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Semantic Analysis

Semantic Analysis. COMP3190: Principle of Programming Languages. Compilation in a Nutshell 1. Source code (character stream). if (b == 0) a = b;. Lexical analysis. if. (. b. ==. 0. ). a. =. b. ;. Token stream. Parsing. if. ;. ==. =. Abstract syntax tree (AST). b. 0. a.

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Semantic Analysis

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  1. Semantic Analysis COMP3190: Principle of Programming Languages

  2. Compilation in a Nutshell 1 Source code (character stream) if (b == 0) a = b; Lexical analysis if ( b == 0 ) a = b ; Token stream Parsing if ; == = Abstract syntax tree (AST) b 0 a b Semantic Analysis if boolean int == = ; Decorated AST int b int 0 int a lvalue intb

  3. fp 4 fp 8 Compilation in a Nutshell 2 if boolean int == = ; Intermediate Code Generation int b int 0 int a lvalue intb CJUMP == MEM CONST MOVE NOP Optimization + 0 MEM MEM fp 8 + + CJUMP == Code generation CX CONST MOVE NOP CMP CX, 0 CMOVZ DX,CX 0 DX CX

  4. Questions to Answer: • Is x a scalar, an array, or a function? • Is x declared before it is used? • Which declaration of x does this reference? • Does the dimension of a reference match the declaration? • Is an array reference in bounds? • Type errors (that can be caught statically)

  5. Attribute Grammars • Context-Free Grammars (CFGs) are used to specify the syntax of programming languages • E.g. arithmetic expressions • How do we tie these rules to mathematical concepts? • Attribute grammars are annotated CFGs in which annotations are used to establish meaning relationships among symbols • Annotations are also known as decorations

  6. Specification of Programming Languages • PLs require precise definitions (i.e. no ambiguity) • Language form (Syntax) • Language meaning (Semantics) • Consequently, PLs are specified using formal notation: • Formal syntax • Tokens • Grammar • Formal semantics • Attribute Grammars • Dynamic Semantics

  7. Each grammar symbols has a set of attributes E.g. the value of E1 is the attribute E1.val Each grammar rule has a set of rules over the symbol attributes Copy rules Semantic Function rules E.g. sum, quotient Attribute GrammarsExample

  8. Attribute Flow • Context-free grammars are not tied to an specific parsing order • E.g. Recursive descent, LR parsing • Attribute grammars are not tied to an specific evaluation order • This evaluation is known as the annotation or decoration of the parse tree

  9. Attribute Flow Example • The figure shows the result of annotating the parse tree for (1+3)*2 • Each symbols has at most one attribute shown in the corresponding box • Numerical value in this example • Operator symbols have no value • Arrows represent attribute flow

  10. Attribute Flow Example

  11. Attribute FlowSynthetic and Inherited Attributes • In the previous example, semantic information is pass up the parse tree • We call this type of attributes are called synthetic attributes • Attribute grammar with synthetic attributes only are said to be S-attributed • Semantic information can also be passed down the parse tree • Using inherited attributes • Attribute grammar with inherited attributes only are said to be non-S-attributed

  12. Attribute FlowInherited Attributes • Semantic information can also be passed down the parse tree • Using inherited attributes • Attribute grammar with inherited attributes only are said to be non-S-attributed • Top-down grammars generally require non-S-attributed flows • The previous annotated grammar was an S-attributed LR(1)

  13. Non-S-Attributed GrammarsExample

  14. Action Routines • Automatic tools can construct a parser for a given context-free grammar • E.g. yacc • Automatic tools can construct a semantic analyzer for an attribute grammar • An ad hoc techniques is to annotate the grammar with executable rules • These rules are known as action routines

  15. Action RoutinesExample

  16. Static and Dynamic Semantics • Attribute grammars add basic semantic rules to the specification of a language • They specify static semantics • But they are limited to the semantic form that can be checked at compile time • Other semantic properties cannot be checked at compile time • They are described using dynamic semantics

  17. Dynamic Semantics • Use to formally specify the behavior of a programming language • Semantic-based error detection • Correctness proofs • There is not a universally accepted notation • Operational semantics • Executing statements that represent changes in the state of a real or simulated machine • Axiomatic semantics • Using predicate calculus (pre and post-conditions) • Denotational semantics • Using recursive function theory

  18. Semantic Specification • The most common way of specifying the semantics of a language is plain english • http://www.python.org/doc/current/ref/binary.html • http://www.python.org/doc/current/ref/while.html • http://java.sun.com/docs/books/jls/first_edition/html/14.doc.html#24588 • There is a lack of formal rigor in the semantic specification of programming languages • Guess why

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