diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
new file mode 100644
index 0000000..100def2
--- /dev/null
+++ b/absl/synchronization/mutex.cc
@@ -0,0 +1,2723 @@
+// 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/mutex.h"
+
+#ifdef _WIN32
+#include <windows.h>
+#ifdef ERROR
+#undef ERROR
+#endif
+#else
+#include <fcntl.h>
+#include <pthread.h>
+#include <sched.h>
+#include <sys/time.h>
+#endif
+
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+
+#include <algorithm>
+#include <atomic>
+#include <cinttypes>
+#include <thread>  // NOLINT(build/c++11)
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/dynamic_annotations.h"
+#include "absl/base/internal/atomic_hook.h"
+#include "absl/base/internal/cycleclock.h"
+#include "absl/base/internal/hide_ptr.h"
+#include "absl/base/internal/low_level_alloc.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/spinlock.h"
+#include "absl/base/internal/sysinfo.h"
+#include "absl/base/internal/thread_identity.h"
+#include "absl/base/port.h"
+#include "absl/debugging/stacktrace.h"
+#include "absl/debugging/symbolize.h"
+#include "absl/synchronization/internal/graphcycles.h"
+#include "absl/synchronization/internal/per_thread_sem.h"
+#include "absl/time/time.h"
+
+using absl::base_internal::CurrentThreadIdentityIfPresent;
+using absl::base_internal::PerThreadSynch;
+using absl::base_internal::ThreadIdentity;
+using absl::synchronization_internal::GetOrCreateCurrentThreadIdentity;
+using absl::synchronization_internal::GraphCycles;
+using absl::synchronization_internal::GraphId;
+using absl::synchronization_internal::InvalidGraphId;
+using absl::synchronization_internal::KernelTimeout;
+using absl::synchronization_internal::PerThreadSem;
+
+extern "C" {
+ABSL_ATTRIBUTE_WEAK void AbslInternalMutexYield() { std::this_thread::yield(); }
+}  // extern "C"
+
+namespace absl {
+
+namespace {
+
+#if defined(THREAD_SANITIZER)
+constexpr OnDeadlockCycle kDeadlockDetectionDefault = OnDeadlockCycle::kIgnore;
+#else
+constexpr OnDeadlockCycle kDeadlockDetectionDefault = OnDeadlockCycle::kAbort;
+#endif
+
+ABSL_CONST_INIT std::atomic<OnDeadlockCycle> synch_deadlock_detection(
+    kDeadlockDetectionDefault);
+ABSL_CONST_INIT std::atomic<bool> synch_check_invariants(false);
+
+// ------------------------------------------ spinlock support
+
+// Make sure read-only globals used in the Mutex code are contained on the
+// same cacheline and cacheline aligned to eliminate any false sharing with
+// other globals from this and other modules.
+static struct MutexGlobals {
+  MutexGlobals() {
+    // Find machine-specific data needed for Delay() and
+    // TryAcquireWithSpinning(). This runs in the global constructor
+    // sequence, and before that zeros are safe values.
+    num_cpus = absl::base_internal::NumCPUs();
+    spinloop_iterations = num_cpus > 1 ? 1500 : 0;
+  }
+  int num_cpus;
+  int spinloop_iterations;
+  // Pad this struct to a full cacheline to prevent false sharing.
+  char padding[ABSL_CACHELINE_SIZE - 2 * sizeof(int)];
+} ABSL_CACHELINE_ALIGNED mutex_globals;
+static_assert(
+    sizeof(MutexGlobals) == ABSL_CACHELINE_SIZE,
+    "MutexGlobals must occupy an entire cacheline to prevent false sharing");
+
+ABSL_CONST_INIT absl::base_internal::AtomicHook<void (*)(int64_t wait_cycles)>
+    submit_profile_data;
+ABSL_CONST_INIT absl::base_internal::AtomicHook<
+    void (*)(const char *msg, const void *obj, int64_t wait_cycles)> mutex_tracer;
+ABSL_CONST_INIT absl::base_internal::AtomicHook<
+    void (*)(const char *msg, const void *cv)> cond_var_tracer;
+ABSL_CONST_INIT absl::base_internal::AtomicHook<
+    bool (*)(const void *pc, char *out, int out_size)>
+    symbolizer(absl::Symbolize);
+
+}  // namespace
+
+static inline bool EvalConditionAnnotated(const Condition *cond, Mutex *mu,
+                                          bool locking, bool trylock,
+                                          bool read_lock);
+
+void RegisterMutexProfiler(void (*fn)(int64_t wait_timestamp)) {
+  submit_profile_data.Store(fn);
+}
+
+void RegisterMutexTracer(void (*fn)(const char *msg, const void *obj,
+                                    int64_t wait_cycles)) {
+  mutex_tracer.Store(fn);
+}
+
+void RegisterCondVarTracer(void (*fn)(const char *msg, const void *cv)) {
+  cond_var_tracer.Store(fn);
+}
+
+void RegisterSymbolizer(bool (*fn)(const void *pc, char *out, int out_size)) {
+  symbolizer.Store(fn);
+}
+
+// spinlock delay on iteration c.  Returns new c.
