Backport fence strengthening from qthreads development version. Surprisingly this appears to improve performance.

Signed-off-by: Ian Henriksen <[email protected]>

Author: insertinterestingnamehere

third-party/qthread/README

@@ -102,3 +102,16 @@ index 2ac887ed..2312c954 100644
    pthread_exit(NULL);
 ```
 
+```
+--- a/include/qthread/qthread.h
++++ b/include/qthread/qthread.h
+@@ -87,7 +87,7 @@ using std::memory_order_relaxed;
+
+ #include "macros.h"
+
+-#define MACHINE_FENCE atomic_thread_fence(memory_order_acq_rel);
++#define MACHINE_FENCE atomic_thread_fence(memory_order_seq_cst);
+
+ #if QTHREAD_ASSEMBLY_ARCH == QTHREAD_AMD64
+ #define QTHREAD_SWAPS_IMPLY_ACQ_REL_FENCES
+```

third-party/qthread/qthread-src/include/qthread/qthread.h

@@ -87,7 +87,7 @@ using std::memory_order_relaxed;
 
 #include "macros.h"
 
-#define MACHINE_FENCE atomic_thread_fence(memory_order_acq_rel);
+#define MACHINE_FENCE atomic_thread_fence(memory_order_seq_cst);
 
 #if QTHREAD_ASSEMBLY_ARCH == QTHREAD_AMD64
 #define QTHREAD_SWAPS_IMPLY_ACQ_REL_FENCES