base/threading/thread_local_storage.cc

// Copyright 2014 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/threading/thread_local_storage.h"

#include <algorithm>
#include <array>
#include <atomic>

#include "base/check_op.h"
#include "base/compiler_specific.h"
#include "base/containers/span.h"
#include "base/memory/raw_ptr_exclusion.h"
#include "base/notreached.h"
#include "base/sampling_heap_profiler/poisson_allocation_sampler.h"
#include "base/synchronization/lock.h"
#include "build/build_config.h"
#include "partition_alloc/buildflags.h"

#if BUILDFLAG(IS_MAC) && defined(ARCH_CPU_X86_64)
#include <pthread.h>

#include <type_traits>
#endif

using base::internal::PlatformThreadLocalStorage;

// Chrome Thread Local Storage (TLS)
//
// This TLS system allows Chrome to use a single OS level TLS slot process-wide,
// and allows us to control the slot limits instead of being at the mercy of the
// platform. To do this, Chrome TLS replicates an array commonly found in the OS
// thread metadata.
//
// Overview:
//
// OS TLS Slots       Per-Thread                 Per-Process Global
//     ...
//     []             Chrome TLS Array           Chrome TLS Metadata
//     [] ----------> [][][][][ ][][][][]        [][][][][ ][][][][]
//     []                      |                          |
//     ...                     V                          V
//                      Metadata Version           Slot Information
//                         Your Data!
//
// Using a single OS TLS slot, Chrome TLS allocates an array on demand for the
// lifetime of each thread that requests Chrome TLS data. Each per-thread TLS
// array matches the length of the per-process global metadata array.
//
// A per-process global TLS metadata array tracks information about each item in
// the per-thread array:
//   * Status: Tracks if the slot is allocated or free to assign.
//   * Destructor: An optional destructor to call on thread destruction for that
//                 specific slot.
//   * Version: Tracks the current version of the TLS slot. Each TLS slot
//              allocation is associated with a unique version number.
//
//              Most OS TLS APIs guarantee that a newly allocated TLS slot is
//              initialized to 0 for all threads. The Chrome TLS system provides
//              this guarantee by tracking the version for each TLS slot here
//              on each per-thread Chrome TLS array entry. Threads that access
//              a slot with a mismatched version will receive 0 as their value.
//              The metadata version is incremented when the client frees a
//              slot. The per-thread metadata version is updated when a client
//              writes to the slot. This scheme allows for constant time
//              invalidation and avoids the need to iterate through each Chrome
//              TLS array to mark the slot as zero.
//
// Just like an OS TLS API, clients of the Chrome TLS are responsible for
// managing any necessary lifetime of the data in their slots. The only
// convenience provided is automatic destruction when a thread ends. If a client
// frees a slot, that client is responsible for destroying the data in the slot.

namespace {
// In order to make TLS destructors work, we need to keep around a function
// pointer to the destructor for each slot. We keep this array of pointers in a
// global (static) array.
// We use the single OS-level TLS slot (giving us one pointer per thread) to
// hold a pointer to a per-thread array (table) of slots that we allocate to
// Chromium consumers.

// g_native_tls_key is the one native TLS that we use. It stores our table.

std::atomic<PlatformThreadLocalStorage::TLSKey> g_native_tls_key{
    PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES};

