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diff --git a/absl/synchronization/internal/per_thread_sem_test.cc b/absl/synchronization/internal/per_thread_sem_test.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
+//
+//      http://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/per_thread_sem.h"
+
+#include <atomic>
+#include <condition_variable>  // NOLINT(build/c++11)
+#include <functional>
+#include <limits>
+#include <mutex>               // NOLINT(build/c++11)
+#include <string>
+#include <thread>              // NOLINT(build/c++11)
+
+#include "absl/base/internal/cycleclock.h"
+#include "absl/base/internal/malloc_extension.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/strings/str_cat.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+#include "gtest/gtest.h"
+
+// In this test we explicitly avoid the use of synchronization
+// primitives which might use PerThreadSem, most notably absl::Mutex.
+
+namespace absl {
+namespace synchronization_internal {
+
+class SimpleSemaphore {
+ public:
+  SimpleSemaphore() : count_(0) {}
+
+  // Decrements (locks) the semaphore. If the semaphore's value is
+  // greater than zero, then the decrement proceeds, and the function
+  // returns, immediately. If the semaphore currently has the value
+  // zero, then the call blocks until it becomes possible to perform
+  // the decrement.
+  void Wait() {
+    std::unique_lock<std::mutex> lock(mu_);
+    cv_.wait(lock, [this]() { return count_ > 0; });
+    --count_;
+    cv_.notify_one();
+  }
+
+  // Increments (unlocks) the semaphore. If the semaphore's value
+  // consequently becomes greater than zero, then another thread
+  // blocked Wait() call will be woken up and proceed to lock the
+  // semaphore.
+  void Post() {
+    std::lock_guard<std::mutex> lock(mu_);
+    ++count_;
+    cv_.notify_one();
+  }
+
+ private:
+  std::mutex mu_;
+  std::condition_variable cv_;
+  int count_;
+};
+
+struct ThreadData {
+  int num_iterations;                 // Number of replies to send.
+  SimpleSemaphore identity2_written;  // Posted by thread writing identity2.
+  base_internal::ThreadIdentity *identity1;  // First Post()-er.
+  base_internal::ThreadIdentity *identity2;  // First Wait()-er.
+  KernelTimeout timeout;
+};
+
+// Need friendship with PerThreadSem.
+class PerThreadSemTest : public testing::Test {
+ public:
+  static void TimingThread(ThreadData* t) {
+    t->identity2 = GetOrCreateCurrentThreadIdentity();
+    t->identity2_written.Post();
+    while (t->num_iterations--) {
+      Wait(t->timeout);
+      Post(t->identity1);
+    }
+  }
+
+  void TestTiming(const char *msg, bool timeout) {
+    static const int kNumIterations = 100;
+    ThreadData t;
+    t.num_iterations = kNumIterations;
+    t.timeout = timeout ?
+        KernelTimeout(absl::Now() + absl::Seconds(10000))  // far in the future
+        : KernelTimeout::Never();
+    t.identity1 = GetOrCreateCurrentThreadIdentity();
+
+    // We can't use the Thread class here because it uses the Mutex
+    // class which will invoke PerThreadSem, so we use std::thread instead.
+    std::thread partner_thread(std::bind(TimingThread, &t));
+
+    // Wait for our partner thread to register their identity.
