1 / 27

Bottom-Up Syntax Analysis

Understand bottom-up syntax analysis using pushdown automata, LR parsing methods, and parser table construction. Learn efficient parsing techniques for context-free grammars to generate syntax trees and detect errors.

hugov
Download Presentation

Bottom-Up Syntax Analysis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Bottom-Up Syntax Analysis Mooly Sagiv html://www.math.tau.ac.il/~msagiv/courses/wcc01.html Textbook:Modern Compiler Implementation in C Chapter 3

  2. Pushdown automata Deterministic Report an error as soon as the input is not a prefix of a valid program Not usable for all context free grammars context free grammar parser tokens Efficient Parsers bison “Ambiguity errors” parse tree

  3. Top-Down (Predictive Parsing) LL Construct parse tree in a top-down matter Find the leftmost derivation For every non-terminal and token predict the next production Bottom-Up LR Construct parse tree in a bottom-up manner Find the rightmost derivation in a reverse order For every potential right hand side and token decide when a production is found Kinds of Parsers

  4. Input A context free grammar A stream of tokens Output A syntax tree or error Method Construct parse tree in a bottom-up manner Find the rightmost derivation in (reversed order) For every potential right hand side and token decide when a production is found Report an error as soon as the input is not a prefix of valid program Bottom-Up Syntax Analysis

  5. Pushdown automata Bottom-up parsing (given a parser table) Constructing the parser table Interesting non LR grammars Plan

  6. Pushdown Automaton input u t w $ V control parser-table $ stack

  7. reduceA   Pop | | symbol from the stack Apply the associated action Push a symbol goto[top, A] on the stack shiftX Push X onto the stack Advance the input accept Parsing is complete error Report an error Bottom-Up Parser Actions

  8. A Parser Table for S a S b|  Manual Construction?

  9. A Parser Table for S AB | AA aB  a

  10. A Parser Table for E E + E| id

  11. The Challenge • How to construct a parser-table from a given grammar • LR(1) grammars • Left to right scanning • Rightmost derivations (reverse) • 1 token • Different solutions • Operator precedence • SLR(1) • Simple LR(1) • CLR(1) • Canonic LR(1) • LALR(1) • Look Ahead LR(1) • Yacc, Bison, JCUP

  12. Grammar Hierarchy Non-ambiguous CFG CLR(1) LL(1) LALR(1) SLR(1)

  13. Constructing an SLR parsing table • Add a production S’  S$ • Construct a finite automaton accepting “valid stack symbols” • The states of the automaton becomes the states of parsing-table • Determine shift operations • Determine goto operations • Construct reduce entries by analyzing the grammar

  14. A finite Automaton for S’  S$ S a S b|  a a S b 0 1 2 3 S 4

  15. Constructing a Finite Automaton • NFA • For X  X1 X2 … Xn • [X  X1 X2 …XiXi+1 … Xn] • “prefixes of rhs (handles)” • X1 X2 … Xi is at the top of the stack and we expect Xi+1 … Xn • The initial state [S’  .S$] • ([X  X1…XiXi+1 … Xn], Xi+1 = [X  X1 …XiXi+1  … Xn] • For every production Xi+1   ([[X  X1 X2 …XiXi+1 … Xn],  ) = [Xi+1   ] • Convert into DFA

  16. a S   b  S  NFA S’  S$ S a S b|  [S .aSb] [S a.Sb] [S aS.b] [S’ .S$] [S .] [S aSb.] [S’ S.$]

  17.  DFA   [S’ .S$] [S .aSb] [S .] [S a.Sb] [S .aSb] [S .] S a [S aS.b] b [S aSb.] S a [S’ S.$] a S [S .aSb] [S a.Sb] [S aS.b] [S’ .S$] b S [S .] [S aSb.] [S’ S.$]

  18. [S’ .S$] [S .aSb] [S .] [S a.Sb] [S .aSb] [S .] S a [S aS.b] b [S aSb.] S a [S’ S.$]

  19. Filling reduce entries • For an item [A .] we need to know the tokens that can follow A in a derivation from S’ • Follow(A) = {t | S’ * At} • See the textbook for an algorithm for constructing Follow from a given grammar

  20. [S’ .S$] [S .aSb] [S .] [S a.Sb] [S .aSb] [S .] S a [S aS.b] b [S aSb.] S a [S’ S.$] Follow(S) = {b, $} r S  r S  r S  r S  r S a S b

  21. Example E  E + E| id

  22. Interesting Non SLR(1) Grammar S’  S$ S  L = R | R L  *R | id R  L Partial DFA [S L=.R] [R .L] [L .*R] [L .id] [S’ .S$] [S .L=R] [S .R] [L .*R] [L .id] [R L] [S L.=R] [R L.] = L Follow(R)= {$, =}

  23. LR(1) Parser • Item [A ., t] •  is at the top of the stack and we are expecting t • LR(1) State • Sets of items • LALR(1) State • Merge items with the same look-ahead

  24. Interesting Non LR(1) Grammars • Ambiguous • Arithmetic expressions • Dangling-else • Common derived prefix • A  B1 a b | B2 a c • B1   • B2   • Optional non-terminals • St  OptLab Ass • OptLab  id : |  • Ass  id := Exp

  25. Summary • LR is a powerful technique • Generates efficient parsers • Generation tools exit • Bison, yacc, CUP • But some grammars need to be tuned • Shift/Reduce conflicts • Reduce/Reduce conflicts • Efficiency of the generated parser

More Related