sdk/runtime/vm/os_thread_win.cc

// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#include "platform/globals.h"  // NOLINT
#if defined(HOST_OS_WINDOWS)

#include "vm/growable_array.h"
#include "vm/lockers.h"
#include "vm/os_thread.h"

#include <process.h>  // NOLINT

#include "platform/assert.h"

namespace dart {

// This flag is flipped by platform_win.cc when the process is exiting.
// TODO(zra): Remove once VM shuts down cleanly.
bool private_flag_windows_run_tls_destructors = true;

class ThreadStartData {
 public:
  ThreadStartData(const char* name,
                  OSThread::ThreadStartFunction function,
                  uword parameter)
      : name_(name), function_(function), parameter_(parameter) {}

  const char* name() const { return name_; }
  OSThread::ThreadStartFunction function() const { return function_; }
  uword parameter() const { return parameter_; }

 private:
  const char* name_;
  OSThread::ThreadStartFunction function_;
  uword parameter_;

  DISALLOW_COPY_AND_ASSIGN(ThreadStartData);
};

// Dispatch to the thread start function provided by the caller. This trampoline
// is used to ensure that the thread is properly destroyed if the thread just
// exits.
static unsigned int __stdcall ThreadEntry(void* data_ptr) {
  ThreadStartData* data = reinterpret_cast<ThreadStartData*>(data_ptr);

  const char* name = data->name();
  OSThread::ThreadStartFunction function = data->function();
  uword parameter = data->parameter();
  delete data;

  MonitorData::GetMonitorWaitDataForThread();

  // Create new OSThread object and set as TLS for new thread.
  OSThread* thread = OSThread::CreateOSThread();
  if (thread != NULL) {
    OSThread::SetCurrent(thread);
    thread->set_name(name);

    // Call the supplied thread start function handing it its parameters.
    function(parameter);
  }

  // Clean up the monitor wait data for this thread.
  MonitorWaitData::ThreadExit();

  return 0;
}

int OSThread::Start(const char* name,
                    ThreadStartFunction function,
                    uword parameter) {
  ThreadStartData* start_data = new ThreadStartData(name, function, parameter);
  uint32_t tid;
  uintptr_t thread = _beginthreadex(NULL, OSThread::GetMaxStackSize(),
                                    ThreadEntry, start_data, 0, &tid);
  if (thread == -1L || thread == 0) {
#ifdef DEBUG
    fprintf(stderr, "_beginthreadex error: %d (%s)\n", errno, strerror(errno));
#endif
    return errno;
  }

  // Close the handle, so we don't leak the thread object.
  CloseHandle(reinterpret_cast<HANDLE>(thread));

  return 0;
}

const ThreadId OSThread::kInvalidThreadId = 0;
const ThreadJoinId OSThread::kInvalidThreadJoinId = NULL;

ThreadLocalKey OSThread::CreateThreadLocal(ThreadDestructor destructor) {
  ThreadLocalKey key = TlsAlloc();
  if (key == kUnsetThreadLocalKey) {
    FATAL1("TlsAlloc failed %d", GetLastError());
  }
  ThreadLocalData::AddThreadLocal(key, destructor);
  return key;
}

void OSThread::DeleteThreadLocal(ThreadLocalKey key) {
  ASSERT(key != kUnsetThreadLocalKey);
  BOOL result = TlsFree(key);
  if (!result) {
    FATAL1("TlsFree failed %d", GetLastError());
  }
  ThreadLocalData::RemoveThreadLocal(key);
}

intptr_t OSThread::GetMaxStackSize() {
  const int kStackSize = (128 * kWordSize * KB);
  return kStackSize;
}

ThreadId OSThread::GetCurrentThreadId() {
  return ::GetCurrentThreadId();
}

#ifndef PRODUCT
ThreadId OSThread::GetCurrentThreadTraceId() {
  return ::GetCurrentThreadId();
}
#endif  // PRODUCT

