# CS 131: Programming Languages and Compilers ## Introduction ![Compiler](/files/-MZlzscUjQLJTLK8_9W0) ### Definition A **Compiler** is "A program that takes a source-code program and translates it into an equivalent program in target language" ### String A **String** is a sequence of characters. * Alphabet: A finite set of symbols （ASCII characters） * String has words and sentences * $$st$$ is the concatenation of $$s$$and $$t$$ * $$s \epsilon = \epsilon s = s$$ * $$\epsilon$$is the empty string * $$s^0 = \epsilon$$ * $$\mid s \mid$$ is the length of s * Suppose s is banana * **Prefix:**前缀 * ban banana * **Suffix:**后缀 * banana, ana * **Substring:**子字符串 * **Subsequence**:子字符 * bnan, nn A **Language** is a set of Strings over a fixed **Alphabet** $$\Sigma$$, constructed using a specific Grammar. * e.g. $${\varepsilon, 0,01,011,0111, \ldots}$$ * Not all Strings of chars in the Alphabet is in the certain Language, only those who satisfy the Grammar rules. * Alphabet $$={0,1}$$ and using Grammar rule $$\mathrm{RE}=01^{\*}$$ , we can specify the above example Language, while $$01$$ isn't. Below is **Operations** and **Examples**. ![Operations of Languages](/files/-MZmlFc0EYCac_0nCzrW)
Example of Laguage Operations
A **Grammar** $$G$$ is the description of method （*rules*） of how to construct a certain Language over a certain Alphabet * Type 0: Turing Machine Recursive Enumerable Gramma * Type 1: Context-sensitive Grammar （CSG） * Type 2: Context-free Grammar (CFG,上下文无关文法）， *mostly recursive* * Type 3: Right-linear Grammar Regular Expressions (RE,正则表达式）， *non-recursive* {% hint style="info" %} Expressiveness: Type 0 > Type 1 > Type 2 > Type 3 {% endhint %} ### Phases ![](/files/-MZzDtySitutcAp8QaOk) ## Lexical Analyzer （词法分析器） Reads the source program character by character and returns the **tokens** of the source program * 分析并把 identifiers 放入符号表 * 利用正则表达式来分析 tokens * 利用有限自动机来分析 Token 以完成词法分析 * 词法分析器 = 读入（scanning） + 词法分析（lexical analysis） ![Lexical Analyzer](/files/-MZmeSjHtfQ1-ogSHzxx) ### Token Describes a pattern of characters having some meaning in the source program (such as identifiers, operators, keywords, numbers, delimiters and so on) * e.g., identifiers, operators, numbers A **Lexeme （词素）** is an instance of a Token, along with its unique attributes. * e.g. `17` * INT * Token.value = 17 ![Process of determing a Token 通常使用双 buffers 来避免 buffer 区太小的问题](/files/-MZmfxNa9GrrRaFbzRSZ) ### Regular Expression （正则表达式） 我们利用**正则表达式**来 描述 **Tokens** 匹配 **Tokens** * 每个**正则表达式** $$r$$ 描述一个语言 $$L(r)$$ * $$r$$被叫做 **regular set** * **正则表达式**是左结合的，从左向右匹配的 * **正则表达式**是一个 **Type-3 Grammar Rule** #### **Properties** ![Properties](/files/-MZmof_B8q_1S9MeftXx) ![Extended Properties](/files/-MZmojydvefc6fY_VzFT) * e.g. * Integers: * Digit = \[0-9]\* * Integer = Digit Digit\* * Identifier * letter = \[a-zA-Z] * identifier = letter （letter + digit）\* * Whitespace * WS = ('\n' + '\t' + ' ')+ * Examples * Even Binary number * 1\[0+1]\*0 | 0 ### Finite Automata （有限自动机） #### Transition Diagram ![Transition Disgram Examples](/files/-MZpxrkV0bNGzQ5kYj-e) 当一个 Token 被识别： * 如果是关键字，那么会返回 **Token of the keyword** * 如果是符号表里的 ID，那么返回 **entry of symbol table** * 如果符号表没有这个ID，那么加入ID并返回新的 **entry of symbol table** ```c switch (state){ case 0: c = nextchar(); if (c == [something]) state = [state], lexeme_beginning ++; else if (c == [something]) [do something] else if (c == [final state]) // terminal retract(1); //forward lexical_value = install_num(); // install something return (NUM); else state = fail(); ``` #### Finite automata A **recognizer** for a language is a program that takes a string x, and answers “yes” if x is a sentence of that language, and “no” otherwise. We call the recognizer of the tokens as a **finite automaton**. **Example** ![Regular expression: (a+b)\*abb](/files/-MZq0pX6wcH7PWX7wenr) * **Start Arrow** * **State** * **Transition Edge** * **Accepting State**:同心圆，接受并结束，状态3 * **Death State:错误**状态，未定义的 transition 指向该状态 * Transition table: | state | a | b | | ----- | ----- | --- | | 0 | {0,1} | {0} | | 1 | -- | {2} | | 2 | -- | {3} | {% hint style="info" %} 注意 empty string 可以被某些自动机接收 {% endhint %} ### NFA Non-Deterministic Finite Automata （NFAs） **easily** represent regular expression, but are **less precise.