Introduction to Compilation

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Introduction to Compilation

• Cheng-Chia Chen

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What is a compiler?

• a program that translates an executable program
  in one language into an executable program in
  another language
• the compiler typically lowers the level of
  abstraction of the program
• for optimizing compilers, we also expect the
  program produced to be better, in some way, than
  the original

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Abstract view of compiler

• Implications
• recognize legal (and illegal) programs
• generate correct code
• manage storage of all variables and code
• need format for object (or assembly) code

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Traditional decomposition of a compiler

• Implications
• intermediate language (il)
• front end maps legal code into il
• back end maps il onto target machine
• simplify retargeting

• allows multiple front ends
• multiple phases gt better code
• Front end is O(n) or O(n log n) Back end is
  NP-Complete

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Advantage of the decomposition
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Components of a Compiler

• Analysis
• Lexical Analysis
• Syntax Analysis
• Semantic Analysis
• Synthesis
• Intermediate Code Generation
• Code Optimization
• Code Generation

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The Structure of a Compiler

• Front-end
• Lexical Analysis
• Parsing
• Semantic Analysis
• intermediate code generation
• back-end
• Optimization
• Code Generation
• The first 3, at least, can be understood by
  analogy to how humans comprehend a natural
  language.

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Responsibilities of Frond End

• recognize legal programs
• report errors
• produce il
• preliminary storage map
• shape the code for the back end
• Much of front end construction can be automated

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Responsibilities of Back-end

• code optimization middle-end
• analyzes and changes il
• goal is to reduce runtime
• must preserve values
• code generation
• translate il into target machine code
• choose instructions for each il operation
• decide what to keep in registers at each point
• ensure conformance with system interfaces

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Lexical Analysis

• First step recognize words.
• Smallest unit above letters
• Compiler is an interesting course.
• Note the
• Capital C (start of sentence symbol)
• Blank (word separator)
• Period . (end of sentence symbol)

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More Lexical Analysis

• Lexical analysis is not trivial. Consider
• ????????????
• Programming languages are typically more cryptic
  than English
• h-gtj -12.345e-5

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And More Lexical Analysis

• Lexical analyzer divides program text into
  words or tokens
• if x y then z 1 else z 2
• Units
• if, x, , y, then, z, , 1, , else, z, , 2,

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Parsing (syntax analysis)

• Once words are understood, the next step is to
  understand sentence structure
• Parsing Diagramming Sentences
• The diagram is a tree

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Diagramming a Sentence
VP
sentence
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Parsing Programs

• Parsing program expressions is the same
• Consider
• If x y then z 1 else z 2
• Diagrammed

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Semantic Analysis

• Once sentence structure is understood, we can try
  to understand meaning
• But meaning is too hard for compilers
• Compilers perform limited analysis to catch
  inconsistencies
• Some do more analysis to improve the performance
  of the program

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Semantic Analysis in Natural Language

• Example
• ????????????.
• ???????? ??,?? or ??? ?
• Even worse
• Jack said Jack left his assignment at home?
• How many Jacks are there?
• Which one left the assignment?

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Semantic Analysis in Programming

• Programming languages define strict rules to
  avoid such ambiguities
• This C code prints 4 the inner definition is
  used
• Illegal in Java.

• int x 3
• int x 4
• cout ltlt x

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More Semantic Analysis

• Compilers perform many semantic checks besides
  variable bindings
• Example
• John loves her sister.
• A type mismatch between her and John we know
  they are different people
• Presumably John is male

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Optimization

• No strong counterpart in English, but akin to
  editing
• Automatically modify programs so that they
• Run faster
• Use less memory
• In general, conserve some resource

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Optimization Example

• X Y 0 is the same as X 0
• X Y 2 is the same as X Y Y
• Assume X and Y are integers

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Code Generation

• Produces assembly code (usually)
• A translation into another language
• Analogous to human translation

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Intermediate Languages

• Many compilers perform translations between
  successive intermediate forms
• All but first and last are intermediate languages
  internal to the compiler
• Typically there is 1 IL
• ILs generally ordered in descending level of
  abstraction
• Highest is source
• Lowest is assembly

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Intermediate Languages (Cont.)

• ILs are useful because lower levels require
  exposure of many features hidden by higher levels
• registers
• memory layout
• etc.
• It is hard to obtain all these hidden features
  directly from the source input.

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Example

• source line a bbabs(c-7)
• a sequence of ASCII characters in a text file.
• The scanner groups characters into tokens
• a bbabs(c-7)
• After scanning, we have the token sequence
• Ida Asg Idbb Plus Idabs Lparen Idc
  Minus IntLiteral7 Rparen Semi

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Example

• The parser groups these tokens into parse tree

note (, ) and disappear in the tree.
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• The type checker resolves types and binds
  declarations within scopes

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• Finally, JVM code is generated for each node in
  the tree (leaves first, then roots)
• iload 3 // push local 3 (bb)
• iload 2 // push local 2 (c)
• ldc 7 // Push literal 7
• isub // compute c-7
• invokestatic java/lang/Math/abs(I)I
• iadd // compute bbabs(c-7)
• istore 1 // store result into local 1(a)

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Issues

• Compiling is almost this simple, but there are
  many pitfalls.
• Example How are erroneous programs handled?
• Language design has big impact on compiler
• Determines what is easy and hard to compile
• Course theme many trade-offs in language design

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Compilers Today

• The overall structure of almost every compiler
  adheres to the outline
• The proportions have changed since FORTRAN
• Early lexing, parsing most complex, expensive
• Today optimization dominates all other phases,
  lexing and parsing are cheap

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Applications of Compilation Techniques

• Editor
• Interpreter
• Debugger
• Word Processing (Tex, Word)
• VLSI Design (VHDL, Verilog)
• Pattern Recognition

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Trends in Compilation

• Compilation for speed is less interesting. But
• scientific programs
• advanced processors (Digital Signal Processors,
  advanced speculative architectures)
• Ideas from compilation used for improving code
  reliability
• memory safety
• detecting data races
• ...