Add comment why we use zero for COMPLETE

2 files changed, 4 insertions, 1 deletions

diff --git a/library/std/src/sys/sync/once/futex.rs b/library/std/src/sys/sync/once/futex.rs
index 827bbf014cd..407fdcebcf5 100644
--- a/library/std/src/sys/sync/once/futex.rs
+++ b/library/std/src/sys/sync/once/futex.rs
@@ -17,6 +17,8 @@ const POISONED: Primitive = 2;
 /// so all future threads need to wait for it to finish.
 const RUNNING: Primitive = 1;
 /// Initialization has completed and all future calls should finish immediately.
+/// By choosing this state as the all-zero state the `is_completed` check can be
+/// a bit faster on some platforms.
 const COMPLETE: Primitive = 0;
 
 // An additional bit indicates whether there are waiting threads:
diff --git a/library/std/src/sys/sync/once/queue.rs b/library/std/src/sys/sync/once/queue.rs
index 7bebb6696a8..49e15d65f25 100644
--- a/library/std/src/sys/sync/once/queue.rs
+++ b/library/std/src/sys/sync/once/queue.rs
@@ -74,7 +74,8 @@ pub struct OnceState {
 }
 
 // Four states that a Once can be in, encoded into the lower bits of
-// `state_and_queue` in the Once structure.
+// `state_and_queue` in the Once structure. By choosing COMPLETE as the all-zero
+// state the `is_completed` check can be a bit faster on some platforms.
 const INCOMPLETE: usize = 0x3;
 const POISONED: usize = 0x2;
 const RUNNING: usize = 0x1;