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Understanding Semantic Analysis in Programming Languages

Explore semantic analysis, attribute grammar, evaluation order, and type checking in programming languages. Learn about static and run-time semantics, symbol tables, and attribute computation during parsing.

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Understanding Semantic Analysis in Programming Languages

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  1. Semantic Analysis Checking what parsers cannot Semantic Analysis

  2. Outline • Overview • Attribute grammar • Attribute • Semantic rule • Computing attributes • Evaluation order • Synthesized and Inherited attribute • Computation of attributes during parsing • Type checking • Type declaration • Type inference • Symbol table Semantic Analysis

  3. Overview • Semantics • Static semantics • Data type (in some languages e.g. C, FORTRAN) • Names • Run-time semantics • Data value • Data type (in other languages e.g. Perl) • Semantic analyzer • Determine if the semantic of the program is correct according to the definition of the language • Concern only static semantics Semantic Analysis

  4. Overview (cont’d) • How to describe semantics • Associated to syntax • Syntax-directed semantics • An attribute represents a semantic concept. • A semantic rule (or attribute equation) is associated to a syntactic rule. • A semantic rule describes the relationship between attributes of symbols in a syntactic rule. Semantic Analysis

  5. Attribute Grammar • Language construct • Variables, function declarations, statements • Attribute • Property of language construct • Name, address, type, value, and scope are attributes of variables. • Return type, parameter types, return address, and scope are attributes of functions. • Syntax tree is an attributes of code sections. • Attribute equation (semantic rule) • Associated with grammar production • Specify the relationship between attributes of construct in a syntactic rule. • Attribute grammar • Set of attribute equations Semantic Analysis

  6. Construct: num digit Attribute: val Grammar num -> num digit num -> digit digit -> 0|1|…|9 Example of Attribute Grammar Semantic Analysis

  7. Parse Tree with Attributes num val = 45*10+5 digit val = 5 num val = 4*10+5 5 num val = 4 digit val = 5 5 digit val = 4 4 Semantic Analysis

  8. Grammar Rule dec -> type varList type -> int type -> float varList1 -> id , varList2 varList -> id Semantic Rule varList.dtype = type.dtype type.dtype = int type.dtype =real id.dtype = varList1.dtype varList2.dtype= varList1.dtype id.dtype = varList.dtype Attribute Grammar for Data Type Declaration dec type dtype= real varList dtype=real dtype=real id varList float , dtype=real dtype=real id Semantic Analysis

  9. Bnum num baseC num digit o digit Another Example of Attribute Grammar val=19 base=8 val=19 base=8 base=8 val=2 base=8 val=3 base=8 val=2 Semantic Analysis

  10. X.a X1.a1 X2.a2 Xn.an … X.a X2.a2 X1.a1 … Xn.an Evaluation Order • From semantic rule X.a=f(X1.a1, X2.a2,…, Xn.an) • Value of a in node X depends on the values of a1in X1, a2in X2,…, and anin Xn. • The order of evaluation can be shown in a dependency graph, which is a directed acyclic graph (DAG). X.a X1.a1 X2.a2 Xn.an … X.a X2.a2 X1.a1 … Xn.an Semantic Analysis

  11. Dependency Graph: Example 1 num val = 45*10+5 digit val = 5 num val = 4*10+5 5 num val = 4 digit val = 5 5 digit val = 4 4 Semantic Analysis

  12. Grammar Rule dec -> type varList type -> int type -> float varList1 -> id , varList2 varList -> id Semantic Rule varList.dtype = type.dtype type.dtype = int type.dtype =real id.dtype = varList1.dtype varList2.dtype= varList1.dtype id.dtype = varList.dtype Dependency Graph: Example 2 dec type dtype= real varList dtype=real dtype=real id varList float , dtype=real dtype=real id Semantic Analysis

  13. Dependency Graph:Example 3 Bnum num baseC num digit o digit Semantic Analysis

  14. Rule-based Attribute Evaluation • Order of attribute evaluation can be fixed at compiler construction • Attribute grammar is to be analyzed in order to find the orger of evaluation • Used often in practice • Not general method • Two types of attributes • Synthesized attributes • Inherited attributes Semantic Analysis

  15. Synthesized Attributes • An attribute ais a synthesized attribute if • for a grammar rule A ->X1 X2 … Xn , anattribute equationwitha on the LHS is of the form A.a = f(X1.a1, X2.a2, … Xn.an), or • all dependencies point from child to parent in the parse tree • If all attributes in an attribute grammar are synthesized attributes, the grammar is called an S-attributed grammar. num num digit num digit 5 digit 5 4 Semantic Analysis

  16. Order of Evaluation for Synthesized Attributes • Use postorder (or bottom-up) evaluation Procedure PostEval (T:node) { for each child C of T { PostEval(C); } compute all synthesized attributes of T } 455 num 45 5 num digit 4 5 5 num digit 5 4 digit 5 4 Semantic Analysis

  17. Inherited Attributes dec type varList float id , varList id • An attribute is an inherited attribute if it is not a synthesized attribute. • An attribute grammar is an L-attributed grammar if for each inherited attribute aj at Xi in each grammar rule X -> X1 X2 … Xndepends on the value of attributes of symbols X, X1, X2,…, Xi-1. Semantic Analysis

  18. Order of Evaluation for Inherited Attributes • Use preorder and inorder evaluation together Procedure Eval (T:node) { case nodeType(T) of dec: { Eval(typeChild(T)); varList.dtype=type.dtype; Eval(varChild(T)); } type: { if child(T) is int then T.dtype=int; if child(T) is float then T.dtype=real; } varList: { leftChild(T).dtype=T.dtype; if (rightmostChild(T) is not null) then {rightmostChild(T).dtype=T.dtype; Eval(rightChild(T));} } } dec float float type varList float float float id varList , float id Semantic Analysis

  19. Another Example of Inherited Attributes Proc Eval(T:node) { case Nodetype(T) of Bnum: { Eval(rightChild(T)); (leftChild(T)).base= (rightChild(T)).base; Eval(leftChild(T)); T.val=leftChild(T).val; } baseC: { if child(T)=o then T.base=8; if child(T)=d thenT.base=10;} num: { leftChild(T).base=T.base; Eval(leftChild(T)); if (rightChild(T)!=null) then { rightChild(T).base=T.base; Eval(rightChild(T); T.val=f(T); } else T.val=leftChild(T).val; cal val} digit: { … } } val=44 Bnum val=44 base=8 base=8 num baseC base=8 base=8 val=5 num digit o val=4 val=5 base=8 digit Semantic Analysis

  20. Evaluation Order for Synthesized + Inherited Attributes Procedure CombinedEval(T:node) { for each child C of T { compute all inherited attributes of C; CombinedEval(C); } compute all synthesized attributes of T; } Semantic Analysis

  21. Attribute Computation During Parsing Semantic Analysis

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