// The OS TLS slot has the following states. The TLS slot's lower 2 bits contain
// the state, the upper bits the TlsVectorEntry*.
//   * kUninitialized: Any call to Slot::Get()/Set() will create the base
//     per-thread TLS state. kUninitialized must be null.
//   * kInUse: value has been created and is in use.
//   * kDestroying: Set when the thread is exiting prior to deleting any of the
//     values stored in the TlsVectorEntry*. This state is necessary so that
//     sequence/task checks won't be done while in the process of deleting the
//     tls entries (see comments in SequenceCheckerImpl for more details).
//   * kDestroyed: All of the values in the vector have been deallocated and
//     the TlsVectorEntry has been deleted.
//
// Final States:
//   * Windows: kDestroyed. Windows does not iterate through the OS TLS to clean
//     up the values.
//   * POSIX: kUninitialized. POSIX iterates through TLS until all slots contain
//     nullptr.
//
// More details on this design:
//   We need some type of thread-local state to indicate that the TLS system has
//   been destroyed. To do so, we leverage the multi-pass nature of destruction
//   of pthread_key.
//
//    a) After destruction of TLS system, we set the pthread_key to a sentinel
//       kDestroyed.
//    b) All calls to Slot::Get() DCHECK that the state is not kDestroyed, and
//       any system which might potentially invoke Slot::Get() after destruction
//       of TLS must check ThreadLocalStorage::ThreadIsBeingDestroyed().
//    c) After a full pass of the pthread_keys, on the next invocation of
//       ConstructTlsVector(), we'll then set the key to nullptr.
//    d) At this stage, the TLS system is back in its uninitialized state.
//    e) If in the second pass of destruction of pthread_keys something were to
//       re-initialize TLS [this should never happen! Since the only code which
//       uses Chrome TLS is Chrome controlled, we should really be striving for
//       single-pass destruction], then TLS will be re-initialized and then go
//       through the 2-pass destruction system again. Everything should just
//       work (TM).

// The state of the tls-entry.
enum class TlsVectorState {
  kUninitialized = 0,

  // In the process of destroying the entries in the vector.
  kDestroying,

  // All of the entries and the vector has been destroyed.
  kDestroyed,

  // The vector has been initialized and is in use.
  kInUse,

  kMaxValue = kInUse
};

// Bit-mask used to store TlsVectorState.
constexpr uintptr_t kVectorStateBitMask = 3;
static_assert(static_cast<int>(TlsVectorState::kMaxValue) <=
                  kVectorStateBitMask,
              "number of states must fit in header");
static_assert(static_cast<int>(TlsVectorState::kUninitialized) == 0,
              "kUninitialized must be null");

// The maximum number of slots in our thread local storage stack.
constexpr size_t kThreadLocalStorageSize = 256;

enum TlsStatus {
  FREE,
  IN_USE,
};

struct TlsMetadata {
  TlsStatus status;
  base::ThreadLocalStorage::TLSDestructorFunc destructor;
  // Incremented every time a slot is reused. Used to detect reuse of slots.
  uint32_t version;
  // Tracks slot creation order. Used to destroy slots in the reverse order:
  // from last created to first created.
  uint32_t sequence_num;
};

struct TlsVectorEntry {
  // `data` is not a raw_ptr<...> for performance reasons (based on analysis of
  // sampling profiler data and tab_search:top100:2020).
  RAW_PTR_EXCLUSION void* data;

  uint32_t version;
};

// This lock isn't needed until after we've constructed the per-thread TLS
// vector, so it's safe to use.
base::Lock* GetTLSMetadataLock() {
  static auto* lock = new base::Lock();
  return lock;
}
std::array<TlsMetadata, kThreadLocalStorageSize> g_tls_metadata;
size_t g_last_assigned_slot = 0;
uint32_t g_sequence_num = 0;

// The maximum number of times to try to clear slots by calling destructors.
// Use pthread naming convention for clarity.
constexpr size_t kMaxDestructorIterations = kThreadLocalStorageSize;

// Sets the value and state of the vector.
void SetTlsVectorValue(PlatformThreadLocalStorage::TLSKey key,
                       TlsVectorEntry* tls_data,
                       TlsVectorState state) {
  DCHECK(tls_data || (state == TlsVectorState::kUninitialized) ||
         (state == TlsVectorState::kDestroyed));
  PlatformThreadLocalStorage::SetTLSValue(
      key, reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(tls_data) |
                                   static_cast<uintptr_t>(state)));
}

// Returns the tls vector and current state from the raw tls value.
TlsVectorState GetTlsVectorStateAndValue(void* tls_value,
                                         TlsVectorEntry** entry = nullptr) {
  if (entry) {
    *entry = reinterpret_cast<TlsVectorEntry*>(
        reinterpret_cast<uintptr_t>(tls_value) & ~kVectorStateBitMask);
  }
  return static_cast<TlsVectorState>(reinterpret_cast<uintptr_t>(tls_value) &
                                     kVectorStateBitMask);
}

