C++ gcc _builtin function gives unexpected answer

Question

Int64_t n=7; after printing __builtin_clz(n) answer is 29 rather than the expected answer 61.

Answer 1

That is the signature for the intrinsic you are using:

int __builtin_clz (unsigned int x)

As you can see it works on 32-bit unsigned. It treat your 64 bit integer as a 32 one. Since 7 has 4 bits set it return 32-3 = 29

Try __builtin_clzl; or __builtin_clzll instead.

details here