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::{CastFrom, Int};
8
9    /// Returns number of trailing binary zeros in `x`.
10    #[allow(dead_code)]
11    pub fn trailing_zeros<I: Int>(x: I) -> usize
12    where
13        u32: CastFrom<I>,
14        u16: CastFrom<I>,
15        u8: CastFrom<I>,
16    {
17        let mut x = x;
18        let mut r: u32 = 0;
19        let mut t: u32;
20
21        const { assert!(I::BITS <= 64) };
22        if I::BITS >= 64 {
23            r += ((u32::cast_from_lossy(x) == 0) as u32) << 5; // if (x has no 32 small bits) t = 32 else 0
24            x >>= r; // remove 32 zero bits
25        }
26
27        if I::BITS >= 32 {
28            t = ((u16::cast_from_lossy(x) == 0) as u32) << 4; // if (x has no 16 small bits) t = 16 else 0
29            r += t;
30            x >>= t; // x = [0 - 0xFFFF] + higher garbage bits
31        }
32
33        const { assert!(I::BITS >= 16) };
34        t = ((u8::cast_from_lossy(x) == 0) as u32) << 3;
35        x >>= t; // x = [0 - 0xFF] + higher garbage bits
36        r += t;
37
38        let mut x: u8 = x.cast_lossy();
39
40        t = (((x & 0x0F) == 0) as u32) << 2;
41        x >>= t; // x = [0 - 0xF] + higher garbage bits
42        r += t;
43
44        t = (((x & 0x3) == 0) as u32) << 1;
45        x >>= t; // x = [0 - 0x3] + higher garbage bits
46        r += t;
47
48        x &= 3;
49
50        r as usize + ((2 - (x >> 1) as usize) & (((x & 1) == 0) as usize).wrapping_neg())
51    }
52}
53
54intrinsics! {
55    /// Returns the number of trailing binary zeros in `x` (32 bit version).
56    pub extern "C" fn __ctzsi2(x: u32) -> usize {
57        trailing_zeros(x)
58    }
59
60    /// Returns the number of trailing binary zeros in `x` (64 bit version).
61    pub extern "C" fn __ctzdi2(x: u64) -> usize {
62        trailing_zeros(x)
63    }
64
65    /// Returns the number of trailing binary zeros in `x` (128 bit version).
66    pub extern "C" fn __ctzti2(x: u128) -> usize {
67        let lo = x as u64;
68        if lo == 0 {
69            64 + __ctzdi2((x >> 64) as u64)
70        } else {
71            __ctzdi2(lo)
72        }
73    }
74}
compiler_builtins/int/trailing_zeros.rs

compiler_builtins/int/
trailing_zeros.rs
