278 lines
11 KiB
C++
278 lines
11 KiB
C++
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// Copyright (c) 2012 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/threading/thread_local_storage.h"
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#include <windows.h>
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#include "base/logging.h"
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namespace {
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// In order to make TLS destructors work, we need to keep function
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// pointers to the destructor for each TLS that we allocate.
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// We make this work by allocating a single OS-level TLS, which
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// contains an array of slots for the application to use. In
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// parallel, we also allocate an array of destructors, which we
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// keep track of and call when threads terminate.
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// g_native_tls_key is the one native TLS that we use. It stores our table.
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long g_native_tls_key = TLS_OUT_OF_INDEXES;
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// g_last_used_tls_key is the high-water-mark of allocated thread local storage.
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// Each allocation is an index into our g_tls_destructors[]. Each such index is
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// assigned to the instance variable slot_ in a ThreadLocalStorage::Slot
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// instance. We reserve the value slot_ == 0 to indicate that the corresponding
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// instance of ThreadLocalStorage::Slot has been freed (i.e., destructor called,
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// etc.). This reserved use of 0 is then stated as the initial value of
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// g_last_used_tls_key, so that the first issued index will be 1.
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long g_last_used_tls_key = 0;
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// The maximum number of 'slots' in our thread local storage stack.
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const int kThreadLocalStorageSize = 64;
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// The maximum number of times to try to clear slots by calling destructors.
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// Use pthread naming convention for clarity.
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const int kMaxDestructorIterations = kThreadLocalStorageSize;
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// An array of destructor function pointers for the slots. If a slot has a
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// destructor, it will be stored in its corresponding entry in this array.
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// The elements are volatile to ensure that when the compiler reads the value
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// to potentially call the destructor, it does so once, and that value is tested
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// for null-ness and then used. Yes, that would be a weird de-optimization,
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// but I can imagine some register machines where it was just as easy to
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// re-fetch an array element, and I want to be sure a call to free the key
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// (i.e., null out the destructor entry) that happens on a separate thread can't
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// hurt the racy calls to the destructors on another thread.
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volatile base::ThreadLocalStorage::TLSDestructorFunc
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g_tls_destructors[kThreadLocalStorageSize];
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void** ConstructTlsVector() {
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if (g_native_tls_key == TLS_OUT_OF_INDEXES) {
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long value = TlsAlloc();
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DCHECK(value != TLS_OUT_OF_INDEXES);
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// Atomically test-and-set the tls_key. If the key is TLS_OUT_OF_INDEXES,
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// go ahead and set it. Otherwise, do nothing, as another
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// thread already did our dirty work.
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if (TLS_OUT_OF_INDEXES != InterlockedCompareExchange(
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&g_native_tls_key, value, TLS_OUT_OF_INDEXES)) {
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// We've been shortcut. Another thread replaced g_native_tls_key first so
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// we need to destroy our index and use the one the other thread got
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// first.
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TlsFree(value);
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}
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}
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DCHECK(!TlsGetValue(g_native_tls_key));
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// Some allocators, such as TCMalloc, make use of thread local storage.
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// As a result, any attempt to call new (or malloc) will lazily cause such a
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// system to initialize, which will include registering for a TLS key. If we
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// are not careful here, then that request to create a key will call new back,
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// and we'll have an infinite loop. We avoid that as follows:
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// Use a stack allocated vector, so that we don't have dependence on our
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// allocator until our service is in place. (i.e., don't even call new until
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// after we're setup)
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void* stack_allocated_tls_data[kThreadLocalStorageSize];
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memset(stack_allocated_tls_data, 0, sizeof(stack_allocated_tls_data));
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// Ensure that any rentrant calls change the temp version.
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TlsSetValue(g_native_tls_key, stack_allocated_tls_data);
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// Allocate an array to store our data.
