diff --git a/absl/synchronization/internal/waiter.cc b/absl/synchronization/internal/waiter.cc
new file mode 100644
index 0000000..f17ea56
--- /dev/null
+++ b/absl/synchronization/internal/waiter.cc
@@ -0,0 +1,457 @@
+// 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/synchronization/internal/waiter.h"
+
+#include "absl/base/config.h"
+
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <pthread.h>
+#include <sys/time.h>
+#include <unistd.h>
+#endif
+
+#ifdef __linux__
+#include <linux/futex.h>
+#include <sys/syscall.h>
+#endif
+
+#ifdef ABSL_HAVE_SEMAPHORE_H
+#include <semaphore.h>
+#endif
+
+#include <errno.h>
+#include <stdio.h>
+#include <time.h>
+
+#include <atomic>
+#include <cassert>
+#include <cstdint>
+#include <new>
+#include <type_traits>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+namespace synchronization_internal {
+
+static void MaybeBecomeIdle() {
+  base_internal::ThreadIdentity *identity =
+      base_internal::CurrentThreadIdentityIfPresent();
+  assert(identity != nullptr);
+  const bool is_idle = identity->is_idle.load(std::memory_order_relaxed);
+  const int ticker = identity->ticker.load(std::memory_order_relaxed);
+  const int wait_start = identity->wait_start.load(std::memory_order_relaxed);
+  if (!is_idle && ticker - wait_start > Waiter::kIdlePeriods) {
+    identity->is_idle.store(true, std::memory_order_relaxed);
+  }
+}
+
+#if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
+
+// Some Android headers are missing these definitions even though they
+// support these futex operations.
+#ifdef __BIONIC__
+#ifndef SYS_futex
+#define SYS_futex __NR_futex
+#endif
+#ifndef FUTEX_WAIT_BITSET
+#define FUTEX_WAIT_BITSET 9
+#endif
+#ifndef FUTEX_PRIVATE_FLAG
+#define FUTEX_PRIVATE_FLAG 128
+#endif
+#ifndef FUTEX_CLOCK_REALTIME
+#define FUTEX_CLOCK_REALTIME 256
+#endif
+#ifndef FUTEX_BITSET_MATCH_ANY
+#define FUTEX_BITSET_MATCH_ANY 0xFFFFFFFF
+#endif
+#endif
+
+class Futex {
+ public:
+  static int WaitUntil(std::atomic<int32_t> *v, int32_t val,
+                       KernelTimeout t) {
+    int err = 0;
+    if (t.has_timeout()) {
+      // https://locklessinc.com/articles/futex_cheat_sheet/
+      // Unlike FUTEX_WAIT, FUTEX_WAIT_BITSET uses absolute time.
+      struct timespec abs_timeout = t.MakeAbsTimespec();
+      // Atomically check that the futex value is still 0, and if it
+      // is, sleep until abs_timeout or until woken by FUTEX_WAKE.
+      err = syscall(
+          SYS_futex, reinterpret_cast<int32_t *>(v),
+          FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME, val,
+          &abs_timeout, nullptr, FUTEX_BITSET_MATCH_ANY);
+    } else {
+      // Atomically check that the futex value is still 0, and if it
+      // is, sleep until woken by FUTEX_WAKE.
+      err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
+                    FUTEX_WAIT | FUTEX_PRIVATE_FLAG, val, nullptr);
+    }
+    if (err != 0) {
+      err = -errno;
+    }
+    return err;
+  }
+
+  static int Wake(std::atomic<int32_t> *v, int32_t count) {
+    int err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
+                      FUTEX_WAKE | FUTEX_PRIVATE_FLAG, count);
+    if (ABSL_PREDICT_FALSE(err < 0)) {
+      err = -errno;
+    }
+    return err;
+  }
+};
+
+void Waiter::Init() {
+  futex_.store(0, std::memory_order_relaxed);
+}
+
+bool Waiter::Wait(KernelTimeout t) {
+  // Loop until we can atomically decrement futex from a positive
+  // value, waiting on a futex while we believe it is zero.
