Auto merge of #28436 - ranma42:faster-partialord, r=bluss

This branch improves the performance of Ord and PartialOrd methods for slices compared to the iter-based implementation.
Based on the approach used in #26884.

Author: bors

src/libcore/cmp.rs

@@ -463,17 +463,33 @@ mod impls {
         }
     }
 
-    partial_ord_impl! { char usize u8 u16 u32 u64 isize i8 i16 i32 i64 f32 f64 }
+    partial_ord_impl! { f32 f64 }
 
     macro_rules! ord_impl {
         ($($t:ty)*) => ($(
+            #[stable(feature = "rust1", since = "1.0.0")]
+            impl PartialOrd for $t {
+                #[inline]
+                fn partial_cmp(&self, other: &$t) -> Option<Ordering> {
+                    Some(self.cmp(other))
+                }
+                #[inline]
+                fn lt(&self, other: &$t) -> bool { (*self) < (*other) }
+                #[inline]
+                fn le(&self, other: &$t) -> bool { (*self) <= (*other) }
+                #[inline]
+                fn ge(&self, other: &$t) -> bool { (*self) >= (*other) }
+                #[inline]
+                fn gt(&self, other: &$t) -> bool { (*self) > (*other) }
+            }
+
             #[stable(feature = "rust1", since = "1.0.0")]
             impl Ord for $t {
                 #[inline]
                 fn cmp(&self, other: &$t) -> Ordering {
-                    if *self < *other { Less }
-                    else if *self > *other { Greater }
-                    else { Equal }
+                    if *self == *other { Equal }
+                    else if *self < *other { Less }
+                    else { Greater }
                 }
             }
         )*)

src/libcore/slice.rs

@@ -1559,30 +1559,41 @@ impl<T: Eq> Eq for [T] {}
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Ord> Ord for [T] {
     fn cmp(&self, other: &[T]) -> Ordering {
-        self.iter().cmp(other.iter())
+        let l = cmp::min(self.len(), other.len());
+
+        // Slice to the loop iteration range to enable bound check
+        // elimination in the compiler
+        let lhs = &self[..l];
+        let rhs = &other[..l];
+
+        for i in 0..l {
+            match lhs[i].cmp(&rhs[i]) {
+                Ordering::Equal => (),
+                non_eq => return non_eq,
+            }
+        }
+
+        self.len().cmp(&other.len())
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: PartialOrd> PartialOrd for [T] {
-    #[inline]
     fn partial_cmp(&self, other: &[T]) -> Option<Ordering> {
-        self.iter().partial_cmp(other.iter())
-    }
-    #[inline]
-    fn lt(&self, other: &[T]) -> bool {
-        self.iter().lt(other.iter())
-    }
-    #[inline]
-    fn le(&self, other: &[T]) -> bool {
-        self.iter().le(other.iter())
-    }
-    #[inline]
-    fn ge(&self, other: &[T]) -> bool {
-        self.iter().ge(other.iter())
-    }
-    #[inline]
-    fn gt(&self, other: &[T]) -> bool {
-        self.iter().gt(other.iter())
+        let l = cmp::min(self.len(), other.len());
+
+        // Slice to the loop iteration range to enable bound check
+        // elimination in the compiler
+        let lhs = &self[..l];
+        let rhs = &other[..l];
+
+        for i in 0..l {
+            match lhs[i].partial_cmp(&rhs[i]) {
+                Some(Ordering::Equal) => (),
+                non_eq => return non_eq,
+            }
+        }
+
+        self.len().partial_cmp(&other.len())
     }
 }