Optimize Ord trait implementation for bool

Casting the booleans to `i8`s and converting their difference
into `Ordering` generates better assembly than casting them to
`u8`s and comparing them.

2 files changed, 18 insertions, 1 deletions

diff --git a/src/libcore/cmp.rs b/src/libcore/cmp.rs
index eea3dc39d34..e72b6117ba8 100644
--- a/src/libcore/cmp.rs
+++ b/src/libcore/cmp.rs
@@ -1006,6 +1006,7 @@ pub fn max_by_key<T, F: FnMut(&T) -> K, K: Ord>(v1: T, v2: T, mut f: F) -> T {
 
 // Implementation of PartialEq, Eq, PartialOrd and Ord for primitive types
 mod impls {
+    use crate::hint::unreachable_unchecked;
     use crate::cmp::Ordering::{self, Less, Greater, Equal};
 
     macro_rules! partial_eq_impl {
@@ -1126,7 +1127,15 @@ mod impls {
     impl Ord for bool {
         #[inline]
         fn cmp(&self, other: &bool) -> Ordering {
-            (*self as u8).cmp(&(*other as u8))
+            // Casting to i8's and converting the difference to an Ordering generates
+            // more optimal assembly.
+            // See <https://github.com/rust-lang/rust/issues/66780> for more info.
+            match (*self as i8) - (*other as i8) {
+                -1 => Less,
+                0 => Equal,
+                1 => Greater,
+                _ => unsafe { unreachable_unchecked() },
+            }
         }
     }
 
diff --git a/src/libcore/tests/cmp.rs b/src/libcore/tests/cmp.rs
index 5e6778e222a..56a2f4acf6e 100644
--- a/src/libcore/tests/cmp.rs
+++ b/src/libcore/tests/cmp.rs
@@ -10,6 +10,14 @@ fn test_int_totalord() {
 }
 
 #[test]
+fn test_bool_totalord() {
+    assert_eq!(true.cmp(&false), Greater);
+    assert_eq!(false.cmp(&true), Less);
+    assert_eq!(true.cmp(&true), Equal);
+    assert_eq!(false.cmp(&false), Equal);
+}
+
+#[test]
 fn test_mut_int_totalord() {
     assert_eq!((&mut 5).cmp(&&mut 10), Less);
     assert_eq!((&mut 10).cmp(&&mut 5), Greater);