ThreadJoinId OSThread::GetCurrentThreadJoinId(OSThread* thread) {
  ASSERT(thread != NULL);
  // Make sure we're filling in the join id for the current thread.
  ThreadId id = GetCurrentThreadId();
  ASSERT(thread->id() == id);
  // Make sure the join_id_ hasn't been set, yet.
  DEBUG_ASSERT(thread->join_id_ == kInvalidThreadJoinId);
  HANDLE handle = OpenThread(SYNCHRONIZE, false, id);
  ASSERT(handle != NULL);
#if defined(DEBUG)
  thread->join_id_ = handle;
#endif
  return handle;
}

void OSThread::Join(ThreadJoinId id) {
  HANDLE handle = static_cast<HANDLE>(id);
  ASSERT(handle != NULL);
  DWORD res = WaitForSingleObject(handle, INFINITE);
  CloseHandle(handle);
  ASSERT(res == WAIT_OBJECT_0);
}

intptr_t OSThread::ThreadIdToIntPtr(ThreadId id) {
  ASSERT(sizeof(id) <= sizeof(intptr_t));
  return static_cast<intptr_t>(id);
}

ThreadId OSThread::ThreadIdFromIntPtr(intptr_t id) {
  return static_cast<ThreadId>(id);
}

bool OSThread::Compare(ThreadId a, ThreadId b) {
  return a == b;
}

bool OSThread::GetCurrentStackBounds(uword* lower, uword* upper) {
// On Windows stack limits for the current thread are available in
// the thread information block (TIB). Its fields can be accessed through
// FS segment register on x86 and GS segment register on x86_64.
#ifdef _WIN64
  *upper = static_cast<uword>(__readgsqword(offsetof(NT_TIB64, StackBase)));
#else
  *upper = static_cast<uword>(__readfsdword(offsetof(NT_TIB, StackBase)));
#endif
  // Notice that we cannot use the TIB's StackLimit for the stack end, as it
  // tracks the end of the committed range. We're after the end of the reserved
  // stack area (most of which will be uncommitted, most times).
  MEMORY_BASIC_INFORMATION stack_info;
  memset(&stack_info, 0, sizeof(MEMORY_BASIC_INFORMATION));
  size_t result_size =
      VirtualQuery(&stack_info, &stack_info, sizeof(MEMORY_BASIC_INFORMATION));
  ASSERT(result_size >= sizeof(MEMORY_BASIC_INFORMATION));
  *lower = reinterpret_cast<uword>(stack_info.AllocationBase);
  ASSERT(*upper > *lower);
  // When the third last page of the reserved stack is accessed as a
  // guard page, the second last page will be committed (along with removing
  // the guard bit on the third last) _and_ a stack overflow exception
  // is raised.
  //
  // http://blogs.msdn.com/b/satyem/archive/2012/08/13/thread-s-stack-memory-management.aspx
  // explains the details.
  ASSERT((*upper - *lower) >= (4u * 0x1000));
  *lower += 4 * 0x1000;
  return true;
}

void OSThread::SetThreadLocal(ThreadLocalKey key, uword value) {
  ASSERT(key != kUnsetThreadLocalKey);
  BOOL result = TlsSetValue(key, reinterpret_cast<void*>(value));
  if (!result) {
    FATAL1("TlsSetValue failed %d", GetLastError());
  }
}

Mutex::Mutex(NOT_IN_PRODUCT(const char* name))
#if !defined(PRODUCT)
    : name_(name)
#endif
{
  // Allocate unnamed semaphore with initial count 1 and max count 1.
  data_.semaphore_ = CreateSemaphore(NULL, 1, 1, NULL);
  if (data_.semaphore_ == NULL) {
#if defined(PRODUCT)
    FATAL1("Mutex allocation failed %d", GetLastError());
#else
    FATAL2("[%s] Mutex allocation failed %d", name_, GetLastError());
#endif
  }
#if defined(DEBUG)
  // When running with assertions enabled we do track the owner.
  owner_ = OSThread::kInvalidThreadId;
#endif  // defined(DEBUG)
}

Mutex::~Mutex() {
  CloseHandle(data_.semaphore_);
#if defined(DEBUG)
  // When running with assertions enabled we do track the owner.
  ASSERT(owner_ == OSThread::kInvalidThreadId);
#endif  // defined(DEBUG)
}

void Mutex::Lock() {
  DWORD result = WaitForSingleObject(data_.semaphore_, INFINITE);
  if (result != WAIT_OBJECT_0) {
    FATAL1("Mutex lock failed %d", GetLastError());
  }
#if defined(DEBUG)
  // When running with assertions enabled we do track the owner.
  owner_ = OSThread::GetCurrentThreadId();
#endif  // defined(DEBUG)
}

