Introduction to Compilers

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Introduction to Compilers
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Related Area

• Programming languages
• Machine architecture
• Language theory
• Algorithms
• Data structures
• Operating systems
• Software engineering

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Compilers

• A compiler is a program that reads a program
  written in one language the source language
  and
• translate it into an equivalent program in
  another language - the target language.
• Early compilers - 1950s

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Machine Language, AssemblyLanguage, High-Level
Language

• Machine language is the native language of the
  computer on which the program is run.
• Native code
• It consists of bit strings which are interpreted
  by the mechanism inside the computer.
• Example in IBM 370
• Binary 0001100000110101
• Hexadecimal 1835
• Copy the content of Register 5 into Register 3
• LR 3, 5
• Assembler, assembly language

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Machine Language, AssemblyLanguage, High-Level
Language

• Example
• High-level language
• X Y Z
• Assembly language
• L 3, Y Load the working register with Y
• A 3, Z Add Z
• ST 3, X Store the result in X

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Terminology

• Source language
• Java, C, C
• Object language
• Machine language
• Object code
• Object file, object module
• Target machine
• The computer on which the program is to be run

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Terminology

• Cross compiler
• A compiler that generates code for a machine that
  is different from the machine on which the
  compiler runs.
• Example
• A compiler which can be run on a IBM PC but which
  compiles to the machine language of a
  special-purpose embedded system.

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Compilers and Interpreters

• Compiler
• Translates the high-level program to the target
  program.
• Interpreter
• Executes the program.

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The Environment of theCompiler
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The Environment of theCompiler

• Example
• COMP myprog Compiles the program
• LINK myprog Links the program
• RUN myprog Runs the program

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Phases of a Compiler

• Five (six) phases of compilation
• Lexical analysis
• Syntactic analysis
• (Semantic analysis)
• Intermediate code generation
• Optimization
• Object code generation

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Phases of a CompilerLanguage Processing System
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Phases of a Compiler
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Two Parts of Compilation

• Analysis
• breaks up the source program
• creates an intermediate representation
• Synthesis
• constructs the desired target program from the
  intermediate representation

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Analysis

• Lexical Analysis
• linear analysis, scanning
• Syntax Analysis
• parsing, hierarchical analysis
• Semantic Analysis
• Intermediate Code Generation
• Advantage of dividing analysis
• simple design
• compiler efficiency
• compiler portability

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Analysis of the SourceProgram

• Lexical analysis (linear analysis)
• the streams of characters making up the source
  program is read from left-to-right and grouped
  into tokens
• Syntax analysis (hierarchical analysis)
• characters or tokens are grouped hierarchically
  into nested collections with collective meaning
• Semantic analysis
• certain checks are performed to ensure that the
  components of a program fit together meaningfully

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Lexical Analysis

• Linear analysis, scanning
• Reads the stream of characters in the source
  program from left to right, and groups into
  tokens
• Tokens
• are sequences of characters having a collective
  meaning

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Example Lexical Analysis

• position initial rate 60
• id1 id2 id3 60

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Syntax Analysis

• Parsing
• Hierarchical analysis
• Groups the tokens of the source program into
  grammatical phrases represented by parse tree
  that are used by the compiler to synthesize
  output.

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Example Syntax Analysis
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Semantic Analysis

• Checks the source program for semantic errors and
  gathers type or semantic information for the
  subsequent code generation phase

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Example Semantic Analysis
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Error Handler

• When each phases of compilation encounters error,
  a phase must somehow deal with that error.
• Error in Lexical Phase
• The characters in the input do not form any token
  of the language.
• Error in Syntax Phase
• The token stream violates the structure rules
  (syntax) of the language.
• Error in Semantic Phase
• Constructs have the right syntactic structure,
  but no meaning to the operation involved.

