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Semantic ProcessingSemantic Processing

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ContentsContents IntroductionIntroduction A Simple CompilerA Simple Compiler Scanning – Theory and PracticeScanning – Theory and Practice Grammars and ParsingGrammars and Parsing LL(1) ParsingLL(1) Parsing Lex and yaccLex and yacc LR ParsingLR Parsing Semantic ProcessingSemantic Processing Symbol TablesSymbol Tables Run-time Storage OrganizationRun-time Storage Organization Code Generation and Local Code OptimizationCode Generation and Local Code Optimization Global OptimizationGlobal Optimization

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Syntax-Directed TranslationSyntax-Directed Translation Semantic processingSemantic processing

To take the To take the sequence of parse actionssequence of parse actions and and use them use them to construct a syntax tree representationto construct a syntax tree representation of the of the input program.input program.

Such a tree is usually referred to as an Such a tree is usually referred to as an abstract abstract syntax treesyntax tree, indicating its relationship to the actual , indicating its relationship to the actual (concrete) syntax and parse tree.(concrete) syntax and parse tree.

One or more traversals of the tree.One or more traversals of the tree. The two semantic processing tasksThe two semantic processing tasks

Static semantic checkingStatic semantic checking IR or code generationIR or code generation

can be accomplished using semantic attributes attached to can be accomplished using semantic attributes attached to the nodes of the syntax tree.the nodes of the syntax tree.

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Syntax-Directed Translation Syntax-Directed Translation (Cont’d.)(Cont’d.)

A tree traversal that visits nodes using a A tree traversal that visits nodes using a post-order traversalpost-order traversal Propagate semantic attributes throughout the tree Propagate semantic attributes throughout the tree

and do static semantic checking at the same time.and do static semantic checking at the same time. Propagation of semantic attributes includesPropagation of semantic attributes includes

Actions as processing declarations to Actions as processing declarations to build a build a symbol tablesymbol table..

Looking up identifiers in the symbol table Looking up identifiers in the symbol table to to attach associated attribute information to attach associated attribute information to appropriate nodesappropriate nodes of the syntax tree. of the syntax tree.

Examining the types of arguments (type Examining the types of arguments (type checking).checking).

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Syntax-Directed Translation Syntax-Directed Translation (Cont’d.)(Cont’d.)

Semantic routines traverse the AST, Semantic routines traverse the AST, computing computing attributesattributes of the nodes of AST.of the nodes of AST.

Initially, only leaves (i.e. terminals, e.g. const, Initially, only leaves (i.e. terminals, e.g. const, id) have attributes.id) have attributes.

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Syntax-Directed Translation Syntax-Directed Translation (Cont’d.)(Cont’d.)

Top-down propagationTop-down propagation Information form declarationsInformation form declarations (the symbol table) is (the symbol table) is

propagated top-down through subtree for propagated top-down through subtree for statements and expressions. statements and expressions.

Bottom-up propagationBottom-up propagation Expression-type informationExpression-type information for semantic for semantic

checking and code generation generally checking and code generation generally propagates bottom-up.propagates bottom-up.

After all the attribute propagation is done, the After all the attribute propagation is done, the tree is said to be tree is said to be decorateddecorated.. Ready for code generation phaseReady for code generation phase

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Syntax-Directed Translation Syntax-Directed Translation (Cont’d.)(Cont’d.)

Two kinds of static semantic checksTwo kinds of static semantic checks Some checks depend entirely on propagated Some checks depend entirely on propagated

semantic attributes.semantic attributes. The check for type compatibility across an assignmentThe check for type compatibility across an assignment

Others combine structural information from the Others combine structural information from the tree with semantic information.tree with semantic information. A comparison of the number of actual parameters with A comparison of the number of actual parameters with

the number of formal parameters.the number of formal parameters.

Attribute grammarAttribute grammar Attributes are associated with each grammar Attributes are associated with each grammar

symbol in a production.symbol in a production. Rules are attached to each production to compute Rules are attached to each production to compute

attribute values.attribute values.

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Syntax-Directed Translation Syntax-Directed Translation (Cont’d.)(Cont’d.)

