Use zero for initialized Once state

library/std/src/sys/sync/once/futex.rs

@@ -8,16 +8,18 @@ use crate::sys::futex::{Futex, Primitive, futex_wait, futex_wake_all};
 // This means we only need one atomic value with 4 states:
 
 /// No initialization has run yet, and no thread is currently using the Once.
-const INCOMPLETE: Primitive = 0;
+const INCOMPLETE: Primitive = 3;
 /// Some thread has previously attempted to initialize the Once, but it panicked,
 /// so the Once is now poisoned. There are no other threads currently accessing
 /// this Once.
-const POISONED: Primitive = 1;
+const POISONED: Primitive = 2;
 /// Some thread is currently attempting to run initialization. It may succeed,
 /// so all future threads need to wait for it to finish.
-const RUNNING: Primitive = 2;
+const RUNNING: Primitive = 1;
 /// Initialization has completed and all future calls should finish immediately.
-const COMPLETE: Primitive = 3;
+/// 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:
 

library/std/src/sys/sync/once/queue.rs

@@ -74,11 +74,12 @@ pub struct OnceState {
 }
 
 // Four states that a Once can be in, encoded into the lower bits of
-// `state_and_queue` in the Once structure.
-const INCOMPLETE: usize = 0x0;
-const POISONED: usize = 0x1;
-const RUNNING: usize = 0x2;
-const COMPLETE: usize = 0x3;
+// `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;
+const COMPLETE: usize = 0x0;
 
 // Mask to learn about the state. All other bits are the queue of waiters if
 // this is in the RUNNING state.