Semantic Analysis

While the Parser ensures the code "looks" right, the Semantic Analyzer ensures the code "is" right. It performs a recursive walk over the Abstract Syntax Tree (AST) to verify that identifiers are used within their proper context. It acts as the final gatekeeper before the compiler commits to generating machine instructions.

Deep-Dive: Semantic Validation Rules

Symbol Table Management: Built during compile time using add_symbol(). Variables are tracked by their current_scope level. There is only one static entry per variable in the table.
Scope Cleanup: When leaving a block or function, exit_scope() loops backward to delete any variable belonging to that scope, keeping memory clean.
The Return Stack Mechanism: Maintains a LIFO Stack (return_stack array) of expected return types.
- On Function Entry: The declared return type (e.g., SYMBOL_INT) is pushed via push_return_type().
- On Return Statement: Validates the type by calling get_current_return_type().
- On Function Exit: The type is removed via pop_return_type().

Compiler Time (Static) vs. Runtime (Dynamic)

In our binarySearch example, the semantic analyzer ensures correct scoping using the Symbol Table. However, during execution, each recursive call creates a separate Activation Record on the call stack.

Symbol Table (Compile Time): Asks "What variables exist?". It records the blueprint of the function exactly once while walking the AST.
Activation Record (Runtime): Asks "Where is the value stored right now?". Every recursive call gets its own copy of local variables like mid at runtime.

Interactive Semantic Simulator

Watch how the compiler analyzes the AST strictly top-to-bottom. It evaluates binarySearch once, verifies its internal logic against the LIFO stack, calls exit_scope() to drop local variables, and then moves on to analyze main().

Target Program (AST Input)

int binarySearch(int arr[], int l, int r, int key) {
    int mid;
    if (l > r) { return 0-1; }
    mid = (l + r) / 2;
    if (arr[mid] == key) { return mid; } 
    else {
        if (key < arr[mid]) { return binarySearch(arr, l, mid - 1, key); }
        else { return binarySearch(arr, mid + 1, r, key); }
    }
}

int main() {
    int arr[5] = {1, 3, 5, 7, 9};
    int n = 5;
    result = binarySearch(arr, 0, n-1, 12);
    return 0;
}

Beyond Syntax: Discovering Intent

Deep-Dive: Semantic Validation Rules

Compiler Time (Static) vs. Runtime (Dynamic)

Interactive Semantic Simulator

Target Program (AST Input)

Active Symbol Table