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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2. Attributes and Attribute Grammars
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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3. 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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4. S-Attributed Grammars Example
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5. The Annotated Parse Tree
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6. Inherited Attributes Inherited Attributes Example
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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7. Inherited Attributes Example
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8. The Annotated Parser Tree
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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.v+B.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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10. Dependency Graphs
Each node represents an attribute of a node in the parse tree
Each arc represents the dependence relationship (flow of values)
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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)
by Neng-Fa Zhou