DOCSLIB.ORG

CS164: Introduction to Programming Languages and Compilers Fall 2010 1

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
CS164: Introduction to Programming Languages and Compilers Fall 2010 1 Parsers, Grammars and Logic Programming Ras Bodik, Thibaud Hottelier, James Ide UC Berkeley CS164: Introduction to Programming Languages and Compilers Fall 2010 1 Administrativia Project regrades: submit a patch to your solution small penalty assessed -- a function of patch size PA extra credit for bugs in solutions, starter kits, handouts we keep track of bugs you find on the Errata wiki page How to obtain a solution to a programming assignment see staff for hardcopy Office hours today: “HW3 clinic” for today, room change to 511 extended hours: 4 to 5:30 2 PA2 feedback: How to desugar Program in source language (text) Program in source language (AST) Program in core language (AST) 3 PA2 feedback: How to specify desugaring Desugaring is a tree-to-tree transformation. you translate abstract syntax to abstract syntax But it’s easier to specify the transformation in text using concrete syntax, as a text-to-text transformation Example: while (E) { StmtList } --> _WhileFun_(lambda(){E}, lambda(){StmtList}) 4 Outline (hidden slide) Goal of the lecture: • Languages vs grammars (a language can be described by many grammars) • CYK and Earley parsers and their complexity (Earley is needed for HW4) • Prolog (backtracking), Datalog (forward evaluation = dynamic programming) Grammars: • generation vs. recognition • random generator and its dual, oracular recognizer (show their symmetry) • Write the backtracking recognizer nicely with amb Parse tree: • result of parsing is parse tree • the oracle reconstructs the parse tree • add tree reconstruction with amb Prolog refresher • Example 1: something cool • Example 2: something with negation Switch to prolog • rewrite amb recognizer (no parse tree reconstruction) in prolog • rewrite amb recognizer (with parse tree reconstruction) in prolog • analyze running time complexity (2^n) Change this algorithm to obtain CYK once evaluated as Datalog • facts should be parse(nonterminal, startPosition, endPosition) • now ask if the evaluation needs to be exponential. How about if we instead compute all the facts bottom up? How much work will we do? 5 Visualize the work as a graph; this is CYK. • analyze CYK running time (N^3) • analyze CYK's inefficiency, motivate Earley Earley algorithm • in-progress edges • the three actions • give pseudocode • explain why it gets you N^2 on non-ambiguous grammars • explain HW3 Recursive definition of a language Language: a set of (desired) strings Example: the language of regular expressions Let’s define this language: – base case: any character c is regular expression – inductive case: if e1, e2 are regular expressions then the following are also regular expressions: e1 | e2 e1 e2 e1 * (e1) 6 Grammars Describing a language in English is tedious. We need a handy formal notation. We have been using it already. It's called a grammar. R ::= c | R R | R'|'R | R'*' | ( R ) 7 Grammars vs. languages Distinct grammars can describe the same language L(G) = language described by grammar G Example: language of all strings of the form bai , i>0 grammar 1: S ::= S a | b grammar 2: S ::= baA A ::= A a | many more grammars exist 8 Generate a string from L(G) Is there a recipe to print all strings from L(G)? Depends if you are willing to wait. L(G)may be infinite. Let’s write function gen(G) that prints a string L(G). Can we do it so that, if L(G) is finite, rerunning gen(G) will eventually print each string in L(G)? 9 gen(G) Grammar G and its language L(G): G: E ::= a | E + E | E * E L(G) = { a, a+a, a*a, a*a+a, … } For simplicity, we hardcoded G into gen() def gen() { E(); print EOF } def E() { switch (choice()): case 1: print “a" case 2: E(); print "+"; E() case 3: E(); print "*"; E() } 10 Visualizing string generation Are we generating the string top-down or bottom-up? The tree that describe string derivation is parse tree. 11 Parsing Parsing is the inverse of string generation: given a string, we want to find the parse tree If parsing is the inverse of generation, let’s obtain the parser mechanically from the generator! def gen() { E(); print EOF } def E() { switch (choice()): case 1: print “a" case 2: E(); print "+"; E() case 3: E(); print "*"; E() } 12 Generator vs. parser def gen() { E(); print EOF } def E() { switch (choice()) { case 1: print ‚a" case 2: E(); print "+"; E() case 3: E(); print "*"; E() }} def parse() { E(); scan(EOF) } def E() { switch (oracle()) { case 1: scan("a‚) case 2: E(); scan("+‚); E() case 3: E(); scan("*"); E() }} def scan(s) { if input starts with s, consume s; else abort } 13 Parsing == reconstruction of the parse tree Why do we need the parse tree? We evaluate it to obtain the AST, or perhaps to directly compute the value of the program. 