Week 1 – Lecture 1

1. Week 1 – Lecture 1 Compiler Construction • Introduction • The Textbook • Assessment • Programming & Tools • A v. small compiler

2. The Big Picture • In this course we will be constructing a compiler! • Moving from a High Level Language to a Low Level Language • Compilers are complex programs • > 10,000 lines of code • Integrate aspects from many different areas of CS • Formal language theory, algorithms, data structures, HLL & LLL (obviously), user interaction (error reporting)

3. L1 L2 Source Target What is a compiler? • A specialization of a language translator • Usually in CS: • the Source is a high level programming language • the Target is a machine code for a micro-processor C x86 processor

4. Applications of Compiler Techniques • Potential Source languages include: • Natural languages (English, French,….) • Circuit layout languages • Mark-up languages (HTML, XML, …) • Command line languages (SQL interface) • Potential Target languages include: • Natural languages • Printer drivers • Markup languages • e.g. HTML to RTF converter • Could involve many of the aspects we will cover in compiler construction

5. Compilers for Programming Languages • If we had 1 compiler for each {Source,Target} pair then we would have a lot of compilers! Source Languages Target Languages Pascal x86 (MMX) Sather C++ ARM C C# AMD K6 Java SPARC Compilers Prolog Fortran PowerPC 750 (G3) Haskell Lisp JVM

6. Source Modularity for Code Generation Compilers x86 ARM G4 Intermediate Representation  Compiler portability (man gcc – lists different target machines)

7. Modularity for Source Languages? Targets Sources Compilers C Java Prolog Intermediate Representation Typically compilers only compile one source language – but the techniques used are very similar and are shared across different compilers

8. Typical Compiler Front-end  Analysis Back-end  Synthesis Independent of Source and Target languages Intermediate Representation Source Target course now week12 Ideally: For a new Source language – we can add a new front-end to an existing back-end For a new Target language – we can add a new back-end to an existing front-end

9. Front End • Knowledge about the source language • Lexical structure (tokens) • Syntax • Programming constructs • Conditionals, iteration etc • Semantics • Type checking • Error-reporting • UI component • Often basic (and unhelpful!) • May vary if part of an IDE or standalone Source program Lexical analyser Symbol table Syntax analyser Error Handler Semantic analyser

10. Back-end • Knowledge about target processor / virtual machine • Instruction set • ‘costs’ of different: • op-codes • instructions • Registers • Memory Semantic analyser Intermediate code generator Symbol table manager Code optimiser Error handler Code generator

11. Putting it together Compiler A language-processing system Source program Skeletal source program Lexical analyser preprocessor Syntax analyser Source program compiler Semantic analyser Error Handler Symbol table Target asse mbly program assembler Intermediate code generator Relocatable machine code Loader link-editor Code optimiser Code generator Absolute machine code

12. The Textbook Compilers: principles, techniques & tools Aho, Sethi & Ullman Addison-Wesley {‘The Dragon Book’}

13. Assessment • Building a compiler for a new language • Front-end • Lexical analysis • Parsing • Back end • Generating assembler code • Some formal and some practical • Formal more at the front-end

14. Programming & Tools • Lexical analysis generator – lex / flex • Parser generator – yacc / bison • C / C++ • To implement the remainder of the compiler • Unix environment • make files will be useful for coordinating lex and yacc

15. History of Compilers • Grace Hopper • A-0 1952 • B-0 (Flow-Matic) 1956 • Fortran compiler – 1957 • (in the top 10 algorithms of 20th century) • (also, quicksort, fast fourier transforms, simplex LP ,..) • BC (before compilers) • Low-level programming • Assemblers were a major advance • Q: can an automatic translation to low-level languages be as efficient as writing it directly? • The only way to show this was to do it - and the Fortran compiler provided a clear ‘yes’ answer

16. Instant Compilation • Consider the program: main() { int a = 3; a = a + 1; } Given a reasonably sensible assembly language a hand-compilation might be: LDA #3 STA 1 LDA 1 ADD a, #1 STA 1

17. & an Instant Compiler could look like … Switch( source_code_construct ) { case INT_DEC: print( “LDA #”, INT.value) print(“STA 1”) break case INT_ADD: print(“LDA 1”) print(“ADD a,#”, ADD.value) print(“STA 1”) break } /* end switch */

18. The Problems …. • Not efficient, (LDA #4; STA 1) • Only works for 1 variable • Only works at one location in memory • (usually let assembler deal with symbolic addresses) • Only has 2 programming constructs! • Not even slightly portable: • 1 instruction set & 1 source language

19. More problems… • No error reporting • type checking? • Assumes: • Program is correct • Recognition of programming language constructs • int a = 3  INT_DEC • Access to values • INT.value, ADD.value • 1:1 relationship between integers and memory locations

20. Solutions • We can View compilers as a solution to all of these problems • E.g. • Only compile correct programs to object code • Recognise all constructs in the language • Improve the efficiency of code • Execution speed • Memory usage • Meaningful error messages to the user • Cope with different target architectures

21. Why are compilers called compilers? • In early compilers one of the main tasks was connecting object program to • standard library functions, I/O devices • collecting information from different sources(e.g. libraries) • OS and processor dependent • This is now performed by ‘linkers’ • Compile – ‘construct by collecting from different sources’