c - literal constant vs variable in math library

Question

So, I know that in C you need to link the code to the math library, libm, to be able to use its functions. Today, while I was trying to demonstrate this to a friend, and explain why you need to do this, I came across the following situation that I do not understand.

Consider the following code:

#include <math.h>
#include <stdio.h>

/* #define VARIABLE */

int main(void)
{
#ifdef VARIABLE
    double a = 2.0;
    double b = sqrt(a);
    printf("b = %lf
",b);
#else
    double b = sqrt(2.0);
    printf("b = %lf
",b);
#endif
    return 0;
}

If VARIABLE is defined, you need to link against libm as you would normally expect; otherwise you get the usual main.c:(.text+0x29): undefined reference to sqrt linking error indicating that the compiler cannot find the definition for the function sqrt. I was surprised to see that if I comment #define VARIABLE, the code runs fine and the result is correct!

Why is it that I need to link to libm when variables are used but I don't need to do so when literal constants are used? How does the compiler find the definition of sqrt when the library is not linked? I'm using gcc 4.4.5 under linux.

Answer 1

GCC can do constant folding for several standard-library functions. Obviously, if the function is folded at compile-time, there is no need for a run-time function call, so no need to link to libm. You could confirm this by taking a looking at the assembler that the compiler produces (using objdump or similar).

I guess these optimizations are only triggered when the argument is a constant expression.