14 Example 1: evaluate an expression (calculator) E (18) Input: 2 * (4 + 5) T (18) T (2) * F (9) F (2) ( E (9) ) int (2) E (4) + T (5) Annotated Parse Tree T (4) F (5) F (4) int (5) int (4) 15 Parse tree vs. abstract syntax tree • Parse tree = concrete syntax tree – contains all syntactic symbols from the input – including those that the parser needs “only” to discover • intended nesting: parentheses, curly braces • statement termination: semicolons • Abstract syntax tree (AST) – abstracts away these artifacts of parsing, – abstraction compresses the parse tree • flattens parse tree hierarchies • drops tokens 16 Ras Bodik, CS 164, Fall 2007, Lecture 6 Add parse tree reconstruction to parser def parse() { root = E(); scan(EOF); return root } def E() { switch (oracle()) { case 1: scan("a") return (‚a‛,) case 2: left = E() scan("+") right = E() return (‚+‛, left, right) case 3: // analogous }} 17 How to implement our oracle? Recall the nondeterministic evaluator from cs61A – (amb 1 2 3 4 5) evaluates to 1 or .. or 5 Which option does amb choose? One leading to success. – in our case, success means parsing successfully How was amb implemented? – backtracking Our parser with amb: def E() { switch (amb(1,2,3)) { case 1: scan("a‚) case 2: E(); scan("+‚); E() case 3: E(); scan("*"); E() }} 18 How do we implement amb in cs164 We won’t. We could implementing with coroutines, but instead we’ll move on to logic programming. We will define a parser as a backtracking logic program with exponential time complexity and then observe that its structure actually permits polynomial time algorithm. 19 Prolog refresher 20 Example with lists 21 Backtracking parser in prolog Our grammar, again: E ::= a | E+E | E*E Backtracking parser for this grammar in Prolog e(In,Out) :- In==[a|Out]. e(In,Out) :- e(In,T1), T1==[+|T2], e(T2,Out) e(In,Out) :- e(In,T1), T1==[*|T2], e(T2,Out) Can be written more concisely as e([a|T],T). e(In,Out) :- e(In, [+|T]), e(T,Out) e(In,Out) :- e(In, [*|T]), e(T,Out) 22 How does this parser work? Let’s start with this (incomplete) program: e([a|T],T). Sample queries: e([a,+,a],Rest). --> Rest = [+,a] e([a],Rest). -->Rest = [] e([a],[]). --> true // parsed successfully 23 A full example Consider input a+a*a e(In,Out) :- In==[a|Out]. e(In,Out) :- e(In,T1), T1==[+|T2], e(T2,Out) e(In,Out) :- e(In,T1), T1==[*|T2], e(T2,Out) 24 Running time of the backtracking parser We can analyze either version, they are the same. amb: def E() { switch (amb(1,2,3)) { case 1: scan("a‚) case 2: E(); scan("+‚); E() case 3: E(); scan("*"); E() }} Prolog: e(In,Out) :- In==[a|Out]. e(In,Out) :- e(In,T1), T1==[+|T2], e(T2,Out) e(In,Out) :- e(In,T1), T1==[*|T2], e(T2,Out) 25 CYK parser Can we run our Prolog parser in polynomial time? Yes, with dynamic programming (an algo technique). Let’s refactor the program a little. Here, e(i,j) is true iff the input[i:j] can be derived (ie generated) from E. e(i,i+1) :- input[i]==‘a’. e(i,j) :- e(i,k), input[k]==‘+’, e(k+1,j) e(i,j) :- e(i,k), input[k]==‘*’, e(k+1,j) Now, instead of top-down backtracking, let’s compute all facts bottom up. 27 Bottom-up evaluation of a prolog program Input: a + a * a Let’s compute gradually which facts we know hold. Step 1: base case (process input segments of length 1) e(0,1) = e(2,3) = e(4,5) = true Step 2: inductive case (input segments of length 3) e(0,3) = true // using rule #2 e(2,5) = true // using rule #3 Step 2 again: inductive case (segments of length 5) e(0,5) = true // using either rule #2 or #3 This evaluation is O(n3) time For grammars with at most two symbols on right-hand side 28 A graphical way to visualize this evaluation Initial graph: the input (terminals) Then: Keep adding non-terminal edges until no more can be added. An edge is added when some adjacent edges form rhs of a production. E 9 E10 E6 E7 E8 a1 +2 a3 *4 a5 E11 Input: a + a * a 29 CYK: the algorithm (can you find the bug?) for i=0,N-1 do add (i,i+1,nonterm(input[i])) to graph -- create nonterminal edges Ad enqueue( (i,i+1,nonterm(input[i])) ) -- nonterm() maps d to A while queue not empty do (j,k,B)=dequeue() for each edge (i,j,A) do -- for each edge “left-adjacent” to (j,k,B) if rule TAB exists then if edge e=(i,k,T) does not exists then add e to graph; enqueue(e) for each edge (k,l,C) do -- for each edge “right-adjacent” to (j,k,B) ..