bool Mutex::TryLock() {
  // Attempt to pass the semaphore but return immediately.
  DWORD result = WaitForSingleObject(data_.semaphore_, 0);
  if (result == WAIT_OBJECT_0) {
#if defined(DEBUG)
    // When running with assertions enabled we do track the owner.
    owner_ = OSThread::GetCurrentThreadId();
#endif  // defined(DEBUG)
    return true;
  }
  if (result == WAIT_ABANDONED || result == WAIT_FAILED) {
    FATAL1("Mutex try lock failed %d", GetLastError());
  }
  ASSERT(result == WAIT_TIMEOUT);
  return false;
}

void Mutex::Unlock() {
#if defined(DEBUG)
  // When running with assertions enabled we do track the owner.
  ASSERT(IsOwnedByCurrentThread());
  owner_ = OSThread::kInvalidThreadId;
#endif  // defined(DEBUG)
  BOOL result = ReleaseSemaphore(data_.semaphore_, 1, NULL);
  if (result == 0) {
    FATAL1("Mutex unlock failed %d", GetLastError());
  }
}

ThreadLocalKey MonitorWaitData::monitor_wait_data_key_ = kUnsetThreadLocalKey;

Monitor::Monitor() {
  InitializeCriticalSection(&data_.cs_);
  InitializeCriticalSection(&data_.waiters_cs_);
  data_.waiters_head_ = NULL;
  data_.waiters_tail_ = NULL;

#if defined(DEBUG)
  // When running with assertions enabled we track the owner.
  owner_ = OSThread::kInvalidThreadId;
#endif  // defined(DEBUG)
}

Monitor::~Monitor() {
#if defined(DEBUG)
  // When running with assertions enabled we track the owner.
  ASSERT(owner_ == OSThread::kInvalidThreadId);
#endif  // defined(DEBUG)

  DeleteCriticalSection(&data_.cs_);
  DeleteCriticalSection(&data_.waiters_cs_);
}

bool Monitor::TryEnter() {
  // Attempt to pass the semaphore but return immediately.
  BOOL result = TryEnterCriticalSection(&data_.cs_);
  if (!result) {
    return false;
  }
#if defined(DEBUG)
  // When running with assertions enabled we do track the owner.
  ASSERT(owner_ == OSThread::kInvalidThreadId);
  owner_ = OSThread::GetCurrentThreadId();
#endif  // defined(DEBUG)
  return true;
}

void Monitor::Enter() {
  EnterCriticalSection(&data_.cs_);

#if defined(DEBUG)
  // When running with assertions enabled we track the owner.
  ASSERT(owner_ == OSThread::kInvalidThreadId);
  owner_ = OSThread::GetCurrentThreadId();
#endif  // defined(DEBUG)
}

void Monitor::Exit() {
#if defined(DEBUG)
  // When running with assertions enabled we track the owner.
  ASSERT(IsOwnedByCurrentThread());
  owner_ = OSThread::kInvalidThreadId;
#endif  // defined(DEBUG)

  LeaveCriticalSection(&data_.cs_);
}

void MonitorWaitData::ThreadExit() {
  if (MonitorWaitData::monitor_wait_data_key_ != kUnsetThreadLocalKey) {
    uword raw_wait_data =
        OSThread::GetThreadLocal(MonitorWaitData::monitor_wait_data_key_);
    // Clear in case this is called a second time.
    OSThread::SetThreadLocal(MonitorWaitData::monitor_wait_data_key_, 0);
    if (raw_wait_data != 0) {
      MonitorWaitData* wait_data =
          reinterpret_cast<MonitorWaitData*>(raw_wait_data);
      delete wait_data;
    }
  }
}

void MonitorData::AddWaiter(MonitorWaitData* wait_data) {
  // Add the MonitorWaitData object to the list of objects waiting for
  // this monitor.
  EnterCriticalSection(&waiters_cs_);
  if (waiters_tail_ == NULL) {
    ASSERT(waiters_head_ == NULL);
    waiters_head_ = waiters_tail_ = wait_data;
  } else {
    waiters_tail_->next_ = wait_data;
    waiters_tail_ = wait_data;
  }
  LeaveCriticalSection(&waiters_cs_);
}