// Returns the tls vector and state using the tls key.
TlsVectorState GetTlsVectorStateAndValue(PlatformThreadLocalStorage::TLSKey key,
                                         TlsVectorEntry** entry = nullptr) {
// Only on x86_64, the implementation is not stable on ARM64. For instance, in
// macOS 11, the TPIDRRO_EL0 registers holds the CPU index in the low bits,
// which is not the case in macOS 12. See libsyscall/os/tsd.h in XNU
// (_os_tsd_get_direct() is used by pthread_getspecific() internally).
#if BUILDFLAG(IS_MAC) && defined(ARCH_CPU_X86_64)
  // On macOS, pthread_getspecific() is in libSystem, so a call to it has to go
  // through PLT. However, and contrary to some other platforms, *all* TLS keys
  // are in a static array in the thread structure. So they are *always* at a
  // fixed offset from the segment register holding the thread structure
  // address.
  //
  // We could use _pthread_getspecific_direct(), but it is not
  // exported. However, on all macOS versions we support, the TLS array is at
  // %gs. This is used in V8 and PartitionAlloc, and can also be seen by looking
  // at pthread_getspecific() disassembly:
  //
  // libsystem_pthread.dylib`pthread_getspecific:
  // libsystem_pthread.dylib[0x7ff800316099] <+0>: movq   %gs:(,%rdi,8), %rax
  // libsystem_pthread.dylib[0x7ff8003160a2] <+9>: retq
  //
  // This function is essentially inlining the content of pthread_getspecific()
  // here.
  //
  // Note that this likely ends up being even faster than thread_local for
  // typical Chromium builds where the code is in a dynamic library. For the
  // static executable case, this is likely equivalent.
  static_assert(
      std::is_same_v<PlatformThreadLocalStorage::TLSKey, pthread_key_t>,
      "The special-case below assumes that the platform TLS implementation is "
      "pthread.");

  intptr_t platform_tls_value;
  asm("movq %%gs:(,%1,8), %0;" : "=r"(platform_tls_value) : "r"(key));

  return GetTlsVectorStateAndValue(reinterpret_cast<void*>(platform_tls_value),
                                   entry);
#else
  return GetTlsVectorStateAndValue(PlatformThreadLocalStorage::GetTLSValue(key),
                                   entry);
#endif
}

// This function is called to initialize our entire Chromium TLS system.
// It may be called very early, and we need to complete most all of the setup
// (initialization) before calling *any* memory allocator functions, which may
// recursively depend on this initialization.
// As a result, we use Atomics, and avoid anything (like a singleton) that might
// require memory allocations.
base::span<TlsVectorEntry> ConstructTlsVector() {
  PlatformThreadLocalStorage::TLSKey key =
      g_native_tls_key.load(std::memory_order_relaxed);
  if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) {
    CHECK(PlatformThreadLocalStorage::AllocTLS(&key));

    // The TLS_KEY_OUT_OF_INDEXES is used to find out whether the key is set or
    // not in NoBarrier_CompareAndSwap, but Posix doesn't have invalid key, we
    // define an almost impossible value be it.
    // If we really get TLS_KEY_OUT_OF_INDEXES as value of key, just alloc
    // another TLS slot.
    if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) {
      PlatformThreadLocalStorage::TLSKey tmp = key;
      CHECK(PlatformThreadLocalStorage::AllocTLS(&key) &&
            key != PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES);
      PlatformThreadLocalStorage::FreeTLS(tmp);
    }
    // Atomically test-and-set the tls_key. If the key is
    // TLS_KEY_OUT_OF_INDEXES, go ahead and set it. Otherwise, do nothing, as
    // another thread already did our dirty work.
    PlatformThreadLocalStorage::TLSKey old_key =
        PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES;
    if (!g_native_tls_key.compare_exchange_strong(old_key, key,
                                                  std::memory_order_relaxed,
                                                  std::memory_order_relaxed)) {
      // We've been shortcut. Another thread replaced g_native_tls_key first so
      // we need to destroy our index and use the one the other thread got
      // first.
      PlatformThreadLocalStorage::FreeTLS(key);
      key = g_native_tls_key.load(std::memory_order_relaxed);
    }
  }
  CHECK_EQ(GetTlsVectorStateAndValue(key), TlsVectorState::kUninitialized);

