Auto merge of #81358 - mcastorina:to-upper-lower-speed, r=joshtriplett

[rust.git] / compiler / rustc_data_structures / src / fingerprint.rs

diff --git a/compiler/rustc_data_structures/src/fingerprint.rs b/compiler/rustc_data_structures/src/fingerprint.rs
index 08c3419a8421df0ef6783d9ee3d555543d0b059d..681b49e2ea97bfaca49751da7d8ab1d79335ad1b 100644 (file)
--- a/compiler/rustc_data_structures/src/fingerprint.rs
+++ b/compiler/rustc_data_structures/src/fingerprint.rs
@@ -7,11 +7,17 @@
 use std::mem::{self, MaybeUninit};
 
 #[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy)]
+#[repr(C)]
 pub struct Fingerprint(u64, u64);
 
 impl Fingerprint {
     pub const ZERO: Fingerprint = Fingerprint(0, 0);
 
+    #[inline]
+    pub fn new(_0: u64, _1: u64) -> Fingerprint {
+        Fingerprint(_0, _1)
+    }
+
     #[inline]
     pub fn from_smaller_hash(hash: u64) -> Fingerprint {
         Fingerprint(hash, hash)
@@ -19,7 +25,12 @@ pub fn from_smaller_hash(hash: u64) -> Fingerprint {
 
     #[inline]
     pub fn to_smaller_hash(&self) -> u64 {
-        self.0
+        // Even though both halves of the fingerprint are expected to be good
+        // quality hash values, let's still combine the two values because the
+        // Fingerprints in DefPathHash have the StableCrateId portion which is
+        // the same for all DefPathHashes from the same crate. Combining the
+        // two halfs makes sure we get a good quality hash in such cases too.
+        self.0.wrapping_mul(3).wrapping_add(self.1)
     }
 
     #[inline]
@@ -92,8 +103,19 @@ impl<H: Hasher> FingerprintHasher for H {
 impl FingerprintHasher for crate::unhash::Unhasher {
     #[inline]
     fn write_fingerprint(&mut self, fingerprint: &Fingerprint) {
-        // `Unhasher` only wants a single `u64`
-        self.write_u64(fingerprint.0);
+        // Even though both halves of the fingerprint are expected to be good
+        // quality hash values, let's still combine the two values because the
+        // Fingerprints in DefPathHash have the StableCrateId portion which is
+        // the same for all DefPathHashes from the same crate. Combining the
+        // two halfs makes sure we get a good quality hash in such cases too.
+        //
+        // Since `Unhasher` is used only in the context of HashMaps, it is OK
+        // to combine the two components in an order-independent way (which is
+        // cheaper than the more robust Fingerprint::to_smaller_hash()). For
+        // HashMaps we don't really care if Fingerprint(x,y) and
+        // Fingerprint(y, x) result in the same hash value. Collision
+        // probability will still be much better than with FxHash.
+        self.write_u64(fingerprint.0.wrapping_add(fingerprint.1));
     }
 }