コンパイルと解釈

The bug that cost a spacecraft, found by a comma

• NASA's Mariner 1 was destroyed 293 seconds after launch in 1962. The failure is often blamed on a single wrong character in the guidance software.
• The story is retold so often because it is the fear every programmer knows: a compiler happily translating a program that says something you did not mean.
• A compiler is not one process but five, and each one catches a different class of mistake. Knowing which stage catches what tells you where any given error comes from.
• This lesson is how an interpreter 解释器 runs a program without producing anything, and the five stages a compiler 编译器 goes through.

How an interpreter runs a program

• An interpreter translates and runs the source at the same time, one statement at a time.
• For each statement it reads the line, analyses it, checks the types, then executes the action immediately, and moves to the next.
• It produces no executable file: the translation exists only in memory and is thrown away. That is the syllabus's exact point, that it can execute a program without producing a translated version.
• It reports an error the moment it reaches the offending line and usually stops, which gives quick feedback while developing. Every run re-translates, so it is slower, and both the source and the interpreter must be present.

Translate once and keep it, or translate and run and forget

Stage 1: lexical analysis

• Lexical analysis 词法分析 groups the individual characters of the source into tokens 词法单元: keywords, identifiers, operators and literals.
• Whitespace and comments are discarded, since they have no meaning to the compiler, and identifiers are entered into a symbol table.
• So total ← count * 2 becomes the token sequence identifier, assignment, identifier, operator, literal. The lexer does not know or care whether that makes sense.

Stage 2: syntax analysis

• Syntax analysis 语法分析, or parsing, checks that the sequence of tokens fits the language's grammar, and builds an abstract syntax tree 抽象语法树 representing the structure.
• A missing bracket, a missing ENDIF or a keyword in the wrong place is caught here: that is what a syntax error 语法错误 is, and it is why the compiler can report one without ever running the program.

The grammar the parser checks against

Stage 3: semantic analysis

• Semantic analysis 语义分析 checks that a syntactically valid program actually makes sense: every variable is declared before use, the types on each side of an assignment are compatible, a function is called with the right number of arguments.
• total ← "seven" * 2 is perfectly good syntax and complete nonsense. Only this stage catches it.
• This is the distinction the exam tests: syntax is form, semantics is meaning.

Stages 4 and 5: code generation and optimisation

• Code generation 代码生成 walks the tree and emits the target machine code, choosing registers and laying out the data.
• Code optimisation 代码优化 improves that code without changing what it does: removing redundant work, computing constant expressions at compile time, and reordering instructions to suit the pipeline.
• The output is a stand-alone executable that runs without the compiler present.

Five stages, and each one catches something the last could not

Worked example: which stage catches which error

Worked example: compare the two translators

Marks that slip away

You've got it