Semantic Analysis

• Abstract syntax tree; nodes
• Symbols and types
• Symbol table
• Varible and function declarations
• Type checking

Type Checking Example: Compact

The package provided here contains the Compact compiler for CDK3.

Note that, even though it does not work (or even compile) with CDK4 and newer, the principles are the same.

This version has been improved to perform type checking in the C generator visitor. This visitor uses a second visitor to check types before deciding on which code to generate. The checks are related with the use of boolean expressions in tests (while, if, and if-else nodes) and with type consistency in operators (in this case, although we could have decided otherwise, operators such as + and - must have arguments of the same type and this type must be integer). In addition, the compiler now checks if a variable has the proper type in certain cases (for instance, in the tests mentioned above).

• Compact with simple type checking (200805111819)

The visitor approach to type checking is justified by the independence of code generation from the details of type checking. In addition, the same type validator may be shared by several visitors performing code generation. In our example, the only changes to the original Compact are located in Cwriter.cpp (in each node we wish to check). The validation code is self-contained and is integrated as the drop-in class TypeValidator (TypeValidator.h and TypeValidator.cpp).

Example 1 from Compact: the original Compact compiler would accept the code, but the new one does not: since x is an integer and conditions must be boolean, it is rejected as a condition for if. <text> program x = 0; if (x) print x; while (x < 30) {

 print x;
 x = x + 1;

} end </text>

Also, notice that type checking is only active for the C target code generator (as intended). The procedure for including the validator calls in the other targets is similar.

Simple Type Checking Example

The following example considers a simple grammar and performs the whole of the semantic analysis process and, finally, generates the corresponding C code. The semantic analysis process must account for variables (they must be declared before they can be used) and for their types (all types must be used correctly).

• Simple Semantic Analysis Example (and C generation)