void MonitorData::RemoveWaiter(MonitorWaitData* wait_data) {
  // Remove the MonitorWaitData object from the list of objects
  // waiting for this monitor.
  EnterCriticalSection(&waiters_cs_);
  MonitorWaitData* previous = NULL;
  MonitorWaitData* current = waiters_head_;
  while (current != NULL) {
    if (current == wait_data) {
      if (waiters_head_ == waiters_tail_) {
        waiters_head_ = waiters_tail_ = NULL;
      } else if (current == waiters_head_) {
        waiters_head_ = waiters_head_->next_;
      } else if (current == waiters_tail_) {
        ASSERT(previous != NULL);
        waiters_tail_ = previous;
        previous->next_ = NULL;
      } else {
        ASSERT(previous != NULL);
        previous->next_ = current->next_;
      }
      // Clear next.
      wait_data->next_ = NULL;
      break;
    }
    previous = current;
    current = current->next_;
  }
  LeaveCriticalSection(&waiters_cs_);
}

void MonitorData::SignalAndRemoveFirstWaiter() {
  EnterCriticalSection(&waiters_cs_);
  MonitorWaitData* first = waiters_head_;
  if (first != NULL) {
    // Remove from list.
    if (waiters_head_ == waiters_tail_) {
      waiters_tail_ = waiters_head_ = NULL;
    } else {
      waiters_head_ = waiters_head_->next_;
    }
    // Clear next.
    first->next_ = NULL;
    // Signal event.
    BOOL result = SetEvent(first->event_);
    if (result == 0) {
      FATAL1("Monitor::Notify failed to signal event %d", GetLastError());
    }
  }
  LeaveCriticalSection(&waiters_cs_);
}

void MonitorData::SignalAndRemoveAllWaiters() {
  EnterCriticalSection(&waiters_cs_);
  // Extract list to signal.
  MonitorWaitData* current = waiters_head_;
  // Clear list.
  waiters_head_ = waiters_tail_ = NULL;
  // Iterate and signal all events.
  while (current != NULL) {
    // Copy next.
    MonitorWaitData* next = current->next_;
    // Clear next.
    current->next_ = NULL;
    // Signal event.
    BOOL result = SetEvent(current->event_);
    if (result == 0) {
      FATAL1("Failed to set event for NotifyAll %d", GetLastError());
    }
    current = next;
  }
  LeaveCriticalSection(&waiters_cs_);
}

MonitorWaitData* MonitorData::GetMonitorWaitDataForThread() {
  // Ensure that the thread local key for monitor wait data objects is
  // initialized.
  ASSERT(MonitorWaitData::monitor_wait_data_key_ != kUnsetThreadLocalKey);

  // Get the MonitorWaitData object containing the event for this
  // thread from thread local storage. Create it if it does not exist.
  uword raw_wait_data =
      OSThread::GetThreadLocal(MonitorWaitData::monitor_wait_data_key_);
  MonitorWaitData* wait_data = NULL;
  if (raw_wait_data == 0) {
    HANDLE event = CreateEvent(NULL, FALSE, FALSE, NULL);
    wait_data = new MonitorWaitData(event);
    OSThread::SetThreadLocal(MonitorWaitData::monitor_wait_data_key_,
                             reinterpret_cast<uword>(wait_data));
  } else {
    wait_data = reinterpret_cast<MonitorWaitData*>(raw_wait_data);
    ASSERT(wait_data->next_ == NULL);
  }
  return wait_data;
}

Monitor::WaitResult Monitor::Wait(int64_t millis) {
#if defined(DEBUG)
  // When running with assertions enabled we track the owner.
  ASSERT(IsOwnedByCurrentThread());
  ThreadId saved_owner = owner_;
  owner_ = OSThread::kInvalidThreadId;
#endif  // defined(DEBUG)

  Monitor::WaitResult retval = kNotified;

  // Get the wait data object containing the event to wait for.
  MonitorWaitData* wait_data = MonitorData::GetMonitorWaitDataForThread();

  // Start waiting by adding the MonitorWaitData to the list of
  // waiters.
  data_.AddWaiter(wait_data);

  // Leave the monitor critical section while waiting.
  LeaveCriticalSection(&data_.cs_);

