compiler Lec02

Lecture 2
Compilers
Principle ,Techniques, and Tools
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Compilation concepts
What is compiler?
A compiler is a program that translates a computer
program(source program) written in H.L.L (such as Pascal,C++)
into an equivalent program (target program) written in L.L.L.
Source program compiler target program
Error messages
Model of Compiler:
The task of constructing a compiler for a particular source
language is complex. The complexity of the compilation process
depend on the source language.A compiler must perform two
major tasks:
1. Analysis :deals with the decomposition of the source program
into its basic parts.
2. Synthesis:builds their equivalent object program using these
basic parts.
To perform these tasks, compiler operates in phases each of
which transforms the source program from one representation to
another. A typical decomposition of a compiler is shown in the
following figure.
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Source program
 

Symbol Ter
Target program
Compiler phases
1.Lexical Analyzer: whose purpose is to separate the incoming
source code into small pieces (tokens) , each representing a
single atomic unit of language, for instance "keywords",
"Constant "," Variable name" and "Operators".
2.Syntax Analyzer : whose purpose is to combine the tokens into
well formed expressions (statements) and program and it check
the syntax error
3.Semantic Analyzer: whose function is to determine the
meaning of the source program.
4.Intermediate Code Generator: at this point an internal form
of a program is usually created.For example:
Y=(a+b)*(c+b)
(+,a,b,t1)
(+,c,d,t2)
(*,t1,t2,t3)
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5.Code Optimizer :Its purpose is to produce a more efficient
object program (Run faster or take less space or both)
6.Code Generator: Finally, the transformed intermediate
representation is translated into the target language.
The grouping of phases : the phases of compiler are collection
into :
1. Front-End :It consists of those phases that depend on the
source language and are largely independent of the target
machine ,those include : (lexical analysis ,syntax analysis ,
semantic analysis, and intermediate code generation )
2. Back-End : Includes those phases of compiler that depend on
the target machine and not depend on the source language .
these include:( code optimization phase and code
generation phase )
The grouping of Compiler Phases
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Symbol–Table Management: An essential function of a compiler
is to record the identifiers used in the source program and collect
information about various attributes of each identifier .These
attributes may provide information about the storage allocated for
an identifier , its type and in case of procedure , the number and
types of its arguments and so on .
Symbol-Table is a data structure containing a record for each
identifier , with fields for the attributes of the identifier.
Error Detection and Reporting
Each phase can encounter errors. However ,after defection an
error a phase must somehow deal with that error, so the
compilation can proceed, allowing further errors in the source
program to be detected .A compiler that stops where it finds the
first error is not as helpful as it could be.
The syntax and semantic analysis phases usually handle a large
fraction of the error detectable by the compiler .
 

Syntax errors: The syntax analysis phase can detect errors
Errors where the token stream violates the structure rules (syntax)
of the language .
Semantic errors: During semantic analysis the compiler tries to
detect constructs that have the right syntactic structure but no
meaning to the operation involved, e.g. to add two identifiers, one
of which is the name of an array, and the other the name of a
procedure .
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Where errors show themselves
Compile-time errors
Many errors are detected by the compiler, the compiler will
generate an error message - Most compiler errors have a file name ,
line number, and Type of error. This tells you where the error was
detected .
File name (fname.java)
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Runtime Errors
Runtime errors occur while the program is running, although the
compilation is successful. The causes of Runtime Errors are [5]:
1) Errors that only become apparent during the course of execution of the
program
2) External Factors – e.g.
Out of memory
Hard disk full
Insufficient i/o privileges
etc.
3) Internal Factors – e.g.
Arithmetic errors
Attempts to read beyond the end of a file
Attempt to open a non-existent file
Attempts to read beyond the end of an array
etc.
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A Language- Processing System :-
In addition to a compiler ,several other programs may be required
to create an executable target program.
(Language- Processing System)
Preprocessing : During this stage , comments ,macros and
directives are processed :
· Comments are removed from the source file.
· Macros :If the language supports macros ,the macros are
replaced with the equivalent text,Example:
# define pi 3.14
When the preprocessor encounter the word(pi) it would replace (pi)
with ( 3.14 )
Source program
Loader / Link editor
Assembler
Compiler
Preprocessor
Machine code
Assembly program
Machine program
Library
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Actions Speak Louder than Words
· Directives : The preprocessor also handles directives. In 'C'
language , including statement looks like:
# include<"file">
this line is replaced by the actual file.
Loader - Link Editor : Is program that performs two functions:
1. Loading :taking relocatable machine code and placed the
altered instructions and data in memory at the proper
locations.
2. Link-Editing :Allows us to make a single program from
several files . these files may have been result of different
compilers and one or more may be library files.
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