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diff --git a/absl/random/internal/pool_urbg.cc b/absl/random/internal/pool_urbg.cc
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+++ b/absl/random/internal/pool_urbg.cc
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+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/random/internal/pool_urbg.h"
+
+#include <algorithm>
+#include <atomic>
+#include <cstdint>
+#include <cstring>
+#include <iterator>
+
+#include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/spinlock.h"
+#include "absl/base/internal/sysinfo.h"
+#include "absl/base/internal/unaligned_access.h"
+#include "absl/base/optimization.h"
+#include "absl/random/internal/randen.h"
+#include "absl/random/internal/seed_material.h"
+#include "absl/random/seed_gen_exception.h"
+
+using absl::base_internal::SpinLock;
+using absl::base_internal::SpinLockHolder;
+
+namespace absl {
+namespace random_internal {
+namespace {
+
+// RandenPoolEntry is a thread-safe pseudorandom bit generator, implementing a
+// single generator within a RandenPool<T>. It is an internal implementation
+// detail, and does not aim to conform to [rand.req.urng].
+//
+// NOTE: There are alignment issues when used on ARM, for instance.
+// See the allocation code in PoolAlignedAlloc().
+class RandenPoolEntry {
+ public:
+  static constexpr size_t kState = RandenTraits::kStateBytes / sizeof(uint32_t);
+  static constexpr size_t kCapacity =
+      RandenTraits::kCapacityBytes / sizeof(uint32_t);
+
+  void Init(absl::Span<const uint32_t> data) {
+    SpinLockHolder l(&mu_);  // Always uncontested.
+    std::copy(data.begin(), data.end(), std::begin(state_));
+    next_ = kState;
+  }
+
+  // Copy bytes into out.
+  void Fill(uint8_t* out, size_t bytes) LOCKS_EXCLUDED(mu_);
+
+  // Returns random bits from the buffer in units of T.
+  template <typename T>
+  inline T Generate() LOCKS_EXCLUDED(mu_);
+
+  inline void MaybeRefill() EXCLUSIVE_LOCKS_REQUIRED(mu_) {
+    if (next_ >= kState) {
+      next_ = kCapacity;
+      impl_.Generate(state_);
+    }
+  }
+
+ private:
+  // Randen URBG state.
+  uint32_t state_[kState] GUARDED_BY(mu_);  // First to satisfy alignment.
+  SpinLock mu_;
+  const Randen impl_;
+  size_t next_ GUARDED_BY(mu_);
+};
+
+template <>
+inline uint8_t RandenPoolEntry::Generate<uint8_t>() {
+  SpinLockHolder l(&mu_);
+  MaybeRefill();
+  return static_cast<uint8_t>(state_[next_++]);
+}
+
+template <>
+inline uint16_t RandenPoolEntry::Generate<uint16_t>() {
+  SpinLockHolder l(&mu_);
+  MaybeRefill();
+  return static_cast<uint16_t>(state_[next_++]);
+}
+
+template <>
+inline uint32_t RandenPoolEntry::Generate<uint32_t>() {
+  SpinLockHolder l(&mu_);
+  MaybeRefill();
+  return state_[next_++];
+}
+
+template <>
+inline uint64_t RandenPoolEntry::Generate<uint64_t>() {
+  SpinLockHolder l(&mu_);
+  if (next_ >= kState - 1) {
+    next_ = kCapacity;
+    impl_.Generate(state_);
+  }
+  auto p = state_ + next_;
+  next_ += 2;
+
+  uint64_t result;
+  std::memcpy(&result, p, sizeof(result));
+  return result;
+}
+
+void RandenPoolEntry::Fill(uint8_t* out, size_t bytes) {
+  SpinLockHolder l(&mu_);
+  while (bytes > 0) {
+    MaybeRefill();
+    size_t remaining = (kState - next_) * sizeof(state_[0]);
+    size_t to_copy = std::min(bytes, remaining);
+    std::memcpy(out, &state_[next_], to_copy);
+    out += to_copy;
+    bytes -= to_copy;
+    next_ += (to_copy + sizeof(state_[0]) - 1) / sizeof(state_[0]);
+  }
+}
+
+// Number of pooled urbg entries.
+static constexpr int kPoolSize = 8;
+
+// Shared pool entries.
+static absl::once_flag pool_once;
+ABSL_CACHELINE_ALIGNED static RandenPoolEntry* shared_pools[kPoolSize];
+
+// Returns an id in the range [0 ... kPoolSize), which indexes into the
+// pool of random engines.
+//
+// Each thread to access the pool is assigned a sequential ID (without reuse)
+// from the pool-id space; the id is cached in a thread_local variable.
