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Semantic Analysis

Semantic Analysis. Chapter 6. Two Flavors. Static (done during compile time) C Ada Dynamic (done during run time) LISP Smalltalk Optimization. Static Semantic Analysis. Build symbol table Keep track of declarations Perform type checking. Static Analysis. Description

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Semantic Analysis

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  1. Semantic Analysis Chapter 6

  2. Two Flavors • Static (done during compile time) • C • Ada • Dynamic (done during run time) • LISP • Smalltalk • Optimization

  3. Static Semantic Analysis • Build symbol table • Keep track of declarations • Perform type checking

  4. Static Analysis • Description • Attributes (properties) • Implementation • Attribute equations (semantic rules) • Application of rules Syntax-directed semantics

  5. General Attribute • Property of the Language • Data type • Value of expressions • Location of variables in memory • Object code of procedure • Number of Significant digits

  6. Specific Attributes • Parameters/Arguments type • Parameters/Arguments number • Array subscript type • Array subscript number • Continue with no place to continue to • Variable undeclared • Variable duplicately declared • Scope • Incorrect structure reference

  7. Specific Attributes Cont. • Break inappropriate • Incorrect Return • Wrong type • Array • None when needed (void) • No main • Two main’s • Constant on left side • Expression types

  8. Binding Time of Attributes • Static - prior to execution • Fortran • Dynamic - during execution • Combination • C • Java • Pascal

  9. Attribute Grammars • X is grammar symbol, Xa is an attribute for this symbol XABCD (grammar) X.x= A.a B.b C.c D.d (attribute grammar)

  10. Attribute Grammar Example • E1 E2 + T E1.type= E2.type+ T.type

  11. Attribute Grammar Example • decl  type var-list var-list.dtype =type.dtype • type  int type.dtype = integer • type  float type.dtype = float • var-list1  id, var-list2 id.dtype = var-list1.dtype var-list2.dtype = var-list1.dtype • var-list  id id.dtype = var-list.dtype

  12. Attribute Grammar Comments • Symbols may have more than one attribute • The grammar is not the master • More of a guide

  13. Attribute Grammar Example • E1 E2 + T E1.tree= mkOpNode(+, E2.tree, T.tree) • E  T E.tree = T.tree • F  number F.tree = mkNumNode(number.lexval)

  14. Attribute Up and DownDependency Tree • Synthesized • Point from child to parent • Inherited • Point child to child or parent to child

  15. Symbol Tables • Lists of Lists • Hash • Collision resolving by use of buckets • Collision resolving by probing • …

  16. Symbol Tables • Keep track of identifiers • Must deal with scope efficiently

  17. Code Fragment int f(int size) { char i, temp; … { double j, i; } { char * j; *j = i = 5; } }

  18. Static vs Dynamic Scopecompile time or run time int i = 1; void f(void) { printf(“%d\n”,i); } void main(void) { int i = 2; f(); return; } What is printed?

  19. Kinds of Declarations • Sequential – each declaration is available starting with the next line • C • Collateral – each declaration is evaluated in the environment preceding the declaration group. Declared identifiers are available only after all finishes. • scheme • ML • Recursive - requires the function name to be added to the symbol table before processing the body of the function. C functions and type declarations are recursive.

  20. Example - Sequential/Colateralorder is not important with in group int i = 1; void f(void) { int i = 2, j = i + 1; … } Is j 2 or 3?

  21. Example - Recursive int gcd(int n, int m) { if (m == 0) return n; else return gcd(m, n%m); } gcd must be added to the symbol table before processing the body

  22. Example - Recursive void f(void) { … g() … } void g(void) { … f() … } Resolved by using prototype. Some languages have issue with using g before g is defined. (pascal)

  23. Data Types – Type Checking • Explicit datatype • int x • Implicit datatype • #define x 5

  24. Implementation of Types • Hardware implementation • int • double • float • Software implementation • boolean • char • enum – can be integers to save space

  25. More Complicated Types • Arrays • base(b)+i*esize • base(ar)+(i1*r2 +i2)*esize • Records • allocate memory sequentially • base+displacement

  26. Type Checking Statements • S  id = E S.type = if id.type = E.type then void else error • S  if E then S1 S.type=if E.type=boolean then S1.type

  27. Equivalence of type Expressions • Structural Equivalence • two expressions are either the same basic type, or are formed by applying the same constructor to structurally equivalent types. I.E. equivalent only if they are identical. • Example typedef link = *cell link next; cell * p; • Name Equivalence • two expressions use the same name

  28. Name Equivalence typedef int t1; typedef int t2; t2 and t1 are not the same type. int typeEqual(t1, t2) { if (t1 and t2 are simple types) return t1 == t2; if (t1 and t2 are type names) return t1 == t2; else return 0;} in case you read the text

  29. Name Equivalence typedef int t1; typedef int t2; t2 x; t2 y; t1 z; x and y are the same type. z is not the same type.

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