** **Accept s:** an Accepting State that spells out **s**. An NFA is a mathematical model that consists of: * S, a **finite** set of **states** * $$\Sigma$$, the symbols of the **input alphabet** * *move*, a **transition function** * move(state, symbol) $$\to$$ sets of states * A state $$s\_0 \in S$$, the **start state** * $$F \subseteq S$$, a set of **final** or **accepting states** * $$\epsilon$$**move: ε- transitions** are allowed in NFAs. In other words, we can move from one state to another one without consuming any symbol {% hint style="info" %} The $$\epsilon$$-Closure of $$S = S \cup {$$ All States that can go to without consuming any input $$}$$ {% endhint %} ### DFA Deterministic Finite Automata （DFAs） require **more complexity** to represent regular expressions but offer **more precision**. **Does not allow** $$\epsilon$$**move,** for every $$s \in S$$, there is ONLY ONE decision for every input *Symbol.* **Accept s:** an Accepting State that spells out **s** *and **ONLY and ONLY ONE** path*. {% hint style="info" %} No $$\epsilon$$-closure !!! {% endhint %} ### Implementation of Lexers One Token, A *Recognizer. There are 4 ways. (r stands for RE)* 1. $$r \to NFA \to Recognizer$$ 2. $$r \to NFA \to DFA \to Recognizer$$ 3. $$r \to DFA \to Recognizer$$ 4. $$r \rightsquigarrow DFA \to Minimized \~ DFA \to Recognizer$$ ### RE to NFA Algorithm is called **Thompson's Construction**. ![](/files/-MZqLle_HFS3AzK_CIg4) There are some requirements on such construction: * $$N(s)$$and $$N(t)$$CANNOT have any intersections * REMEMBER to assign unique names to all states Properties of the resulting NFA: * Exactly 1 Start State & 1 Accepting State * $$#$$ of States in NFA $$\leq 2 \times$$(# of Symbols + # of Operators) in $$r$$ * States do not have multiple outgoing edges with the same input symbol * States have at most 2 outgoing $$\epsilon$$ edges ### RE to DFA **\[Step 1]**: We make Augmented RE: concatenate with symbol # (meaning "finish"). * e.g. (a+b)\*a# * Ensures at least one operator in the RE **\[Step 2]:** Build syntax tree for this Augmented RE:
* $$\epsilon$$, # and $$a \in \Sigma$$ all are at leaves * All other operators are inner nodes * Non-$$\epsilon$$ leaves get its position number, increasing from left $$\to$$ right **\[Step 3]:** Compute `nullable()`, `firstpos()` & `lastpos()` for ALL nodes. 1. `firstpos(n)`: Function returning the set of positions where the *first* Symbol can be at, in the *sub-RE* rooted at `n` 2. `lastpos(n)`: Function returning the set of Positions where the *last* Symbol can be at, in the sub-RE rooted at `n` 3. `nullable(n)`: Function judging whether the *sub-RE* rooted at `n` can generate $$\epsilon$$ ![](/files/-MZqOknizrDpl08vfeyI) **\[Step 4]:** Compute `followpos()` for Leaf positions `followpos(i)`: Function returning the set of positions *which can follow* position i in the generated String Conduct a *Post-order* *Depth First Traversal* on the syntax tree, and do the following oprations when leaving $$\cdot$$ / \* nodes: * $$c{1} \cdot c{2}:$$ For all $$i \in$$ `lastpos(c1)` , `followpos(i)`= `followpo(i)` $$\cup$$ `firstpos(c2)` * $$c^{\*}:$$ For all $$i \in$$ `lastpos(c)`, `followpos(i)`$$=$$ `followpos(i)` $$\cup$$ `firstpos(c)` **\[Step 5]:** Construct the DFA. ```c void construct() { S0=firstpos(root); DStates= {(S0, unmarked)}; while (DStates has an unmarked State U) { Mark State U; for (each possible input char c) { V= {}; for (each position p in U whose symbol is c) V=UnionofVandfollowpos(p); if (V is not empty) { if (V is not in DStates) Include V in DStates, unmarked; Add the Transition U--c->V; } } } } ``` * A State $$S$$in resulting DFA is an Accepting State iff # node $$\in S$$ * Start State of the resulting DFA is $$S\_0$$ #### Calculate \epsilon-Closure ```c set epsClosure(set S) { for (each State s in S) Push s onto stack; closure = S; while (stack is not empty) { Pop State u; for (each State v that u->v is an epsilon Transition) { if (v is not in closure) { Include v in closure; Push v onto stack; } } } return closure; } ``` #### Implement NFA as Recognizer ```c bool recognizer() { S=epsClosure(s0); while ((c=getchar()) !