Load more

Recommended publications
  • Derivatives of Parsing Expression Grammars
    Derivatives of Parsing Expression Grammars Aaron Moss Cheriton School of Computer Science University of Waterloo Waterloo, Ontario, Canada [email protected] This paper introduces a new derivative parsing algorithm for recognition of parsing expression gram- mars. Derivative parsing is shown to have a polynomial worst-case time bound, an improvement on the exponential bound of the recursive descent algorithm. This work also introduces asymptotic analysis based on inputs with a constant bound on both grammar nesting depth and number of back- tracking choices; derivative and recursive descent parsing are shown to run in linear time and constant space on this useful class of inputs, with both the theoretical bounds and the reasonability of the in- put class validated empirically. This common-case constant memory usage of derivative parsing is an improvement on the linear space required by the packrat algorithm. 1 Introduction Parsing expression grammars (PEGs) are a parsing formalism introduced by Ford [6]. Any LR(k) lan- guage can be represented as a PEG [7], but there are some non-context-free languages that may also be represented as PEGs (e.g. anbncn [7]). Unlike context-free grammars (CFGs), PEGs are unambiguous, admitting no more than one parse tree for any grammar and input. PEGs are a formalization of recursive descent parsers allowing limited backtracking and infinite lookahead; a string in the language of a PEG can be recognized in exponential time and linear space using a recursive descent algorithm, or linear time and space using the memoized packrat algorithm [6]. PEGs are formally defined and these algo- rithms outlined in Section 3.
    [Show full text]
  • Abstract Syntax Trees & Top-Down Parsing
    Abstract Syntax Trees & Top-Down Parsing Review of Parsing • Given a language L(G), a parser consumes a sequence of tokens s and produces a parse tree • Issues: – How do we recognize that s ∈ L(G) ? – A parse tree of s describes how s ∈ L(G) – Ambiguity: more than one parse tree (possible interpretation) for some string s – Error: no parse tree for some string s – How do we construct the parse tree? Compiler Design 1 (2011) 2 Abstract Syntax Trees • So far, a parser traces the derivation of a sequence of tokens • The rest of the compiler needs a structural representation of the program • Abstract syntax trees – Like parse trees but ignore some details – Abbreviated as AST Compiler Design 1 (2011) 3 Abstract Syntax Trees (Cont.) • Consider the grammar E → int | ( E ) | E + E • And the string 5 + (2 + 3) • After lexical analysis (a list of tokens) int5 ‘+’ ‘(‘ int2 ‘+’ int3 ‘)’ • During parsing we build a parse tree … Compiler Design 1 (2011) 4 Example of Parse Tree E • Traces the operation of the parser E + E • Captures the nesting structure • But too much info int5 ( E ) – Parentheses – Single-successor nodes + E E int 2 int3 Compiler Design 1 (2011) 5 Example of Abstract Syntax Tree PLUS PLUS 5 2 3 • Also captures the nesting structure • But abstracts from the concrete syntax a more compact and easier to use • An important data structure in a compiler Compiler Design 1 (2011) 6 Semantic Actions • This is what we’ll use to construct ASTs • Each grammar symbol may have attributes – An attribute is a property of a programming language construct
    [Show full text]
  • AST Indexing: a Near-Constant Time Solution to the Get-Descendants-By-Type Problem Samuel Livingston Kelly Dickinson College