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void** tls_data = new void*[kThreadLocalStorageSize];
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memcpy(tls_data, stack_allocated_tls_data, sizeof(stack_allocated_tls_data));
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TlsSetValue(g_native_tls_key, tls_data);
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return tls_data;
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}
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// Called when we terminate a thread, this function calls any TLS destructors
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// that are pending for this thread.
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void WinThreadExit() {
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if (g_native_tls_key == TLS_OUT_OF_INDEXES)
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return;
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void** tls_data = static_cast<void**>(TlsGetValue(g_native_tls_key));
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// Maybe we have never initialized TLS for this thread.
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if (!tls_data)
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return;
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// Some allocators, such as TCMalloc, use TLS. As a result, when a thread
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// terminates, one of the destructor calls we make may be to shut down an
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// allocator. We have to be careful that after we've shutdown all of the
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// known destructors (perchance including an allocator), that we don't call
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// the allocator and cause it to resurrect itself (with no possibly destructor
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// call to follow). We handle this problem as follows:
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// Switch to using a stack allocated vector, so that we don't have dependence
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// on our allocator after we have called all g_tls_destructors. (i.e., don't
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// even call delete[] after we're done with destructors.)
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void* stack_allocated_tls_data[kThreadLocalStorageSize];
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memcpy(stack_allocated_tls_data, tls_data, sizeof(stack_allocated_tls_data));
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// Ensure that any re-entrant calls change the temp version.
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TlsSetValue(g_native_tls_key, stack_allocated_tls_data);
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delete[] tls_data; // Our last dependence on an allocator.
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int remaining_attempts = kMaxDestructorIterations;
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bool need_to_scan_destructors = true;
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while (need_to_scan_destructors) {
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need_to_scan_destructors = false;
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// Try to destroy the first-created-slot (which is slot 1) in our last
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// destructor call. That user was able to function, and define a slot with
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// no other services running, so perhaps it is a basic service (like an
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// allocator) and should also be destroyed last. If we get the order wrong,
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// then we'll itterate several more times, so it is really not that
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// critical (but it might help).
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for (int slot = g_last_used_tls_key; slot > 0; --slot) {
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void* value = stack_allocated_tls_data[slot];
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if (value == NULL)
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continue;
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base::ThreadLocalStorage::TLSDestructorFunc destructor =
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g_tls_destructors[slot];
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if (destructor == NULL)
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continue;
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stack_allocated_tls_data[slot] = NULL; // pre-clear the slot.
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destructor(value);
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// Any destructor might have called a different service, which then set
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// a different slot to a non-NULL value. Hence we need to check
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// the whole vector again. This is a pthread standard.
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need_to_scan_destructors = true;
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}
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if (--remaining_attempts <= 0) {
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NOTREACHED(); // Destructors might not have been called.
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break;
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}
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}
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// Remove our stack allocated vector.
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TlsSetValue(g_native_tls_key, NULL);
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}
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} // namespace
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namespace base {
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ThreadLocalStorage::Slot::Slot(TLSDestructorFunc destructor) {
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initialized_ = false;
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slot_ = 0;
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Initialize(destructor);
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}
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bool ThreadLocalStorage::StaticSlot::Initialize(TLSDestructorFunc destructor) {
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if (g_native_tls_key == TLS_OUT_OF_INDEXES || !TlsGetValue(g_native_tls_key))
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ConstructTlsVector();
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// Grab a new slot.
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slot_ = InterlockedIncrement(&g_last_used_tls_key);
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DCHECK_GT(slot_, 0);
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if (slot_ >= kThreadLocalStorageSize) {
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NOTREACHED();
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return false;
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}
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// Setup our destructor.
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g_tls_destructors[slot_] = destructor;
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initialized_ = true;
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return true;
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}
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void ThreadLocalStorage::StaticSlot::Free() {
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// At this time, we don't reclaim old indices for TLS slots.
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// So all we need to do is wipe the destructor.
