Overview of Compilation

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Overview of Compilation
Programming Language Concepts Lecture 2

• Prepared by
• Manuel E. Bermúdez, Ph.D.
• Associate Professor
• University of Florida

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Overview of Translation

• Definition A translator is an algorithm that
  converts source programs into equivalent target
  programs.
• Definition A compiler is a translator whose
  target language is at a lower level than its
  source language.

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Overview of Translation (contd)

• When is one languages level lower than
  anothers?
• Definition An interpreter is an algorithm that
  simulates the execution of programs written in a
  given source language.

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Overview of Translation (contd)

• Definition An implementation of a programming
  language consists of a translator (or compiler)
  for that language, and an interpreter for the
  corresponding target language.

input
Target
Source
Interpreter
Compiler
output
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Translation

• A source program may be translated an arbitrary
  number of times before the target program is
  generated.

Source
Translator1
Translator2
...
TranslatorN
Target
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Translation (contd)

• Each of these translations is called a phase, not
  to be confused with a pass, i.e., a disk dump.
• Q How should a compiler be divided into phases?
• A So that each phase can be easily described by
  some formal model of computation, and so the
  phase can be carried out efficiently.

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Translation (contd)

• Q How is a compiler usually divided?
• A Two major phases, with many possibilities for
  subdivision.
• Phase 1 Analysis (determine correctness)
• Phase 2 Synthesis (produce target code)
• Another criterion
• Phase 1 Syntax (form).
• Phase 2 Semantics (meaning).

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Typical Compiler Breakdown

• Scanning (Lexical analysis).
• Goal Group sequences of characters that occur on
  the source, into logical atomic units called
  tokens.
• Examples of tokens Identifiers, keywords,
  integers, strings, punctuation marks, white
  spaces, end-of-line characters, comments, etc.,

Scanner (Lexical analysis)
Source
Sequence of Tokens
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Lexical Analysis

• Must deal with end-of-line and end-of-file
  characters.
• A preliminary classification of tokens is made.
  For example, both program and Ex are
  classified as Identifier.
• Someone must give unambiguous rules for forming
  tokens.

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Screening

• Goals
• Remove unwanted tokens.
• Classify keywords.
• Merge/simplify tokens.

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Screening

• Keywords recognized.
• White spaces (and comments) discarded.
• The screener acts as an interface between the
  scanner and the next phase, the parser.

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Parsing (Syntax Analysis)

• Goals
• To group together the tokens, into the correct
  syntactic structures, if possible.
• To determine whether the tokens appear in
  patterns that syntactically correct.

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Parsing (Syntax Analysis)

• Syntactic structures
• Expressions
• Statements
• Procedures
• Functions
• Modules
• Methodology
• Use re-write rules (a.k.a. BNF).

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String-To-Tree Transduction

• Goal To build a syntax tree from the sequence
  of rewrite rules. The tree will be the functional
  representation of the source.
• Method Build tree bottom-up, as the rewrite
  rules are emitted. Use a stack of trees.

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Contextual Constraint Analysis

• Goal To analyze static semantics, e.g.,
• Are variables declared before they are used?
• Is there assignment compatibility?
• e.g., a3
• Is there operator type compatibility?
• e.g., a3
• Do actual and formal parameter types match?
• e.g. int f(int n, char c)
• ...
• f('x', 3)
• Enforcement of scope rules.

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Contextual Constraint Analysis

• Method Traverse the tree recursively, deducing
  type information at the bottom, and passing it
  up.
• Make use of a DECLARATION TABLE,
• to record information about names.
• Decorate tree with reference information.

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Example

• Chronologically,
• Enter x into the DCLN table, with its type.
• Check type compatibility for x5.
• X2 not declared!
• Verify type of is boolean.
• Check type compatibility for .
• Check type compatibility between x and int, for
  assignment.

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Code Generation

• Goal Convert syntax tree to target code.
• Target code could be
• Machine language.
• Assembly language.
• Quadruples for a fictional machine
• label
• opcode
• operands (1 or 2)

?
?
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Code Generation

• Example
• pc on UNIX generates assembly code
• pi on UNIX generates code for the p machine,
  which is interpreted by an interpreter.
• pc slow compilation, fast running code.
• pi fast compilation, slow running code.
• Method Traverse the tree again.

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Code (for a stack machine)

• LOAD 5
• STORE X
• LOAD X
• LOAD 10
• BGT
• COND L1 L2
• L1 LOAD X
• LOAD 1
• BADD
• STORE X
• GOTO L3
• L2
• . . .
• L3

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Code Optimization

• Goals
• Reduce the size of the target program.
• Decrease the running time of the target.
• Note Optimization is a misnomer. Code
  improvement would be better.
• Two types of optimization
• Peephole optimization (local).
• Global optimization (improve loops, etc.).

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Code Optimization (contd)

• Example (from previous slide)
• LOAD 5 can be LOAD 5
• STORE X replaced STND X
• LOAD X with

Store non-destructively, i.e., store in X, but do
not destroy value on top of stack.
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Summary
Source
Scanner
Tokens
Screener
Tokens
Error Routines
Table Routines
Parser
Tree
Constrainer
Tree
Code Generator
Code (for an abstract machine)
Interpreter
Input
Output
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Overview of Compilation
Programming Language Concepts Lecture 2

• Prepared by
• Manuel E. Bermúdez, Ph.D.
• Associate Professor
• University of Florida