Compiler Construction Sohail Aslam Lecture 31

2 Beyond Syntax  These questions are part of context-sensitive analysis  Answers depend on values, not parts of speech  Answers may involve computation

3 Beyond Syntax  These questions are part of context-sensitive analysis  Answers depend on values, not parts of speech  Answers may involve computation

4 Beyond Syntax  These questions are part of context-sensitive analysis  Answers depend on values, not parts of speech  Answers may involve computation

5 Beyond Syntax How can we answer these questions?  Use formal methods Context-sensitive grammars Attribute grammars

6 Beyond Syntax How can we answer these questions?  Use formal methods Context-sensitive grammars Attribute grammars

7 Beyond Syntax How can we answer these questions?  Use formal methods Context-sensitive grammars Attribute grammars

8 Beyond Syntax  Use ad-hoc techniques Symbol tables ad-hoc code

9 Attribute Grammars  A CFG is augmented with a set of rules  Each symbol in the derivation has a set of values or attributes  Rules specify how to compute a value for each attribute

10 Attribute Grammars  A CFG is augmented with a set of rules  Each symbol in the derivation has a set of values or attributes  Rules specify how to compute a value for each attribute

11 Attribute Grammars  A CFG is augmented with a set of rules  Each symbol in the derivation has a set of values or attributes  Rules specify how to compute a value for each attribute

12 Attribute Grammars  grammar for signed binary numbers (SBN) Number → Sign List Sign → +  – List → List Bit | Bit Bit → 0  1

13 Attribute Grammars derivation for “ –1 ” Number → Sign List → – List → – Bit → – 1

14 Attribute Grammars For “-101” Number → Sign List → Sign List Bit → Sign List 1 → Sign List Bit 1 → Sign List 0 1 → Sign Bit 0 1 → Sign → – 1 0 1

15 Attribute Grammars For an attributed version of SBN, the following attributes are needed SymbolAttributes Number val Sign neg List pos, val Bit pos, val

16 Attribute Grammars  We will add rules to compute decimal value of a signed binary number

17 Attribute Grammars ProductionsAttribution Rules Number → Sign List List.pos  0 if Sign.neg then Number.val  – List.val else Number.val  List.val Sign → + Sign.neg  false Sign → – Sign.neg  true

18 Attribute Grammars ProductionsAttribution Rules Number → Sign List List.pos  0 if Sign.neg then Number.val  – List.val else Number.val  List.val Sign → + Sign.neg  false Sign → – Sign.neg  true

19 Attribute Grammars ProductionsAttribution Rules List 0 → List 1 Bit List 1.pos  List 0.pos + 1 Bit.pos  List 0.pos List 0.val  List 1.val + Bit.val List → Bit Bit.pos  List.pos List.val  Bit.val Bit → 0 Bit.val  0 Bit → 1 Bit.val  2 Bit.pos

20 Attribute Grammars  Attributes are associated with nodes in parse tree  Rules are value assignments associated with productions

21 Attribute Grammars  Attributes are associated with nodes in parse tree  Rules are value assignments associated with productions

22 Attribute Grammars  Rules and parse tree define an attribute dependence graph Graph must be acyclic

23 Attribute Grammars Number Sign List Bit – 1 List.pos  0 List.val  Bit.val = 1 Number.val  – List.val = –1 Bit.pos  0 Bit.val  2 Bit.pos = 1 Sign.neg  true

24 AttributesAttributes Attributes are distinguished based on the direction of value flow 1.Synthesized attributes 2.Inherited attributes

25 Synthesized Attributes  Attributes of a node whose values are defined wholly in terms of attributes of node’s children and from constants are called synthesized attributes

26 Synthesized Attributes  Values used to compute synthesized attributes flow bottom-up in the parse tree  Good match to LR parsing

27 Inherited Attributes  Attributes whose values are defined in terms of node’s own attributes, node’s siblings node’s parent  Values flow top-down and laterally in the parse tree

28 Inherited Attributes  Attributes whose values are defined in terms of node’s own attributes, node’s siblings node’s parent  Values flow top-down and laterally in the parse tree