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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at https://mozilla.org/MPL/2.0/. */
#ifndef mozilla_freestanding_SafeThreadLocal_h
#define mozilla_freestanding_SafeThreadLocal_h
#include <type_traits>
#include "mozilla/NativeNt.h"
#include "mozilla/ThreadLocal.h"
namespace mozilla {
namespace freestanding {
// We cannot fall back to the Tls* APIs because kernel32 might not have been
// loaded yet.
#if defined(__MINGW32__) && !defined(HAVE_THREAD_TLS_KEYWORD)
# error "This code requires the compiler to have native TLS support"
#endif // defined(__MINGW32__) && !defined(HAVE_THREAD_TLS_KEYWORD)
/**
* This class holds data as a thread-local variable, or as a global variable
* if the thread local storage is not initialized yet. It should be safe
* because in that early stage we assume there is no more than a single thread.
*/
template <typename T>
class SafeThreadLocal final {
static MOZ_THREAD_LOCAL(T) sThreadLocal;
static T sGlobal;
static bool sIsTlsUsed;
// In normal cases, TLS is always available and the class uses sThreadLocal
// without changing sMainThreadId. So sMainThreadId is likely to be 0.
//
// If TLS is not available, we use sGlobal instead and update sMainThreadId
// so that that thread keeps using sGlobal even after TLS is initialized
// later.
static DWORD sMainThreadId;
// Need non-inline accessors to prevent the compiler from generating code
// accessing sThreadLocal before checking a condition.
MOZ_NEVER_INLINE static void SetGlobalValue(T aValue) { sGlobal = aValue; }
MOZ_NEVER_INLINE static T GetGlobalValue() { return sGlobal; }
public:
static void set(T aValue) {
static_assert(std::is_pointer_v<T>,
"SafeThreadLocal must be used with a pointer");
if (sMainThreadId == mozilla::nt::RtlGetCurrentThreadId()) {
SetGlobalValue(aValue);
} else if (sIsTlsUsed) {
MOZ_ASSERT(mozilla::nt::RtlGetThreadLocalStoragePointer(),
"Once TLS is used, TLS should be available till the end.");
sThreadLocal.set(aValue);
} else if (mozilla::nt::RtlGetThreadLocalStoragePointer()) {
sIsTlsUsed = true;
sThreadLocal.set(aValue);
} else {
MOZ_ASSERT(sMainThreadId == 0,
"A second thread cannot be created before TLS is available.");
sMainThreadId = mozilla::nt::RtlGetCurrentThreadId();
SetGlobalValue(aValue);
}
}
static T get() {
if (sMainThreadId == mozilla::nt::RtlGetCurrentThreadId()) {
return GetGlobalValue();
} else if (sIsTlsUsed) {
return sThreadLocal.get();
}
return GetGlobalValue();
}
};
template <typename T>
MOZ_THREAD_LOCAL(T)
SafeThreadLocal<T>::sThreadLocal;
template <typename T>
T SafeThreadLocal<T>::sGlobal = nullptr;
template <typename T>
bool SafeThreadLocal<T>::sIsTlsUsed = false;
template <typename T>
DWORD SafeThreadLocal<T>::sMainThreadId = 0;
} // namespace freestanding
} // namespace mozilla
#endif // mozilla_freestanding_SafeThreadLocal_h
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