Compilers

Introduction

Alex Aiken

Intro to Compilers

Main Text Book:Compilers: Principles, Techniques & Tools, 2nd ed.,Aho, Lam, Sethi, and Ullman, 2007

Evaluation:Midterm Exam 32.5%Final Exam 32.5%Assignments and Quizzes 15%Project 20%

Intro to Compilers

• Compilers

• Interpreters

• a program that translates an executable program in a source language (usually high level)into an equivalent executable program in a target language(usually low level)

• a program that reads an executable program and produces the results of running that program

• usually, this involves executing the source program in some fashion

Intro to Compilers

• Compilers

CompilerSource

Program

Target

Program

Errors

Target ProgramInput Output

Intro to Compilers

• Interpreters

Interpreter

Input

Output

Source

Program

Translates line by line

Executes each translated line immediately

Execution is slower because translation is repeated

But, usually give better error diagnostics than a compiler

Intro to Compilers

• 1954 IBM develops the 704

– Successor to the 701

– customers found that the software costs exceeded the

hardware costs.

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Intro to Compilers

• “Speedcoding”

JohnBackus

Image used with permission of The History of Computing Project(www.thocp.net)Alex Aiken

– an early example of an interpreter

– developed in 1953 by John Backus

– much faster way of developing programs

– programs were ten to twenty times slower

Intro to Compilers

• FORTRAN I

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– FORmula TRANslation Project

– FORTRAN ran from 1954 To1957

– By 1958, over 50 percent of all of programs

were in FORTRAN

JohnBackus

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Intro to Compilers

• The first compiler

– Huge impact on computer science

• Led to an enormous body of theoreticalwork

• Modern compilers preserve the outline of FORTRAN I

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Intro to Compilers

1. LexicalAnalysis

2. Parsing

3. SemanticAnalysis

4. Optimization

5. Code Generation

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Intro to Compilers

• First step: recognize words.

– Smallest unit aboveletters

This is a sentence.

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Intro to Compilers

ist his ase nte nce

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Intro to Compilers

• Lexical analysis divides program text into “words” or

“tokens”

if x == y then z = 1; else z = 2;

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Intro to Compilers

• Once words are understood, the next step is tounderstand sentence structure

• Parsing = Diagramming Sentences

– The diagram is a tree

Intro to Compilers

This sentenceline is a longer

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Intro to Compilers

This sentence

article

line is

noun verb

a longer

article adjective noun

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Intro to Compilers

This sentence

article

line is

noun verb

a longer

article adjective noun

subject object

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Intro to Compilers

This sentence

article

line is

noun verb

a longer

article adjective noun

subject object

sentence

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Intro to Compilers

if x == y then z = 1; else z = 2;

if-then-elseAlex Aiken

Intro to Compilers

predicate else-stmtthen-stmt

if-then-elseAlex Aiken

if x == y then z = 1; else z = 2;

Intro to Compilers

x == y

relation

predicate

z 2

assign

else-stmt

z 1

assign

then-stmt

if-then-elseAlex Aiken

if x == y then z = 1; else z = 2;

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Intro to Compilers

• Once sentence structure is understood, we can try to

understand “meaning”

– This is hard!

• Compilers perform limited semantic analysis to catch

inconsistencies

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Intro to Compilers

• Example:

Jack said Jerry left his assignment at home.

• Even worse:

Jack said Jack left his assignment at home?

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Intro to Compilers

• Programming languages

define strict rules to

avoid such ambiguities

{

int Jack = 3 ;

{

int Jack = 4;

cout << Jack;

}

}

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Intro to Compilers

• Compilers perform many semantic checks besides variable bindings

• Example:

Jack left her homework at home.

• A “type mismatch” between her and Jack; we know they are different people

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Intro to Compilers

• Optimization has no strong counterpart in English

– But a little bit likeediting

– But akin to editing

• Automatically modify programs so that they– Runfaster

– Use less memory

– Use less power

– Use less network messages, database accesses, etc.

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Intro to Compilers

X = Y *0 is the same as X =0

NOT ALWAYS CORRECT

NaN

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Intro to Compilers

• Produces assembly code (usually)

• A translation into another language

– Analogous to humantranslation

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Intro to Compilers

• The overall structure of almost everycompiler

adheres to our outline

• The proportions have changed since FORTRAN

L P S O C

L P S O C

Intro to Compilers

Intro to Compilers

Errors

position = initial + rate * 60

lexical analyzer

syntax analyzer

semantic analyzer

intermediate code generator

<id, 1> < = > <id, 2> < + > <id, 3> < * > < 60 >

:=

<id, 1><id, 2>

<id, 3>

+

*

60

:=

<id, 1>

<id, 2>

<id, 3>

+

*

inttofloat

60

Symbol Table

1 position ....

2 initial ….

3 rate….

Intro to Compilers

Errors

intermediate code generator

code optimizer

final code generator

t1 := inttofloat(60)

t2 := id3 * t1

t3 := id2 + t2

id1 := t3

t1 := id3 * 60.0

id1 := id2 + t1

LDF R2, id3

MULF R2, R2, #60.0

LDF R1, id2

ADDF R1, R1, R2

STF id1, R1

1 position ....

2 initial ….

3 rate….

Symbol Table

3 address codes

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Economy

• Why are there so many programming languages?

• Why are there new programminglanguages?

• What is a good programminglanguage?

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Economy

Why are there so many programming languages?

Application domains have distinctive/conflictingneeds

o Scientific computing

• good floating point support

• good arrays

• parallelism

o Business applications

• persistence

• report generation

• data analysis

o Systems programming

• control of resources

• Real time constraints

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Economy

Why are there new programminglanguages?

Claim:Programmertraining is thedominantcost fora programming language.

o widely used languages are slow to change

o it's easy to start a new language

o new languages are born to fill a void

New languages tend to look like old languages

o Java vsC++

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Economy

What is a goodprogramming language?

o There is NO universally accepted metric for language

• Popularity? VB was once a very popular language

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Economy

Application domains have conflictingneeds.

Programmer training isthe dominant cost for a

programming language.