=EOF) S=epsClosure(move(S, c)); if (S and F has intersections) return ACCEPT; return REJECT; } ``` {% hint style="info" %} Performance of NFA-type Recognizers: Space $$O(|r|)$$; Time $$O(|r| \times |s|)$$ {% endhint %} #### Implement DFA as Recognizer ```c bool recognizer() { s=s_0; while ((c=getchar()) !=EOF) s=move(s, c); if (s is in F) return ACCEPT; return REJECT; } ``` {% hint style="info" %} Performance of DFA-type Recognizers: Space $$O(|2^{|r|})$$; Time $$O(|s|)$$ {% endhint %} #### Convert NFA to DFA Algorithm is called **Subset Construction(子集构造法)**, since we make subset of States in original NFA into a single State in resulting DFA ```c void subsetConstruction() { S0=epsClosure({s0}); DStates= {(S0, unmarked)}; while (DStates has any unmarked State U) { MarkState U; for (each possible inputchar c) { V=epsClosure(move(U, c)); if (V is not empty) { if (V is not in DStates) Include V in DStates, unmarked; Add the Transition U--c->V; } } } } ``` * A State $$S$$ in resulting DFA is an Accepting State iff $$\exists s \in S, s$$ is an Accepting State in original NFA * Start State of the resulting DFA is $$S\_0$$ ### Minimize DFA ```c void minimize() { PI = {G_A, G_n}; do { for (every group G in PI){ for (every pair of States (s,t) in G){ if (for every possible input char c, transition s--c -> and t--c-> go to states in the same group) s,t are in the same subgroup; else s,t should split into different subgroups; } Split G according to the above information; } }while (PI changed in this iteration) Every Group in PI is a state in the minimal DFA; } ``` * A State S in the minimal DFA is an Accepting State iff $$\exists s \in S$$, s is an Accepting State in original DFA * Start State of the minimal DFA is the one containing original Starting State ### Other Issues for Lexers #### Look ahead #### Comment Skip #### Symbol Table ## Syntax Analyzer （句法分析） 如果说词法分析这一步提供了可供计算机识别的**词**，那么句法分析是为了理解句子结构。 通常这一步会生成 **parse tree**, **parse tree** 用以描述句法结构。 ### Difference with Lexical Analyzer * The syntax analyzer deals with **recursive** constructs of the language * Both do similar things; But the lexical analyzer deals with simple **non-recursive** constructs of the language. * The lexical analyzer recognizes the smallest meaningful units （**tokens**） in a source program. * The syntax analyzer works on the smallest meaningful units （**tokens**） in a source program to recognize meaningful structures （**sentences**） in our programming language. ### Parse Tree Abstraction A **Parse Tree / Syntax Tree (语法树)** is a graphical representation of the structure of a program, where leaf nodes are Tokens. ### CFG （上下文无关文法） A **Context-free Grammar (CFG)** is a **Type-2** Grammar rule, which serves the construction of a Parse Tree from a streamof Tokens. We use a set of Production Rules to characterize a CFG ![](/files/-MZvRarzF4mg2I3Uq7KL) A **Terminal (终结符号)** is a Token; A **Non-terminal (非终结符号)** is a syntactic variable. * The Start Symbol is the first one of Non-terminals; Usually represents the whole program * A Sentence is a string of Terminals such that Start Symbol $$S \Rightarrow{ }^{+} s$$ A **Production Rule (生成规则)** is a law of production, from a Non-terminal to a sequence of Terminals & Non-terminals. * e.g. $$A \rightarrow \alpha A \mid \beta$$, where $$A$$ is a Non-terminal and $$\alpha, \beta$$ are Terminals * May be *recursive* * The procedure of applying these rules to get a sentence of Terminals is called **Sentential Form** / **Derivation** {% hint style="info" %} $$|$$ Context-free Languages $$|>|$$ Regular Languages $$|$$, e.g. $${(^{i})^{i}: i \geq 0 }$$. {% endhint %} ### Derivation