Attribute Grammars

1
Attribute Grammars

• Recall the yacc program
• Parse a given expression
• Evaluate it

expr expr '' expr 1 3
expr '-' expr 1 - 3 expr ''
expr 1 3 expr '/' expr
1 / 3 '(' expr ')' 2
NUMBER
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Attributes and Attribute Grammars

• Attributes
• variables (type,offset,first or last
  occurrence,...)
• constants (type, value, scope, ...)
• Attribute grammar(Syntax-directed definition)
• A generalization of CFG in which each grammar
  symbol has an associated set of attributes
• Semantics rules for computing attribute values

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Synthesized Attributes

• Synthesized Attributes
• The value is computed from the values of
  attributes of the children
• S-attributed grammar - synthesized attributes
  only
• Bottom-up propagation
• Example
• values of expressions
• types of expressions

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S-Attributed GrammarsExample
5
The Annotated Parse Tree
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Inherited Attributes

• Inherited Attributes
• The value is computed from the values of
  attributes of the siblings and parent
• Top-down propagation
• Example
• type information
• where does a variable occurs? lhs or rhs

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Inherited AttributesExample
8
The Annotated Parser Tree
9
Converting Binary to Decimal
N BIN N.v BIN.v BIN.r 0 BIN0
BIN1 B BIN1.r BIN0.r 1 B.r
BIN0.r BIN0.v BIN1.vB.v BIN
B BIN.v B.v B.r BIN.r B '1' B.v
2B.r B '0' B.v 0
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Dependency Graphs

• Each node represents an attribute of a node in
  the parse tree
• Each arc represents the dependence relationship
  (flow of values)

11
Computing Attribute Values

• 1.Compute while parsing
• Oblivious methods
• Fast (one pass)
• Complicated
• Some attribute values cannot be computed
• 2. Compute after parsing
• Parse-tree methods (sort dependency graphs)
• Rule-based methods (data-flow or fix-point)