Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -1- Compiler Construction Principles & Implementation.

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -1- Compiler Construction Principles & Implementation Techniques Dr. Zheng Xiaojuan Associate Professor Software College of Northeast Normal University April. 2008

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -2- Knowledge Relation Graph Develop a Parser Syntax definition basing on Context Free Grammar using implement Top-down Bottom-up

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -3- §4 Top-down Parsing 4.1 Overview of Top-down Parsing 4.2 Three Important Sets 4.3 Left Recursion Removal & Left Factoring 4.4 Recursive-Descent Parsing 4.5 LL(1) Parsing    

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -4- 4.5 LL(1) Parsing Main idea of LL(1) Parsing Method LL(1) Grammar (LL(1) 文法 ) LL(1) Parsing Table (LL(1) 分析表 ) LL(1) Parsing Engine (LL(1) 分析驱动程序 ) LL(1) Parsing Process (LL(1) 分析过程 ) LL(1) Parser Generator – LLGen (LL(1) 分析程序的自动生成器 )

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -5- Main idea of LL(1) Parsing Method LL(1) Parsing –Left-to-right parsing, Left-most derivation, 1-symbol look ahead ; –Requires the same precondition( 和递归下降法要求相同的前提条件 ) G = (V T, V N, S, P) For any A  V N, For any two productions of A, Predict(A   1)  Predict(A   2) = 

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -6- Main idea of LL(1) Parsing Method LL(1) Parsing –LL(1) parsing table to record predict sets for each production; (LL(1) 分析表 ) –A general engine( 一个通用的驱动程序 )

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -7- Example P: (1) Z  aBd (2) B  d (3) B  c (4) B  bB a b c d 句型输入动作 ZabcdDerive (1) aBdabcdMatch BdbcdDerive(4) bBdbcdMatch BdcdDerive(3) cd Match dd Succeed

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -8- LL(1) Parsing Mechanism Stack Input #dcba 驱动程序 Z LL[1] 分析表

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -9- LL(1) Parsing Mechanism Stack Input #………a 驱动程序 :  栈为空情形的处理  X  V T 情形的处理  X  V N 情形的处理 X … … LL[1] 分析表

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -11- LL(1) Grammar If a CFG G meets the precondition below, we will call G is a LL(1) Grammar; G = (V T, V N, S, P) For any A  V N, For any two productions of A, Predict(A   1 )  Predict(A   2 ) = 

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -12- LL(1) Grammar Some features of a LL(1) Grammar –No ambiguity( 无二义性 ) –No left recursion( 无左递归 ) –For a non-terminal symbol, there is at most one empty production( 对于一个非终极符来讲, 最多只有一个空产生式 )

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -14- LL(1) Parsing Table (LL(1) 分析表 ) The purpose of LL(1) Parsing Table –According to, decide that which production of current non-terminal symbol can be used to derive; – 根据当前的非终极符和当前的输入符决定用哪一个产生式来进行推导 ; If current non-terminal symbol is X, current input symbol is a, we can use X  if and only if a  predict(X  );

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -15- LL(1) Parsing Table (LL(1) 分析表 ) How to build LL(1) Parsing Table for a LL(1) Grammar? –For a LL(1) Grammar G = (V T, V N, S, P) –V T = {a 1, …, a n } –V N = {A 1, …, A n } –LL(A i, a i ) = [A i   ], if a i  predict(A i   ) –LL(A i, a i ) = error, if a i not belong to the predict set of any production of A i a1a1 …anan # A1A1 ……. … AnAn

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -16- Example 产生式 Predict 集 (1){a} (2){d} (3){c} (4){b} P: (1) Z  aBd (2) B  d (3) B  c (4) B  bB abcd# Z(1) B(4)(3)(2)

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -17- Example (1) E  TE’ { i, n, ( } (2) E’  + TE’ {+} (3) E’   {#, )} (4) T  FT’ {i,n,(} (5) T’  * F T’ {*} (6) T’   {),+, # } (7) F  (E) { ( } (8) F  i {i} (9) F  n {n} +*()in# E(1) E’(2)(3) T(4) T’(6)(5)(6) F(7)(8)(9)

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -19- LL(1) Parsing Engine 分析驱动程序 Stack Input #………a 驱动程序 :  栈为空情形的处理  X  V T 情形的处理  X  V N 情形的处理 X … … LL[1] 分析表

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -20- LL(1) Parsing Engine Configuration( 格局 ): 可能的情形 – 栈为空情形的处理 :, succeed, if a=#, else error; –X  V T 情形的处理 :, match; else error; –X  V N 情形的处理 :, derive if there is a production X  such that a  predict(X  ), else error;

