Compiler construction

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COMPILER CONSTRUCTION

Jeena Thomas, Asst Professor, CSE, SJCET Palai

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MAIN TOPICS COVERED

» Phases of a compiler» Analysis and synthesis phases

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WHAT IS A COMPILER?

» A compiler is a kind of translator.

TRANSLATORSoftware that accepts text in certain language

(SOURCE LANGUAGE)

Text in another language,preserving the meaning

of text(TARGET/OBJECT

LANGUAGE)

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» A translator, is a generalized form of compiler.» When the object language is a low level language,

such a translator is called a compiler.» This conversion process is essential for the hardware

to interpret and perform the semantics of the input program.

» As an important part of this translation process, the compiler reports to its user the presence of errors in source program.

Error message by compiler


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» Compiler is a program written in source language and translates it into an equivalent target language.

example

» Source code» a=(b+c)*(b+c)*2

Target codeMOV b,R2ADD R2,cMUL R2,R2MUL R2, #2.0MOV R2,a

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The first real compiler

» FORTRAN compilers of the late 1950s» 18 person-years to build

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WHY STUDY COMPILERS?» Writing a compiler gives a student experience with large-scale

applications development. Your compiler program may be the largest program you write as a student. Experience working with really big data structures and complex interactions between algorithms will help you out on your next big programming project.

» Compiler writing is one of the shining triumphs of CS theory. It demonstrates the value of theory over the impulse to just "hack up" a solution.

» Compiler writing is a basic element of programming language research. Many language researchers write compilers for the languages they design.

» Many applications have similar properties to one or more phases of a compiler, and compiler expertise and tools can help an application programmer working on other projects besides compiler

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» Throughout the 1950’s, compilers were considered difficult programs to write.

» The first Fortan compiler took 18 staff-years o implement.

» Good implementation languages, programming environments, and software tool has been developed as the systematic techniques for handling many of important tasks that occur during compilation.

» With these advances, a substantial compiler can be implemented even as a student project in a one-semester compiler-design course.


Compiler technology -applications

» is more broadly applicable and has been employed in rather unexpected areas.

» Text-formatting languages, preprocessor packages

» Silicon compiler for the creation of VLSI circuits» Command languages of OS» Query languages of Database systems

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ISSUES IN COMPILATION» Hierarchy of operations to be maintained -to determine the correct order of evaluation of the expressions. Maintaining data type integrity -each part of complex expression can be made of different types. Compiler as prior knowledge about the nature of

user defined data types. - struct, enum, union, Appropriate storage mappings for data structures - allocation of memory for data


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» The compiler must resolve the occurrence of each

variable name in a program to determine the name space to which a referenced variable belongs to.(Symbol table)

» Compiler should have facilities to handle different control structures like ‘if-then-else’, ‘for’, ‘while’ etc. The compiler should have the facilities to increment the loop variable and terminate the loop.

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PHASES OF COMPILATION

» Process of compilation is highly complex, it is split into a series of subprocesses called phases.

» A phase is a logically cohesive operation that takes as input one representation of source program and produces as output another representation.

» Activities of compilation split into two parts1) Analysis part

2) Synthesis part


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The Structure of a Compiler

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» Analysis of source program» is done by the front end of compiler» It determines meaning of source string.» Synthesis of target program» Is done by the back end of the compiler.» An equivalent target string is constructed from

the output given by the front end of compiler.

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Analysis

» In compiling, analysis has three phases:» Linear analysis: stream of characters read from

left-to-right and grouped into tokens; known as lexical analysis or scanning

» Hierarchical analysis: tokens grouped hierarchically with collective meaning; known as parsing or syntax analysis

» Semantic analysis: check if the program components fit together meaningfully

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Sub-steps of synthesis part

» Optimization of code» Allocation of memory» Generation of code

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PHASES OF A COMPILER


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PHASES OF COMPILATION

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1.LEXICAL ANALYSIS» Performs the linear analysis on the source

program.» It reads a stream of characters making up the

source program from left to right and groups them into tokens.

» A token is defined as a sequence of characters that have a collective meaning.

» For each token identified, this phase also determines the category of the token as identifier, constant or reserved words and its attribute that identifies the symbol’ position in the symbol table

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LEXICAL ANALYZER/SCANNER

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Symbol Table

» Identifiers are names of variables, constants, functions, data types, etc.

» Store information associated with identifiers» Information associated with different types of

identifiers can be different» Information associated with variables are name,

type, address, size (for array), etc.» Information associated with functions are name

, type of return value, parameters, address, etc.

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EXAMPLE » Consider the following statement

» a=(b+c)*(b+c)*2--------------------------------(1)

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Tokens for the expression 1Symbol Category Attribute

a Identifier #1

= operator Assignment(1)b Identifier #2

+ operator Arithmetic(1)

c Identifier #3

* operator Arithmetic(2)

( operator Open parenthesis(1)) operator Closed parenthesis(1)

2 Constant #4

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2.SYNTAX ANALYSIS» This phase performs hierarchical analysis on the

source program.» Here, the tokens are grouped into hierarchically

nested collections with collective meaning called expressions or statements.

» It determines structure of source language.» Represents the grammar / syntax of the language.» These grammatical phrases are represented in the

form of parse tree.

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Parse tree» Describes the syntactic structure of input» The terminal nodes represent the tokens and

interior nodes represent the expressions.

