Intro to Compilers

Compilers

Interpreters

FORTRAN 1

• The first Compiler
  • Huge impact o computer science.
• Led to an enormous body of theoretical work
• Modern compilers preserve the outline of FORTRAN 1

Structure of FORTRAN 1

1. Lexical Analysis（词法分析）
2. Parsing （语法分析）
3. Semantic Analysis （语义分析）
4. Optimization（优化）
5. Code Generation

First step: recognize words.

smallest unit above letters

    This is a sentence.

• Lexical analysics divides program text into "words" or "tokens"

    if x == y then z = 1;
    
    else z = 2;
  

• Parsing = Diagrmming Sentences.

  • The diagram is a tree.

• Once sentence structure is understood, we can try to understand "meaning"
  • This is too hard

• Compilers perform limited semantic analysis to catch inconsistencies.

• Expmple: Jack said Jerry left his assignment at home.

• Even worse: Jack said Jack left his assignment at home?

• Question? Which one "his" is referring to whether it's jack or it's Jerry.

• Programming languages define strict rules to avoid such ambiguities.

    int main {
        int jack = 3;
        {
            int jack = 4;
            cout << jack;
        }
    }
  

Compilers perform many semantic checks besides variable bindings.

• Example: Jack left her homework at home.
• A "type mismatch" between her and jack; we know they are different people.

Optimization has no strong counterpart in English.

• But a little bit like editing.

Automatically modify programs so that they

• Run faster
• Use less memory

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

• NO!
• valid for integers
• invalid for floating point (NaN)
• NaN, Standing for Not a Number, is a numeric data type value representing an undefined or unrepresentable value.
• NaN * 0 = NaN

Produces assembly code (usually)

A translation into another language

• Analogous to human translation

• The overall structure of almost every compiler adheres to our outline

• The proportions has changed since FORTRAN

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