4 files changed, 59 insertions, 6 deletions
diff --git a/src/corelib/thread/qthread.cpp b/src/corelib/thread/qthread.cpp
index 9fd1dd059d..eeaec7f24d 100644
--- a/src/corelib/thread/qthread.cpp
+++ b/src/corelib/thread/qthread.cpp
@@ -844,7 +844,7 @@ bool QThread::event(QEvent* event)
     return QObject::event(event);
 }
 
-Qt::HANDLE QThread::currentThreadId() noexcept
+Qt::HANDLE QThread::currentThreadIdImpl() noexcept
 {
     return Qt::HANDLE(currentThread());
 }
diff --git a/src/corelib/thread/qthread.h b/src/corelib/thread/qthread.h
index c7a6dc8f1a..8141f945b6 100644
--- a/src/corelib/thread/qthread.h
+++ b/src/corelib/thread/qthread.h
@@ -161,6 +161,7 @@ private:
 #if QT_CONFIG(cxx11_future)
     static QThread *createThreadImpl(std::future<void> &&future);
 #endif
+    static Qt::HANDLE currentThreadIdImpl() noexcept Q_DECL_PURE_FUNCTION;
 
     friend class QCoreApplication;
     friend class QThreadData;
@@ -236,6 +237,38 @@ QThread *QThread::create(Function &&f)
 
 #endif // QT_CONFIG(cxx11_future)
 
+/*
+    On architectures and platforms we know, interpret the thread control
+    block (TCB) as a unique identifier for a thread within a process. Otherwise,
+    fall back to a slower but safe implementation.
+
+    As per the documentation of currentThreadId, we return an opaque handle
+    as a thread identifier, and application code is not supposed to use that
+    value for anything. In Qt we use the handle to check if threads are identical,
+    for which the TCB is sufficient.
+
+    So we use the fastest possible way, rathern than spend time on returning
+    some pseudo-interoperable value.
+*/
+inline Qt::HANDLE QThread::currentThreadId() noexcept
+{
+    Qt::HANDLE tid; // typedef to void*
+    Q_STATIC_ASSERT(sizeof(tid) == sizeof(void*));
+    // See https://akkadia.org/drepper/tls.pdf for x86 ABI
+#if defined(Q_PROCESSOR_X86_32) && defined(Q_OS_LINUX) // x86 32-bit always uses GS
+    __asm__("movl %%gs:0, %0" : "=r" (tid) : : );
+#elif defined(Q_PROCESSOR_X86_64) && defined(Q_OS_DARWIN64)
+    // 64bit macOS uses GS, see https://github.com/apple/darwin-xnu/blob/master/libsyscall/os/tsd.h
+    __asm__("movq %%gs:0, %0" : "=r" (tid) : : );
+#elif defined(Q_PROCESSOR_X86_64) && (defined(Q_OS_LINUX) || defined(Q_OS_FREEBSD))
+    // x86_64 Linux, BSD uses FS
+    __asm__("movq %%fs:0, %0" : "=r" (tid) : : );
+#else
+    tid = currentThreadIdImpl();
+#endif
+    return tid;
+}
+
 QT_END_NAMESPACE
 
 #endif // QTHREAD_H
diff --git a/src/corelib/thread/qthread_unix.cpp b/src/corelib/thread/qthread_unix.cpp
index cb3c0d6bb1..21abe372eb 100644
--- a/src/corelib/thread/qthread_unix.cpp
+++ b/src/corelib/thread/qthread_unix.cpp
@@ -415,16 +415,36 @@ void QThreadPrivate::finish(void *arg)
 }
 
 
-
-
 /**************************************************************************
  ** QThread
  *************************************************************************/
 
-Qt::HANDLE QThread::currentThreadId() noexcept
+/*
+    Since each thread is guaranteed to have its own copy of
+    currenThreadData, the address is guaranteed to be unique for each
+    running thread (but likely to be reused for newly started threads).
+
+    CI tests fails on ARM architectures if we try to use the assembler,
+    or the address of the thread_local (even with a recent gcc version), so
+    stick to the pthread version there. The assembler would be
+
+    // http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0344k/Babeihid.html
+    asm volatile ("mrc p15, 0, %0, c13, c0, 3" : "=r" (tid));
+
+    and
+
+    // see glibc/sysdeps/aarch64/nptl/tls.h
+    asm volatile ("mrs %0, tpidr_el0" : "=r" (tid));
+
+    for 32 and 64bit versions, respectively.
+*/
+Qt::HANDLE QThread::currentThreadIdImpl() noexcept
 {
-    // requires a C cast here otherwise we run into trouble on AIX
+#if defined(Q_PROCESSOR_ARM)
     return to_HANDLE(pthread_self());
+#else
+    return &currentThreadData;
+#endif
 }
 
 #if defined(QT_LINUXBASE) && !defined(_SC_NPROCESSORS_ONLN)
diff --git a/src/corelib/thread/qthread_win.cpp b/src/corelib/thread/qthread_win.cpp
index a72df2fc40..996bcf0a71 100644
--- a/src/corelib/thread/qthread_win.cpp
+++ b/src/corelib/thread/qthread_win.cpp
@@ -449,7 +449,7 @@ void QThreadPrivate::finish(void *arg, bool lockAnyway) noexcept
  ** QThread
  *************************************************************************/
 
-Qt::HANDLE QThread::currentThreadId() noexcept
+Qt::HANDLE QThread::currentThreadIdImpl() noexcept
 {
     return reinterpret_cast<Qt::HANDLE>(quintptr(GetCurrentThreadId()));
 }