GCC Wunused-variable warns for used variable

Question

I am using GCC version 8.2 On several pieces of code, I use small functions. On each one of the functions, I have tests (i.e. Unity framework tests). The tests are defined as #define macros, testing very specific things. For instance, if a number if positive.

Now, when compiling the code using -Wextra flag, I am getting warning about unused variables, although I am using them on the defined macros.

The question is, GCC does not recognize a macro as using a variable, or am I missing something?

Example:

    #define compare(a,b) ( ((a) == (b)) ? 1 : 0 )
    ...
    void f() {
        int a;
        a = f1();
        if(compare(a,123))
            printf("It works");
    }

In this case, GCC would warning about unused variable a, although it is being used by the macro (besides being attributed a value by function f1()).

Answer 1

This is not the case, at least with the example you supplied. Here is a Minimal, Complete, and Verifiable demonstration:

#include <stdio.h>

#define compare(a,b) ( ((a) == (b)) ? 1 : 0 )

int f1() {
    return 42;
}

void f() { // your code
    int a;
    a = f1();
    if (compare(a, 123))
        printf("It works");
}

int main(int argc, char *argv[]) {
    f();
    return 0;
}

When compiled with gcc -Wall -Wunused (yes, this is redundant) using gcc 8.2 or 7.3 there are no warnings or errors.