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Make Fingerprint::combine_commutative associative

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The previous implementation swapped lower and upper 64-bits of a result
of modular addition, so the function was non-associative.
This commit is contained in:
Tomasz Miąsko 2022-01-03 00:00:00 +00:00
parent ddabe0775c
commit 1d64b59664
2 changed files with 18 additions and 1 deletions
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5
compiler/rustc_data_structures/src/fingerprint.rs
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@ -3,6 +3,9 @@
use std::convert::TryInto;
use std::hash::{Hash, Hasher};
#[cfg(test)]
mod tests;
#[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy)]
#[repr(C)]
pub struct Fingerprint(u64, u64);
@ -54,7 +57,7 @@ pub fn combine_commutative(self, other: Fingerprint) -> Fingerprint {
let c = a.wrapping_add(b);
Fingerprint((c >> 64) as u64, c as u64)
Fingerprint(c as u64, (c >> 64) as u64)
}
pub fn to_hex(&self) -> String {

14
compiler/rustc_data_structures/src/fingerprint/tests.rs Normal file
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@ -0,0 +1,14 @@
use super::*;
// Check that `combine_commutative` is order independent.
#[test]
fn combine_commutative_is_order_independent() {
let a = Fingerprint::new(0xf6622fb349898b06, 0x70be9377b2f9c610);
let b = Fingerprint::new(0xa9562bf5a2a5303c, 0x67d9b6c82034f13d);
let c = Fingerprint::new(0x0d013a27811dbbc3, 0x9a3f7b3d9142ec43);
let permutations = [(a, b, c), (a, c, b), (b, a, c), (b, c, a), (c, a, b), (c, b, a)];
let f = a.combine_commutative(b).combine_commutative(c);
for p in &permutations {
assert_eq!(f, p.0.combine_commutative(p.1).combine_commutative(p.2));
}
}