  // Some allocators, such as TCMalloc, make use of thread local storage. As a
  // result, any attempt to call new (or malloc) will lazily cause such a system
  // to initialize, which will include registering for a TLS key. If we are not
  // careful here, then that request to create a key will call new back, and
  // we'll have an infinite loop. We avoid that as follows: Use a stack
  // allocated vector, so that we don't have dependence on our allocator until
  // our service is in place. (i.e., don't even call new until after we're
  // setup)
  std::array<TlsVectorEntry, kThreadLocalStorageSize> stack_allocated_tls_data;
  std::ranges::fill(stack_allocated_tls_data, TlsVectorEntry{});
  // Ensure that any rentrant calls change the temp version.
  // The state is set to kDestroying to ensure that if the thread terminates
  // during this phase, OnThreadExit() will skip deleting this stack pointer.
  SetTlsVectorValue(key, stack_allocated_tls_data.data(),
                    TlsVectorState::kInUse);

  // Allocate an array to store our data.
  // SAFETY: `tls_entries` is a raw pointer to a heap-allocated array of size
  // kThreadLocalStorageSize.
  base::span<TlsVectorEntry> UNSAFE_BUFFERS(tls_data(
      new TlsVectorEntry[kThreadLocalStorageSize], kThreadLocalStorageSize));
  tls_data.copy_from(stack_allocated_tls_data);
  SetTlsVectorValue(key, tls_data.data(), TlsVectorState::kInUse);
  return tls_data;
}

void OnThreadExitInternal(TlsVectorEntry* tls_data) {
  DCHECK(tls_data);
  // Some allocators, such as TCMalloc, use TLS. As a result, when a thread
  // terminates, one of the destructor calls we make may be to shut down an
  // allocator. We have to be careful that after we've shutdown all of the known
  // destructors (perchance including an allocator), that we don't call the
  // allocator and cause it to resurrect itself (with no possibly destructor
  // call to follow). We handle this problem as follows: Switch to using a stack
  // allocated vector, so that we don't have dependence on our allocator after
  // we have called all g_tls_metadata destructors. (i.e., don't even call
  // delete[] after we're done with destructors.)
  std::array<TlsVectorEntry, kThreadLocalStorageSize> stack_allocated_tls_data;
  // SAFETY: This was allocated in ConstructTlsVector() from using
  // kThreadLocalStorageSize.
  base::span<TlsVectorEntry> data_span =
      UNSAFE_BUFFERS(base::span(tls_data, kThreadLocalStorageSize));
  std::ranges::copy(data_span, stack_allocated_tls_data.begin());
  // Ensure that any re-entrant calls change the temp version.
  PlatformThreadLocalStorage::TLSKey key =
      g_native_tls_key.load(std::memory_order_relaxed);
  SetTlsVectorValue(key, stack_allocated_tls_data.data(),
                    TlsVectorState::kDestroying);
  delete[] tls_data;  // Our last dependence on an allocator

  size_t remaining_attempts = kMaxDestructorIterations + 1;
  bool need_to_scan_destructors = true;
  while (need_to_scan_destructors) {
    need_to_scan_destructors = false;

    // Snapshot the TLS Metadata so we don't have to lock on every access.
    std::array<TlsMetadata, kThreadLocalStorageSize> tls_metadata;
    {
      base::AutoLock auto_lock(*GetTLSMetadataLock());
      std::ranges::copy(g_tls_metadata, tls_metadata.begin());
    }

    // We destroy slots in reverse order (i.e. destroy the first-created slot
    // last), for the following reasons:
    // 1) Slots that are created early belong to basic services (like an
    // allocator) and might have to be recreated by destructors of other
    // services. So we save iterations here by destroying them last.
    // 2) Perfetto tracing service allocates a slot early and relies on it to
    // keep emitting trace events while destructors of other slots are called,
    // so it's important to keep it live to avoid use-after-free errors.
    // To achieve this, we sort all slots in the order of decreasing sequence
    // numbers.
    struct OrderedSlot {
      uint32_t sequence_num;
      uint16_t slot;
    };
    std::array<OrderedSlot, kThreadLocalStorageSize> slot_destruction_order;
    for (uint16_t i = 0; i < kThreadLocalStorageSize; ++i) {
      slot_destruction_order[i].sequence_num = tls_metadata[i].sequence_num;
      slot_destruction_order[i].slot = i;
    }
    std::ranges::sort(slot_destruction_order,
                      [](const OrderedSlot& s1, const OrderedSlot& s2) {
                        return s1.sequence_num > s2.sequence_num;
                      });

