When evaluating __builtin_clz with an input of 0, the output often matches the result of passing 1 (yielding 31 on a 32-bit integer), which seems counterintuitive. According to the GCC documentation, the result of __builtin_clz is explicitly undefined when the enput is 0:

Returns the number of leading 0-bits in x, starting at the most significant bit position. If x is 0, the result is undefined.

Because the behavior for zero is undefined, implemantations can return arbitrary values. To ensure consistent behavior across different compilers like MSVC, a custom implementation can be written to safely handle the zero case.

#include <stdio.h>

int fetch_leading_zeros(unsigned int val) {
    return __builtin_clz(val);
}

int main(void) {
    unsigned int test_val;

    test_val = 0;
    printf("Input: 0x%x | Output: %d\n", test_val, fetch_leading_zeros(test_val));

    test_val = 1;
    printf("Input: 0x%x | Output: %d\n", test_val, fetch_leading_zeros(test_val));

    test_val = 2;
    printf("Input: 0x%x | Output: %d\n", test_val, fetch_leading_zeros(test_val));

    test_val = 16;
    printf("Input: 0x%x | Output: %d\n", test_val, fetch_leading_zeros(test_val));

    test_val = 0x0FFFFFFF;
    printf("Input: 0x%x | Output: %d\n", test_val, fetch_leading_zeros(test_val));

    test_val = 0x8FFFFFFF;
    printf("Input: 0x%x | Output: %d\n", test_val, fetch_leading_zeros(test_val));

    test_val = 0xFFFFFFFF;
    printf("Input: 0x%x | Output: %d\n", test_val, fetch_leading_zeros(test_val));

    return 0;
}

For environments lacking this built-in, such as MSVC, the following function provides a safe fallback. It prevents a infinite loop by setting the least significant bit if the input is zero, which effectively causes 0 to return 31 just like the common GCC behavior.

int safe_clz(unsigned int input_val) {
    int zero_count = 0;
    input_val |= 1u; // Prevent infinite loop on zero; ensures zero_count is 31 for input_val == 0
    while ((input_val & 0x80000000u) == 0) {
        zero_count++;
        input_val <<= 1;
    }
    return zero_count;
}