Directions（派生文法）\&Ambiguity（二义性） **Left-most Derivation** **(左递归)**$$\left(\Rightarrow\_{l m}\right)$$ means to replace the leftmost Non-terminal at each step. * If $$\beta A \gamma \Rightarrow \operatorname{lm} \beta \delta \gamma$$, then NO Non-terminals in $$\mathcal{\beta}$$ * Corresponds to *Top Down Parsing* **Right-most Derivation** $$(\Rightarrow r m)$$means Replace the rightmost Non-terminal at each step. * If $$\beta A \gamma \Rightarrow\_{r m} \beta \delta \gamma$$, then NO Non-terminals in $$\gamma$$ * Corresponds to *Bottom Up Parsing*, in reversed manner ![](/files/-MZzG4b-kgANk-Vstyqs) A CFG is **Ambiguous** when it produces more than one Parse Tree for the same sentence. Must remove Ambiguity for apractical CFG, by: * Enforce *Precedence (优先级)* and *Associativity (结合律)* * e.g. $$\* > +$$ , then $$+$$ gets expanded first * Grammar Rewritten ![](/files/-MZzFc923KMiyxDum2ST) ### Top-Down Parsers Construction of the parse tree starts at the root, and proceeds towards the leaves. * Recursive Predictive Parsing * Non-Recursive Predictive Parsing （**LL Parsing**）. （**L**-left to right; **L**-leftmost derivation） * 语法构架能力弱 #### Implement 1. Eliminate Left Recursion $$\to$$ Recursive-descent Parsing 2. Eliminate Left Recursion $$\to$$ Left Factoring $$\to$$Recursive Predictive Parsing 3. Eliminate Left Recursion $$\to$$Left Factoring $$\to$$Construct Parsing Table $$\to$$Non-recursive Predictive Parsing #### Left Recursion Elimination (消除左递归) $$A \Rightarrow^{+} A\_{\alpha}$$: Left Recursion * Top Down Parsing **CANNOT** handle Left-recursive Grammars * Can be eliminated by rewriting For *Immediate* Left Recursions (Left Recursion that may appear in a single step), eliminate by:
立即左递归的消除
```c /* Non-terminals arranged in order: A1, A2, ... An. */ void eliminate() { for (i from 1 to n) { for (j from 1 to i-1) Replace Aj with its products in every Prodcution Rule Ai->Aj ...; Eliminate Immediate Left Recursions Ai->Ai ...; } } ``` ![左递归的消除](/files/-MZzH1KO5UIk5U0j1bRY) #### Implementing Recursive-descent Parsing ```c /* Example: * E -> T | T + E* T -> int | int * T | ( E ) */ bool term(TOKENtok) { return*ptr++==tok; } bool E1() { returnT(); } bool E2() { returnT() &&term(PLUS) &&E(); } bool E() { TOKEN*save=ptr; return (ptr=save, E1()) || (ptr=save, E2()); } bool T1() { returnterm(INT); } bool T2() { returnterm(INT) &&term(TIMES) &&T(); } bool T3() { returnterm (OPEN) &&E() &&term(CLOSE); } bool T() { TOKEN*save=ptr; return (ptr=save, T1()) || (ptr=save, T2()) || (ptr=save, T3()); } ``` #### Left Factoring: Produce LL(1) Grammar LL(1) means Only 1 Token Look-ahead ensures which Pruduction Rule to expand now. To convert LL(1) to a CFG, for each Non-terminal :
{% hint style="info" %} || LL(1) || < || CFG ||, so not all Grammar can be convert to LL(1) * Such Grammar will have an entry with multiple Production Rules to use in the Parsing Table, thusWill be inappropriate for Predictive Parsing {% endhint %} #### Implementing Recursive Predictive Parsing {% hint style="info" %} This part stongly suggest to see for better understanding. {% endhint %} ![](/files/-MZzIAL51Xb-Tx5XjVz8) #### Parsing Table Construction {% hint style="info" %} This part stongly suggest to see for better understanding. {% endhint %} #### Implementing LL(1) Parsing {% hint style="info" %} This part stongly suggest to see for better understanding. {% endhint %} ### Bottom-Up Parsers Construction of the parse tree starts at the leaves, and proceeds towards the root. * Bottom-up parsing is also known as **shift-reduce parsing** * **LR Parsing** – much general form of shift-reduce parsing: **LR**, **SLR**, **LALR** (**L**-left to right; **R**-rightmost derivation) {% hint style="info" %} This part stongly suggest to see for better understanding. {% endhint %} ## IR ## Thanks * \[Guanzhou HU's Notes]\() * TAs: 季杨彪, 杨易为, 尤存翰 --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://notes.aaaab3n.moe/cs-131-programming-languages-and-compilers-1.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.