    Dickinson College Dickinson Scholar Student Honors Theses By Year Student Honors Theses 5-18-2014 AST Indexing: A Near-Constant Time Solution to the Get-Descendants-by-Type Problem Samuel Livingston Kelly Dickinson College Follow this and additional works at: http://scholar.dickinson.edu/student_honors Part of the Software Engineering Commons Recommended Citation Kelly, Samuel Livingston, "AST Indexing: A Near-Constant Time Solution to the Get-Descendants-by-Type Problem" (2014). Dickinson College Honors Theses. Paper 147. This Honors Thesis is brought to you for free and open access by Dickinson Scholar. It has been accepted for inclusion by an authorized administrator. For more information, please contact [email protected]. AST Indexing: A Near-Constant Time Solution to the Get-Descendants-by-Type Problem by Sam Kelly Submitted in partial fulfillment of Honors Requirements for the Computer Science Major Dickinson College, 2014 Associate Professor Tim Wahls, Supervisor Associate Professor John MacCormick, Reader Part-Time Instructor Rebecca Wells, Reader April 22, 2014 The Department of Mathematics and Computer Science at Dickinson College hereby accepts this senior honors thesis by Sam Kelly, and awards departmental honors in Computer Science. Tim Wahls (Advisor) Date John MacCormick (Committee Member) Date Rebecca Wells (Committee Member) Date Richard Forrester (Department Chair) Date Department of Mathematics and Computer Science Dickinson College April 2014 ii Abstract AST Indexing: A Near-Constant Time Solution to the Get- Descendants-by-Type Problem by Sam Kelly In this paper we present two novel abstract syntax tree (AST) indexing algorithms that solve the get-descendants-by-type problem in near constant time.
    [Show full text]
  • Syntactic Analysis, Or Parsing, Is the Second Phase of Compilation: the Token File Is Converted to an Abstract Syntax Tree
    Syntactic Analysis Syntactic analysis, or parsing, is the second phase of compilation: The token file is converted to an abstract syntax tree. Analysis Synthesis Compiler Passes of input program of output program (front -end) (back -end) character stream Intermediate Lexical Analysis Code Generation token intermediate stream form Syntactic Analysis Optimization abstract intermediate syntax tree form Semantic Analysis Code Generation annotated target AST language 1 Syntactic Analysis / Parsing • Goal: Convert token stream to abstract syntax tree • Abstract syntax tree (AST): – Captures the structural features of the program – Primary data structure for remainder of analysis • Three Part Plan – Study how context-free grammars specify syntax – Study algorithms for parsing / building ASTs – Study the miniJava Implementation Context-free Grammars • Compromise between – REs, which can’t nest or specify recursive structure – General grammars, too powerful, undecidable • Context-free grammars are a sweet spot – Powerful enough to describe nesting, recursion – Easy to parse; but also allow restrictions for speed • Not perfect – Cannot capture semantics, as in, “variable must be declared,” requiring later semantic pass – Can be ambiguous • EBNF, Extended Backus Naur Form, is popular notation 2 CFG Terminology • Terminals -- alphabet of language defined by CFG • Nonterminals -- symbols defined in terms of terminals and nonterminals • Productions -- rules for how a nonterminal (lhs) is defined in terms of a (possibly empty) sequence of terminals
    [Show full text]
  • Lecture 3: Recursive Descent Limitations, Precedence Climbing
    Lecture 3: Recursive descent limitations, precedence climbing David Hovemeyer September 9, 2020 601.428/628 Compilers and Interpreters Today I Limitations of recursive descent I Precedence climbing I Abstract syntax trees I Supporting parenthesized expressions Before we begin... Assume a context-free struct Node *Parser::parse_A() { grammar has the struct Node *next_tok = lexer_peek(m_lexer); following productions on if (!next_tok) { the nonterminal A: error("Unexpected end of input"); } A → b C A → d E struct Node *a = node_build0(NODE_A); int tag = node_get_tag(next_tok); (A, C, E are if (tag == TOK_b) { nonterminals; b, d are node_add_kid(a, expect(TOK_b)); node_add_kid(a, parse_C()); terminals) } else if (tag == TOK_d) { What is the problem node_add_kid(a, expect(TOK_d)); node_add_kid(a, parse_E()); with the parse function } shown on the right? return