  // Perform the actual wait on the event.
  DWORD result = WAIT_FAILED;
  if (millis == 0) {
    // Wait forever for a Notify or a NotifyAll event.
    result = WaitForSingleObject(wait_data->event_, INFINITE);
    if (result == WAIT_FAILED) {
      FATAL1("Monitor::Wait failed %d", GetLastError());
    }
  } else {
    // Wait for the given period of time for a Notify or a NotifyAll
    // event.
    result = WaitForSingleObject(wait_data->event_, millis);
    if (result == WAIT_FAILED) {
      FATAL1("Monitor::Wait with timeout failed %d", GetLastError());
    }
    if (result == WAIT_TIMEOUT) {
      // No longer waiting. Remove from the list of waiters.
      data_.RemoveWaiter(wait_data);
      // Caveat: wait_data->event_ might have been signaled between
      // WaitForSingleObject and RemoveWaiter because we are not in any critical
      // section here. Leaving it in a signaled state would break invariants
      // that Monitor::Wait code relies on. We assume that when
      // WaitForSingleObject(wait_data->event_, ...) returns successfully then
      // corresponding wait_data is not on the waiters list anymore.
      // This is guaranteed because we only signal these events from
      // SignalAndRemoveAllWaiters/SignalAndRemoveFirstWaiter which
      // simultaneously remove MonitorWaitData from the list.
      // Now imagine that wait_data->event_ is left signaled here. In this case
      // the next WaitForSingleObject(wait_data->event_, ...) will immediately
      // return while wait_data is still on the waiters list. This would
      // leave waiters list in the inconsistent state.
      // To prevent this from happening simply reset the event.
      // Note: wait_data is no longer on the waiters list so it can't be
      // signaled anymore at this point so there is no race possible from
      // this point onward.
      ResetEvent(wait_data->event_);
      retval = kTimedOut;
    }
  }

  // Reacquire the monitor critical section before continuing.
  EnterCriticalSection(&data_.cs_);

#if defined(DEBUG)
  // When running with assertions enabled we track the owner.
  ASSERT(owner_ == OSThread::kInvalidThreadId);
  owner_ = OSThread::GetCurrentThreadId();
  ASSERT(owner_ == saved_owner);
#endif  // defined(DEBUG)
  return retval;
}

Monitor::WaitResult Monitor::WaitMicros(int64_t micros) {
  // TODO(johnmccutchan): Investigate sub-millisecond sleep times on Windows.
  int64_t millis = micros / kMicrosecondsPerMillisecond;
  if ((millis * kMicrosecondsPerMillisecond) < micros) {
    // We've been asked to sleep for a fraction of a millisecond,
    // this isn't supported on Windows. Bumps milliseconds up by one
    // so that we never return too early. We likely return late though.
    millis += 1;
  }
  return Wait(millis);
}

void Monitor::Notify() {
  // When running with assertions enabled we track the owner.
  ASSERT(IsOwnedByCurrentThread());
  data_.SignalAndRemoveFirstWaiter();
}

void Monitor::NotifyAll() {
  // When running with assertions enabled we track the owner.
  ASSERT(IsOwnedByCurrentThread());
  // If one of the objects in the list of waiters wakes because of a
  // timeout before we signal it, that object will get an extra
  // signal. This will be treated as a spurious wake-up and is OK
  // since all uses of monitors should recheck the condition after a
  // Wait.
  data_.SignalAndRemoveAllWaiters();
}

void ThreadLocalData::AddThreadLocal(ThreadLocalKey key,
                                     ThreadDestructor destructor) {
  ASSERT(thread_locals_ != NULL);
  if (destructor == NULL) {
    // We only care about thread locals with destructors.
    return;
  }
  MutexLocker ml(mutex_, false);
#if defined(DEBUG)
  // Verify that we aren't added twice.
  for (intptr_t i = 0; i < thread_locals_->length(); i++) {
    const ThreadLocalEntry& entry = thread_locals_->At(i);
    ASSERT(entry.key() != key);
  }
#endif
  // Add to list.
  thread_locals_->Add(ThreadLocalEntry(key, destructor));
}

void ThreadLocalData::RemoveThreadLocal(ThreadLocalKey key) {
  ASSERT(thread_locals_ != NULL);
  MutexLocker ml(mutex_, false);
  intptr_t i = 0;
  for (; i < thread_locals_->length(); i++) {
    const ThreadLocalEntry& entry = thread_locals_->At(i);
    if (entry.key() == key) {
      break;
    }
  }
  if (i == thread_locals_->length()) {
    // Not found.
    return;
  }
  thread_locals_->RemoveAt(i);
}