    for (const auto& ordered_slot : slot_destruction_order) {
      size_t slot = ordered_slot.slot;
      void* tls_value = stack_allocated_tls_data[slot].data;
      if (!tls_value || tls_metadata[slot].status == TlsStatus::FREE ||
          stack_allocated_tls_data[slot].version !=
              tls_metadata[slot].version) {
        continue;
      }

      base::ThreadLocalStorage::TLSDestructorFunc destructor =
          tls_metadata[slot].destructor;
      if (!destructor) {
        continue;
      }
      stack_allocated_tls_data[slot].data = nullptr;  // pre-clear the slot.
      destructor(tls_value);
      // Any destructor might have called a different service, which then set a
      // different slot to a non-null value. Hence we need to check the whole
      // vector again. This is a pthread standard.
      need_to_scan_destructors = true;
    }

    if (--remaining_attempts == 0) {
      NOTREACHED();  // Destructors might not have been called.
    }
  }

  // Remove our stack allocated vector.
  SetTlsVectorValue(key, nullptr, TlsVectorState::kDestroyed);
}

}  // namespace

namespace base {

namespace internal {

#if BUILDFLAG(IS_WIN)
void PlatformThreadLocalStorage::OnThreadExit() {
  PlatformThreadLocalStorage::TLSKey key =
      g_native_tls_key.load(std::memory_order_relaxed);
  if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) {
    return;
  }
  TlsVectorEntry* tls_vector = nullptr;
  const TlsVectorState state = GetTlsVectorStateAndValue(key, &tls_vector);

  // On Windows, thread destruction callbacks are only invoked once per module,
  // so there should be no way that this could be invoked twice.
  DCHECK_NE(state, TlsVectorState::kDestroyed);

  // Maybe we have never initialized TLS for this thread.
  if (state == TlsVectorState::kUninitialized) {
    return;
  }
  OnThreadExitInternal(tls_vector);
}
#elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
void PlatformThreadLocalStorage::OnThreadExit(void* value) {
  // On posix this function may be called twice. The first pass calls dtors and
  // sets state to kDestroyed. The second pass sets kDestroyed to
  // kUninitialized.
  TlsVectorEntry* tls_vector = nullptr;
  const TlsVectorState state = GetTlsVectorStateAndValue(value, &tls_vector);
  if (state == TlsVectorState::kDestroyed) {
    PlatformThreadLocalStorage::TLSKey key =
        g_native_tls_key.load(std::memory_order_relaxed);
    SetTlsVectorValue(key, nullptr, TlsVectorState::kUninitialized);
    return;
  }

  OnThreadExitInternal(tls_vector);
}
#endif  // BUILDFLAG(IS_WIN)

}  // namespace internal

// static
bool ThreadLocalStorage::HasBeenDestroyed() {
  PlatformThreadLocalStorage::TLSKey key =
      g_native_tls_key.load(std::memory_order_relaxed);
  if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) {
    return false;
  }
  const TlsVectorState state = GetTlsVectorStateAndValue(key);
  return state == TlsVectorState::kDestroying ||
         state == TlsVectorState::kDestroyed;
}

void ThreadLocalStorage::Slot::Initialize(TLSDestructorFunc destructor) {
  // The heap sampler uses TLS internally. Disable allocation sampling before
  // allocating TLS-internal structures, to safeguard against reentrancy.
#if BUILDFLAG(IS_IOS) && !PA_BUILDFLAG(USE_ALLOCATOR_SHIM)
  // Heap sampler is only built on IOS when the allocator shim is enabled.
#else
  base::PoissonAllocationSampler::ScopedMuteThreadSamples mute_heap_sampler;
#endif

  PlatformThreadLocalStorage::TLSKey key =
      g_native_tls_key.load(std::memory_order_relaxed);
  if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES ||
      GetTlsVectorStateAndValue(key) == TlsVectorState::kUninitialized) {
    ConstructTlsVector();
  }

