mpmc: use i8 to calculate dif

Remove the boundary check introduced by #132
Now it passes full_at_wrapped_pos0.

Author: debug-ito

src/mpmc.rs

@@ -479,7 +479,7 @@ unsafe fn dequeue<T>(buffer: *mut Cell<T>, dequeue_pos: &AtomicU8, mask: u8) ->
     loop {
         cell = buffer.add(usize::from(pos & mask));
         let seq = (*cell).sequence.load(Ordering::Acquire);
-        let dif = i16::from(seq) - i16::from(pos.wrapping_add(1));
+        let dif = (seq as i8) - ((pos.wrapping_add(1)) as i8);
 
         if dif == 0 {
             if dequeue_pos
@@ -496,11 +496,7 @@ unsafe fn dequeue<T>(buffer: *mut Cell<T>, dequeue_pos: &AtomicU8, mask: u8) ->
         } else if dif < 0 {
             return None;
         } else {
-            if pos == 255 && dif == 255 {
-                return None;
-            } else {
-                pos = dequeue_pos.load(Ordering::Relaxed);
-            }
+            pos = dequeue_pos.load(Ordering::Relaxed);
         }
     }
 
@@ -523,7 +519,7 @@ unsafe fn enqueue<T>(
     loop {
         cell = buffer.add(usize::from(pos & mask));
         let seq = (*cell).sequence.load(Ordering::Acquire);
-        let dif = i16::from(seq) - i16::from(pos);
+        let dif = (seq as i8) - (pos as i8);
 
         if dif == 0 {
             if enqueue_pos