a; } Limitations of recursive descent Recall: a better infix expression grammar Grammar (start symbol is A): A → i = A T → T*F A → E T → T/F E → E + T T → F E → E-T F → i E → T F → n Precedence levels: Nonterminal Precedence Meaning Operators Associativity A lowest Assignment = right E Expression + - left T Term * / left F highest Factor No Parsing infix expressions Can we write a recursive descent parser for infix expressions using this grammar? Parsing infix expressions Can we write a recursive descent parser for infix expressions using this grammar? No Left recursion Left-associative operators want to have left-recursive productions, but recursive descent parsers can’t handle left recursion
    [Show full text]
  • Understanding Source Code Evolution Using Abstract Syntax Tree Matching
    Understanding Source Code Evolution Using Abstract Syntax Tree Matching Iulian Neamtiu Jeffrey S. Foster Michael Hicks [email protected] [email protected] [email protected] Department of Computer Science University of Maryland at College Park ABSTRACT so understanding how the type structure of real programs Mining software repositories at the source code level can pro- changes over time can be invaluable for weighing the merits vide a greater understanding of how software evolves. We of DSU implementation choices. Second, we are interested in present a tool for quickly comparing the source code of dif- a kind of “release digest” for explaining changes in a software ferent versions of a C program. The approach is based on release: what functions or variables have changed, where the partial abstract syntax tree matching, and can track sim- hot spots are, whether or not the changes affect certain com- ple changes to global variables, types and functions. These ponents, etc. Typical release notes can be too high level for changes can characterize aspects of software evolution use- developers, and output from diff can be too low level. ful for answering higher level questions. In particular, we To answer these and other software evolution questions, consider how they could be used to inform the design of a we have developed a tool that can quickly tabulate and sum- dynamic software updating system. We report results based marize simple changes to successive versions of C programs on measurements of various versions of popular open source by partially matching their abstract syntax trees. The tool programs, including BIND, OpenSSH, Apache, Vsftpd and identifies the changes, additions, and deletions of global vari- the Linux kernel.
    [Show full text]
  • Parsing with Earley Virtual Machines
    Communication papers of the Federated Conference on DOI: 10.15439/2017F162 Computer Science and Information Systems, pp. 165–173 ISSN 2300-5963 ACSIS, Vol. 13 Parsing with Earley Virtual Machines Audrius Saikˇ unas¯ Institute of Mathematics and Informatics Akademijos 4, LT-08663 Vilnius, Lithuania Email: [email protected] Abstract—Earley parser is a well-known parsing method used EVM is capable of parsing context-free languages with data- to analyse context-free grammars. While being less efficient in dependant constraints. The core idea behind of EVM is to practical contexts than other generalized context-free parsing have two different grammar representations: one is user- algorithms, such as GLR, it is also more general. As such it can be used as a foundation to build more complex parsing algorithms. friendly and used to define grammars, while the other is used We present a new, virtual machine based approach to parsing, internally during parsing. Therefore, we end up with grammars heavily based on the original Earley parser. We present several that specify the languages in a user-friendly way, which are variations of the Earley Virtual Machine, each with increasing then compiled into grammar programs that are executed or feature set. The final version of the Earley Virtual Machine is interpreted by the EVM to match various input sequences. capable of parsing context-free grammars with data-dependant constraints and support grammars with regular right hand sides We present several iterations of EVM: and regular lookahead. • EVM0 that is equivalent to the original Earley parser in We show how to translate grammars into virtual machine it’s capabilities.