+  while (true) {
+    int32_t x = futex_.load(std::memory_order_relaxed);
+    if (x != 0) {
+      if (!futex_.compare_exchange_weak(x, x - 1,
+                                        std::memory_order_acquire,
+                                        std::memory_order_relaxed)) {
+        continue;  // Raced with someone, retry.
+      }
+      return true;  // Consumed a wakeup, we are done.
+    }
+
+    const int err = Futex::WaitUntil(&futex_, 0, t);
+    if (err != 0) {
+      if (err == -EINTR || err == -EWOULDBLOCK) {
+        // Do nothing, the loop will retry.
+      } else if (err == -ETIMEDOUT) {
+        return false;
+      } else {
+        ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
+      }
+    }
+
+    MaybeBecomeIdle();
+  }
+}
+
+void Waiter::Post() {
+  if (futex_.fetch_add(1, std::memory_order_release) == 0) {
+    // We incremented from 0, need to wake a potential waker.
+    Poke();
+  }
+}
+
+void Waiter::Poke() {
+  // Wake one thread waiting on the futex.
+  const int err = Futex::Wake(&futex_, 1);
+  if (ABSL_PREDICT_FALSE(err < 0)) {
+    ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
+  }
+}
+
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
+
+class PthreadMutexHolder {
+ public:
+  explicit PthreadMutexHolder(pthread_mutex_t *mu) : mu_(mu) {
+    const int err = pthread_mutex_lock(mu_);
+    if (err != 0) {
+      ABSL_RAW_LOG(FATAL, "pthread_mutex_lock failed: %d", err);
+    }
+  }
+
+  PthreadMutexHolder(const PthreadMutexHolder &rhs) = delete;
+  PthreadMutexHolder &operator=(const PthreadMutexHolder &rhs) = delete;
+
+  ~PthreadMutexHolder() {
+    const int err = pthread_mutex_unlock(mu_);
+    if (err != 0) {
+      ABSL_RAW_LOG(FATAL, "pthread_mutex_unlock failed: %d", err);
+    }
+  }
+
+ private:
+  pthread_mutex_t *mu_;
+};
+
+void Waiter::Init() {
+  const int err = pthread_mutex_init(&mu_, 0);
+  if (err != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_mutex_init failed: %d", err);
+  }
+
+  const int err2 = pthread_cond_init(&cv_, 0);
+  if (err2 != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_cond_init failed: %d", err2);
+  }
+
+  waiter_count_.store(0, std::memory_order_relaxed);
+  wakeup_count_.store(0, std::memory_order_relaxed);
+}
+
+bool Waiter::Wait(KernelTimeout t) {
+  struct timespec abs_timeout;
+  if (t.has_timeout()) {
+    abs_timeout = t.MakeAbsTimespec();
+  }
+
+  PthreadMutexHolder h(&mu_);
+  waiter_count_.fetch_add(1, std::memory_order_relaxed);
+  // Loop until we find a wakeup to consume or timeout.
+  while (true) {
+    int x = wakeup_count_.load(std::memory_order_relaxed);
+    if (x != 0) {
+      if (!wakeup_count_.compare_exchange_weak(x, x - 1,
+                                               std::memory_order_acquire,
+                                               std::memory_order_relaxed)) {
+        continue;  // Raced with someone, retry.
+      }
+      // Successfully consumed a wakeup, we're done.
+      waiter_count_.fetch_sub(1, std::memory_order_relaxed);
+      return true;
+    }
+
+    // No wakeups available, time to wait.
