vectorize slice::is_sorted

Author: LaihoE

library/core/src/slice/mod.rs

@@ -4093,7 +4093,23 @@ impl<T> [T] {
     where
         T: PartialOrd,
     {
-        self.is_sorted_by(|a, b| a <= b)
+        // This odd number works the best. 32 + 1 extra due to overlapping chunk boundaries.
+        const CHUNK_SIZE: usize = 33;
+        if self.len() < CHUNK_SIZE {
+            return self.windows(2).all(|w| w[0] <= w[1]);
+        }
+        let mut i = 0;
+        // Check in chunks for autovectorization.
+        while i < self.len() - CHUNK_SIZE {
+            let chunk = &self[i..i + CHUNK_SIZE];
+            if !chunk.windows(2).fold(true, |acc, w| acc & (w[0] <= w[1])) {
+                return false;
+            }
+            // We need to ensure that chunk boundaries are also sorted.
+            // Overlap the next chunk with the last element of our last chunk.
+            i += CHUNK_SIZE - 1;
+        }
+        self[i..].windows(2).all(|w| w[0] <= w[1])
     }
 
     /// Checks if the elements of this slice are sorted using the given comparator function.