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diff --git a/abseil-cpp/absl/synchronization/internal/futex.h b/abseil-cpp/absl/synchronization/internal/futex.h
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+++ b/abseil-cpp/absl/synchronization/internal/futex.h
@@ -0,0 +1,177 @@
+// Copyright 2020 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.
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_H_
+
+#include "absl/base/config.h"
+
+#ifndef _WIN32
+#include <sys/time.h>
+#include <unistd.h>
+#endif
+
+#ifdef __linux__
+#include <linux/futex.h>
+#include <sys/syscall.h>
+#endif
+
+#include <errno.h>
+#include <stdio.h>
+#include <time.h>
+
+#include <atomic>
+#include <cstdint>
+#include <limits>
+
+#include "absl/base/optimization.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX
+#error ABSL_INTERNAL_HAVE_FUTEX may not be set on the command line
+#elif defined(__BIONIC__)
+// Bionic supports all the futex operations we need even when some of the futex
+// definitions are missing.
+#define ABSL_INTERNAL_HAVE_FUTEX
+#elif defined(__linux__) && defined(FUTEX_CLOCK_REALTIME)
+// FUTEX_CLOCK_REALTIME requires Linux >= 2.6.28.
+#define ABSL_INTERNAL_HAVE_FUTEX
+#endif
+
+#ifdef ABSL_INTERNAL_HAVE_FUTEX
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+// 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
+
+#if defined(__NR_futex_time64) && !defined(SYS_futex_time64)
+#define SYS_futex_time64 __NR_futex_time64
+#endif
+
+#if defined(SYS_futex_time64) && !defined(SYS_futex)
+#define SYS_futex SYS_futex_time64
+using FutexTimespec = struct timespec;
+#else
+// Some libc implementations have switched to an unconditional 64-bit `time_t`
+// definition. This means that `struct timespec` may not match the layout
+// expected by the kernel ABI on 32-bit platforms. So we define the
+// FutexTimespec that matches the kernel timespec definition. It should be safe
+// to use this struct for 64-bit userspace builds too, since it will use another
+// SYS_futex kernel call with 64-bit tv_sec inside timespec.
+struct FutexTimespec {
+  long tv_sec;   // NOLINT
+  long tv_nsec;  // NOLINT
+};
+#endif
+
+class FutexImpl {
+ public:
+  // Atomically check that `*v == val`, and if it is, then sleep until the until
+  // woken by `Wake()`.
+  static int Wait(std::atomic<int32_t>* v, int32_t val) {
+    return WaitAbsoluteTimeout(v, val, nullptr);
+  }
+
+  // Atomically check that `*v == val`, and if it is, then sleep until
+  // CLOCK_REALTIME reaches `*abs_timeout`, or until woken by `Wake()`.
+  static int WaitAbsoluteTimeout(std::atomic<int32_t>* v, int32_t val,
+                                 const struct timespec* abs_timeout) {
+    FutexTimespec ts;
+    // https://locklessinc.com/articles/futex_cheat_sheet/
+    // Unlike FUTEX_WAIT, FUTEX_WAIT_BITSET uses absolute time.
+    auto err = syscall(
+        SYS_futex, reinterpret_cast<int32_t*>(v),
+        FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME, val,
+        ToFutexTimespec(abs_timeout, &ts), nullptr, FUTEX_BITSET_MATCH_ANY);
+    if (err != 0) {
+      return -errno;
+    }
+    return 0;
+  }
+
+  // Atomically check that `*v == val`, and if it is, then sleep until
+  // `*rel_timeout` has elapsed, or until woken by `Wake()`.
+  static int WaitRelativeTimeout(std::atomic<int32_t>* v, int32_t val,
+                                 const struct timespec* rel_timeout) {
+    FutexTimespec ts;
+    // Atomically check that the futex value is still 0, and if it
+    // is, sleep until abs_timeout or until woken by FUTEX_WAKE.
+    auto err =
+        syscall(SYS_futex, reinterpret_cast<int32_t*>(v), FUTEX_PRIVATE_FLAG,
+                val, ToFutexTimespec(rel_timeout, &ts));
+    if (err != 0) {
+      return -errno;
+    }
+    return 0;
+  }
+
+  // Wakes at most `count` waiters that have entered the sleep state on `v`.
+  static int Wake(std::atomic<int32_t>* v, int32_t count) {
+    auto err = syscall(SYS_futex, reinterpret_cast<int32_t*>(v),
+                       FUTEX_WAKE | FUTEX_PRIVATE_FLAG, count);
+    if (ABSL_PREDICT_FALSE(err < 0)) {
+      return -errno;
+    }
+    return 0;
+  }
+
+ private:
+  static FutexTimespec* ToFutexTimespec(const struct timespec* userspace_ts,
+                                        FutexTimespec* futex_ts) {
+    if (userspace_ts == nullptr) {
+      return nullptr;
+    }
+
+    using FutexSeconds = decltype(futex_ts->tv_sec);
+    using FutexNanoseconds = decltype(futex_ts->tv_nsec);
+
+    constexpr auto kMaxSeconds{(std::numeric_limits<FutexSeconds>::max)()};
+    if (userspace_ts->tv_sec > kMaxSeconds) {
+      futex_ts->tv_sec = kMaxSeconds;
+    } else {
+      futex_ts->tv_sec = static_cast<FutexSeconds>(userspace_ts->tv_sec);
+    }
+    futex_ts->tv_nsec = static_cast<FutexNanoseconds>(userspace_ts->tv_nsec);
+    return futex_ts;
+  }
+};
+
+class Futex : public FutexImpl {};
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_INTERNAL_HAVE_FUTEX
+
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_FUTEX_H_