michael/integer-sqrt-rs
michael
/
integer-sqrt-rs
1
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

Use leading_zeros to compute initial bit 

This increases performance on processors with lzcnt instructions.

Signed-off-by: Joe Richey <joerichey@google.com>
This commit is contained in:
Joe Richey 2020-09-07 20:44:32 -07:00
parent 0445b3e5c1
commit 1e02bdb415
1 changed files with 9 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
src/lib.rs
View File

@ -46,17 +46,18 @@ pub trait IntegerSquareRoot {
#[inline(always)]
fn integer_sqrt_impl<T: num_traits::PrimInt>(mut n: T) -> Option<T> {
// Hopefully this will be stripped for unsigned numbers (impossible condition)
if n < T::zero() {
return None;
use core::cmp::Ordering;
match n.cmp(&T::zero()) {
// Hopefully this will be stripped for unsigned numbers (impossible condition)
Ordering::Less => return None,
Ordering::Equal => return Some(T::zero()),
_ => {}
}
// Compute bit, the largest power of 4 <= n
use core::mem::size_of;
let mut bit = T::one().unsigned_shl(size_of::<T>() as u32 * 8 - 2);
while bit > n {
bit = bit.unsigned_shr(2);
}
let max_shift: u32 = T::zero().leading_zeros() - 1;
let shift: u32 = (max_shift - n.leading_zeros()) & !1;
let mut bit = T::one().unsigned_shl(shift);
// Algorithm based on the implementation in:
// https://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Binary_numeral_system_(base_2)