Compiler organization Compiler organization alternativesalternatives

AnalysisAnalysis ScannerScanner ParserParser Static semantic checkingStatic semantic checking

SynthesisSynthesis Synthesis task of Synthesis task of

Semantic RoutinesSemantic Routines IRIR Code optimizationCode optimization Code generationCode generation

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Syntax-Directed Translation Syntax-Directed Translation (Cont’d.)(Cont’d.)

Compiler organization alternativesCompiler organization alternatives A single pass for analysis and synthesis (Chap 2)A single pass for analysis and synthesis (Chap 2) One-pass compiler plus peephole optimization One-pass compiler plus peephole optimization

(Chap 15)(Chap 15) One-pass analysis and IR synthesis plus a code One-pass analysis and IR synthesis plus a code

generation pass (Chap 10-13)generation pass (Chap 10-13) Multipass analysisMultipass analysis Multipass synthesisMultipass synthesis Multilanguage and multitargeted compilersMultilanguage and multitargeted compilers

The GNU C compiler, uses two intermediate formsThe GNU C compiler, uses two intermediate forms The first is a high-level, tree-oriented intermediate form.The first is a high-level, tree-oriented intermediate form. The second is called RTL (Register Transfer Language), The second is called RTL (Register Transfer Language),

which is more machine-oriented.which is more machine-oriented.

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Syntax-Directed Translation Syntax-Directed Translation (Cont’d.)(Cont’d.)

Parsing, checking, and translation in a single Parsing, checking, and translation in a single passpass

There are two principal advantages of such There are two principal advantages of such an approachan approach The front-end of the compiler is simpler because The front-end of the compiler is simpler because

no tree building or tree traversal code is required.no tree building or tree traversal code is required. Much less storage space is required to process a Much less storage space is required to process a

program if an entire syntax tree is not explicitly program if an entire syntax tree is not explicitly built.built.

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Attribute GrammarsAttribute Grammars Please refer Chap 14Please refer Chap 14

If only synthesized If only synthesized attributes are used, the attributes are used, the SDT is said to be SDT is said to be S-S-attributedattributed..

In practice, this means In practice, this means that a parse tree may that a parse tree may be “decorated” as it is be “decorated” as it is built bottom-up.built bottom-up.

Production Semantic Rules

L En Print(E.val)

E E1 + T E.val := E1.val + T.val

E T E.val := T.val

T T1 * F T.val := T1.val * F.val

T F T.val := F.val

F (E) F.val := E.val

F digit F.val := digit.lexval

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Attribute Grammars Attribute Grammars (Cont’d.)(Cont’d.)

Annotated parse tree for Annotated parse tree for 3*5+4n3*5+4nL

E.val = 19

T.val = 4T.val = 15

T.val = 3 F.val = 5

F.val = 3

digit

digit

*

+

F.val = 4

digit

n

3

5 4

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Semantic Processing TechniquesSemantic Processing Techniques LL parsers and action symbolsLL parsers and action symbols LR parsers and action symbolsLR parsers and action symbols

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

Semantic record Semantic record representationsrepresentations

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

Implementation action-controlled semantic stacksImplementation action-controlled semantic stacks

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

LL parser-controlled semantic stacksLL parser-controlled semantic stacks

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

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Semantic Processing Techniques Semantic Processing Techniques (Cont’d.)(Cont’d.)

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Intermediate Representation and Code Intermediate Representation and Code generationgeneration

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Intermediate Representation and Code Intermediate Representation and Code generation generation (Cont’d.)(Cont’d.)

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Intermediate Representation and Code Intermediate Representation and Code generation generation (Cont’d.)(Cont’d.)

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Intermediate Representation and Code Intermediate Representation and Code generation generation (Cont’d.)(Cont’d.)

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Intermediate Representation and Code Intermediate Representation and Code generation generation (Cont’d.)(Cont’d.)

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Intermediate Representation and Code Intermediate Representation and Code generation generation (Cont’d.)(Cont’d.)

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Intermediate Representation and Code Intermediate Representation and Code generation generation (Cont’d.)(Cont’d.)