+// This id is assigned based on the arrival-order of the thread to the
+// GetPoolID call; this has no binary, CL, or runtime stability because
+// on subsequent runs the order within the same program may be significantly
+// different. However, as other thread IDs are not assigned sequentially,
+// this is not expected to matter.
+int GetPoolID() {
+  static_assert(kPoolSize >= 1,
+                "At least one urbg instance is required for PoolURBG");
+
+  ABSL_CONST_INIT static std::atomic<int64_t> sequence{0};
+
+#ifdef ABSL_HAVE_THREAD_LOCAL
+  static thread_local int my_pool_id = -1;
+  if (ABSL_PREDICT_FALSE(my_pool_id < 0)) {
+    my_pool_id = (sequence++ % kPoolSize);
+  }
+  return my_pool_id;
+#else
+  static pthread_key_t tid_key = [] {
+    pthread_key_t tmp_key;
+    int err = pthread_key_create(&tmp_key, nullptr);
+    if (err) {
+      ABSL_RAW_LOG(FATAL, "pthread_key_create failed with %d", err);
+    }
+    return tmp_key;
+  }();
+
+  // Store the value in the pthread_{get/set}specific. However an uninitialized
+  // value is 0, so add +1 to distinguish from the null value.
+  intptr_t my_pool_id =
+      reinterpret_cast<intptr_t>(pthread_getspecific(tid_key));
+  if (ABSL_PREDICT_FALSE(my_pool_id == 0)) {
+    // No allocated ID, allocate the next value, cache it, and return.
+    my_pool_id = (sequence++ % kPoolSize) + 1;
+    int err = pthread_setspecific(tid_key, reinterpret_cast<void*>(my_pool_id));
+    if (err) {
+      ABSL_RAW_LOG(FATAL, "pthread_setspecific failed with %d", err);
+    }
+  }
+  return my_pool_id - 1;
+#endif
+}
+
+// Allocate a RandenPoolEntry with at least 32-byte alignment, which is required
+// by ARM platform code.
+RandenPoolEntry* PoolAlignedAlloc() {
+  constexpr size_t kAlignment =
+      ABSL_CACHELINE_SIZE > 32 ? ABSL_CACHELINE_SIZE : 32;
+
+  // Not all the platforms that we build for have std::aligned_alloc, however
+  // since we never free these objects, we can over allocate and munge the
+  // pointers to the correct alignment.
+  void* memory = std::malloc(sizeof(RandenPoolEntry) + kAlignment);
+  auto x = reinterpret_cast<intptr_t>(memory);
+  auto y = x % kAlignment;
+  void* aligned =
+      (y == 0) ? memory : reinterpret_cast<void*>(x + kAlignment - y);
+  return new (aligned) RandenPoolEntry();
+}
+
+// Allocate and initialize kPoolSize objects of type RandenPoolEntry.
+//
+// The initialization strategy is to initialize one object directly from
+// OS entropy, then to use that object to seed all of the individual
+// pool instances.
+void InitPoolURBG() {
+  static constexpr size_t kSeedSize =
+      RandenTraits::kStateBytes / sizeof(uint32_t);
+  // Read the seed data from OS entropy once.
+  uint32_t seed_material[kPoolSize * kSeedSize];
+  if (!random_internal::ReadSeedMaterialFromOSEntropy(
+          absl::MakeSpan(seed_material))) {
+    random_internal::ThrowSeedGenException();
+  }
+  for (int i = 0; i < kPoolSize; i++) {
+    shared_pools[i] = PoolAlignedAlloc();
+    shared_pools[i]->Init(
+        absl::MakeSpan(&seed_material[i * kSeedSize], kSeedSize));
+  }
+}
+
+// Returns the pool entry for the current thread.
+RandenPoolEntry* GetPoolForCurrentThread() {
+  absl::call_once(pool_once, InitPoolURBG);
+  return shared_pools[GetPoolID()];
+}
+
+}  // namespace
+
+template <typename T>
+typename RandenPool<T>::result_type RandenPool<T>::Generate() {
+  auto* pool = GetPoolForCurrentThread();
+  return pool->Generate<T>();
+}
+
+template <typename T>
+void RandenPool<T>::Fill(absl::Span<result_type> data) {
+  auto* pool = GetPoolForCurrentThread();
+  pool->Fill(reinterpret_cast<uint8_t*>(data.data()),
+             data.size() * sizeof(result_type));
+}
+
+template class RandenPool<uint8_t>;
+template class RandenPool<uint16_t>;
+template class RandenPool<uint32_t>;
+template class RandenPool<uint64_t>;
+
+}  // namespace random_internal
+}  // namespace absl