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 // This file is part of Eigen, a lightweight C++ template library
 // for linear algebra.
 //
 // Copyright (C) 2016 Benoit Steiner <benoit.steiner.goog@gmail.com>
 //
 // 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 http://mozilla.org/MPL/2.0/.
 
 #ifndef EIGEN_CXX11_THREADPOOL_THREAD_LOCAL_H
 #define EIGEN_CXX11_THREADPOOL_THREAD_LOCAL_H
 
 #ifdef EIGEN_AVOID_THREAD_LOCAL
 
 #ifdef EIGEN_THREAD_LOCAL
 #undef EIGEN_THREAD_LOCAL
 #endif
 
 #else
 
 #if EIGEN_MAX_CPP_VER >= 11 &&                         \
     ((EIGEN_COMP_GNUC && EIGEN_GNUC_AT_LEAST(4, 8)) || \
      __has_feature(cxx_thread_local)                || \
      (EIGEN_COMP_MSVC >= 1900) )
 #define EIGEN_THREAD_LOCAL static thread_local
 #endif
 
 // Disable TLS for Apple and Android builds with older toolchains.
 #if defined(__APPLE__)
 // Included for TARGET_OS_IPHONE, __IPHONE_OS_VERSION_MIN_REQUIRED,
 // __IPHONE_8_0.
 #include <Availability.h>
 #include <TargetConditionals.h>
 #endif
 // Checks whether C++11's `thread_local` storage duration specifier is
 // supported.
 #if defined(__apple_build_version__) &&     \
     ((__apple_build_version__ < 8000042) || \
      (TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_9_0))
 // Notes: Xcode's clang did not support `thread_local` until version
 // 8, and even then not for all iOS < 9.0.
 #undef EIGEN_THREAD_LOCAL
 
 #elif defined(__ANDROID__) && EIGEN_COMP_CLANG
 // There are platforms for which TLS should not be used even though the compiler
 // makes it seem like it's supported (Android NDK < r12b for example).
 // This is primarily because of linker problems and toolchain misconfiguration:
 // TLS isn't supported until NDK r12b per
 // https://developer.android.com/ndk/downloads/revision_history.html
 // Since NDK r16, `__NDK_MAJOR__` and `__NDK_MINOR__` are defined in
 // <android/ndk-version.h>. For NDK < r16, users should define these macros,
 // e.g. `-D__NDK_MAJOR__=11 -D__NKD_MINOR__=0` for NDK r11.
 #if __has_include(<android/ndk-version.h>)
 #include <android/ndk-version.h>
 #endif  // __has_include(<android/ndk-version.h>)
 #if defined(__ANDROID__) && defined(__clang__) && defined(__NDK_MAJOR__) && \
     defined(__NDK_MINOR__) &&                                               \
     ((__NDK_MAJOR__ < 12) || ((__NDK_MAJOR__ == 12) && (__NDK_MINOR__ < 1)))
 #undef EIGEN_THREAD_LOCAL
 #endif
 #endif  // defined(__ANDROID__) && defined(__clang__)
 
 #endif  // EIGEN_AVOID_THREAD_LOCAL
 
 namespace Eigen {
 
 namespace internal {
 template <typename T>
 struct ThreadLocalNoOpInitialize {
   void operator()(T&) const {}
 };
 
 template <typename T>
 struct ThreadLocalNoOpRelease {
   void operator()(T&) const {}
 };
 
