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Compiler Construction LR(1)

Compiler Construction LR(1). Rina Zviel-Girshin and Ohad Shacham School of Computer Science Tel-Aviv University. LR Grammar Hierarchy. Non-ambiguous CFG. CLR(1). LALR(1). SLR(1). LR(0). Example. S’  S$ S  A | xb A  aAb | B B  x. S 0.

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Compiler Construction LR(1)

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  1. Compiler ConstructionLR(1) Rina Zviel-Girshinand Ohad Shacham School of Computer Science Tel-Aviv University

  2. LR Grammar Hierarchy Non-ambiguous CFG CLR(1) LALR(1) SLR(1) LR(0)

  3. Example S’  S$ S  A | xb A  aAb | B B  x

  4. S0 S‘S$S AS xbA aAbA  BB  x S7 S4 S’  S$ S  A | xb A  aAb | B B  x B  x A  B B B S3 x A  aAbA aAbA BB x a a S1 A S  A A S6 S x A  aAb S2 S  xbB x S’  S$ b S9 b S8 T S5 S10 A  aAb S  xb S’  S$

  5. S0 S‘S$S AS xbA aAbA  BB  x SLR(1) • Calculate FOLLOW sets for each non terminal • Use the FOLLOW set to break conflicts x S2 S  xbB x b S5 FOLLOW(B) = {b,$} S  xb

  6. LR(1) • Keeps the specific look-ahead with each item • Use the look-ahead to break conflicts • More accurate than use the FOLLOW set

  7. LR(1) S7 S11 S4 B  x {b} A  B {$} A  B {b} B B x S0 S3 S12 x B S‘S$S A {$}S xb {$}A aAb {$}A  B {$}B  x {$} A  aAb {$}A aAb {b}A B {b}B x {b} A  aAb {b}A aAb {b}A B {b}B x {b} a a a A A S1 A S6 S x S13 S  A {$} A  aAb {$} A  aAb {b} S2 S  xb {$}B x {$} S’  S$ S9 b b b T S8 S14 S5 S10 A  aAb {$} A  aAb {b} S  xb {$} S’  S$

  8. LR(0) S0 S‘S$S AS xbA aAbA  BB  x S7 S4 S’  S$ S  A | xb A  aAb | B B  x B  x A  B B B S3 x A  aAbA aAbA BB x a a S1 A S  A A S6 S x A  aAb S2 S  xbB x S’  S$ b S9 b S8 T S5 S10 A  aAb S  xb S’  S$

  9. LR(1) S7 S11 S4 B  x {b} A  B {$} A  B {b} B B x S0 S3 S12 x B S‘S$S A {$}S xb {$}A aAb {$}A  B {$}B  x {$} A  aAb {$}A aAb {b}A B {b}B x {b} A  aAb {b}A aAb {b}A B {b}B x {b} a a a A A S1 A S6 S x S13 S  A {$} A  aAb {$} A  aAb {b} S2 S  xb {$}B x {$} S’  S$ S9 b b b T S8 S14 S5 S10 A  aAb {$} A  aAb {b} S  xb {$} S’  S$

  10. LALR(1) S7 S4 B  x {b} A  B {$,b} B B S3,12 S0 x B S‘S$S A {$}S xb {$}A aAb {$}A  B {$}B  x {$} A  aAb {$,b}A aAb {b}A B {b}B x {b} a a A A S1 S6,13 S x S  A {$} A  aAb {$,b} S2 S  xb {$}B x {$} S’  S$ S9 b b T S8,14 S5 S10 A  aAb {$,b} S  xb {$} S’  S$

  11. LALR(1) • Combines states with the same items • Much smaller parsing tables • May introduce reduce-reduce conflicts • Cannot introduce shift-reduce conflict S  AtB {$} C  x {ty}

  12. LR Grammar Hierarchy Non-ambiguous CFG CLR(1) LALR(1) SLR(1) LR(0)

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