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Prof. Roy Levow

COP 4620 / 5625 Programming Language Translation / Compiler Writing Fall 2003 Lecture 7, 10/09/2003. Prof. Roy Levow. Bottom-up Parsing. Almost always automatic generation Because algorithm is complex Uses Push-down Automata

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Prof. Roy Levow

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  1. COP 4620 / 5625Programming Language Translation /Compiler WritingFall 2003Lecture 7, 10/09/2003 Prof. Roy Levow

  2. Bottom-up Parsing • Almost always automatic generation • Because algorithm is complex • Uses Push-down Automata • Basic principles are can be seen with LR(0) though it is of little practical value • “handle” is complete sequence of children that can be “reduced”

  3. LR(0) • Using dotted notation • A shift item has dot anywhere but end • A reduce item has dot at end • Item sets are sets of items that can occur in a state

  4. Sample Grammar for LR(0)

  5. Transition Diagram

  6. LR(0) PDA

  7. Example LR(0) Parse

  8. LR Parsing with Lookahead • There are a number of variants for LR parsing with lookahead • Problem is that tables for direct approaches can be very large • 1M for LR(1) • 10K for SLR(1) • 1K for LALR(1) • Smaller size reduces class of languages that can be parsed

  9. Example for LR(1)

  10. LALR(1) Automaton, fig. 2.100

  11. Conflicts & Error Recovery • Grammars that are not LR(1) exhibit • Shift-Reduce Conflicts • Reduce-Reduce Conflicts • Adding actions often generates conflicts • Error recovery • Add symbol in input • Pop items from stack and input to recovery point

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