+    t.identity2_written.Wait();
+
+    int64_t min_cycles = std::numeric_limits<int64_t>::max();
+    int64_t total_cycles = 0;
+    for (int i = 0; i < kNumIterations; ++i) {
+      absl::SleepFor(absl::Milliseconds(20));
+      int64_t cycles = base_internal::CycleClock::Now();
+      Post(t.identity2);
+      Wait(t.timeout);
+      cycles = base_internal::CycleClock::Now() - cycles;
+      min_cycles = std::min(min_cycles, cycles);
+      total_cycles += cycles;
+    }
+    std::string out =
+        StrCat(msg, "min cycle count=", min_cycles, " avg cycle count=",
+               absl::SixDigits(static_cast<double>(total_cycles) /
+                               kNumIterations));
+    printf("%s\n", out.c_str());
+
+    partner_thread.join();
+  }
+
+ protected:
+  static void Post(base_internal::ThreadIdentity *id) {
+    PerThreadSem::Post(id);
+  }
+  static bool Wait(KernelTimeout t) {
+    return PerThreadSem::Wait(t);
+  }
+
+  // convenience overload
+  static bool Wait(absl::Time t) {
+    return Wait(KernelTimeout(t));
+  }
+
+  static void Tick(base_internal::ThreadIdentity *identity) {
+    PerThreadSem::Tick(identity);
+  }
+};
+
+namespace {
+
+TEST_F(PerThreadSemTest, WithoutTimeout) {
+  PerThreadSemTest::TestTiming("Without timeout: ", false);
+}
+
+TEST_F(PerThreadSemTest, WithTimeout) {
+  PerThreadSemTest::TestTiming("With timeout:    ", true);
+}
+
+TEST_F(PerThreadSemTest, Timeouts) {
+  absl::Time timeout = absl::Now() + absl::Milliseconds(50);
+  EXPECT_FALSE(Wait(timeout));
+  EXPECT_LE(timeout, absl::Now());
+
+  absl::Time negative_timeout = absl::UnixEpoch() - absl::Milliseconds(100);
+  EXPECT_FALSE(Wait(negative_timeout));
+  EXPECT_LE(negative_timeout, absl::Now());  // trivially true :)
+
+  Post(GetOrCreateCurrentThreadIdentity());
+  // The wait here has an expired timeout, but we have a wake to consume,
+  // so this should succeed
+  EXPECT_TRUE(Wait(negative_timeout));
+}
+
+// Test that idle threads properly register themselves as such with malloc.
+TEST_F(PerThreadSemTest, Idle) {
+  // We can't use gmock because it might use synch calls.  So we do it
+  // by hand, messily.  I don't bother hitting every one of the
+  // MallocExtension calls because most of them won't get made
+  // anyway--if they do we can add them.
+  class MockMallocExtension : public base_internal::MallocExtension {
+   public:
+    MockMallocExtension(base_internal::MallocExtension *real,
+                        base_internal::ThreadIdentity *id,
+                        std::atomic<int> *idles, std::atomic<int> *busies)
+        : real_(real), id_(id), idles_(idles), busies_(busies) {}
+    void MarkThreadIdle() override {
+      if (base_internal::CurrentThreadIdentityIfPresent() != id_) {
+        return;
+      }
+      idles_->fetch_add(1, std::memory_order_relaxed);
+    }
+
+    void MarkThreadBusy() override {
+      if (base_internal::CurrentThreadIdentityIfPresent() != id_) {
+        return;
+      }
+      busies_->fetch_add(1, std::memory_order_relaxed);
+    }
+    size_t GetAllocatedSize(const void* p) override {
+      return real_->GetAllocatedSize(p);
+    }
+
+   private:
+    MallocExtension *real_;
+    base_internal::ThreadIdentity *id_;
+    std::atomic<int>* idles_;
+    std::atomic<int>* busies_;
+  };
+
+  base_internal::ThreadIdentity *id = GetOrCreateCurrentThreadIdentity();
+  std::atomic<int> idles(0);
+  std::atomic<int> busies(0);
+  base_internal::MallocExtension *old =
+      base_internal::MallocExtension::instance();
+  MockMallocExtension mock(old, id, &idles, &busies);
+  base_internal::MallocExtension::Register(&mock);
+  std::atomic<int> sync(0);
+
+  std::thread t([id, &idles, &sync]() {
+    // Wait for the main thread to begin the wait process
+    while (0 == sync.load(std::memory_order_relaxed)) {
+      SleepFor(absl::Milliseconds(1));
+    }
+    // Wait for main thread to become idle, then wake it
+    // pretend time is passing--enough of these should cause an idling.
+    for (int i = 0; i < 100; ++i) {
+      Tick(id);
+    }
+    while (0 == idles.load(std::memory_order_relaxed)) {
+      // Keep ticking, just in case.
+      Tick(id);
+      SleepFor(absl::Milliseconds(1));
+    }
+    Post(id);
+  });
+
+  idles.store(0, std::memory_order_relaxed);  // In case we slept earlier.
+  sync.store(1, std::memory_order_relaxed);
+  Wait(KernelTimeout::Never());
+
+  // t will wake us once we become idle.
+  EXPECT_LT(0, busies.load(std::memory_order_relaxed));
+  t.join();
+  base_internal::MallocExtension::Register(old);
+}
+
+}  // namespace
+
+}  // namespace synchronization_internal
+}  // namespace absl