+namespace {
+  enum DelayMode { AGGRESSIVE, GENTLE };
+};
+static int Delay(int32_t c, DelayMode mode) {
+  // If this a uniprocessor, only yield/sleep.  Otherwise, if the mode is
+  // aggressive then spin many times before yielding.  If the mode is
+  // gentle then spin only a few times before yielding.  Aggressive spinning is
+  // used to ensure that an Unlock() call, which  must get the spin lock for
+  // any thread to make progress gets it without undue delay.
+  int32_t limit = (mutex_globals.num_cpus > 1) ?
+      ((mode == AGGRESSIVE) ? 5000 : 250) : 0;
+  if (c < limit) {
+    c++;               // spin
+  } else {
+    ABSL_TSAN_MUTEX_PRE_DIVERT(nullptr, 0);
+    if (c == limit) {  // yield once
+      AbslInternalMutexYield();
+      c++;
+    } else {           // then wait
+      absl::SleepFor(absl::Microseconds(10));
+      c = 0;
+    }
+    ABSL_TSAN_MUTEX_POST_DIVERT(nullptr, 0);
+  }
+  return (c);
+}
+
+// --------------------------Generic atomic ops
+// Ensure that "(*pv & bits) == bits" by doing an atomic update of "*pv" to
+// "*pv | bits" if necessary.  Wait until (*pv & wait_until_clear)==0
+// before making any change.
+// This is used to set flags in mutex and condition variable words.
+static void AtomicSetBits(std::atomic<intptr_t>* pv, intptr_t bits,
+                          intptr_t wait_until_clear) {
+  intptr_t v;
+  do {
+    v = pv->load(std::memory_order_relaxed);
+  } while ((v & bits) != bits &&
+           ((v & wait_until_clear) != 0 ||
+            !pv->compare_exchange_weak(v, v | bits,
+                                       std::memory_order_release,
+                                       std::memory_order_relaxed)));
+}
+
+// Ensure that "(*pv & bits) == 0" by doing an atomic update of "*pv" to
+// "*pv & ~bits" if necessary.  Wait until (*pv & wait_until_clear)==0
+// before making any change.
+// This is used to unset flags in mutex and condition variable words.
+static void AtomicClearBits(std::atomic<intptr_t>* pv, intptr_t bits,
+                            intptr_t wait_until_clear) {
+  intptr_t v;
+  do {
+    v = pv->load(std::memory_order_relaxed);
+  } while ((v & bits) != 0 &&
+           ((v & wait_until_clear) != 0 ||
+            !pv->compare_exchange_weak(v, v & ~bits,
+                                       std::memory_order_release,
+                                       std::memory_order_relaxed)));
+}
+
+//------------------------------------------------------------------
+
+// Data for doing deadlock detection.
+static absl::base_internal::SpinLock deadlock_graph_mu(
+    absl::base_internal::kLinkerInitialized);
+
+// graph used to detect deadlocks.
+static GraphCycles *deadlock_graph ABSL_GUARDED_BY(deadlock_graph_mu)
+    ABSL_PT_GUARDED_BY(deadlock_graph_mu);
+
+//------------------------------------------------------------------
+// An event mechanism for debugging mutex use.
+// It also allows mutexes to be given names for those who can't handle
+// addresses, and instead like to give their data structures names like
+// "Henry", "Fido", or "Rupert IV, King of Yondavia".
+
+namespace {  // to prevent name pollution
+enum {       // Mutex and CondVar events passed as "ev" to PostSynchEvent
+             // Mutex events
+  SYNCH_EV_TRYLOCK_SUCCESS,
+  SYNCH_EV_TRYLOCK_FAILED,
+  SYNCH_EV_READERTRYLOCK_SUCCESS,
+  SYNCH_EV_READERTRYLOCK_FAILED,
+  SYNCH_EV_LOCK,
+  SYNCH_EV_LOCK_RETURNING,
+  SYNCH_EV_READERLOCK,
+  SYNCH_EV_READERLOCK_RETURNING,
+  SYNCH_EV_UNLOCK,
+  SYNCH_EV_READERUNLOCK,
+
+  // CondVar events
+  SYNCH_EV_WAIT,
+  SYNCH_EV_WAIT_RETURNING,
+  SYNCH_EV_SIGNAL,
+  SYNCH_EV_SIGNALALL,
+};
+
+enum {                    // Event flags
+  SYNCH_F_R = 0x01,       // reader event
+  SYNCH_F_LCK = 0x02,     // PostSynchEvent called with mutex held
+  SYNCH_F_TRY = 0x04,     // TryLock or ReaderTryLock
+  SYNCH_F_UNLOCK = 0x08,  // Unlock or ReaderUnlock
+
+  SYNCH_F_LCK_W = SYNCH_F_LCK,
+  SYNCH_F_LCK_R = SYNCH_F_LCK | SYNCH_F_R,
+};
+}  // anonymous namespace
+
+// Properties of the events.
