gcc - What does !!(x) mean in C (esp. the Linux kernel)?

Question

I've been reading through the Linux kernel (specifically, 2.6.11). I came across the following definition:

#define unlikely(x)     __builtin_expect(!!(x), 0)

(from linux-2.6.11/include/linux/compiler.h:61 lxr link)

What does !! accomplish? Why not just use (x)?

See also:

• How does logical negation work in C?
• Double Negation in C++ code.

Answer 1

!!(x) forces it to be either 0 or 1. 0 remains 0, but any non-zero value (which would be 'true' in a boolean context) becomes 1.