 }  // namespace internal
 
 // Thread local container for elements of type T, that does not use thread local
 // storage. As long as the number of unique threads accessing this storage
 // is smaller than `capacity_`, it is lock-free and wait-free. Otherwise it will
 // use a mutex for synchronization.
 //
 // Type `T` has to be default constructible, and by default each thread will get
 // a default constructed value. It is possible to specify custom `initialize`
 // callable, that will be called lazily from each thread accessing this object,
 // and will be passed a default initialized object of type `T`. Also it's
 // possible to pass a custom `release` callable, that will be invoked before
 // calling ~T().
 //
 // Example:
 //
 //   struct Counter {
 //     int value = 0;
 //   }
 //
 //   Eigen::ThreadLocal<Counter> counter(10);
 //
 //   // Each thread will have access to it's own counter object.
 //   Counter& cnt = counter.local();
 //   cnt++;
 //
 // WARNING: Eigen::ThreadLocal uses the OS-specific value returned by
 // std::this_thread::get_id() to identify threads. This value is not guaranteed
 // to be unique except for the life of the thread. A newly created thread may
 // get an OS-specific ID equal to that of an already destroyed thread.
 //
 // Somewhat similar to TBB thread local storage, with similar restrictions:
 // https://www.threadingbuildingblocks.org/docs/help/reference/thread_local_storage/enumerable_thread_specific_cls.html
 //
 template <typename T,
           typename Initialize = internal::ThreadLocalNoOpInitialize<T>,
           typename Release = internal::ThreadLocalNoOpRelease<T>>
 class ThreadLocal {
   // We preallocate default constructed elements in MaxSizedVector.
   static_assert(std::is_default_constructible<T>::value,
                 "ThreadLocal data type must be default constructible");
 
  public:
   explicit ThreadLocal(int capacity)
       : ThreadLocal(capacity, internal::ThreadLocalNoOpInitialize<T>(),
                     internal::ThreadLocalNoOpRelease<T>()) {}
 
   ThreadLocal(int capacity, Initialize initialize)
       : ThreadLocal(capacity, std::move(initialize),
                     internal::ThreadLocalNoOpRelease<T>()) {}
 
   ThreadLocal(int capacity, Initialize initialize, Release release)
       : initialize_(std::move(initialize)),
         release_(std::move(release)),
         capacity_(capacity),
         data_(capacity_),
         ptr_(capacity_),
         filled_records_(0) {
     eigen_assert(capacity_ >= 0);
     data_.resize(capacity_);
     for (int i = 0; i < capacity_; ++i) {
       ptr_.emplace_back(nullptr);
     }
   }
 
   T& local() {
     std::thread::id this_thread = std::this_thread::get_id();
     if (capacity_ == 0) return SpilledLocal(this_thread);
 
     std::size_t h = std::hash<std::thread::id>()(this_thread);
     const int start_idx = h % capacity_;
 
     // NOTE: From the definition of `std::this_thread::get_id()` it is
     // guaranteed that we never can have concurrent insertions with the same key
     // to our hash-map like data structure. If we didn't find an element during
     // the initial traversal, it's guaranteed that no one else could have
     // inserted it while we are in this function. This allows to massively
     // simplify out lock-free insert-only hash map.
 
     // Check if we already have an element for `this_thread`.
     int idx = start_idx;
     while (ptr_[idx].load() != nullptr) {
       ThreadIdAndValue& record = *(ptr_[idx].load());
       if (record.thread_id == this_thread) return record.value;
 
       idx += 1;
       if (idx >= capacity_) idx -= capacity_;
       if (idx == start_idx) break;
     }
 
     // If we are here, it means that we found an insertion point in lookup
     // table at `idx`, or we did a full traversal and table is full.
 
     // If lock-free storage is full, fallback on mutex.
     if (filled_records_.load() >= capacity_) return SpilledLocal(this_thread);
 
     // We double check that we still have space to insert an element into a lock
     // free storage. If old value in `filled_records_` is larger than the
     // records capacity, it means that some other thread added an element while
     // we were traversing lookup table.
     int insertion_index =
         filled_records_.fetch_add(1, std::memory_order_relaxed);
     if (insertion_index >= capacity_) return SpilledLocal(this_thread);
 
     // At this point it's guaranteed that we can access to
     // data_[insertion_index_] without a data race.
     data_[insertion_index].thread_id = this_thread;
     initialize_(data_[insertion_index].value);
 
     // That's the pointer we'll put into the lookup table.
     ThreadIdAndValue* inserted = &data_[insertion_index];
 
     // We'll use nullptr pointer to ThreadIdAndValue in a compare-and-swap loop.
     ThreadIdAndValue* empty = nullptr;
 