// This function is executed on the thread that is exiting. It is invoked
// by |OnDartThreadExit| (see below for notes on TLS destructors on Windows).
void ThreadLocalData::RunDestructors() {
  ASSERT(thread_locals_ != NULL);
  ASSERT(mutex_ != NULL);
  MutexLocker ml(mutex_, false);
  for (intptr_t i = 0; i < thread_locals_->length(); i++) {
    const ThreadLocalEntry& entry = thread_locals_->At(i);
    // We access the exiting thread's TLS variable here.
    void* p = reinterpret_cast<void*>(OSThread::GetThreadLocal(entry.key()));
    // We invoke the constructor here.
    entry.destructor()(p);
  }
}

Mutex* ThreadLocalData::mutex_ = NULL;
MallocGrowableArray<ThreadLocalEntry>* ThreadLocalData::thread_locals_ = NULL;

void ThreadLocalData::InitOnce() {
  mutex_ = new Mutex();
  thread_locals_ = new MallocGrowableArray<ThreadLocalEntry>();
}

void ThreadLocalData::Shutdown() {
  if (mutex_ != NULL) {
    delete mutex_;
    mutex_ = NULL;
  }
  if (thread_locals_ != NULL) {
    delete thread_locals_;
    thread_locals_ = NULL;
  }
}

}  // namespace dart

// The following was adapted from Chromium:
// src/base/threading/thread_local_storage_win.cc

// Thread Termination Callbacks.
// Windows doesn't support a per-thread destructor with its
// TLS primitives.  So, we build it manually by inserting a
// function to be called on each thread's exit.
// This magic is from http://www.codeproject.com/threads/tls.asp
// and it works for VC++ 7.0 and later.

// Force a reference to _tls_used to make the linker create the TLS directory
// if it's not already there.  (e.g. if __declspec(thread) is not used).
// Force a reference to p_thread_callback_dart to prevent whole program
// optimization from discarding the variable.
#ifdef _WIN64

#pragma comment(linker, "/INCLUDE:_tls_used")
#pragma comment(linker, "/INCLUDE:p_thread_callback_dart")

#else  // _WIN64

#pragma comment(linker, "/INCLUDE:__tls_used")
#pragma comment(linker, "/INCLUDE:_p_thread_callback_dart")

#endif  // _WIN64

// Static callback function to call with each thread termination.
void NTAPI OnDartThreadExit(PVOID module, DWORD reason, PVOID reserved) {
  if (!dart::private_flag_windows_run_tls_destructors) {
    return;
  }
  // On XP SP0 & SP1, the DLL_PROCESS_ATTACH is never seen. It is sent on SP2+
  // and on W2K and W2K3. So don't assume it is sent.
  if (DLL_THREAD_DETACH == reason || DLL_PROCESS_DETACH == reason) {
    dart::ThreadLocalData::RunDestructors();
    dart::MonitorWaitData::ThreadExit();
  }
}

// .CRT$XLA to .CRT$XLZ is an array of PIMAGE_TLS_CALLBACK pointers that are
// called automatically by the OS loader code (not the CRT) when the module is
// loaded and on thread creation. They are NOT called if the module has been
// loaded by a LoadLibrary() call. It must have implicitly been loaded at
// process startup.
// By implicitly loaded, I mean that it is directly referenced by the main EXE
// or by one of its dependent DLLs. Delay-loaded DLL doesn't count as being
// implicitly loaded.
//
// See VC\crt\src\tlssup.c for reference.

// extern "C" suppresses C++ name mangling so we know the symbol name for the
// linker /INCLUDE:symbol pragma above.
extern "C" {
// The linker must not discard p_thread_callback_dart.  (We force a reference
// to this variable with a linker /INCLUDE:symbol pragma to ensure that.) If
// this variable is discarded, the OnDartThreadExit function will never be
// called.
#ifdef _WIN64

// .CRT section is merged with .rdata on x64 so it must be constant data.
#pragma const_seg(".CRT$XLB")
// When defining a const variable, it must have external linkage to be sure the
// linker doesn't discard it.
extern const PIMAGE_TLS_CALLBACK p_thread_callback_dart;
const PIMAGE_TLS_CALLBACK p_thread_callback_dart = OnDartThreadExit;

// Reset the default section.
#pragma const_seg()

#else  // _WIN64

#pragma data_seg(".CRT$XLB")
PIMAGE_TLS_CALLBACK p_thread_callback_dart = OnDartThreadExit;

// Reset the default section.
#pragma data_seg()

#endif  // _WIN64
}  // extern "C"

#endif  // defined(HOST_OS_WINDOWS)