compiler_builtins/int/
trailing_zeros.rs

1#[cfg(feature = "unstable-public-internals")]
2pub use implementation::trailing_zeros;
3#[cfg(not(feature = "unstable-public-internals"))]
4pub(crate) use implementation::trailing_zeros;
5
6mod implementation {
7    use crate::int::{CastInto, Int};
8
9    /// Returns number of trailing binary zeros in `x`.
10    #[allow(dead_code)]
11    pub fn trailing_zeros<T: Int + CastInto<u32> + CastInto<u16> + CastInto<u8>>(x: T) -> usize {
12        let mut x = x;
13        let mut r: u32 = 0;
14        let mut t: u32;
15
16        const { assert!(T::BITS <= 64) };
17        if T::BITS >= 64 {
18            r += ((CastInto::<u32>::cast(x) == 0) as u32) << 5; // if (x has no 32 small bits) t = 32 else 0
19            x >>= r; // remove 32 zero bits
20        }
21
22        if T::BITS >= 32 {
23            t = ((CastInto::<u16>::cast(x) == 0) as u32) << 4; // if (x has no 16 small bits) t = 16 else 0
24            r += t;
25            x >>= t; // x = [0 - 0xFFFF] + higher garbage bits
26        }
27
28        const { assert!(T::BITS >= 16) };
29        t = ((CastInto::<u8>::cast(x) == 0) as u32) << 3;
30        x >>= t; // x = [0 - 0xFF] + higher garbage bits
31        r += t;
32
33        let mut x: u8 = x.cast();
34
35        t = (((x & 0x0F) == 0) as u32) << 2;
36        x >>= t; // x = [0 - 0xF] + higher garbage bits
37        r += t;
38
39        t = (((x & 0x3) == 0) as u32) << 1;
40        x >>= t; // x = [0 - 0x3] + higher garbage bits
41        r += t;
42
43        x &= 3;
44
45        r as usize + ((2 - (x >> 1) as usize) & (((x & 1) == 0) as usize).wrapping_neg())
46    }
47}
48
49intrinsics! {
50    /// Returns the number of trailing binary zeros in `x` (32 bit version).
51    pub extern "C" fn __ctzsi2(x: u32) -> usize {
52        trailing_zeros(x)
53    }
54
55    /// Returns the number of trailing binary zeros in `x` (64 bit version).
56    pub extern "C" fn __ctzdi2(x: u64) -> usize {
57        trailing_zeros(x)
58    }
59
60    /// Returns the number of trailing binary zeros in `x` (128 bit version).
61    pub extern "C" fn __ctzti2(x: u128) -> usize {
62        let lo = x as u64;
63        if lo == 0 {
64            64 + __ctzdi2((x >> 64) as u64)
65        } else {
66            __ctzdi2(lo)
67        }
68    }
69}
compiler_builtins/int/trailing_zeros.rs

compiler_builtins/int/
trailing_zeros.rs
