Looking for a lightweight solution to systematically instrument the if-conditions of C functions

Question

I need to use lightweight instrumentation tools (say, using some existing Python or maybe C pre-processor framework) to systematically insert statements in if-conditions of C functions:

For example, let foo be the C function:

int foo(int x){
   if (x>3) 
        return x;
}

I need to transform it to

int foo(int x){
   if (x>3){ 
        print ("%d", x); 
        return x;
  }
}

I have tried using LLVM, but it does not integrate better with other parts of the code. Here, I would prefer a Python/C preprocessor framework solution. Thanks for your ideas.

Answer 1

You could consider using GCC MELT or perhaps the GCC Python Plugin to customize the GCC compiler to transform its Gimple representation. It would be a complete solution, but I am not sure it qualifies as "lightweight" (because you need to understand the details of GCC internals), but neither is a Clang/LLVM approach. You'll probably need more than a week of work (notably to learn about the internals of GCC, or of Clang/LLVM)

BTW, your problem is less easy than what you believe, since

int foobar(int x) { return (x>3)?(x*6):(x*x); } 

contains a conditional. Also, your original function is exactly equivalent to

int foo(int x){
    while (x>3) // this while is done 0 or 1 time
      return x;
}

and it is easy to find many other ways to express it.

So a purely syntactic approach would never work completely. Hence, you cannot hope for something "light".

^{(my opinion is that C code instrumentation is never a lightweight problem)}