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -21- LL(1) Parsing Engine [1] 初始化： Stack := [ ] ； Push(S) ； [2] 读下一个输入符： Read(a) ； [3] 若当前格局是 (, # ) ，则成功结束；否则转下； [4] 设当前格局为（..... X, a.....) ，则  若 X  V T & X= a ，则 { Pop(1) ； Read(a) ； goto [3] }  若 X  V T & X  a ，则 Error ；  若 X  V N ，则： if LL(X, a)=X→Y 1 Y 2...... Y n then { Pop(1) ； Push(Y n,.....,Y 1 ) ； goto[3] } else Error

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -23- LL(1) Parsing Process P: (1) Z  aBd (2) B  d (3) B  c (4) B  bB abcd# Z(1) B(4)(3)(2) a b c d a c c d

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -25- Dealing with If-Then-Else CFG for If-Then-Else V T = {if, then, else, other, ;, exp} V N = {S, Stm, ElsePart} P: (1) S  Stm ; (2) Stm  if exp then Stm ElsePart (3) Stm  other (4) ElsePart  else Stm (5) ElsePart   (1){if, other} (2){if} (3){other} (4){else} (5){;, else} Predict set:

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -26- Dealing with If-Then-Else LL(1) parsing table ifthenelseotherexp;# S(1) Stm(2)(3) ElsePart(4) (5)(5) (4) 每个 else 与其前面未匹配的最近的 then 相匹配 !

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -27- Dealing with If-Then-Else stackinputaction Sif exp then if exp then other else other ;(1) Stm ;if exp then if exp then other else other;(2) if exp then Stm ElsePart;if exp then if exp then other else other; Stm ElsePart ElsePart; if exp then other else other; if exp then Stm ElsePart ElsePart; if exp then other else other; Match(3 次 ) (2) Match(3 次 ) other else other;(3) other ElsePart ElsePart;other else other;Match ElsePart ElsePart;else other;(4) Stm ElsePart;

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -28- Dealing with If-Then-Else stackinputaction succeed ElsePart ElsePart;else other;(4) else Stm ElsePart;else other;match Stm ElsePart;other; other ElsePart;other;match ElsePart;; (3) (5) ;;match

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -29- S Stm; if expthenStm ElsePart ifexp then StmElsePart other else Stm other

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -30- Building Parse Tree During LL(1) [1] 初始化： Stack := [ ] ； root=BuildOneNode(S); Push(S, root) ； [2] 读下一个输入符： Read(a) ； [3] 若当前格局是 (, # ) ，则成功结束；否则转下； [4] 设当前格局为（..... X, a.....) ，则  若 X  V T & X= a ，则 { Pop(1) ； Read(a) ； goto [3] }  若 X  V T & X  a ，则 Error ；  若 X  V N ，则： if LL(X, a)=X→Y 1 Y 2...... Y n then { (X, ptr) = Pop(1); for i=n to 1 { p[i] = BuildOneNode(Yi), Push(Y i, p[i]) ； } AddSons(ptr, p, n); goto[3] } else Error

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -31- LL(1) Parser Generator – LLGen LLGen 终极符 / 产生式个数产生式右部长度 LL( 分析表 ) 可导空串的符号表文法中所有符号非 LL(1) 表的冲突

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -32- §4 Top-down Parsing 4.1 Overview of Top-down Parsing 4.2 Three Important Sets 4.3 Left Recursion Removal & Left Factoring 4.4 Recursive-Descent Parsing 4.5 LL(1) Parsing

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -33- Knowledge Relation Graph Top-down parsing Syntax definition basing on CFG using check precondition Predict(A  ) first(  ) follow(A) Yes Recursive-descent LL(1) Implement 文法等价变换

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -34- Summary What is the main idea of Top-down parsing? What is the precondition of recursive-descent and LL(1) parsing? The definitions of three sets ? For a given CFG, calculate three sets? The main idea of recursive-descent parsing? ※

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -35- Summary For a given CFG, develop recursive-descent parser? –Calculate predict set for each production; –Check whether meets the precondition; –If yes, develop function for each non-terminal symbol; –Build parse tree during parsing;

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -36- Summary The main idea of LL(1) parsing? LL(1) Grammar? The mechanism of LL(1) parsing? For a given CFG, develop LL(1) parser? –Calculate predict set for each production; –Check whether meets the precondition; –If yes, generate LL(1) parsing table; –LL(1) parser = LL(1) parsing table + LL(1) parsing engine –Build parse tree; Give the LL(1) parsing process?

Software College of Northeast Normal University Compiler Construction Principles & Implementation Techniques -37- Assignment G = {V T, V N, S, P} V T = {-, (, ), id} V N = {E, ET, V, VT} S = E P = { E  -E ET   E  (E) V  id VT E  V ET VT  (E) ET  -E VT   } (1)Generate LL(1) Parsing table; (2) “ -(id--id) ” 的分析过程 ;

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