QSemaphore: Improve waking up on 64-bit Linux

By judiciously positioning of the bits, we can optimize for the case of
threads trying to acquire a single token, which is what QSemaphore
should be mostly used for, as it matches the POSIX Semaphore API
(sem_wait, sem_timedwait and sem_trywait). If there are only waiters
waiting for a single token, we know that adding n tokens means n threads
can wake up.

This optimizes for multi-token waiters too. For example, if we have 50
single-token waiters and 50 multi-token waiters, a sem.release(5) will
wake up 55 threads instead of 100.

Change-Id: I209fcd5dbc2b4e5381cffffd14de5550c75d2600
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/corelib/thread/qsemaphore.h b/src/corelib/thread/qsemaphore.h
index 03ffa033d8..c41f258577 100644
--- a/src/corelib/thread/qsemaphore.h
+++ b/src/corelib/thread/qsemaphore.h
@@ -68,7 +68,7 @@ private:
 
     union {
         QSemaphorePrivate *d;
-        QBasicAtomicInteger<quint32> u;
+        QBasicAtomicInteger<quintptr> u;        // ### Qt6: make 64-bit
     };
 };