1. CSC 338: Compiler design and implementation
Dr. Mohamed Ben Othman

2. 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.

3. Introduction Terminology Compilers implementation languages
Compiler structure
Example

4. 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 Code
C Compiler
C Program

5. 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.

6. Translation operation
Compiler
Source Code
Executable Code
Linker
Object Code
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.

7. Compiler structure
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.
Front
End
Source
Code
Machine
Back
Intermediate
Object Code

8. The Front End Intermediate Source code Program Lexical analyzer Tokens
Syntactic analyzer
Semantic analyzer
Intermediate code
generator
Source
Program
Tokens
Parse tree
Abstract
Syntax tree
Intermediate
code

9. The Back End Machine Independent optimizer Machine dependent optimizer
Object code Generator
Intermediate
code
Optimized
Object code

10. 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.

11. Syntactic Analysis
Syntactic Analyzer (or parser): takes as input the Token stream produced by the lexical analyzer.
The parser produces a Parse Tree

12. Semantic Analysis
The 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.

13. 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

14. 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.

15. 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

16. Lexical analysis example: (Pascal program)
PROGRAM AverageNumbers(Input, Output);
CONST Amount = 3;
VAR Average : Real;
x : ARRAY[1..Amount] OF Integer;
i, Sum : Integer;
BEGIN
x[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 }

17. Lexical analysis result
PROGRAM ID (ID , ID );
CONST ID = NUMLITERAL;
VAR ID : ID ;
ID : ARRAY[1 .. ID ] OF ID ;
ID, ID : ID ;
BEGIN
and so on

18. Miscellaneous Symbol Table : contains all keywords and symbols.
Symbol Table Handler: manages the symbol table.
Error Handling : gives a clear description of errors.

19. Resume: Lexical analysis Source Program Syntax analysis
Inter. code generation
code optimization
code generation
Error handling
Symbol table
Target program