     // Now we have to find an insertion point into the lookup table. We start
     // from the `idx` that was identified as an insertion point above, it's
     // guaranteed that we will have an empty record somewhere in a lookup table
     // (because we created a record in the `data_`).
     const int insertion_idx = idx;
 
     do {
       // Always start search from the original insertion candidate.
       idx = insertion_idx;
       while (ptr_[idx].load() != nullptr) {
         idx += 1;
         if (idx >= capacity_) idx -= capacity_;
         // If we did a full loop, it means that we don't have any free entries
         // in the lookup table, and this means that something is terribly wrong.
         eigen_assert(idx != insertion_idx);
       }
       // Atomic CAS of the pointer guarantees that any other thread, that will
       // follow this pointer will see all the mutations in the `data_`.
     } while (!ptr_[idx].compare_exchange_weak(empty, inserted));
 
     return inserted->value;
   }
 
   // WARN: It's not thread safe to call it concurrently with `local()`.
   void ForEach(std::function<void(std::thread::id, T&)> f) {
     // Reading directly from `data_` is unsafe, because only CAS to the
     // record in `ptr_` makes all changes visible to other threads.
     for (auto& ptr : ptr_) {
       ThreadIdAndValue* record = ptr.load();
       if (record == nullptr) continue;
       f(record->thread_id, record->value);
     }
 
     // We did not spill into the map based storage.
     if (filled_records_.load(std::memory_order_relaxed) < capacity_) return;
 
     // Adds a happens before edge from the last call to SpilledLocal().
     std::unique_lock<std::mutex> lock(mu_);
     for (auto& kv : per_thread_map_) {
       f(kv.first, kv.second);
     }
   }
 
   // WARN: It's not thread safe to call it concurrently with `local()`.
   ~ThreadLocal() {
     // Reading directly from `data_` is unsafe, because only CAS to the record
     // in `ptr_` makes all changes visible to other threads.
     for (auto& ptr : ptr_) {
       ThreadIdAndValue* record = ptr.load();
       if (record == nullptr) continue;
       release_(record->value);
     }
 
     // We did not spill into the map based storage.
     if (filled_records_.load(std::memory_order_relaxed) < capacity_) return;
 
     // Adds a happens before edge from the last call to SpilledLocal().
     std::unique_lock<std::mutex> lock(mu_);
     for (auto& kv : per_thread_map_) {
       release_(kv.second);
     }
   }
 
  private:
   struct ThreadIdAndValue {
     std::thread::id thread_id;
     T value;
   };
 
   // Use unordered map guarded by a mutex when lock free storage is full.
   T& SpilledLocal(std::thread::id this_thread) {
     std::unique_lock<std::mutex> lock(mu_);
 
     auto it = per_thread_map_.find(this_thread);
     if (it == per_thread_map_.end()) {
       auto result = per_thread_map_.emplace(this_thread, T());
       eigen_assert(result.second);
       initialize_((*result.first).second);
       return (*result.first).second;
     } else {
       return it->second;
     }
   }
 
   Initialize initialize_;
   Release release_;
   const int capacity_;
 
   // Storage that backs lock-free lookup table `ptr_`. Records stored in this
   // storage contiguously starting from index 0.
   MaxSizeVector<ThreadIdAndValue> data_;
 
   // Atomic pointers to the data stored in `data_`. Used as a lookup table for
   // linear probing hash map (https://en.wikipedia.org/wiki/Linear_probing).
   MaxSizeVector<std::atomic<ThreadIdAndValue*>> ptr_;
 
   // Number of records stored in the `data_`.
   std::atomic<int> filled_records_;
 
   // We fallback on per thread map if lock-free storage is full. In practice
   // this should never happen, if `capacity_` is a reasonable estimate of the
   // number of threads running in a system.
   std::mutex mu_;  // Protects per_thread_map_.
   std::unordered_map<std::thread::id, T> per_thread_map_;
 };
 
 }  // namespace Eigen
 
 #endif  // EIGEN_CXX11_THREADPOOL_THREAD_LOCAL_H

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