Inspecting Compiler-Generated Template Instantiations in C

In C , template functions and classes allow for code reuse by defining generic functionality that can be specialized for different types. To understand the code generated by the compiler for a template instantiation, it is helpful to have visibility into these instantiated functions or classes.

Clang's AST Printing Capability

One tool that provides this visibility is the Abstract Syntax Tree (AST) printing feature of Clang, a widely used compiler for C . The AST represents the internal representation of the code before compilation, including the generated code for template instantiations.

To print the instantiated AST for a C template, invoke Clang with the -Xclang -ast-print flag along with the -fsyntax-only flag to prevent actual compilation.

For example, consider the following code:

template  T add(T a, T b) {
    return a   b;
}

void tmp() {
    add(10, 2); // Call the template with int specialization
}

To view the AST for this code, run the following command:

$ clang   -Xclang -ast-print -fsyntax-only test.cpp

Example Output

The output will contain the AST, including the instantiated add function:

template  T add(T a, T b) {
    return a   b;
}
template int add(int a, int b) {
    return a   b;
}
void tmp() {
    add(10, 2);
}

In this output, the instantiated add function is shown as a template specialization, indicating the specific type (int) for which the function was generated.

Conclusion

Clang's AST printing capability provides a useful way to inspect the code generated by the compiler for template instantiations. This can be invaluable for understanding the implementation details, debugging, and optimizing template-based code in C .