CS 3240: Languages and Computation

1
CS 3240 Languages and Computation

• Course Overview
• Santosh Pande

2
Personnel

• Instructor Prof. Santosh Pande
• Email santosh_at_cc.gatech.edu
• Office KACB 2338
• Office Hours
• Tue. Thur. at 300 to 400 pm
• Or by appointment
• TAs Philip Wright and Aditya Devurkar
• Email pwright_at_gatech.edu and agdevurkar_at_gmail.co
  m
• Office Hours TBA
• Subject to change

3
Required Textbooks

• Bundle ISBN 1418879746, including
• Compiler Construction Principles and Practice
  by Kenneth C. Louden, Thompson Course Technology,
  1997, ISBN 0534939724
• Introduction to Theory of Computation, Second
  Edition by Michael Sipser, Thompson Course
  Technology, 2005, ISBN 0534950973
• Both available on reserve in the library

4
Course Objectives

• Formal languages
• Understand definitions of regular and
  context-free languages and their corresponding
  machines
• Understand their computational powers and
  limitations
• Compiler concepts
• Understand their applications in compilers
• Front-end of compiler
• Lexical analysis, parsing, semantic analysis
• Theory of computation
• Understand Turing machines
• Understand decidability

5
Course Syllabus

• Lexical analysis, scanners, pattern matching
• Regular expressions, DFAs, NFAs and automata
• Limits on regular expressions, pumping lemma
• Practical parsing, LL and LR parsing
• Context-free languages, grammars, Chomsky
  Hierarchy
• Pushdown automata, deterministic vs.
  non-deterministic
• Attribute grammars, type inferencing
• Context-free vs. context-sensitive grammars
• Decidable vs. Undecidable problems, Turing
  Machines, Halting Problem
• Complexity of computation, classes of languages
  P/NP, space and time completeness
• Future topics Quantum

6
Grading

• Homeworks 25
• Pop-quizzes in class 5
• Mini-project 15
• Midterm 25
• Final 30
• Homeworks to be submitted in class - hardcopy
• No late homework or assignments without prior
  approval of instructor
• Homework should be concise, complete, and precise
• Tests will be in class. Closed book, closed notes

7
Class Policies

• Students must write solutions to assignments
  completely independently
• General discussions are allowed on assignments
  among students, but names of collaborators must
  be reported
• No walking in or out during the lecture
• Cell phones off, silence please
• All absences must be vetted by Dean of Students
  5 credit pop quizzes!

8
Resources

• Class webpage and Swiki
• http//www.cc.gatech.edu/classes/AY2009/cs3240_spr
  ing/
• Newsgroup git.cc.class.cs3240
• Check often for announcements and schedule
  changes.

9
Introduction toCompiler Concepts
10
Compilers

• What is a compiler?
• A program that translates an executable program
  from source language into target language
• Usually source language is high-level language,
  and target language is object (or machine) code
• Related to interpreters
• Why compilers?
• Programming in machine (or assembly) language is
  tedious, error prone, and machine dependent
• Historical note In 1954, IBM started developing
  FORTRAN language and its compiler

11
Why study theory of compiler?

• Besides it is required
• Prerequisite for developing advanced compilers,
  which continues to be active as new computer
  architectures emerge
• Useful to develop software tools that parse
  computer codes or strings
• E.g., editors, debuggers, interpreters,
  preprocessors,
• Important to understand how compliers work to
  program more effectively

12
How Does Compiler Work?
Scanner
Request Token
Get Token
Parser
Start

• Front End Analysis of program syntax and
  semantics

Semantic Action
Semantic Error
Checking
Intermediate Representation
13
Parts of Compilers
Focus of this class.
Analysis
1. Lexical Analysis 2. Syntax Analysis 3.
Semantic Analysis
Front End
Synthesis
4. Code Generation 5. Optimization
Back End
14
The Big Picture

• Parsing Translating code to rules of grammar.
  Building representation of code.
• Scanning Converting input text into stream of
  known objects called tokens. Simplifies parsing
  process.
• Grammar dictates syntactic rules of language
  i.e., how legal sentence could be formed
• Lexical rules of language dictate how legal word
  is formed by concatenating alphabet.

