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Semantic Analysis

Semantic Analysis. Aaron Bloomfield CS 415 Fall 2005. 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.

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Semantic Analysis

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  1. Semantic Analysis Aaron Bloomfield CS 415 Fall 2005

  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. Context-Sensitive Analysis • Two solutions: Attribute grammars – augment CFG with rules, calculate attributes for grammar symbols ad hoc techniques – augment grammar with arbitrary code, execute at corresponding reduction, store information in attributes, symbol tables values

  6. Attribute Grammars • Generalization of context-free grammars • Each grammar symbol has an associated set of attributes • Augment grammar with rules that define values • Not allowed to refer to any variables or attributes outside the current production

  7. Attribute Types • Values are computed from constants and other types: • Synthesized attribute – value computed from children • Inherited attribute – value computed from siblings, parent, and own attributes

  8. Attribute Flow S-attributed grammar • Uses only synthesized types • Bottom-up attribute flow L-attributed grammar • Attributes can be evaluated in a single left-to-right pass over the input • Each synthesized attribute of LHS depends only on that symbol’s own inherited attributes or on attributes (synthesized or inherited) of the production’s RHS symbols

  9. Action Routines • We need a translation scheme - an algorithm that invokes the attributes in a order that respects attribute flow. • Action Routines = an Ad hoc translation scheme that is interleaved with parsing • An action routine = Semantic function that programmer instructs the compiler to execute at a particular point in the parse. • What most production compilers use.

  10. Abstract Syntax Tree • An abstract syntax tree is the procedure’s parse tree with the nodes for most non-terminal symbols removed E.g., “a + 3 * b”

  11. Symbol Table • A “Dictionary” that maps names to info the compiler knows about that name. • What names? • Variable and procedure names • Literal constants and strings • What info? Textual name Data type Declaring procedure Lexical level of declaration If array, number and size of dimensions If procedure, number and type of parameters

  12. Sample program Program gcd (input, output); Var I,j: integer; Begin Read(I,j); While I <> j to If I > j then I := I – j Else j := j – I; Writeln (i) End.

  13. Symbol Table Implementation • Usually implemented as hash tables • Return closest lexical declaration to handle nested lexical scoping • How to handle multiple scopes?

  14. One option • Use one symbol table per scope • Chain tables to enclosing scope • Insert names in tables for current scope • Start name lookup in current table, checking enclosing scopes in order if needed

  15. LeBlanc-Cook symbol tables • Give each scope a number, • All names in a hash table, keyed by name • Also have a scope stack – to show current referencing environment. • As analyzer looks at programs, it pushes and pops this stack as it enters and leaves scopes. • To look up – scan down the appropriate hash chain, for each matching entry, scan down the scope stack to see if that is visible. We look no deeper than the top-most scope.

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