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

10. Attribute Grammar Example  E 1  E 2 + T E 1. type = E 2.type + T.type E 1. type = E 2.type + T.type

11. Attribute Grammar Example  decl  type var-list var-list.dtype =type.dtype 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 id.dtype = var-list1.dtype var-list2.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  E 1  E 2 + T E 1. tree = E 1. tree = mkOpNode(+, E 2.tree, T.tree) mkOpNode(+, E 2.tree, T.tree)  E  T E.tree = T.tree E.tree = T.tree  F  number F.tree = mkNumNode(number.lexval) F.tree = mkNumNode(number.lexval)

14. Attribute Up and Down Dependency 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; { double j, i; } { char * j; { char * j; *j = i = 5; *j = i = 5; }}

18. Static vs Dynamic Scope compile time or run time int i = 1; void f(void) { printf(“%d\n”,i); } void main(void) { int i = 2; f(); f(); return; 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/Colateral order is not important with in group int i = 1; void f(void) { int i = 2, j = i + 1; 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); 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 else error  S  if E then S 1 S.type=if E.type=boolean then S 1.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; link next; cell * p; 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. t2 and t1 are not the same type. int typeEqual(t1, t2) { if (t1 and t2 are simple types) return t1 == t2; return t1 == t2; if (t1 and t2 are type names) if (t1 and t2 are type names) return t1 == t2; return t1 == t2; else return 0;} in case you read the text else return 0;} in case you read the text

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