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DCHECK_GT(slot_, 0);
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DCHECK_LT(slot_, kThreadLocalStorageSize);
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g_tls_destructors[slot_] = NULL;
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slot_ = 0;
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initialized_ = false;
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}
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void* ThreadLocalStorage::StaticSlot::Get() const {
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void** tls_data = static_cast<void**>(TlsGetValue(g_native_tls_key));
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if (!tls_data)
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tls_data = ConstructTlsVector();
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DCHECK_GT(slot_, 0);
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DCHECK_LT(slot_, kThreadLocalStorageSize);
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return tls_data[slot_];
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}
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void ThreadLocalStorage::StaticSlot::Set(void* value) {
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void** tls_data = static_cast<void**>(TlsGetValue(g_native_tls_key));
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if (!tls_data)
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tls_data = ConstructTlsVector();
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DCHECK_GT(slot_, 0);
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DCHECK_LT(slot_, kThreadLocalStorageSize);
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tls_data[slot_] = value;
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}
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} // namespace base
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// Thread Termination Callbacks.
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// Windows doesn't support a per-thread destructor with its
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// TLS primitives. So, we build it manually by inserting a
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// function to be called on each thread's exit.
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// This magic is from http://www.codeproject.com/threads/tls.asp
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// and it works for VC++ 7.0 and later.
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// Force a reference to _tls_used to make the linker create the TLS directory
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// if it's not already there. (e.g. if __declspec(thread) is not used).
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// Force a reference to p_thread_callback_base to prevent whole program
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// optimization from discarding the variable.
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#ifdef _WIN64
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#pragma comment(linker, "/INCLUDE:_tls_used")
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#pragma comment(linker, "/INCLUDE:p_thread_callback_base")
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#else // _WIN64
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#pragma comment(linker, "/INCLUDE:__tls_used")
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#pragma comment(linker, "/INCLUDE:_p_thread_callback_base")
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#endif // _WIN64
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// Static callback function to call with each thread termination.
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void NTAPI OnThreadExit(PVOID module, DWORD reason, PVOID reserved) {
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// On XP SP0 & SP1, the DLL_PROCESS_ATTACH is never seen. It is sent on SP2+
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// and on W2K and W2K3. So don't assume it is sent.
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if (DLL_THREAD_DETACH == reason || DLL_PROCESS_DETACH == reason)
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WinThreadExit();
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}
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// .CRT$XLA to .CRT$XLZ is an array of PIMAGE_TLS_CALLBACK pointers that are
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// called automatically by the OS loader code (not the CRT) when the module is
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// loaded and on thread creation. They are NOT called if the module has been
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// loaded by a LoadLibrary() call. It must have implicitly been loaded at
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// process startup.
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// By implicitly loaded, I mean that it is directly referenced by the main EXE
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// or by one of its dependent DLLs. Delay-loaded DLL doesn't count as being
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// implicitly loaded.
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//
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// See VC\crt\src\tlssup.c for reference.
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// extern "C" suppresses C++ name mangling so we know the symbol name for the
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// linker /INCLUDE:symbol pragma above.
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extern "C" {
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// The linker must not discard p_thread_callback_base. (We force a reference
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// to this variable with a linker /INCLUDE:symbol pragma to ensure that.) If
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// this variable is discarded, the OnThreadExit function will never be called.
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#ifdef _WIN64
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// .CRT section is merged with .rdata on x64 so it must be constant data.
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#pragma const_seg(".CRT$XLB")
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// When defining a const variable, it must have external linkage to be sure the
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// linker doesn't discard it.
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extern const PIMAGE_TLS_CALLBACK p_thread_callback_base;
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const PIMAGE_TLS_CALLBACK p_thread_callback_base = OnThreadExit;
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// Reset the default section.
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#pragma const_seg()
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#else // _WIN64
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#pragma data_seg(".CRT$XLB")
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PIMAGE_TLS_CALLBACK p_thread_callback_base = OnThreadExit;
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// Reset the default section.
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#pragma data_seg()
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#endif // _WIN64
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} // extern "C"
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