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CSC 338: Compiler design and implementation

Dr. Mohamed Ben Othman

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Goals:

- Compilers are used everyday in most computers.

- Allow students to implement big programming projects.

- Parts of most projects (specially those containing command language) can be built in the same way compilers are built.

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Introduction

1. Terminology

2. Compilers implementation languages

3. Compiler structure

4. Example

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Terminology

- Compilers: translate programs written in high level languages. The entire program has to be translated before execution (compared to a book translation).

- Interpreters: translate statement by statement (or line by line) and then this statement is given to the CPU to be executed before the next statement in being translated (compared to a man translator).

- High Level Language- Source Language- Source Code- Implementation language- Target machine: the machine to which the translation

will be done. 8086 Machine CodeC CompilerC Program

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How to choose the compiler implementation language

- The first compiler was written in assembly language

- The compiler of Fortran77 programming language is Pascal.

- It is preferable to write the compiler in the source language from which it will translate to machine language.

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Translation operation

The are some compilers do not respect this pattern. - Load-and-go Compilers: gives programs ready for

execution.- Cross-language compilers: compilers between

high level languages.

CompilerSource Code Executable CodeLinkerObject Code

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Compiler is divided into two parts: Front End and Back End.

The Front End translates a program from source language to an intermediate language.

The back End translates from the intermediate language to the machine language.

Compiler structure

Front

End

Source

Code

Machine

Code

Back

End

Intermediate

Object Code

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The Front End

Lexical analyzer

Syntactic analyzerSemantic analyzer

Intermediate code

generator

Source

Program

Tokens

Parse tree

Abstract

Syntax tree

Intermediate

code

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The Back End

Machine Independent optimizer

Machine dependent optimizerObject code Generator

Intermediate

code

Optimized

Intermediate

code

Object code

Optimized

Object code

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Lexical analysis

The lexical analyzer reads the input program as a character stream and produces a stream of lexemes (or token strings) as output.

The lexical analyzer reads the input program character by character until it reads a word (symbol).

The lexical analyzer searches the current work in a table (called symbol table) and adds it if not found. The lexical analyzer produces an output for each symbol called token. Tokens are generally integer numbers.

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Syntactic Analysis

Syntactic Analyzer (or parser): takes as input the Token stream produced by the lexical analyzer.

The parser produces a Parse Tree

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Semantic AnalysisThe semantic analyzer determines if the

meaning is respected in the user program.The syntax rules may be respected without

respecting the meaning.Semantic analysis is mainly to be sure that

data types are used properly. Semantic analysis is part of syntactic

analysis.

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Detecting errors in the source program

In all phases above the main goal is to determine if the program source respects the source language rules.

Example:for (int i $ 1; i<n; i++) x++;

if x > N Y -= 3 else Y += 3;

This error may be detected at lexical analysis

This is a syntax error

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Intermediate code generation

The intermediate code generator produces a code that is not related to the target machine. The intermediate code has to be very close to the and very easy to translate to machine language.

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Object Code

The target code is the machine code.

The machine code generation is not the same as the intermediate code generation.

A assembly language code generation may be done in the same time

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Lexical analysis example: (Pascal program)PROGRAM AverageNumbers(Input, Output);CONST Amount = 3;VAR Average : Real;

x : ARRAY[1..Amount] OF Integer;i, Sum : Integer;

BEGINx[1]:=3;x[2]:=6;x[3]:=10;Sum := 0;FOR i := 1 TO Amount DO Sum := Sum + x[i];Average := Sum/Amount

END. { AverageNumbers }

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Lexical analysis result

PROGRAM ID (ID , ID );CONST ID = NUMLITERAL;VAR ID : ID ;

ID : ARRAY[1 .. ID ] OF ID ; ID, ID : ID ;

BEGINand so on

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Miscellaneous

Symbol Table : contains all keywords and symbols.

Symbol Table Handler: manages the symbol table.Error Handling : gives a clear description of

errors.

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Resume:

Lexical analysis

Source Program

Syntax analysis

Inter. code generation

code optimization

code generation

Error handlingSymbol table

Target program