From c847f0091d484de7b3fb1049838eee3f2ac3dfc5 Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@qt.io>
Date: Sat, 30 Apr 2022 10:21:26 +0200
Subject: QFutureInterface: use (new) qYieldCpu() instead of _mm_pause()

This loop here was a lonesome instance of a CAS loop in which adding
_mm_pause() was simple, because the code didn't use the usual pattern

  do {
     construct new value
  } while (!testAndSet)

we use everywhere else in Qt.

In search of an elegant pattern that would allow to apply
qYieldCpu()/_mm_pause() to those idiomatic CAS loops, too, I've
reached for a lambda to construct the new value. This should apply to
all (tight) CAS loops, and may form the basis of an API extension
whereby we take that lambda as a function argument to encapsulate the
CAS loop in an algorithm (a function).

Pick-to: 6.3
Change-Id: Id4a8f174dd812aa26f0b163e943bd4558e5e6a7b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
---
 src/corelib/thread/qfutureinterface.cpp | 21 ++++++---------------
 1 file changed, 6 insertions(+), 15 deletions(-)

(limited to 'src/corelib/thread/qfutureinterface.cpp')

diff --git a/src/corelib/thread/qfutureinterface.cpp b/src/corelib/thread/qfutureinterface.cpp
index a54caa878b..efbcf9ce58 100644
--- a/src/corelib/thread/qfutureinterface.cpp
+++ b/src/corelib/thread/qfutureinterface.cpp
@@ -43,12 +43,9 @@
 
 #include <QtCore/qatomic.h>
 #include <QtCore/qthread.h>
+#include <QtCore/private/qsimd_p.h> // for qYieldCpu()
 #include <private/qthreadpool_p.h>
 
-#ifdef Q_PROCESSOR_X86
-#  include <immintrin.h>        // for _mm_pause()
-#endif
-
 #ifdef interface
 #  undef interface
 #endif
@@ -108,17 +105,11 @@ static inline int switch_off(QAtomicInt &a, int which)
 
 static inline int switch_from_to(QAtomicInt &a, int from, int to)
 {
-    int newValue;
-    int expected = a.loadRelaxed();
-    for (;;) {
-        newValue = (expected & ~from) | to;
-        if (a.testAndSetRelaxed(expected, newValue, expected))
-            break;
-#ifdef Q_PROCESSOR_X86
-        _mm_pause();
-#endif
-    }
-    return newValue;
+    const auto adjusted = [&](int old) { return (old & ~from) | to; };
+    int value = a.loadRelaxed();
+    while (!a.testAndSetRelaxed(value, adjusted(value), value))
+        qYieldCpu();
+    return value;
 }
 
 void QFutureInterfaceBase::cancel()
-- 
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