CODEWITHSUNDEEP
gccc++14built-in
ruchitesh22
ruchitesh22

Reputation: 27

C++ gcc _builtin function gives unexpected answer

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

Upvotes: 0

Views: 277

Answers (2)

Davide Spataro
Davide Spataro

Reputation: 7482

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

Upvotes: 4

melpomene
melpomene

Reputation: 85777

https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html states:

Built-in Function: int __builtin_clz (unsigned int x)

I.e. n is implicitly converted to an unsigned int because that's what the function takes.

There's also int __builtin_clzll (unsigned long long) if you need more bits.

Upvotes: 5

Related Questions