Compilers

1
Compilers

• Book Crafting a Compiler with C
• Author Charles N. Fischer and Richard J.
  LeBlanc, Jr.
• The Benjamin/Cumming Publishing Company, Inc

2
Gain Score

• Homework 10
• Project 40 (Two members in a team)
• Lexical analysis 10
• Syntax analysis 20
• Code generation 10
• Mid Exam 25
• Final Exam 25

3
Contents

• Introduction
• A Simple Compiler
• Scanning Theory and Practice
• Grammars and Parsing
• LL(1) Parsing
• LR Parsing
• Semantic Processing
• Symbol Tables
• Run-time Storage Organization

4
Contents (Contd.)

• Processing Declarations
• Processing Expressions and Data Structure
  References
• Translating Control Structure
• Translating Procedures and Functions
• Attribute Grammars and Multipass Translation
• Code Generation and Local Code Optimization
• Global Optimization
• Parsing in the Real World

5
Chapter 1 Introduction
6
Contents

• Overview and History
• What Do Compilers Do?
• The Structure of a Compiler
• The Syntax and Semantics of Programming Languages
• Compiler Design and Programming Language Design
• Compiler Classifications
• Influences on Computer Design

7
Overview and History

• Compilers are fundamental to modern computing.
• They act as translators, transforming
  human-oriented programming languages into
  computer-oriented machine languages.

Programming Language (Source)
Machine Language (Target)
Compiler
8
Overview and History (Contd.)

• The first real compiler
• FORTRAN compilers of the late 1950s
• 18 person-years to build
• Today, we can build a simple compiler in a few
  month.
• Crafting an efficient and reliable compiler is
  still challenging.

9
Overview and History (Contd.)

• Compiler technology is more broadly applicable
  and has been employed in rather unexpected areas.
• Text-formatting languages, like nroff and troff
  preprocessor packages like eqn, tbl, pic
• Silicon compiler for the creation of VLSI
  circuits
• Command languages of OS
• Query languages of Database systems

10
What Do Compilers Do?

• Compilers may be distinguished according to the
  kind of target code they generate
• Pure Machine Code
• Assume there is no run-time OS support.
• For systems implementation or embedded systems
• Run on bare machines
• Augmented Machine Code
• For hardware OS language-specific support
  routines, e.g., I/O, math functions, storage
  allocation, and data transfer.
• Virtual Machine Code
• JVM, P-code
• Portable
• 4-times slower
• Code is interpreted.

11
What Do Compilers Do? (Contd.)

• Another way that compilers differ from one
  another is in the format of the target machine
  code they generate
• Assembly Language Format
• Simplify compilation
• Use symbolic labels rather than calculating
  address
• Pro good for smaller machines
• Con need an additional pass

12
What Do Compilers Do? (Contd.)

• Relocatable Binary Format
• A linkage step is required
• Similar to the output of assembler
• Need a linking step before execution
• Good for modular compilation, cross-language
  references, and libraries
• Memory-Image (Load-and-Go) Format
• Fast
• Very limited linking capabilities
• Good for debugging (frequent changes)

13
What Do Compilers Do? (Contd.)

• Another kind of language processor, called an
  interpreter, differs from a compiler in that it
  executes programs without explicitly performing a
  translation
• Advantages and Disadvantages of an interpreter
• See page 6 7

Output
Interpreter
Source Program Encoding
Data
14
What Do Compilers Do? (Contd.)

• Advantage
• Modification to program during execution
• Interactive debugging
• Not for every language, e.g., Basic, Pascal
• Dynamic-typed languages
• Variable types may change at run time, e.g.,
  LISP.
• Difficult to compile
• Better diagnostics
• Source code is available.
• Machine independence
• However, the interpreter itself must be portable.

15
What Do Compilers Do? (Contd.)

• Disadvantage
• Slower execution due to repeated examination
• Dynamic (LISP) 1001
• Static (BASIC) 101
• Substantial space overhead

16
The Structure of a Compiler

• Modern compilers are syntax-directed
• Compilation is driven the syntactic structure of
  programs i.e., actions are associated with the
  structures.
• Any compiler must perform two major tasks
• Analysis of the source program
• Synthesis of a machine-language program

17
The Structure of a Compiler (Contd.)
Source Program
Syntactic
Tokens
Scanner
Parser
Semantic Routines
Structure
(Character Stream)
Intermediate Representation
Optimizer
Symbol and Attribute Tables
(Used by all Phases of The Compiler)
Code Generator
The structure of a Syntax-Directed Compiler
Target Machine Code
18
The Structure of a Compiler (Contd.)

• Scanner
• The scanner begins the analysis of the source
  program by reading the input, character by
  character, and grouping characters into
  individual words and symbols (tokens)
• The tokens are encoded and then are fed to the
  parser for syntactic analysis
• For details, see the bottom of page 8.
• Scanner generators

finite automata as programs
regular exp for tokens
lex or scangen
19
The Structure of a Compiler (Contd.)

• Parser
• Given a formal syntax specification (typically as
  a context-free grammar CFG), the parse reads
  tokens and groups them into units as specified by
  the productions of the CFG being used.
• While parsing, the parser verifies correct
  syntax, and if a syntax error is found, it issues
  a suitable diagnostic.
• As syntactic structure is recognized, the parser
  either calls corresponding semantic routines
  directly or builds a syntax tree.

yacc or llgen
grammar
parser
20
The Structure of a Compiler (Contd.)

• Semantic Routines
• Perform two functions
• Check the static semantics of each construct
• Do the actual translation for generating IR
• The heart of a compiler
• Optimizer
• The IR code generated by the semantic routines is
  analyzed and transformed into functionally
  equivalent but improved IR code.
• This phase can be very complex and slow
• Peephole optimization

21
The Structure of a Compiler (Contd.)

• One-pass compiler
• No optimization is required
• To merge code generation with semantic routines
  and eliminate the use of an IR
• Retargetable compiler
• Many machine description files, e.g., gcc
• Match IR against target machine patterns.
• Compiler writing tools
• Compiler generators or compiler-compilers
• Lex and Yacc
• E.g., scanner and parser generators

22
Compiler Design and Programming Language Design

• An interesting aspect is how programming language
  design and compiler design influence one another.
• Programming languages that are easy to compiler
  have many advantages
• See the 2nd paragraph of page 16.

23
Compiler Design and Programming Language Design
(Contd.)

• Languages such as Snobol and APL are usually
  considered noncompilable
• What attributes must be found in a programming
  language to allow compilation?
• Can the scope and binding of each identifier
  reference be determined before execution begins
• Can the type of object be determined before
  execution begins?
• Can existing program text be changed or added to
  during execution?

24
Compiler Classifications

• Diagnostic compilers
• Report and repair compile-time errors.
• Add run-time checks, e.g., array subscripts.
• should be used in real world.
• vs. production compiler
• Optimizing compilers
• Re-targetable compiler
• Localize machine dependence.
• difficult to implement
• less efficient object code
• Integrated programming environments
• integrated E-C-D