15
Overall Operation

• Parser is in control of the overall operation
• Demands scanner to produce a token
• Scanner reads input file into token buffer
  forms a token (How?)
• Token is returned to parser
• Parser attempts to match the token (How?)
• Failure Syntax Error!
• Success
• Does nothing and returns to get next token, or
• Takes semantic action

16
Overall Operation

• Semantic action look up variable name
• If found okay
• If not put in symbol table
• If semantic checks succeed, do code-generation
  (How?)
• Continue to get next token
• No more tokens? Done!

17
Scanning/Tokenization
Input File
Token Buffer

• What does the Token Buffer contain?
• Token being identified
• Why a two-way ( ) street?
• Characters can be read
• and unread
• Termination of a token

18
Example
main()
m
19
Example
main()
am
20
Example
main()
iam
21
Example
main()
niam
22
Example
main()
(niam
23
Example
main()
niam
Keyword main
24
Parser

• Translating code to rules of a grammar
• Control the overall operation
• Demands scanner to produce a token
• Failure Syntax Error!
• Success
• Does nothing and returns to get next token, or
• Takes semantic action

25
Grammar Rules

• ltC-PROGgt ? MAIN OPENPAR ltPARAMSgt CLOSEPAR
  ltMAIN-BODYgt
• ltPARAMSgt ? NULL
• ltPARAMSgt ? VAR ltVAR-LISTgt
• ltVARLISTgt ? , VAR ltVARLISTgt
• ltVARLISTgt ? NULL
• ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt ltASSIGN-STMTgt
  CURLYCLOSE
• ltDECL-STMTgt ? ltTYPEgt VAR ltVAR-LISTgt
• ltASSIGN-STMTgt ? VAR ltEXPRgt
• ltEXPRgt ? VAR
• ltEXPRgt ? VARltOPgtltEXPRgt
• ltOPgt ?
• ltOPgt ? -
• ltTYPEgt ? INT
• ltTYPEgt ? FLOAT

26
Demo
main() int a,b a b
Scanner
Token Buffer
Parser
27
Demo
main() int a,b a b
Scanner
Token Buffer
"Please, get me the next token"
Parser
28
Demo
main() int a,b a b
Scanner
m
Parser
29
Demo
main() int a,b a b
Scanner
am
Parser
30
Demo
main() int a,b a b
Scanner
iam
Parser
31
Demo
main() int a,b a b
Scanner
niam
Parser
32
Demo
main() int a,b a b
Scanner
(niam
Parser
33
Demo
main() int a,b a b
Scanner
niam
Parser
34
Demo
main() int a,b a b
Scanner
Token Buffer
Token main
Parser
35
Demo
main() int a,b a b
Scanner
Token Buffer
Parser
"I recognize this"
36
Parsing (Matching)

• Start matching using a rule
• When match takes place at certain position, move
  further (get next token repeat)
• If expansion needs to be done, choose appropriate
  rule (How to decide which rule to choose?)
• If no rule found, declare error
• If several rules found, the grammar (set of
  rules) is ambiguous

