Topic 5: semantic analysis. 5.2 Attribute Grammars

Transcription

1 Topi 5: semanti analysis 5.2 Semanti Analysis Attribute Grammar: An attribute grammar is an extension of a ontextfree grammar, with two extensions: 1.Attributes on Symbols: ah grammar symbol S, terminal or nonterminal, is speified to have various attributes. ah attribute is speified both with a name (e.g., type, value, and also a type, restriting the range of fillers the attribute an take. 2.Attribute evaluation rules: ah prodution rule in the grammar an have assoiated with it a number of semanti rules, eah of whih speifies how an attribute of one symbol an be alulated from the attributes of other symbols in the prodution. 2 1

2 Semanti Analysis Attribute valuation Rules: Sometimes alled semanti rules A mehanism for propagating attributes between nodes. ah CF Grammar rule an have assoiated one or more semanti rules whih state how attributes are to be propagated between the symbls. CFG Rule Mode :- int Del :- Mode IDList IDList :- id Semanti Rule Mode.dtyp := integer Del.type := Mode.dtyp; IDList.type := Del.type id.type = IDList.type 3 Semanti analysis Indexing of Symbols: In normal CFG, a symbol an our multiple times in the rule: IDList :- id, IDList In (AGs), where the same symbol appears more than one in the prodution (on either side), we need to refer uniquely to eah appearane of the symbol. We will add subsripts to those on the RHS: IDList :- id, IDList 1 This allows the semanti rules to refer distintly to eah of the symbols whih have the same name: IDList 1.type := IDList.type 4 2

3 Semanti analysis From Grammar to Tree: Semanti Rules are assoiated with eah prodution of the CFG. ah node of the parse tree orresponds to a prodution rule: IDList IDList :- id, IDList id, IDList So, as we build the parse tree, we also attah the semanti rules of the prodution with the node of the tree. 5 Semanti analysis xample 1 revisited: Lets now look at the semanti rules behind example () 6 3

4 Semanti analysis xample 1 revisited: First, we add in indexes for repeated symbols ( 1 ) i 7 Semanti analysis xample 1 revisited: Then, we add in attribute propagation rules {.val:= 1.val+ 2.val } 1-2 {.val:= 1.val- 2.val } - 1 {.val:= - 1.val } ( 1 ) {.val:= 1.val } i {.val:=symbtable(i.name,type)} {.val:=.val } 8 4

5 Attributes: informal desription xample 1 revisited.val (given by lexial analysis) v:3 9 Attributes: informal desription xample 1 revisited.val:=.val (from rule for :-) v:3 v:3 10 5

6 Attributes: informal desription xample 1 revisited.val:= 1.val+ 2.val v:7 v:3 v:3 11 Attributes: informal desription xample 1 revisited.val:= 1.val v:7 v:7 v:3 v:3 12 6

7 Semanti analysis Definition: An annotated parse tree is the parse tree with the attributes at eah node shown. v:7 v:35 v:5 v:7 v:3 v:3 v:5 13 Semanti analysis & side effets: Attribute evaluation rules have an abstrat form like: X.attrib := f(y.attrib, Z.attrib) where f() is a funtion over attributes. In a strit attribute grammar, the funtions on the RHS of attribute evaluation rules should not have side effets, i.e., they should not hange the struture of the parse tree, nor the attributes of symbols (attributes should only be hanged by assignment). Later, we will look at extensions of attribute grammars whih allow suh side effets. 14 7

8 Semanti analysis Synthesized and Inherited Attribute valuation Rules: A evaluation rule for a synthesized attribute will always have the LHS of the prodution on the LHS of the evaluation rule, e.g., xpr :- xpr 1 + xpr 2 {xpr.val = xpr 1.val + xpr 2.val } A evaluation rule for a inherited attribute will always a symbol from the RHS of the prodution on the LHS of the evaluation rule, e.g., L :- L 1, id { L 1. type = L.type } 15 Semanti analysis Formal Statement of Grammars: A CFG is a quadruple < Σ T, Σ N, rules,axiom>.g, G={ {+,*,(,),,i}, {}, { +, -, -, * /, ^, (), i, }, } 16 8

9 Semanti analysis Formal Statement for an Attribute Grammar (Note that it details the attributes for eah terminal and nonterminal) G ={{+,*,(,),{value,type},i{value,type}}, {{value,type}}, { }, } i + d {.value:= i.value+ d.value,.type:= i i - d {.value:= i.value- d.value,.type:= i - d, {.value:= - d.value,.type:= d * {.value:= i.value* d.value,.type:= i i / d, {.value:= i.value/ d.value,.type:= i.type} i^ d {.value:= i.value^ d.value,.type:= i ( d ) {.value:= d.value,.type:= i i {.value:= i.value,.type:= i {.value:=.value,.type:=.type} 17 9