Auto merge of #107925 - thomcc:sip13, r=cjgillot

Use SipHash-1-3 instead of SipHash-2-4 for StableHasher

Noticed this, and it seems easy and likely a perf win. IIUC we don't need DDOS resistance (just collision) so we ideally would have an even faster hash, but it's hard to beat this SipHash impl here, since it's been so highly tuned for the interface.

It wouldn't surprise me if there's some subtle reason changing this sucks, as it's so obvious it seems likely to have been done. Still, SipHash-1-3 seems to still have the guarantees StableHasher should need (and seemingly more), and is clearly less work. So it's worth a shot.

Not fully tested locally.

compiler/rustc_data_structures/src/sip128.rs

@@ -247,7 +247,7 @@ impl SipHasher128 {
         for i in 0..BUFFER_CAPACITY {
             let elem = self.buf.get_unchecked(i).assume_init().to_le();
             self.state.v3 ^= elem;
-            Sip24Rounds::c_rounds(&mut self.state);
+            Sip13Rounds::c_rounds(&mut self.state);
             self.state.v0 ^= elem;
         }
 
@@ -327,7 +327,7 @@ impl SipHasher128 {
         for i in 0..last {
             let elem = self.buf.get_unchecked(i).assume_init().to_le();
             self.state.v3 ^= elem;
-            Sip24Rounds::c_rounds(&mut self.state);
+            Sip13Rounds::c_rounds(&mut self.state);
             self.state.v0 ^= elem;
         }
 
@@ -340,7 +340,7 @@ impl SipHasher128 {
         for _ in 0..elems_left {
             let elem = (msg.as_ptr().add(processed) as *const u64).read_unaligned().to_le();
             self.state.v3 ^= elem;
-            Sip24Rounds::c_rounds(&mut self.state);
+            Sip13Rounds::c_rounds(&mut self.state);
             self.state.v0 ^= elem;
             processed += ELEM_SIZE;
         }
@@ -368,7 +368,7 @@ impl SipHasher128 {
         for i in 0..last {
             let elem = unsafe { self.buf.get_unchecked(i).assume_init().to_le() };
             state.v3 ^= elem;
-            Sip24Rounds::c_rounds(&mut state);
+            Sip13Rounds::c_rounds(&mut state);
             state.v0 ^= elem;
         }
 
@@ -392,15 +392,15 @@ impl SipHasher128 {
         let b: u64 = ((length as u64 & 0xff) << 56) | elem;
 
         state.v3 ^= b;
-        Sip24Rounds::c_rounds(&mut state);
+        Sip13Rounds::c_rounds(&mut state);
         state.v0 ^= b;
 
         state.v2 ^= 0xee;
-        Sip24Rounds::d_rounds(&mut state);
+        Sip13Rounds::d_rounds(&mut state);
         let _0 = state.v0 ^ state.v1 ^ state.v2 ^ state.v3;
 
         state.v1 ^= 0xdd;
-        Sip24Rounds::d_rounds(&mut state);
+        Sip13Rounds::d_rounds(&mut state);
         let _1 = state.v0 ^ state.v1 ^ state.v2 ^ state.v3;
 
         (_0, _1)
@@ -477,20 +477,18 @@ impl Hasher for SipHasher128 {
 }
 
 #[derive(Debug, Clone, Default)]
-struct Sip24Rounds;
+struct Sip13Rounds;
 
-impl Sip24Rounds {
+impl Sip13Rounds {
     #[inline]
     fn c_rounds(state: &mut State) {
         compress!(state);
-        compress!(state);
     }
 
     #[inline]
     fn d_rounds(state: &mut State) {
         compress!(state);
         compress!(state);
         compress!(state);
-        compress!(state);
     }
 }