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SYNTAX TREES

» Syntactic structures also represented using syntax trees.

» A syntax tree is a compressed representation of the parse tree, where the operators appear as interior nodes and operands for this operator as their children

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PARSE TREE Vs. SYNTAX TREE» Syntax tree is a compressed representation of a

parse tree.» The interior node in a syntax tree represent an

operator, whereas the interior nodes in a parse tree represent an expression.

» The leaf node of a syntax tree represent the operand, whereas leaf node in a parse tree represent the tokens.

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3.SEMANTIC ANALYSIS

» Goal- is to determine the meaning of a source string.

» It checks the source program for semantic errors and gathers the type of information that can be used in subsequent phases of compilation.

» Type checking for operations also performed during this phase.

» Output- Annotated tree

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4.INTERMEDIATE CODE GENERATION» It is a part of the synthesis process of the

compiler.» The intermediate code is the representation for

an abstract machine.» Using the intermediate code, optimization and

code generation can be performed.

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PROPERTIES OF AN INTERMEDIATE CODE

» It should be easily generated from semantic representation of the source program.

» It should be easy to translate the intermediate code to target language.

» It should be capable of holding the values computed during translation.

» It should maintain precedence ordering of the source language.

» It should be capable of holding the correct number of operands of the instruction.

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example

» T1=intoreal(2)» T2=b+c» T3=b+c» T4=T2*T3» T5=T4*T1» a=T5

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5.CODE OPTIMIZATION» The main aim of this phase is to improve on the

intermediate code to generate a code that runs faster and/or occupies less space.

» It is used to establish trade off between compilation speed and execution speed.

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EXAMPLE

» T1=inttoreal(2)» T2=b+c» T3=T2*T2» T4=T3*T1» a=T4

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6.CODE GENERATION

» The main aim of this phase is to allocate storage and generate a relocatable machine/ assembly code.

» Memory locations and registers are allocated for variables.

» The instructions in intermediate code format are converted into machine instructions.

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example

» MOV R2, b» ADD R2,c» MUL R2,R2» MUL R2, #2.0» MOV R2, a

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7.TARGET CODE OPTIMIZATION

» The compiler also attempts to improve the target code generated by the code generator by choosing proper addressing modes to improve the performance, replacing slow instructions by fast ones and eliminating redundant instructions.

» MUL R2, #2.0-------------------SHL(Shift Left Instruction)

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Example

» MOV b, R2» ADD R2,c» MUL R2,R2» SHL R2» MOV R2, a

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OTHER MODULES INVOLVED

» 1.)Symbol Table Management» 2.) Literal Table Management» 3.) Error Detection and Reporting

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1.SYMBOL TABLE MANAGEMENT

» A symbol table is a data structure that contains a record for each identifier with fields for the attributes of the identifier.

» This data structure has facilities to manipulate(add/delete) the elements in it.

» The type information about the identifier is detected during lexical analysis phase and is entered into the symbol table.

» This information is used during the intermediate code generation and code generation phases of compiler to verify type information.

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SYMBOL TABLE

Address Symbol Attribute Memory Location

1 A id,real 1000

2 B id,real 1100

3 C id,real 1110

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2.LITERAL TABLE MANAGEMENT

» literal table maintains the details of constants and strings used in the program.

» It reduces the size of a program in memory by allowing reuse of constants and strings.

» It is also needed by the code generator to construct symbolic addresses for literals.

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LITERAL TABLE

Address Literal Attribute Memory Location

4 2 const,int 1200

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3.ERROR DETECTION & REPORTING

» Each phase encounters errors.» After detecting the errors, this phase must deal

with the errors to continue with the process of compilation.

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LIST OF ERRORS ENCOUTERED BY VARIOUS PHASES

» 1. Lexical analyzer: Misspelled tokens» 2.Syntax analyzer: syntax errors like missing

parenthesis» 3.Intermediate code generator: Incompatible

operands for an operator» 4. Code Optimizer: Unreachable statements» 5. Code Generator :Memory restrictions to store a

variable. For example, when the value of an integer variable exceeds its size.

» Symbol tables: Multiply declared identifiers

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Exercise

» Show the output of all the phases of he compiler for the following line o code

» A[index]=4+2+index

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The Grouping of Phases

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COMPILER CONSTRUCTION TOOLS

» Scanner generators» Parser generators» Syntax-directed translation engines» Automatic code generators» Data-flow engines

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» Scanner generators: » generate lexical analyzers automatically from

the language specifications written using regular expressions.

» It generates a finite automaton to recognize the regular expression.

» Example-lex

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» parser generators» They produce syntax analyzers from Context

Free Grammar(CFG).» As syntax analysis phase is highly complex and

consumes manual and compilation time, these parser generators are highly useful.

» Example-yacc

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» Syntax-directed translation engines» These engines have routines to traverse the

parse tree and produce intermediate code.» The basic idea is that one or more translations

are associated with each node of parse tree.


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» Automatic code generators» These tools convert the intermediate language

into machine language for the target machine using a collection of rules.

» Template matching process is used.» An intermediate language statement is replaced

by its equivalent machine language statement

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» Data-flow engines» It is used in code optimization.» These tools perform good code optimization

using “data-flow analysis” which gathers information that flows from one part of the program to another.

Date post:	15-Apr-2017
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