Attribute GrammarsSemantic AnalysisSemantic AnalysisSemantic AnalysisSemantic AnalysisSemantic AnalysisFrom Code Form To Program MeaningCompiler or InterpreterCompiler or InterpreterTranslation Translation ExecutionExecutionSource CodeSource CodeTarget CodeTarget CodeInterpreInterpre--tationtationPhases of CompilationSpecification of Programming Languages• PLs require precise definitions (i.e. no ambiguity)– Languageform (Syntax)– LanguageLanguageLanguageLanguagemeaningmeaningmeaningmeaning(Semantics)(Semantics)(Semantics)(Semantics)• Consequently, PLs are specified using formal notation:– Formal syntax• Tokens• Grammar– Formal semanticsFormal semanticsFormal semanticsFormal semantics• Attribute Grammars (static semantics)Attribute Grammars (static semantics)Attribute Grammars (static semantics)Attribute Grammars (static semantics)• Dynamic SemanticsDynamic SemanticsDynamic SemanticsDynamic SemanticsThe Semantic Analyzer• The principal job of the semantic analyzer is to enforce static semantic rules. • In general, anything that requires the requires the compiler to compare things that are separate by a long distance or to count things ends up being a matter of semantics.• The semantic analyzer also commonly constructs a syntax tree (usually first), and much of the information it gathers is needed by the code generator.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 grammarsare annotated CFGs in which annotationsare used to establish meaning relationships among symbols– Annotations are also known as decorationsAttribute GrammarsExample• Each grammar symbols has a set of attributes–E.g.the value of E1is the attribute E1.val• Each grammar rule has a set of rules over the symbol attributes–Copy rules–Semantic Function rules•E.g. sum, quotientAttribute 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 annotationor decorationof the parse treeAttribute 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 flowAttribute Flow ExampleAttribute 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-attributedAttribute FlowInherited Attributes•L-attributed grammars, such as the one on the next slide, can still be evaluated in a single left-to-right pass over the input.•Each synthetic attribute of a LHS symbol (by definition of synthetic)depends only on attributes of its RHS symbols.•Each inherited attribute of a RHS symbol (by definition of L-attributed) depends only on inherited attributes of the LHS symbol or on synthetic or inherited attributes of symbols to itsleft in the RHS.•Top-down grammars generally require non-S-attributed flows– The previous annotated grammar was an S-attributed LR(1)– L-attributed grammars are the most general class of attribute grammars that can be evaluated during an LL parse.LL GrammarNon-S-Attributed GrammarsExampleSyntax Tree• There is considerable variety in the extent to which parsing, semantic analysis, and intermediate code generation are interleaved. • A one-passcompiler interleaves scanning, parsing, semantic analysis, and code generation in a single traversal of the input.• A common approach interleaves construction of a syntax tree with parsing (eliminating the need to build an explicit parse tree), then follows with separate, sequential phases for semantic analysis and code generation.Bottom-up Attribute Grammar to Construct a Syntax TreeConstruction of the Syntax TreeAction 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 routinesAction Rules for the Previous LL(1) attribute grammarE => T { TT.st := T.v } TT { E.v := TT.v }TT => + T { TT2.st := TT1.st + T.v } TT { TT1.v := TT2.v }TT => - T { TT2.st := TT1.st - T.v } TT { TT1.v := TT2.v }TT => { TT.v := TT.st }T => F { FT.st := F.v } FT { T.v := FT.v }FT => * F { FT2.st := FT1.st * F.v } FT { FT1.v := FT2.v }FT => / F { FT2.st := FT1.st / F.v } FT { FT1.v := FT2.v }FT => { FT.v := FT.st }F => - F { F1.v := - F2.v }F => ( E ) { F.v := E.v }F => const { F.v := C.v }Action Rules• The ease with which rules were incorporated in the grammar is due to the fact that the attribute grammar is L-attributed.• The action rules forL-attributedgrammars, in which the attribute flow is depth-first left-to-right, can be evaluated in the order of the parse tree prediction for LL grammars.• Action rules for S-attributedgrammars can be incorporated at the end of the right-hand sides of LR grammars. But, if action rules are responsible for a significant part of the semantic analysis, they will need more contextual information to do their job.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 semanticsDynamic 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 semanticsOperational semanticsOperational semanticsOperational semantics• Executing statements that represent changes in the state of a real or simulated machine– Axiomatic semanticsAxiomatic semanticsAxiomatic semanticsAxiomatic semantics• Using predicate calculus (pre and