Chapter 1.pptx

Chapter one
1. Introduction Compiler

Computer Language Representation
• A computer is a logical assembly of Software and Hardware.
• The hardware understands a language, which humans cannot
understand.
• So we write programs in high-level language, which is easier
for us to understand and remember. These programs are then
fed into a series of tools and OS components to get the desired
code that can be used by the machine. This is known as
Language Processing System.

• The program written in a high-level language is known as a source
program, and the program converted into low-level language is
known as an object (or target) program.
• High-Level Language: If a program contains #define or #include
directives such as #include or #define it is called HLL. These (#)
tags are called preprocessor directives. They direct the pre-processor
about what to do.
• Pre-Processor: The pre-processor removes all the #include
directives by including the files called file inclusion and all the
#define directives using macro(A macro is a piece of code that is
given a name) expansion.

• Assembly Language: It is an intermediate state that is a
combination of machine instructions and some other useful data
needed for execution.
• Relocatable Machine Code: It can be loaded at any point and can
be run. All these files might have been compiled by separate
assemblers.
• Loader/Linker: It converts the relocatable code into absolute code
.Linker loads a variety of object files into a single file to make it
executable. Then loader loads it in memory and executes it.

Computer Language Representation
Translator:
It is a program that translates one language to another.
source code Translator target code
Types of Translator:
1.Interpreter
2.Compiler
3.Assembler
1.Interpreter:
It is one of the translators that translate high level language to low level language.
high level language Interpreter low level language
During execution, it checks line by line for errors.
Example: Basic, Lower version of Pascal.

2.Assembler:
It translates assembly level language to machine code.
assembly language Assembler machine code
Example: Microprocessor 8085, 8086.
3.Compiler:
It is a program that translates one language(source code) to another language (target
code).
source code Compiler target code
It executes the whole program and then displays the errors.
Example: C, C++, COBOL, higher version of Pascal.

PARTS OF COMPILATION
There are 2 parts to compilation:
1. Analysis
2. Synthesis
• Analysis part breaks down the source program into constituent pieces and creates
an intermediate representation of the source program.
• Synthesis part constructs the desired target program from the intermediate
representation.

PHASES OF COMPILER
A Compiler operates in phases, each of which transforms the
source program from one representation into another. The
following are the phases of the compiler:
Main phases:
1) Lexical analysis
2) Syntax analysis
3) Semantic analysis
4) Intermediate code generation
5)Code optimization
6)Code generation

Sub-Phases:
1)Symbol table management 2)Error handling

LEXICAL ANALYSIS:
• It is the first phase of the compiler. It gets input from the source program and
produces tokens as output.
• It reads the characters one by one, starting from left to right and forms the
tokens.
• Token : It represents a logically cohesive sequence of characters such as
keywords, operators, identifiers, special symbols etc.
Example: a +b =20
Here, a,b,+,=,20 are all separate tokens.
• Group of characters forming a token is called the Lexeme.
• The lexical analyzer not only generates a token but also enters the lexeme into
the symbol table if it is not already there.

SYNTAX ANALYSIS:
• It is the second phase of the compiler. It is also known as parser.
• It gets the token stream as input from the lexical analyzer of the compiler and
generates syntax tree as the output.
• Syntax tree:
• It is a tree in which interior nodes are operators and exterior nodes are operands.
Example: For a=b+c*2, syntax tree is

• It is the third phase of the compiler.
• It gets input from the syntax analysis as parse tree and checks whether the
given syntax is correct or not.
• It performs type conversion of all the data types into real data types.
INTERMEDIATE CODE GENERATION:
• It is the fourth phase of the compiler.
• It gets input from the semantic analysis and converts the input into output as
intermediate code such as three-address code.
• The three-address code consists of a sequence of instructions, each of which
has at most three operands.
• Example: t1=t2+t3
SEMANTIC ANALYSIS

CODE OPTIMIZATION
• It is the fifth phase of the compiler.
• It gets the intermediate code as input and produces optimized intermediate code as
output.
• This phase reduces the redundant code and attempts to improve the intermediate
code so that faster-running machine code will result.
• During the code optimization, the result of the program is not affected.
• To improve the code generation, the optimization involves :
- deduction and removal of dead code (unreachable code).
- calculation of constants in expressions and terms.
- collapsing of repeated expression into temporary string.
- loop unrolling.
- moving code outside the loop.
- removal of unwanted temporary variables.

CODE GENERATION
• It is the final phase of the compiler. It gets input from code optimization phase
and produces the target code or object code as result.
• Intermediate instructions are translated into a sequence of machine instructions
that perform the same task.
The code generation involves
- allocation of register and memory
- generation of correct references
- generation of correct data types
- generation of missing code

SYMBOL TABLE MANAGEMENT:
• Symbol table is used to store all the information about
identifiers used in the program.
• It is a data structure containing a record for each
identifier, with fields for the attributes of the identifier.
• It allows to find the record for each identifier quickly and
to store or retrieve data from that record.
• Whenever an identifier is detected in any of the phases, it
is stored in the symbol table.

ERROR HANDLING
• Each phase can encounter errors. After detecting an error, a phase must
handle the error so that compilation can proceed.
• In lexical analysis, errors occur in separation of tokens.
• In syntax analysis, errors occur during construction of syntax tree.
• In semantic analysis, errors occur when the compiler detects constructs
with right syntactic structure but no meaning and during type conversion.
• In code optimization, errors occur when the result is affected by the
optimization.
• In code generation, it shows error when code is missing etc.

• To illustrate the translation of source code through each phase, consider the statement
a=b+c*2. The figure shows the representation of this statement after each phase:

GROUPING OF THE PHASES
• Compiler can be grouped into front and back ends:
- Front end: analysis (machine independent)
• These normally include lexical and syntactic analysis, the
creation of the symbol table, semantic analysis and the
generation of intermediate code. It also includes error handling
that goes along with each of these phases.
- Back end: synthesis (machine dependent)
• It includes code optimization phase and code generation along
with the necessary error handling and symbol table operations.

Compiler passes
• Total number of times the compiler goes through the source
code before converting it into the target machine.
• A collection of phases is done only once (single pass) or
multiple times (multi pass)
• Single pass: a compiler that passes through the source code of
each compilation unit only once.
• Multi pass: a compiler that processes the source code or
abstract syntax tree of a program several times.

Cross compiler
• A cross compiler that runs one platform and generate target
code for other platform.
• A compiler that runs on platform (A) and is capable of
generating executable code for platform (B) is called a cross-
compiler.
• The process of creating executable code for different machines
is called retargeting.

COMPILER CONSTRUCTION TOOLS
These are specialized tools that have been developed for helping implement various
phases of a compiler. The following are the compiler construction tools:
1) Parser Generators:
-These produce syntax analyzers, normally from input that is based on a context-free
grammar.
-It consumes a large fraction of the running time of a compiler. -Example-YACC
(Yet Another Compiler-Compiler).
2) Scanner Generator:
-These generate lexical analyzers, normally from a specification based on regular
expressions. -The basic organization of lexical analyzers is based on finite automation.

3) Syntax-Directed Translation:
-These produce routines that walk the parse tree and as a result generate intermediate
code. -Each translation is defined in terms of translations at its neighbor nodes in the
tree.
4) Automatic Code Generators:
-It takes a collection of rules to translate intermediate language into machine language.
The rules must include sufficient details to handle different possible access methods for
data.
5) Data-Flow Engines:
-It does code optimization using data-flow analysis, that is, the gathering of information
about how values are transmitted from one part of a program to each other part.

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