compiler - introd

8/2/2019 Compiler - Introd

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COMPILERS

Module 3 begins..



Compiler

Introduction to Compilers

Translator

A translator is a program that takes a program writtenin one programming language as input and produces aprogram in another language as output. If the source

language is a high level language and the object language is alow level language , then such a translator is called acompiler.

Source

Program

Object

Program



Structure of compiler

Lexical Analysis

Tablemanagement

Errorhandling

Codegeneration

Code

optimization

Intermediatecode generation

S yntax Analysis

Source program

Target program (Fig) Phases of compiler



Passes- Portions of one or more phases

- R eads the source program or o/p of previous pass,make transformation and writes o/p into anintermediate file, which then be read by a subsequentpass

Number of passes and grouping of phases, depends

Programming language & machine

Structure of source language Environment where compiler works



Compilers are some times classified into multi pass,Single pass, Load and go, debugging or optimizing compilerdepending on

How they have been constructed

Or what functions they have are supposed to perform

There are two part of compilation

Analysis and S ynthesis

Analysis part breaks the source program into cnstituent piecesand creates intermediate representation of source programThe synthesis part constructs the desired target program from

the intermediate code



Lexical Analysis

- token formation ± interface between the sourceprogram and the compiler

Consider the statement,

IF (5 .EQ. Max) GOTO 100

Here the tokens are IF; (; 5; EQ; MAX; ); GOTO; 100.



Specific strings

-eg: if, semicolon etc

Tokens

Class of strings ± eg: keywords, constants,labels

Worked with parser as a co routine or under theparser

Finding of tokens



The token stream for the above statement is

IF ( [const,341] EQ [id,729] ) GOTO [label,554]

Constant, integer, value=5

variable, integer, value= MAX

Label, value= 100

341

554

729

.

.

.

.

.

.

Fig : S ymbol table



S yntax Analysis

Functions

Checking the syntax (eg: A+/B )

imposes on tokens a tree like structure

(i) eg: A/B*C

it has two interpretations

a) divide A by B and then multiply by C

b) multiply B by C and then use the result to divide A



Each of these interpretations can be represented

by a parse tree which is a tree representation thatexhibits the syntactic structure of the expression.

The rules for a programming language form thesyntactic specification of the language for whichcontext free grammars are used.



expression

expression

expressionexpression

A

expression

/ B * C

(a)

expression

expression

A / B

(b)

expression

expression

* C

expression



Intermediate Code Generation

Popularly used method is Three address code

So, T1 := A/B;

T2 := T1 *C;Eg : Consider While A>B & A<=2*B-5 do

A := A + B

Token form,

While [id,n1] > [id,n2] & [id,n1] <= [const,n3] *

[id,n2] - [const,n4] do [id,n1] := [id,n1] + [id,n2] ;



Statement

while

While statement

Id (A)

do

> <=

&

exp

relation

statementcondition

condition condition

relation

exp expexp reloprelop

Id (B)Const(2)

*

-Id (A)Id (B) expexp

exp exp Const(5)

assignment

location :=

Id(A)

+ expexp

exp

Id(B)

Id(A)



Intermediate Code

L1: if A> B goto L2goto L3

L2: T1 := 2*BT2 := T1 ± 5If A <= T2 goto L4goto L3

L4: A:= A + Bgoto L1

L3 :Intermediate code for while stmt



Code Optimization

This phase tries to apply transformations to the o/pof the intermediate code generatior, in an attempt toproduce a more improved intermediate languagerepresentation

L1: if A> B goto L2goto L3

L2: T1 := 2*BT2 := T1 ± 5

If A <= T2 goto L4goto L3

L4: A:= A + Bgoto L1

L3 :Intermediate code for while stmt



Optimization Techniques

Local Optimization

If A>B goto L2Goto L3

L2:

It can be replaced by If A <= B goto L3

Elimination of common subexpressions

A:= B+C+D

E:= B+C+F can be evaluated as

T1 := B+C;

A := T1 +D



Loop Optimization

It move a computation that produces the same resulteach time around the loop, to a point in the program just

before the loop is entered. Such a computation is calledLoop Invariant.

Code generationThis phase converts the intermediate code ito a

sequence of m/c insructions.

A simple code generator may map the statement A:= B + C

into the m/c code sequence

Load B

Add C

Store A



Book keeping

Collects all the information about the data objects that

appear in the source program.

Error Handling

One of the most important functions of a compiler isthe detection and reporting of errors in the sourceprogram.



Errors can be encountered by all of the phases of thecompiler.

For eg,

L A ma y be una ble to proceed b¶se of the next tokenis misspelled

S A m

a y un

a ble to infer

astructure for its i/p b¶se

aof

a syntactic error has occurred««

compiler - introd

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