unroll slice::equal impl

Author: the8472

library/core/src/slice/cmp.rs

@@ -60,7 +60,22 @@ where
             return false;
         }
 
-        self.iter().zip(other.iter()).all(|(x, y)| x == y)
+        let (chunks_a, residual_a) = self.as_chunks::<4>();
+        let (chunks_b, residual_b) = other.as_chunks::<4>();
+
+        // check the residual first to bail out fast if there's a mismatch and comparisons
+        // happen to be expensive
+        let mut result = residual_a.into_iter().zip(residual_b).all(|(a, b)| a == 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.iter())
+                .all(|(a, b)| (a[0] == b[0]) & (a[1] == b[1]) & (a[2] == b[2]) & (a[3] == b[3]));
+
+        result
     }
 }