Auto merge of rust-lang#116422 - the8472:chunked-generic-slice-eq, r=<try>

Chunked generic slice eq

looks nice in a microbenchmark, let's see if perf agrees

```
OLD:
    slice::slice_cmp_generic 54.00ns/iter +/- 1.00ns
NEW:
    slice::slice_cmp_generic 20.00ns/iter +/- 2.00ns
```

Author: bors

library/core/benches/slice.rs

@@ -171,3 +171,17 @@ fn fold_to_last(b: &mut Bencher) {
     let slice: &[i32] = &[0; 1024];
     b.iter(|| black_box(slice).iter().fold(None, |_, r| Some(NonNull::from(r))));
 }
+
+#[bench]
+fn slice_cmp_generic(b: &mut Bencher) {
+    #[derive(PartialEq, Clone, Copy)]
+    struct Foo(u32, u32);
+
+    let left = [Foo(128, 128); 128];
+    let right = [Foo(128, 128); 128];
+
+    b.iter(|| {
+        let (left, right) = (black_box(&left), black_box(&right));
+        left.as_slice() == right.as_slice()
+    });
+}

library/core/src/slice/cmp.rs

@@ -60,10 +60,50 @@ where
             return false;
         }
 
-        self.iter().zip(other.iter()).all(|(x, y)| x == y)
+        // at least 8 items for unrolling to make sense (4 peeled + 4+ unrolled)
+        if self.len() < 8 {
+            return eq_small(self, other);
+        }
+
+        eq_unroll(self, other)
     }
 }
 
+#[inline]
+fn eq_small<A, B>(a: &[A], b: &[B]) -> bool
+where
+    A: PartialEq<B>,
+{
+    a.iter().zip(b).all(|(a, b)| a == b)
+}
+
+fn eq_unroll<A, B>(a: &[A], b: &[B]) -> bool
+where
+    A: PartialEq<B>,
+{
+    let (mut chunks_a, residual_a) = a.as_chunks::<4>();
+    let (mut chunks_b, residual_b) = b.as_chunks::<4>();
+    let peeled_a = chunks_a.take_first().unwrap();
+    let peeled_b = chunks_b.take_first().unwrap();
+
+    // peel the first chunk and do a short-circuiting comparison to bail early on mismatches
+    // in case comparisons are expensive
+    let mut result = eq_small(peeled_a, peeled_b);
+
+    // then check the residual, another chance to bail early
+    result = result && eq_small(residual_a, residual_b);
+
+    // iter.all short-circuits which means the backend can't unroll the loop due to early exits.
+    // So we unroll it manually.
+    result = result
+        && chunks_a
+            .iter()
+            .zip(chunks_b)
+            .all(|(a, b)| (a[0] == b[0]) & (a[1] == b[1]) & (a[2] == b[2]) & (a[3] == b[3]));
+
+    result
+}
+
 // When each element can be compared byte-wise, we can compare all the bytes
 // from the whole size in one call to the intrinsics.
 impl<A, B> SlicePartialEq<B> for [A]