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 (compile time) Value of expressions (run time) Location of variables (compile time) Object code of procedure (compile time) Number of Significant digits (design time)

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 E1  E2 + T E1.tree = mkOpNode(+, E2.tree, T.tree) E  T E.tree = T.tree F  number F.tree = mkNumNode(number.lexval)

Symbol Tables Keep track of identifiers Must deal with scope efficiently

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(); return; What is printed?

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.

Example - Sequential/Colateral order is not important with in group 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. Some languages have issue with using g before g is defined. (pascal)

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

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

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

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.