+static const struct {
+  int flags;
+  const char *msg;
+} event_properties[] = {
+    {SYNCH_F_LCK_W | SYNCH_F_TRY, "TryLock succeeded "},
+    {0, "TryLock failed "},
+    {SYNCH_F_LCK_R | SYNCH_F_TRY, "ReaderTryLock succeeded "},
+    {0, "ReaderTryLock failed "},
+    {0, "Lock blocking "},
+    {SYNCH_F_LCK_W, "Lock returning "},
+    {0, "ReaderLock blocking "},
+    {SYNCH_F_LCK_R, "ReaderLock returning "},
+    {SYNCH_F_LCK_W | SYNCH_F_UNLOCK, "Unlock "},
+    {SYNCH_F_LCK_R | SYNCH_F_UNLOCK, "ReaderUnlock "},
+    {0, "Wait on "},
+    {0, "Wait unblocked "},
+    {0, "Signal on "},
+    {0, "SignalAll on "},
+};
+
+static absl::base_internal::SpinLock synch_event_mu(
+    absl::base_internal::kLinkerInitialized);
+// protects synch_event
+
+// Hash table size; should be prime > 2.
+// Can't be too small, as it's used for deadlock detection information.
+static const uint32_t kNSynchEvent = 1031;
+
+static struct SynchEvent {     // this is a trivial hash table for the events
+  // struct is freed when refcount reaches 0
+  int refcount ABSL_GUARDED_BY(synch_event_mu);
+
+  // buckets have linear, 0-terminated  chains
+  SynchEvent *next ABSL_GUARDED_BY(synch_event_mu);
+
+  // Constant after initialization
+  uintptr_t masked_addr;  // object at this address is called "name"
+
+  // No explicit synchronization used.  Instead we assume that the
+  // client who enables/disables invariants/logging on a Mutex does so
+  // while the Mutex is not being concurrently accessed by others.
+  void (*invariant)(void *arg);  // called on each event
+  void *arg;            // first arg to (*invariant)()
+  bool log;             // logging turned on
+
+  // Constant after initialization
+  char name[1];         // actually longer---null-terminated std::string
+} * synch_event[kNSynchEvent] ABSL_GUARDED_BY(synch_event_mu);
+
+// Ensure that the object at "addr" has a SynchEvent struct associated with it,
+// set "bits" in the word there (waiting until lockbit is clear before doing
+// so), and return a refcounted reference that will remain valid until
+// UnrefSynchEvent() is called.  If a new SynchEvent is allocated,
+// the string name is copied into it.
+// When used with a mutex, the caller should also ensure that kMuEvent
+// is set in the mutex word, and similarly for condition variables and kCVEvent.
+static SynchEvent *EnsureSynchEvent(std::atomic<intptr_t> *addr,
+                                    const char *name, intptr_t bits,
+                                    intptr_t lockbit) {
+  uint32_t h = reinterpret_cast<intptr_t>(addr) % kNSynchEvent;
+  SynchEvent *e;
+  // first look for existing SynchEvent struct..
+  synch_event_mu.Lock();
+  for (e = synch_event[h];
+       e != nullptr && e->masked_addr != base_internal::HidePtr(addr);
+       e = e->next) {
+  }
+  if (e == nullptr) {  // no SynchEvent struct found; make one.
+    if (name == nullptr) {
+      name = "";
+    }
+    size_t l = strlen(name);
+    e = reinterpret_cast<SynchEvent *>(
+        base_internal::LowLevelAlloc::Alloc(sizeof(*e) + l));
+    e->refcount = 2;    // one for return value, one for linked list
+    e->masked_addr = base_internal::HidePtr(addr);
+    e->invariant = nullptr;
+    e->arg = nullptr;
+    e->log = false;
+    strcpy(e->name, name);  // NOLINT(runtime/printf)
+    e->next = synch_event[h];
+    AtomicSetBits(addr, bits, lockbit);
+    synch_event[h] = e;
+  } else {
+    e->refcount++;      // for return value
+  }
+  synch_event_mu.Unlock();
+  return e;
+}
+
+// Deallocate the SynchEvent *e, whose refcount has fallen to zero.
+static void DeleteSynchEvent(SynchEvent *e) {
+  base_internal::LowLevelAlloc::Free(e);
+}
+
+// Decrement the reference count of *e, or do nothing if e==null.