+    if (!t.has_timeout()) {
+      const int err = pthread_cond_wait(&cv_, &mu_);
+      if (err != 0) {
+        ABSL_RAW_LOG(FATAL, "pthread_cond_wait failed: %d", err);
+      }
+    } else {
+      const int err = pthread_cond_timedwait(&cv_, &mu_, &abs_timeout);
+      if (err == ETIMEDOUT) {
+        waiter_count_.fetch_sub(1, std::memory_order_relaxed);
+        return false;
+      }
+      if (err != 0) {
+        ABSL_RAW_LOG(FATAL, "pthread_cond_wait failed: %d", err);
+      }
+    }
+    MaybeBecomeIdle();
+  }
+}
+
+void Waiter::Post() {
+  wakeup_count_.fetch_add(1, std::memory_order_release);
+  Poke();
+}
+
+void Waiter::Poke() {
+  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
+    return;
+  }
+  // Potentially a waker. Take the lock and check again.
+  PthreadMutexHolder h(&mu_);
+  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
+    return;
+  }
+  const int err = pthread_cond_signal(&cv_);
+  if (err != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_cond_signal failed: %d", err);
+  }
+}
+
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
+
+void Waiter::Init() {
+  if (sem_init(&sem_, 0, 0) != 0) {
+    ABSL_RAW_LOG(FATAL, "sem_init failed with errno %d\n", errno);
+  }
+  wakeups_.store(0, std::memory_order_relaxed);
+}
+
+bool Waiter::Wait(KernelTimeout t) {
+  struct timespec abs_timeout;
+  if (t.has_timeout()) {
+    abs_timeout = t.MakeAbsTimespec();
+  }
+
+  // Loop until we timeout or consume a wakeup.
+  while (true) {
+    int x = wakeups_.load(std::memory_order_relaxed);
+    if (x != 0) {
+      if (!wakeups_.compare_exchange_weak(x, x - 1,
+                                          std::memory_order_acquire,
+                                          std::memory_order_relaxed)) {
+        continue;  // Raced with someone, retry.
+      }
+      // Successfully consumed a wakeup, we're done.
+      return true;
+    }
+
+    // Nothing to consume, wait (looping on EINTR).
+    while (true) {
+      if (!t.has_timeout()) {
+        if (sem_wait(&sem_) == 0) break;
+        if (errno == EINTR) continue;
+        ABSL_RAW_LOG(FATAL, "sem_wait failed: %d", errno);
+      } else {
+        if (sem_timedwait(&sem_, &abs_timeout) == 0) break;
+        if (errno == EINTR) continue;
+        if (errno == ETIMEDOUT) return false;
+        ABSL_RAW_LOG(FATAL, "sem_timedwait failed: %d", errno);
+      }
+    }
+    MaybeBecomeIdle();
+  }
+}
+
+void Waiter::Post() {
+  wakeups_.fetch_add(1, std::memory_order_release);  // Post a wakeup.
+  Poke();
+}
+
+void Waiter::Poke() {
+  if (sem_post(&sem_) != 0) {  // Wake any semaphore waiter.
+    ABSL_RAW_LOG(FATAL, "sem_post failed with errno %d\n", errno);
+  }
+}
+
+#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
+
+class Waiter::WinHelper {
+ public:
+  static SRWLOCK *GetLock(Waiter *w) {
+    return reinterpret_cast<SRWLOCK *>(&w->mu_storage_);
+  }
+
+  static CONDITION_VARIABLE *GetCond(Waiter *w) {
+    return reinterpret_cast<CONDITION_VARIABLE *>(&w->cv_storage_);
+  }
+
+  static_assert(sizeof(SRWLOCK) == sizeof(Waiter::SRWLockStorage),
+                "SRWLockStorage does not have the same size as SRWLOCK");
+  static_assert(
+      alignof(SRWLOCK) == alignof(Waiter::SRWLockStorage),
+      "SRWLockStorage does not have the same alignment as SRWLOCK");
+
+  static_assert(sizeof(CONDITION_VARIABLE) ==
+                    sizeof(Waiter::ConditionVariableStorage),
+                "ABSL_CONDITION_VARIABLE_STORAGE does not have the same size "
+                "as CONDITION_VARIABLE");
+  static_assert(alignof(CONDITION_VARIABLE) ==
+                    alignof(Waiter::ConditionVariableStorage),
+                "ConditionVariableStorage does not have the same "
+                "alignment as CONDITION_VARIABLE");
+
+  // The SRWLOCK and CONDITION_VARIABLE types must be trivially constuctible
+  // and destructible because we never call their constructors or destructors.
