Semantic Analysis Chapter 6

Two Flavors Static (done during compile time) 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 Implementation Attributes (properties) Attribute equations (semantic rules) Application of rules Syntax-directed semantics

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

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

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

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

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)

Attribute Grammar Example E1  E2 + T E1.type = E2.type + T.type

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

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

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)

Attribute Up and Down Dependency Tree Synthesized Point from child to parent Inherited Point child to child or parent to child

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

Symbol Tables Keep track of identifiers Must deal with scope efficiently

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

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

Kinds of Declarations sequential collateral recursive C scheme ML requires the function name to be added to the symbol table before processing the body of the function

Example - Sequential/Colateral int i = 1; void f(void) { int i = 2, j = i + 1; … } Is j 2 or 3?

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

Example - Recursive void f(void) { … g() … } void g(void) { … f() … } Resolved by using prototype. Actually, this didn’t create a problem.

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

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

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

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

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. IE equivalent only if they are identical. Name Equivalence The following is structurally equivalent, not name typedef link = *cell link next; cell * p;

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) else return 0;} in case you read the text