Long live qYieldCpu()!

This is a small wrapper around PAUSE (x86) (and, eventually, YIELD
(ARM) and whatever MIPS and Sparc have for this purpose) instructions
to improve the performance of tight CAS loops.

Pick-to: 6.3
Change-Id: I51e58484c7f02fdbdc491fd1f4a2e9a34f7c2cd0
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qsimd_p.h

@@ -394,6 +394,35 @@ static inline uint64_t qCpuFeatures()
 #define qCpuHasFeature(feature)     (((qCompilerCpuFeatures & CpuFeature ## feature) == CpuFeature ## feature) \
                                      || ((qCpuFeatures() & CpuFeature ## feature) == CpuFeature ## feature))
 
+/*
+    Small wrapper around x86's PAUSE instruction.
+
+    This is completely different from QThread::yieldCurrentThread(), which is
+    an OS-level operation that takes the whole thread off the CPU.
+
+    This is just preventing one SMT thread from filling a core's pipeline with
+    speculated further loop iterations (which need to be expensively flushed on
+    final success) when it could just give those pipeline slots to a second SMT
+    thread that can do something useful with the core, such as unblocking this
+    SMT thread :)
+
+    So, instead of
+
+        while (!condition)
+            ;
+
+    it's better to use
+
+        while (!condition)
+            qYieldCpu();
+*/
+static inline void qYieldCpu()
+{
+#if defined(Q_PROCESSOR_X86)
+    _mm_pause();
+#endif
+}
+
 #ifdef __cplusplus
 } // extern "C"