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Compilers

Compilers. ABHISHEK REDDY PAM (11cs30002) date : 07/10/2013. Viable Prefix. The set of prefixes of right sentential forms that can appear on the stack of a shift-reduce parser are called viable prefixes.

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Compilers

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  1. Compilers ABHISHEK REDDY PAM (11cs30002) date : 07/10/2013

  2. Viable Prefix • The set of prefixes of right sentential forms that can appear on the stack of a shift-reduce parser are called viable prefixes. • Right-Sentential Form : A sentential form that can be derived by a rightmost derivation • The prefix of the sentential form may not be the content of the stack.

  3. Example Consider: S  aABe A  Abc | b B  d Rightmost Derivation of the string abbcde: S  aABe  aAde  aAbcde  abbcde The (unique) handle is underlined for each step.

  4. Continuation… A viable prefix is : (1) a string that equals a prefix of a right-sentential form up to (and including) its unique handle. (2) any prefix of a string that satisfies (1) For given example : Viable prefixes: a, aA, aAd, aAbc, ab, aAb,… Not viable prefixes: aAde, Abc, aAA,…

  5. Shift/Reduce Parser All strings in the stack are viable prefixes

  6. When to shift/reduce ? • Sometimes on top of the stack something appears to be a handle (i.e., matches the RHS of a production). • But: maybe we have not shifted enough elements to identify the handle. • Observe the correct sequence of Shift and Reducesteps preserves the property that the stack IS a viable prefix. Example : $aAbcde$ Shift or Reduce? • If we shift we obtain aAbcin the stack (aA is the valid item for the viable prefix aAbc). Recall that Abcis a handle. • Instead if we reduce we obtain aAAin the stack. (this is NOT a viable prefix!!!)

  7. Valid Items DEFINITION: A α.β,{a} is a valid LR(1)-item for a viable prefix γα if there exists a right-most derivation. S γAy  γαβy Such that • a is the first symbol of y (or) • y = ε and a = $

  8. THE END • A DFA accepting viable prefixes. Its states are sets of LR(1)-items (that are valid for the corresponding viable prefixes) • Canonical LR Parser(LR(1) Parser) , States and encoding of valid items and viable prefixes are discussed in the next lecture.

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