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Always const-eval the gcd in slice::align_to_offsets

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This commit is contained in:
Markus Everling 2023-05-06 17:31:51 +00:00
parent 333b920fee
commit b5ee324d79
1 changed files with 6 additions and 37 deletions
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43
library/core/src/slice/mod.rs
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@ -3474,44 +3474,13 @@ fn align_to_offsets<U>(&self) -> (usize, usize) {
// Ts = size_of::<U> / gcd(size_of::<T>, size_of::<U>)
//
// Luckily since all this is constant-evaluated... performance here matters not!
#[inline]
fn gcd(a: usize, b: usize) -> usize {
use crate::intrinsics;
// iterative stein’s algorithm
// We should still make this `const fn` (and revert to recursive algorithm if we do)
// because relying on llvm to consteval all this is… well, it makes me uncomfortable.
// SAFETY: `a` and `b` are checked to be non-zero values.
let (ctz_a, mut ctz_b) = unsafe {
if a == 0 {
return b;
}
if b == 0 {
return a;
}
(intrinsics::cttz_nonzero(a), intrinsics::cttz_nonzero(b))
};
let k = ctz_a.min(ctz_b);
let mut a = a >> ctz_a;
let mut b = b;
loop {
// remove all factors of 2 from b
b >>= ctz_b;
if a > b {
mem::swap(&mut a, &mut b);
}
b = b - a;
// SAFETY: `b` is checked to be non-zero.
unsafe {
if b == 0 {
break;
}
ctz_b = intrinsics::cttz_nonzero(b);
}
}
a << k
const fn gcd(a: usize, b: usize) -> usize {
if b == 0 { a } else { gcd(b, a % b) }
}
let gcd: usize = gcd(mem::size_of::<T>(), mem::size_of::<U>());
// Explicitly wrap the function call in a const block so it gets
// constant-evaluated even in debug mode.
let gcd: usize = const { gcd(mem::size_of::<T>(), mem::size_of::<U>()) };
let ts: usize = mem::size_of::<U>() / gcd;
let us: usize = mem::size_of::<T>() / gcd;