    [Show full text]
  • To an Abstract Syntax Tree
    School of Computing Science @ Simon Fraser University Parsing Lecture 09: Data Abstraction ++ ➡ Parsing A parser transforms a source program (in concrete • Parsing is the process of translating a sequence of characters (a string) into syntax) toana abstractn abs syntaxtract tree.syn tax tree (AST): program AST −−−−−−→ Parser −−−→ Processor text • Compilers (and some interpreters) analyse abstract syntax trees. • Compilers very well understood • In fact, there exist “compiler compilers” • Example: YACC (yet another compiler compiler) http://dinosaur.compilertools.net/ An Abstraction for Inductive Data Types – p.34/43 Copyright © Bill Havens 1 School of Computing Science @ Simon Fraser University Expression Syntax Extended • An abstract syntax expresses the structure of the concrete syntax without the details. • Example: extended syntax for Scheme expressions • Includes literals (constant numbers) <exp> ::= <number> | <symbol> | (lambda (<symbol>) <exp>) | (<exp> <exp>) • We would define an abstract datatype for this grammar as follows: (define-datatype expression expression? (lit-exp (datum number?)) (var-exp (id symbol?)) (lambda-exp (id symbol?) (body expression?)) (app-exp (rator expression?) (rand expression?))) • Now lets parse sentences in the language specified by the BNF grammar into abstract syntax trees Copyright © Bill Havens 2 School of Computing Science @ Simon Fraser University Parser for Extended Expressions • Here is an example parser for the lambda expression language above: (define parse-expression (lambda (datum) (cond ((number?
    [Show full text]
  • CSCI 742 - Compiler Construction
    CSCI 742 - Compiler Construction Lecture 12 Recursive-Descent Parsers Instructor: Hossein Hojjat February 12, 2018 Recap: Predictive Parsers • Predictive Parser: Top-down parser that looks at the next few tokens and predicts which production to use • Efficient: no need for backtracking, linear parsing time • Predictive parsers accept LL(k) grammars • L means “left-to-right” scan of input • L means leftmost derivation • k means predict based on k tokens of lookahead 1 Implementations Analogous to lexing: Recursive descent parser (manual) • Each non-terminal parsed by a procedure • Call other procedures to parse sub-nonterminals recursively • Typically implemented manually Table-driven parser (automatic) • Push-down automata: essentially a table driven FSA, plus stack to do recursive calls • Typically generated by a tool from a grammar specification 2 Making Grammar LL • Recall the left-recursive grammar: S ! S + E j E E ! num j (E) • Grammar is not LL(k) for any number of k • Left-recursion elimination: rewrite left-recursive productions to right-recursive ones S ! EE0 S ! S + E j E E0 ! +EE0 j E ! num j (E) E ! num j (E) 3 Making Grammar LL • Recall the grammar: S ! E + E j E E ! num j (E) • Grammar is not LL(k) for any number of k • Left-factoring: Factor common prefix E, add new non-terminal E0 for what follows that prefix S ! EE0 S ! E + E j E E0 ! +E j E ! num j (E) E ! num j (E) 4 Parsing with new grammar S ! EE0 E0 ! +E j E ! num j (E) Partly-derived String Lookahead parsed part unparsed part S ( (num)+num ) E E0 ( (num)+num ) (E)
    [Show full text]
  • Syntax and Parsing
    Syntax and Parsing COMS W4115 Prof. Stephen A. Edwards Spring 2002 Columbia University Department of Computer Science Last Time Administrivia Class Project Types of Programming Languages: Imperative, Object-Oriented, Functional, Logic, Dataflow This Time Interpreters and Compilers Structure of a Compiler Lexical Analysis Syntax Parsing The Compilation Process Interpreters Source Program # Input ! Interpreter ! Output Compilers Source Program # Compiler # Input ! Executable Program ! Output Structure of a Compiler # Program Text Lexer # Token Stream Parser # Abstract Syntax Tree Static semantics (type checking) # Annotated AST Translation to intermediate form # Three-address code Code generation # Assembly Code Compiling a Simple Program int gcd(int a, int b) { while (a != b) { if (a > b) a -= b; else b -= a; } return a; } What the Compiler Sees int gcd(int a, int b) { while (a != b) { if (a > b) a -= b; else b -= a; } return a; } i n t sp g c d ( i n t sp a , sp i n t sp b ) nl { nl sp sp w h i l e sp ( a sp ! = sp b ) sp { nl sp sp sp sp i f sp ( a sp > sp b ) sp a sp - = sp b ; nl sp sp sp sp e l s e sp b sp - = sp a ; nl sp sp } nl sp sp r e t u r n sp a ; nl } nl Text file is a sequence of characters After Lexical Analysis int gcd(int a, int b) { while (a != b) { if (a > b) a -= b; else b -= a; } return a; } int gcd ( int a , int b ) – while ( a != b ) – if ( a > b ) a -= b ; else b -= a ; ˝ return a ; ˝ A stream of tokens.