  // Grab a new slot.
  {
    base::AutoLock auto_lock(*GetTLSMetadataLock());
    for (size_t i = 0; i < kThreadLocalStorageSize; ++i) {
      // Tracking the last assigned slot is an attempt to find the next
      // available slot within one iteration. Under normal usage, slots remain
      // in use for the lifetime of the process (otherwise before we reclaimed
      // slots, we would have run out of slots). This makes it highly likely the
      // next slot is going to be a free slot.
      size_t slot_candidate =
          (g_last_assigned_slot + 1 + i) % kThreadLocalStorageSize;
      if (g_tls_metadata[slot_candidate].status == TlsStatus::FREE) {
        g_tls_metadata[slot_candidate].status = TlsStatus::IN_USE;
        g_tls_metadata[slot_candidate].destructor = destructor;
        g_tls_metadata[slot_candidate].sequence_num = ++g_sequence_num;
        g_last_assigned_slot = slot_candidate;
        DCHECK_EQ(kInvalidSlotValue, slot_);
        slot_ = slot_candidate;
        version_ = g_tls_metadata[slot_candidate].version;
        break;
      }
    }
  }
  CHECK_LT(slot_, kThreadLocalStorageSize);
}

void ThreadLocalStorage::Slot::Free() {
  DCHECK_LT(slot_, kThreadLocalStorageSize);
  {
    base::AutoLock auto_lock(*GetTLSMetadataLock());
    g_tls_metadata[slot_].status = TlsStatus::FREE;
    g_tls_metadata[slot_].destructor = nullptr;
    ++(g_tls_metadata[slot_].version);
  }
  slot_ = kInvalidSlotValue;
}

void* ThreadLocalStorage::Slot::Get() const {
  TlsVectorEntry* tls_vector = nullptr;
  const TlsVectorState state = GetTlsVectorStateAndValue(
      g_native_tls_key.load(std::memory_order_relaxed), &tls_vector);
  // SAFETY: The span is either empty or points to `kThreadLocalStorageSize`
  // elements allocated in `ConstructTlsVector`.
  //
  // Constructing this span (repeatedly) shows up hot in Speedometer3
  // profiling (by way of `blink::HarfBuzzGetNominalGlyph()`). To
  // remedy, exempt from bounds checking. (N.B. this is only relevant
  // when build support for Checked Span is actually enabled.)
  base::span<TlsVectorEntry> UNSAFE_BUFFERS(tls_data(
      base::unchecked, tls_vector, tls_vector ? kThreadLocalStorageSize : 0));
  DCHECK_NE(state, TlsVectorState::kDestroyed);
  if (tls_data.empty()) {
    return nullptr;
  }
  DCHECK_LT(slot_, kThreadLocalStorageSize);
  // Version mismatches means this slot was previously freed.
  if (tls_data[slot_].version != version_) {
    return nullptr;
  }
  return tls_data[slot_].data;
}

void ThreadLocalStorage::Slot::Set(void* value) {
  TlsVectorEntry* tls_vector = nullptr;
  const TlsVectorState state = GetTlsVectorStateAndValue(
      g_native_tls_key.load(std::memory_order_relaxed), &tls_vector);
  // SAFETY: The span is either empty or points to `kThreadLocalStorageSize`
  // elements allocated in `ConstructTlsVector`.
  //
  // Constructing this span (repeatedly) shows up hot in Speedometer3
  // profiling (by way of `blink::HarfBuzzGetNominalGlyph()`). To
  // remedy, exempt from bounds checking. (N.B. this is only relevant
  // when build support for Checked Span is actually enabled.)
  base::span<TlsVectorEntry> UNSAFE_BUFFERS(tls_data(
      base::unchecked, tls_vector, tls_vector ? kThreadLocalStorageSize : 0));
  DCHECK_NE(state, TlsVectorState::kDestroyed);
  if (tls_data.empty()) [[unlikely]] {
    if (!value) {
      return;
    }
    tls_data = ConstructTlsVector();
  }
  DCHECK_LT(slot_, kThreadLocalStorageSize);
  tls_data[slot_].data = value;
  tls_data[slot_].version = version_;
}

ThreadLocalStorage::Slot::Slot(TLSDestructorFunc destructor) {
  Initialize(destructor);
}

ThreadLocalStorage::Slot::~Slot() {
  Free();
}

}  // namespace base