How to give the compiler a hint about the maximum runtime

Question

// if I know that in_x will never be bigger than Max
template <unsigned Max>
void foo(unsigned in_x)
{
    unsigned cap = Max;

    // I can tell the compiler this loop will never run more than log(Max) times
    for (; cap != 0 && in_x != 0; cap >>= 1, in_x >>= 1)
    {
    }
}

As shown in the above code, I assume that if I just write

for (; in_x! = 0; in_x → = 1)

the compiler will not expand the loop because it cannot be sure of the maximum possible in_x.

I want to know if I am right or not, and if there are any more effective ways to deal with such things.

---

Or maybe the problem can be generalized as if some code could be written to tell the compiler a range of some runtime value, and such code does not have to be compiled into a binary runtime file.

---

Truly fighting the XD compiler

// with MSC
// if no __forceinline here, unrolling is ok, but the function will not be inlined
// if I add __forceinline here, lol, the entire loop is unrolled (or should I say the tree is expanded)...
// compiler freezes when Max is something like 1024
template <int Max>
__forceinline void find(int **in_a, int in_size, int in_key)
{
    if (in_size == 0)
    {
        return;
    }

    if (Max == 0)
    {
        return;
    }

    {
        int m = in_size / 2;

        if ((*in_a)[m] >= in_key)
        {
            find<Max / 2>(in_a, m, in_key);
        }
        else
        {
            *in_a = *in_a + m + 1;

            find<Max - Max / 2 - 1>(in_a, in_size - (m + 1), in_key);
        }
    }
}

Answer 1

- , TMP. ( ). , , :

template <unsigned char MaxRec>
inline void foo(unsigned in_x)
{
    if (MaxRec == 0) // will be eliminated at compile time
        return; // tells the compiler to stop the pseudo recursion

    if (in_x == 0) {
        // TODO : end recursion;
        return;
    };

    // TODO: Process for iteration rec

    // Note: NOT recursion, the compiler would not be able to inline
    foo<MaxRec-1>(in_x >> 1);
}

// Usage:
foo<5>(in_x); // doubt the compiler will inline 32 times, but you can try.