    [Show full text]
  • Top-Down Parsing
    CS 4120 Introduction to Compilers Andrew Myers Cornell University Lecture 5: Top-down parsing 5 Feb 2016 Outline • More on writing CFGs • Top-down parsing • LL(1) grammars • Transforming a grammar into LL form • Recursive-descent parsing - parsing made simple CS 4120 Introduction to Compilers 2 Where we are Source code (character stream) Lexical analysis Token stream if ( b == 0 ) a = b ; Syntactic Analysis if Parsing/build AST == = ; Abstract syntax tree (AST) b 0 a b Semantic Analysis CS 4120 Introduction to Compilers 3 Review of CFGs • Context-free grammars can describe programming-language syntax • Power of CFG needed to handle common PL constructs (e.g., parens) • String is in language of a grammar if derivation from start symbol to string • Ambiguous grammars a problem CS 4120 Introduction to Compilers 4 Top-down Parsing • Grammars for top-down parsing • Implementing a top-down parser (recursive descent parser) • Generating an abstract syntax tree CS 4120 Introduction to Compilers 5 Parsing Top-down S → E + S | E E → num | ( S ) Goal: construct a leftmost derivation of string while reading in token stream Partly-derived String Lookahead parsed part unparsed part S ( (1+2+(3+4))+5 → E+S ( (1+2+(3+4))+5 → (S) +S 1 (1+2+(3+4))+5 → (E+S)+S 1 (1+2+(3+4))+5 → (1+S)+S 2 (1+2+(3+4))+5 → (1+E+S)+S 2 (1+2+(3+4))+5 → (1+2+S)+S 2 (1+2+(3+4))+5 → (1+2+E)+S ( (1+2+(3+4))+5 → (1+2+(S))+S 3 (1+2+(3+4))+5 → (1+2+(E+S))+S 3 (1+2+(3+4))+5 6 Problem S → E + S | E E → num | ( S ) • Want to decide which production to apply based on next symbol (1) S
    [Show full text]
  • Codegenassem.Java) Lexical Analysis (Scanning)
    Undergraduate Compilers Review Structure of a Typical Interpreter Compiler Analysis Synthesis Announcements character stream – Send me email if you have set up a group. Include account names for group. lexical analysis IR code generation – Each project is 10% of grade tokens “words” IR Today syntactic analysis optimization – Outline of planned topics for course AST “sentences” IR – Overall structure of a compiler semantic analysis code generation – Lexical analysis (scanning) – Syntactic analysis (parsing) annotated AST target language interpreter CS553 Lecture Undergrad Compilers Review 2 CS553 Lecture Undergrad Compilers Review 3 Structure of the MiniJava Compiler (CodeGenAssem.java) Lexical Analysis (Scanning) Analysis Synthesis Break character stream into tokens (“words”) character stream – Tokens, lexemes, and patterns – Lexical analyzers are usually automatically generated from patterns Lexer lexical analysis IR code generation Translate (regular expressions) (e.g., lex) tokens “words” IRT Tree/ Examples Parser.parse() syntactic analysis instruction selection Mips/Codegen token lexeme(s) pattern AST “sentences” Assem const const const Project 4 if if if BuildSymTable semantic analysis optimization relation <,<=,=,!=,... < | <= | = | != | ... CheckTypes identifier foo,index [a-zA-Z_]+[a-zA-Z0-9_]* Assem CodeGenAssem number 3.14159,570 [0-9]+ | [0-9]*.[0-9]+ string “hi”, “mom” “.*” AST and symbol table code generation minijava.node/ MIPS const pi := 3.14159 ⇒ const, identifier(pi), assign,number(3.14159) SymTable/ CS553 Lecture Undergrad
    [Show full text]