37
Scanning Parsing Combined
main() int a,b a b
Scanner
"Please, get me the next token"
Parser
38
Scanning Parsing Combined
main() int a,b a b
Scanner
Token MAIN
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt
39
Scanning Parsing Combined
main() int a,b a b
Scanner
"Please, get me the next token"
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt
40
Scanning Parsing Combined
main() int a,b a b
Scanner
Token OPENPAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt
41
Scanning Parsing Combined
main() int a,b a b
Scanner
Token CLOSEPAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltPARAMETERSgt ? NULL
42
Scanning Parsing Combined
main() int a,b a b
Scanner
Token CLOSEPAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltPARAMETERSgt ? NULL
43
Scanning Parsing Combined
main() int a,b a b
Scanner
Token CLOSEPAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt
44
Scanning Parsing Combined
main() int a,b a b
Scanner
Token CURLYOPEN
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE
45
Scanning Parsing Combined
main() int a,b a b
Scanner
Token INT
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltTYPEgt ? INT
46
Scanning Parsing Combined
main() int a,b a b
Scanner
Token INT
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltTYPEgt ? INT
47
Scanning Parsing Combined
main() int a,b a b
Scanner
Token INT
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltTYPEgt ? INT
48
Scanning Parsing Combined
main() int a,b a b
Scanner
Token VAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltVARLISTgt ? , VAR
ltVARLISTgt ltVARLISTgt ? NULL
49
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ',' COMMA
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltVARLISTgt ? , VAR
ltVARLISTgt ltVARLISTgt ? NULL
50
Scanning Parsing Combined
main() int a,b a b
Scanner
Token VAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltVARLISTgt ? , VAR
ltVARLISTgt ltVARLISTgt ? NULL
51
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ''
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltVARLISTgt ? , VAR
ltVARLISTgt ltVARLISTgt ? NULL
52
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ''
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltVARLISTgt ? , VAR
ltVARLISTgt ltVARLISTgt ? NULL
53
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ''
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltVARLISTgt ? , VAR
ltVARLISTgt ltVARLISTgt ? NULL
54
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ''
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt ltVARLISTgt ? , VAR
ltVARLISTgt ltVARLISTgt ? NULL
55
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ''
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt
56
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ''
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltDECL-STMTgt ?
ltTYPEgtVARltVAR-LISTgt
57
Scanning Parsing Combined
main() int a,b a b
Scanner
Token VAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltASSIGN-STMTgt ? VAR
ltEXPRgt ltEXPRgt ? VAR
58
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ''
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltASSIGN-STMTgt ? VAR
ltEXPRgt ltEXPRgt ? VAR
59
Scanning Parsing Combined
main() int a,b a b
Scanner
Token VAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltASSIGN-STMTgt ? VAR
ltEXPRgt ltEXPRgt ? VAR
60
Scanning Parsing Combined
main() int a,b a b
Scanner
Token VAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltASSIGN-STMTgt ? VAR
ltEXPRgt ltEXPRgt ? VAR
61
Scanning Parsing Combined
main() int a,b a b
Scanner
Token VAR
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltASSIGN-STMTgt ? VAR
ltEXPRgt ltEXPRgt ? VAR
62
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ''
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltASSIGN-STMTgt ? VAR
ltEXPRgt
63
Scanning Parsing Combined
main() int a,b a b
Scanner
Token ''
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE ltASSIGN-STMTgt ? VAR
ltEXPRgt
64
Scanning Parsing Combined
main() int a,b a b
Scanner
Token CURLYCLOSE
Parser
ltC-PROGgt ? MAIN OPENPAR ltPARAMETERSgt CLOSEPAR
ltMAIN-BODYgt ltMAIN-BODYgt ? CURLYOPEN ltDECL-STMTgt
ltASSIGN-STMTgt CURLYCLOSE
65
What Is Happening?

• During/after parsing?
• Tokens get gobbled
• Symbol tables
• Variables have attributes
• Declaration attached attributes to variables
• Semantic actions
• What are semantic actions?
• Semantic checks

66
Symbol Table

• int a,b
• Declares a and b
• Within current scope
• Type integer
• Use of a and b now legal

67
Typical Semantic Actions

• Enter variable declaration into symbol table
• Look up variables in symbol table
• Do binding of looked-up variables (scoping rules,
  etc.)
• Do type checking for compatibility
• Keep the semantic context of processing
• a b c ? t1 a b
• t2 t1 c

Semantic Context
68
How Are Semantic Actions Called?

• Action symbols embedded in the grammar
• Each action symbol represents a semantic
  procedure
• These procedures do things and/or return values
• Semantic procedures are called by parser at
  appropriate places during parsing
• Semantic stack implements stores semantic
  records

69
Semantic Actions

• ltdecl-stmtgt ? lttypegtput-typeltvar-listgtdo-decl
• lttypegt ? int float
• ltvar-listgt ? ltvargtadd-decl ltvar-listgt
• ltvar-listgt ? ltvargtadd-decl
• ltvargt ? IDproc-decl
• put-type puts given type on semantic stack
• proc-decl builds decl record for var on stack
• add-decl builds decl-chain
• do-decl traverses chain on semantic stack
  using
• backwards pointers entering each var
  into
• symbol table

decl record
id3
Name
Type
Scope
id2
id1
1
3
do-decl ?
id1
id2
1
3
type
id3
1
3
70
Semantic Actions

• What else can semantic actions do in addition to
  storing and looking up names in a symbol table?
• Two type of semantic actions
• Checking (binding, type compatibility, scoping,
  etc.)
• Translation (generate temporary values, propagate
  them to keep semantic context).

71
Full Compiler Structure
Scanner

• Most compilers have two pass

Parser
Start
Semantic Action
Semantic Error
Code Generation
CODE
72
Summary

• Front-end of compiler scanner and parser
• Translation takes place in back end
• Scanner, parser and code generator are automated
• How? We will answer this question in this class