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// Copyright (C) 2023 The Qt Company Ltd.
// Copyright (C) 2023 Intel Corporation.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\fn qYieldCpu()
\inmodule QtCore
\ingroup thread
\relates QAtomicInteger
//! \relatesalso QAtomicPointer
\since 6.7
Pauses the execution of the current thread for an unspecified time, using
hardware instructions, without de-scheduling this thread. This function is
meant to be used in high-throughput loops where the code expects another
thread to modify an atomic variable. This is completely different from
QThread::yieldCurrentThread(), which is an OS-level operation that may take
the whole thread off the CPU and allow other threads (possibly belonging to
other processes) to run.
So, instead of
\code
while (!condition)
;
\endcode
one should write
\code
while (!condition)
qYieldCpu();
\endcode
This is useful both with and without hardware multithreading on the same
core. In the case of hardware threads, it serves to prevent further
speculative execution filling up the pipeline, which could starve the
sibling thread of resources. Across cores and higher levels of separation,
it allows the cache coherency protocol to allocate the cache line being
modified and inspected to the logical processor whose result this code is
expecting.
It is also recommended to loop around code that does not modify the global
variable, to avoid contention in exclusively obtaining the memory location.
Therefore, an atomic modification loop such as a spinlock acquisition
should be:
\code
while (true) {
while (!readOnlyCondition(atomic))
qYieldCpu();
if (modify(atomic))
break;
}
\endcode
On x86 processors and on RISC-V processors with the \c{Zihintpause}
extension, this will emit the \c PAUSE instruction, which is ignored on
processors that don't support it; on ARMv7 or later ARM processors, it will
emit the \c{YIELD} instruction.
*/
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