+  static_assert(std::is_trivially_constructible<SRWLOCK>::value,
+                "The SRWLOCK type must be trivially constructible");
+  static_assert(std::is_trivially_constructible<CONDITION_VARIABLE>::value,
+                "The CONDITION_VARIABLE type must be trivially constructible");
+  static_assert(std::is_trivially_destructible<SRWLOCK>::value,
+                "The SRWLOCK type must be trivially destructible");
+  static_assert(std::is_trivially_destructible<CONDITION_VARIABLE>::value,
+                "The CONDITION_VARIABLE type must be trivially destructible");
+};
+
+class LockHolder {
+ public:
+  explicit LockHolder(SRWLOCK* mu) : mu_(mu) {
+    AcquireSRWLockExclusive(mu_);
+  }
+
+  LockHolder(const LockHolder&) = delete;
+  LockHolder& operator=(const LockHolder&) = delete;
+
+  ~LockHolder() {
+    ReleaseSRWLockExclusive(mu_);
+  }
+
+ private:
+  SRWLOCK* mu_;
+};
+
+void Waiter::Init() {
+  auto *mu = ::new (static_cast<void *>(&mu_storage_)) SRWLOCK;
+  auto *cv = ::new (static_cast<void *>(&cv_storage_)) CONDITION_VARIABLE;
+  InitializeSRWLock(mu);
+  InitializeConditionVariable(cv);
+  waiter_count_.store(0, std::memory_order_relaxed);
+  wakeup_count_.store(0, std::memory_order_relaxed);
+}
+
+bool Waiter::Wait(KernelTimeout t) {
+  SRWLOCK *mu = WinHelper::GetLock(this);
+  CONDITION_VARIABLE *cv = WinHelper::GetCond(this);
+
+  LockHolder h(mu);
+  waiter_count_.fetch_add(1, std::memory_order_relaxed);
+
+  // Loop until we find a wakeup to consume or timeout.
+  while (true) {
+    int x = wakeup_count_.load(std::memory_order_relaxed);
+    if (x != 0) {
+      if (!wakeup_count_.compare_exchange_weak(x, x - 1,
+                                               std::memory_order_acquire,
+                                               std::memory_order_relaxed)) {
+        continue;  // Raced with someone, retry.
+      }
+      // Successfully consumed a wakeup, we're done.
+      waiter_count_.fetch_sub(1, std::memory_order_relaxed);
+      return true;
+    }
+
+    // No wakeups available, time to wait.
+    if (!SleepConditionVariableSRW(cv, mu, t.InMillisecondsFromNow(), 0)) {
+      // GetLastError() returns a Win32 DWORD, but we assign to
+      // unsigned long to simplify the ABSL_RAW_LOG case below.  The uniform
+      // initialization guarantees this is not a narrowing conversion.
+      const unsigned long err{GetLastError()};  // NOLINT(runtime/int)
+      if (err == ERROR_TIMEOUT) {
+        waiter_count_.fetch_sub(1, std::memory_order_relaxed);
+        return false;
+      } else {
+        ABSL_RAW_LOG(FATAL, "SleepConditionVariableSRW failed: %lu", err);
+      }
+    }
+
+    MaybeBecomeIdle();
+  }
+}
+
+void Waiter::Post() {
+  wakeup_count_.fetch_add(1, std::memory_order_release);
+  Poke();
+}
+
+void Waiter::Poke() {
+  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
+    return;
+  }
+  // Potentially a waker. Take the lock and check again.
+  LockHolder h(WinHelper::GetLock(this));
+  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
+    return;
+  }
+  WakeConditionVariable(WinHelper::GetCond(this));
+}
+
+#else
+#error Unknown ABSL_WAITER_MODE
+#endif
+
+}  // namespace synchronization_internal
+}  // namespace absl