MIT CS143 lecture 01 course overview

课程首页

http://web.stanford.edu/class/cs143/

课程要求 Open the lid of compilers and see inside

• Understand what they do
• Understand how they work
• Understand how to build them

Correctness over performance

• Correctness is essential in compilers
• They must produce correct code
• CS143 is more like CS103+CS110 than CS107
• Other classes focus on performance (CS149, CS243)

课程目标

You will write your own compiler in 4 parts.

语言是如何实现的？ 有两种主要策略

• Interpreters run your program
• Compilers translate your program

Compilers dominate low-level languages

• C, C++, Go, Rust

Interpreters dominate high-level languages

• Python, Ruby

Some language implementations provide both

• Java, Javascript, WebAssembly
• Interpreter + Just in Time (JIT) compiler

编译器架构

1. Lexical Analysis — identify words
2. Parsing — identify sentences
3. Semantic Analysis — analyse sentences
4. Optimization — editing
5. Code Generation — translation

词法分析

• 识别单词：Smallest unit above letters
• Lexical analyzer divides program text into “words” or “tokens”

Parsing

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

Diagramming a Sentence

Parsing program expressions is the same，consider：

If x == y then z = 1; else z = 2; text{If x == y then z = 1; else z = 2;} If x == y then z = 1; else z = 2;

Diagrammed:

Semantic Analysis

• Once sentence structure is understood, we can try to understand “meaning”
• Compilers perform limited semantic analysis to catch inconsistencies

一些英语语境中语法分析的例子：

• Jack said Jerry left his assignment at home.（What does “his” refer to? Jack or Jerry?）
• Jack said Jack left his assignment at home?（How many Jacks are there? Which one left the assignment?）

编程语言中的语法分析：

• Programming languages define strict rules to avoid such ambiguities
• 如下的C++代码会打印4，使用内层的Jack定义

{
	int Jack = 3;
	{
		int Jack = 4;
		cout << Jack;
	}
}

Compilers perform many semantic checks besides variable bindings

例子：

Jack left her homework at home.（Possible type mismatch between her and Jack，Jack是一个男性）

Optimization

Akin to editing：

• Minimize reading time
• Minimize items the reader must keep in short-term memory

Automatically modify programs so that they

• Run faster
• Use less memory
• In general, to use or conserve some resource

（CS143课程中并没有涉及到优化的部分）

优化的例子：

X = Y * 0 is the same as X = 0

Code Generation

• Typically produces assembly code
• Generally a translation into another language

Intermediate Representations

Many compilers perform translations between successive intermediate languages

All but first and last are intermediate representations (IR) internal to the compiler

IRs are generally ordered in descending level of abstraction

• Highest is source
• Lowest is assembly

IRs are useful because lower levels expose features hidden by higher levels：

• registers
• memory layout
• raw pointers
• etc

But lower levels obscure high-level meaning：

• Classes
• Higher-order functions
• Even loops
• etc

Issues

• Compiling is almost this simple, but there are many pitfalls
• Example: How to handle erroneous programs?

Language design has big impact on compiler：

• Determines what is easy and hard to compile
• Course theme: many trade-offs in language design