Auto merge of #45319 - michaelwoerister:use-128bit-siphash, r=nikomat…

…sakis

incr.comp.: Use 128bit SipHash for fingerprinting

This PR switches incr. comp. result fingerprinting from 128 bit BLAKE2 to 128 bit SipHash. When we started using BLAKE2 for fingerprinting, the 128 bit version of SipHash was still experimental. Now that it isn't anymore we should be able to get a nice performance boost without significantly increasing collision probability.

~~I'm going to start a try-build for this, so we can gauge the performance impact before merging (hence the `WIP` in the title).~~

EDIT: Performance improvements look as expected. Tests seem to be passing.

Fixes #41215.

src/libcore/hash/sip.rs

@@ -72,6 +72,7 @@ struct Hasher<S: Sip> {
 }
 
 #[derive(Debug, Clone, Copy)]
+#[repr(C)]
 struct State {
     // v0, v2 and v1, v3 show up in pairs in the algorithm,
     // and simd implementations of SipHash will use vectors

src/librustc/ich/fingerprint.rs

@@ -9,8 +9,6 @@
 // except according to those terms.
 
 use rustc_data_structures::stable_hasher;
-use std::mem;
-use std::slice;
 
 #[derive(Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Clone, Copy, RustcEncodable, RustcDecodable)]
 pub struct Fingerprint(u64, u64);
@@ -54,16 +52,9 @@ impl ::std::fmt::Display for Fingerprint {
 }
 
 impl stable_hasher::StableHasherResult for Fingerprint {
-    fn finish(mut hasher: stable_hasher::StableHasher<Self>) -> Self {
-        let hash_bytes: &[u8] = hasher.finalize();
-
-        assert!(hash_bytes.len() >= mem::size_of::<u64>() * 2);
-        let hash_bytes: &[u64] = unsafe {
-            slice::from_raw_parts(hash_bytes.as_ptr() as *const u64, 2)
-        };
-
-        // The bytes returned bytes the Blake2B hasher are always little-endian.
-        Fingerprint(u64::from_le(hash_bytes[0]), u64::from_le(hash_bytes[1]))
+    fn finish(hasher: stable_hasher::StableHasher<Self>) -> Self {
+        let (_0, _1) = hasher.finalize();
+        Fingerprint(_0, _1)
     }
 }
 

src/librustc_data_structures/lib.rs

@@ -28,6 +28,7 @@
 #![feature(fn_traits)]
 #![feature(unsize)]
 #![feature(i128_type)]
+#![feature(i128)]
 #![feature(conservative_impl_trait)]
 #![feature(specialization)]
 
@@ -54,6 +55,7 @@ pub mod graph;
 pub mod indexed_set;
 pub mod indexed_vec;
 pub mod obligation_forest;
+pub mod sip128;
 pub mod snapshot_map;
 pub mod snapshot_vec;
 pub mod stable_hasher;