the same front end and intermediate tier.
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28. Comparing Compilers and Interpreters
A compiler generates object code, but an interpreter does not.
Executing the source program from object code can be several orders of magnitude faster than executing the program by interpreting the intermediate code and the symbol table.
But an interpreter requires less effort to get a source program to execute = faster turnaround time.
An interpreter maintains control of the source program’s execution.
Interpreters often come with interactive source-level debuggers that allow you to refer to source program elements, such as variable names.
AKA symbolic debugger _
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29. Compilers and Interpreters, cont’d
Therefore ...
Interpreters are useful during program development.
Compilers are useful to run released programs in a production environment.
In this course, you will ...
Develop a compiler/an interpretor for a language of your choice.
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30. A Pascal Interpreter
The interpreter parses and executes a Pascal program without first generating object code.
The parser generates intermediate code (icode) and a symbol table (symtab).
The executor interprets the icode and the symtab in order to execute the Pascal program. _
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31. Sample Pascal Program
PROGRAM Newton;
CONST
epsilon = 1e-6;
VAR
number : integer;
root, sqroot : real;
BEGIN
REPEAT
writeln;
write('Enter new number (0 to quit): ');
read(number);
IF number = 0 THEN BEGIN
writeln(number:12, 0.0:12:6);
END
ELSE IF number < 0 THEN BEGIN
writeln('*** ERROR: number < 0');
ELSE BEGIN
sqroot := sqrt(number);
writeln(number:12, sqroot:12:6);
root := 1;
root := (number/root + root)/2;
writeln(root:24:6,
100*abs(root - sqroot)/sqroot:12:2,
'%')
UNTIL abs(number/sqr(root) - 1) < epsilon;
UNTIL number = 0
END.
read, write, and writeln are standard (built-in) Pascal procedures.
abs, sqr, and sqrt are standard Pascal functions.
Command line:
java –classpath classes
Pascal execute newton.pas
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32. Why Study Compilers and Interpreters?
Compilers and interpreters are complex programs.
The subject matter has a well-established body of theory and techniques.
Don’t take compilers and interpreters for granted!
Understand how they work and you’ll become a better programmer. _
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33. Why Study Compilers and Interpreters?
Programming challenge.
This is one of the largest, most complex programs you’ll ever write.
Develop it incrementally and have working code at each step.
Use good object-oriented design and software engineering practices.
Useful and fun.
Create a compiler or an interpreter for a programming language that you invent that’s suitable for your application domain.
a procedure-oriented language like Pascal
a scripting language like the Linux shell
Learn programming techniques and algorithms that you can apply to other applications. _
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34. Small Teams
Compilers and interpreters are generally written by small teams.
Form your own teams of 2 or 3 students each.
Choose your team members wisely!
Be sure you’ll be able to meet and communicate with each other and work together well.
Each team member will receive the same score on the team assignments and on the team compiler project.
me your team name and the list of team members by Tuesday, September 5.
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35. An Open Source Pascal IDE
You can use Lazarus for Pascal development.
Runs on Windows, Mac OS X, and Linux.
It’s an integrated development environment (IDE).
Your task # 1.
Use Lazarus to develop your first Pascal program that displays “Hello World!”.
Crash course to learning Pascal.
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36. Compiler Project
Each team develops a working compiler for a procedure-oriented source language.
A subset of c.
A subset of any other procedure-oriented languages.
Your own procedure-oriented language.
Use the JavaCC compiler-compiler to generate a parser and a scanner for your source language.
You will need to provide a grammar for the language.
Re-use any other compiler code from the class.
Start thinking about and planning for your project early in the semester.
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37. Postmortem Report
At the end of the semester, each student will individually turn in a short (1 or 2 pp.) report:
A brief description of what you learned in the course.
An assessment of your personal accomplishments for your project team.
An assessment of each of your other project team members. _
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