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S/W ToolsPerforming Analysis

• Structure editors
• a sequence of command gt a source program
• Pretty printers
• indentation, fonts
• Static checkers
• a program gt discover bugs without run
• Interpreters
• performing operations

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Performing Analysis

• Text formatters
• typeset text
• Silicon compilers
• circuit design
• Query interpreters
• DB

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Intermediate Code Generation

• Explicit intermediate representation
• A program for an abstract machine
• Two properties of intermediate code
• Easy to produce
• Easy to translate into the target program
• Intermediate form
• Three address form (quadruples, triples)
• Two address form

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Three-Address Code

• Has at most three operands
• Each three-address instruction has at most one
  operator in addition to the assignment
• The compiler must generate a temporary name to
  hold the value computed by each instruction
• May have fewer than 3 operands

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Example Three-Address Code
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ExampleIntermediate Code
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Synthesis Part

• The synthesis part constructs the desired target
  program from the intermediate representation.

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Code Optimization

• Improve the intermediate code to get the
  fast-running machine code
• Optimizing compiler

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Example Code Optimization
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Code Generation

• Generates the target codes
• re-locatable machine code
• assembly code

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Example Code Generation
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System Support

• There is a certain amount of supporting code to
  be supplied to the compilation.
• Symbol table management
• Error handling

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System Support

• Symbol table handler
• The central repository of information about the
  names or identifiers in the program
• Error handling
• Implements the compilers response to errors in
  the code it is compiling.
• Diagnostics
• Where the error was found and what kind of error
  it was

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Passes, Front End, Back End

• The compiler makes one or more passes through the
  program.
• A pass consists of reading a version of the
  program from a file and writing a new version of
  it to an output file.
• A pass normally comprises more than one phase,
  but the number of passes, and the phases they
  cover, varies.

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Passes, Front End, Back End

• Front End
• Dependent on the source language and have little
  or no concern with the target machine
• Lexical analysis
• (Semantic analysis)
• Intermediate code generation
• Back End
• Machine-dependent
• Code optimization
• Target code generation

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Writing a Compiler

• The first compiler was written in assembly
  language there was no other alternative.
• High-level language compilers
• Cross compiler
• Useful tools compiler compilers
• Lex
• Yacc

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Retargetable Compilers

• In many cases, a compiler writer will want to
  adapt a compiler for use with a new target
• A compiler that can be modified in this way is
  said to be retargetable.
• Cross compiler
• Alternative approaches
• Distinction between Front End and Back End
• Compiler for imaginary machine (virtual machine)

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Cousins of the Compiler

• Preprocessors
• Assemblers
• Loaders and Link-Editors

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Preprocessor

• A preprocessor is a simple translator that is
  applied to the source program before it is
  submitted to the compiler.
• Before the program is compiled it is passed
  through the preprocessor, which replaces all
  occurrences of the pre-defined expression with
  the defined sequence of instructions.
• Example

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Functions of Preprocessors

• Macro processing
• File inclusion
• Language extensions
• DB query languages embedded in highlevel
  languages

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Example Preprocessors

• The C Programming Language

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Assemblers

• Assemblers
• Two-pass assembly
• Loaders and Link-Editors

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Assemblers

• Assembly code
• a mnemonic version of machine code, in which
• names are used instead of binary codes for
  operations, and
• names are also given to memory addresses

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Two-Pass Assembly(1)

• In the first pass,
• All the identifiers that denote storage locations
  are found and stored in a symbol table
• Identifiers are assigned storage locations as
  they are encountered for the first time
• Example b a 2

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Two-Pass Assembly (2)

• In the second pass,
• The assembler scans the input again.
• It translates each operation code into the
  sequence of bits representing that operation in
  machine language
• It translates each identifier representing a
  location into the address given for that
  identifier in the symbol table
• The output of the second pass is usually
  relocatable machine code

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Example Relocatable Addresses

• Altering the relocatable address to absolute or
  unrelocatable machine code
• Suppose that the address space containing the
  data is to be loaded starting at location L
  00001111
• L must be added to the address of the instruction

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Loaders and Link-Editors

• Loader
• Performs the two functions of loading and
  link-editing
• Loading
• Consists of taking relocatable machine code,
• altering the relocatable addresses, and
• placing the altered instructions and data in
  memory at the proper locations

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Link-Editors

• Link-editor
• Allows us to make a single program from several
  files of relocatable machine code.
• These files may have been the result of several
  different compilations, and
• One or more may be library files.
• External references
• In which the code of one file refers to a
  location in another file.

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Summary

• A quick overall picture of
• what a compiler does,
• what goes into it, and
• how it is organized