Compilers Crash Course

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Egbert Fletcher
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1 Compilers Crash Course Prof. Michael Clarkson CSci Spring 2014 Slides Acknowledgment: Prof. Andrew Myers (Cornell) What are Compilers? Translators from one representation of program code to another Typically: high-level source code to machine language (object code) Not always: Java compiler: Java to interpretable bytecodes Java JIT: bytecode to executable image 2

2 Source Code Source code: optimized for human readability expressive: matches human notions of grammar redundant to help avoid programming errors computation possibly not fully determined by code int expr(int n) { int d; d = 4 * n * n * (n + 1) * (n + 1); return d; } 3 Machine Code Optimized for hardware expr: Redundancy, ambiguity reduced Information about intent and reasoning lost Assembly code machine code pushl subl sall leave ret %ebp %esp, %ebp $4, %esp %eax, %edx 8(%ebp), %edx %eax %eax, %edx %eax $2, %eax %eax, -4(%ebp) -4(%ebp), %eax e5 83 ec 04 8b c2 0f af b f af d0 8b f af c2 c1 e fc 8b 45 fc c9 c3 4

3 Example (Output assembly code) Unoptimized Code Optimized Code expr: pushl subl sall leave ret %ebp %esp, %ebp $4, %esp %eax, %edx 8(%ebp), %edx %eax %eax, %edx %eax $2, %eax %eax, -4(%ebp) -4(%ebp), %eax expr: pushl sall leave ret %ebp %esp, %ebp 8(%ebp), %edx %edx $2, %eax 5 How to translate? Source code and machine code mismatch Goals: source-level expressiveness for task best performance for concrete computation reasonable translation efficiency (< O(n 3 )) maintainable compiler code 6

4 How to translate correctly? Programming languages describe computation precisely erefore: translation can be precisely described (a compiler can be correct) Correctness is very important! hard to debug programs with broken compiler non-trivial: programming languages are expressive implications for development cost, security some compilers have been proven correct! [X. Leroy, Formal Verification of a Realistic Compiler, CACM 52(7), 2009] 7 How to translate effectively? High-level source code? Low-level machine code 8

5 Idea: translate in steps Compiler uses a series of different intermediate representations (IRs) of programs. Different IRs are good for different phases of compilation 9 Compilation in a Nutshell 1 Source code (character stream) Token stream Abstract syntax tree (AST) if (b == 0) a = b; if ( b == 0 ) a = b ; == b 0 if boolean == Decorated AST int b int 0 if a = b int = int a int b lvalue Lexical analysis Parsing Semantic Analysis 10

6 Compilation in a Nutshell 2 if boolean == int b int 0 int = int a int b lvalue Intermediate Code Generation if b == 0 goto L1 else L2 L1: a = b L2: cmp r b, 0 jnz L2 L1: mov r a, r b L2: Assembly Code Generation Register allocation, Optimization cmp ecx, 0 cmovz [ebp+8],ecx 11 Compilation in a Nutshell 2 if boolean == int b int 0 int = int a int b lvalue Intermediate Code Generation if b == 0 goto L1 else L2 L1: a = b L2: cmp r b, 0 jnz L2 L1: mov r a, r b L2: Assembly Code Generation Register allocation, Optimization cmp ecx, 0 cmovz [ebp+8],ecx 12

7 Simplified Compiler Structure Source code (character stream) if (b == 0) a = b; Program analysis & Optimization Assembly code cmp 0, %rcx cmovz %rcx, %rdx Token stream Lexical analysis Parsing Abstract syntax tree Intermediate Code Generation Intermediate code Control flow graphs Assembly Code generation Front end (machine-independent) Back end (machine-dependent) 13 Even Bigger Picture Assembly code Source code Compiler Fully-resolved object code (machine code + symbol tables, relocation info) Assembler Linker Loader Executable image in memory Object code (machine code + symbol tables) 14

8 Where to Learn More Compilers Principles, Techniques and Tools. Aho, Lam, Sethi and Ullman ( e Dragon Book) (strength: parsing) Modern Compiler Implementation in Java. Andrew Appel. (strength: translation) Advanced Compiler Design and Implementation. Steve Muchnick. (strength: optimization) 15

CS 4120 and 5120 are really the same course. CS 4121 (5121) is required! Outline CS 4120 / 4121 CS 5120/ = 5 & 0 = 1. Course Information

CS 4120 and 5120 are really the same course. CS 4121 (5121) is required! Outline CS 4120 / 4121 CS 5120/ = 5 & 0 = 1. Course Information CS 4120 / 4121 CS 5120/5121 Introduction to Compilers Fall 2011 Andrew Myers Lecture 1: Overview Outline About this course Introduction to compilers What are compilers? Why should we learn about them?