+static void UnrefSynchEvent(SynchEvent *e) {
+  if (e != nullptr) {
+    synch_event_mu.Lock();
+    bool del = (--(e->refcount) == 0);
+    synch_event_mu.Unlock();
+    if (del) {
+      DeleteSynchEvent(e);
+    }
+  }
+}
+
+// Forget the mapping from the object (Mutex or CondVar) at address addr
+// to SynchEvent object, and clear "bits" in its word (waiting until lockbit
+// is clear before doing so).
+static void ForgetSynchEvent(std::atomic<intptr_t> *addr, intptr_t bits,
+                             intptr_t lockbit) {
+  uint32_t h = reinterpret_cast<intptr_t>(addr) % kNSynchEvent;
+  SynchEvent **pe;
+  SynchEvent *e;
+  synch_event_mu.Lock();
+  for (pe = &synch_event[h];
+       (e = *pe) != nullptr && e->masked_addr != base_internal::HidePtr(addr);
+       pe = &e->next) {
+  }
+  bool del = false;
+  if (e != nullptr) {
+    *pe = e->next;
+    del = (--(e->refcount) == 0);
+  }
+  AtomicClearBits(addr, bits, lockbit);
+  synch_event_mu.Unlock();
+  if (del) {
+    DeleteSynchEvent(e);
+  }
+}
+
+// Return a refcounted reference to the SynchEvent of the object at address
+// "addr", if any.  The pointer returned is valid until the UnrefSynchEvent() is
+// called.
+static SynchEvent *GetSynchEvent(const void *addr) {
+  uint32_t h = reinterpret_cast<intptr_t>(addr) % kNSynchEvent;
+  SynchEvent *e;
+  synch_event_mu.Lock();
+  for (e = synch_event[h];
+       e != nullptr && e->masked_addr != base_internal::HidePtr(addr);
+       e = e->next) {
+  }
+  if (e != nullptr) {
+    e->refcount++;
+  }
+  synch_event_mu.Unlock();
+  return e;
+}
+
+// Called when an event "ev" occurs on a Mutex of CondVar "obj"
+// if event recording is on
+static void PostSynchEvent(void *obj, int ev) {
+  SynchEvent *e = GetSynchEvent(obj);
+  // logging is on if event recording is on and either there's no event struct,
+  // or it explicitly says to log
+  if (e == nullptr || e->log) {
+    void *pcs[40];
+    int n = absl::GetStackTrace(pcs, ABSL_ARRAYSIZE(pcs), 1);
+    // A buffer with enough space for the ASCII for all the PCs, even on a
+    // 64-bit machine.
+    char buffer[ABSL_ARRAYSIZE(pcs) * 24];
+    int pos = snprintf(buffer, sizeof (buffer), " @");
+    for (int i = 0; i != n; i++) {
+      pos += snprintf(&buffer[pos], sizeof (buffer) - pos, " %p", pcs[i]);
+    }
+    ABSL_RAW_LOG(INFO, "%s%p %s %s", event_properties[ev].msg, obj,
+                 (e == nullptr ? "" : e->name), buffer);
+  }
+  const int flags = event_properties[ev].flags;
+  if ((flags & SYNCH_F_LCK) != 0 && e != nullptr && e->invariant != nullptr) {
+    // Calling the invariant as is causes problems under ThreadSanitizer.
+    // We are currently inside of Mutex Lock/Unlock and are ignoring all
+    // memory accesses and synchronization. If the invariant transitively
+    // synchronizes something else and we ignore the synchronization, we will
+    // get false positive race reports later.
+    // Reuse EvalConditionAnnotated to properly call into user code.
+    struct local {
+      static bool pred(SynchEvent *ev) {
+        (*ev->invariant)(ev->arg);
+        return false;
+      }
+    };
+    Condition cond(&local::pred, e);
+    Mutex *mu = static_cast<Mutex *>(obj);
+    const bool locking = (flags & SYNCH_F_UNLOCK) == 0;
+    const bool trylock = (flags & SYNCH_F_TRY) != 0;
+    const bool read_lock = (flags & SYNCH_F_R) != 0;
+    EvalConditionAnnotated(&cond, mu, locking, trylock, read_lock);
+  }
+  UnrefSynchEvent(e);
+}
+
+//------------------------------------------------------------------
+
+// The SynchWaitParams struct encapsulates the way in which a thread is waiting:
+// whether it has a timeout, the condition, exclusive/shared, and whether a
+// condition variable wait has an associated Mutex (as opposed to another
+// type of lock).  It also points to the PerThreadSynch struct of its thread.
+// cv_word tells Enqueue() to enqueue on a CondVar using CondVarEnqueue().
+//
+// This structure is held on the stack rather than directly in
+// PerThreadSynch because a thread can be waiting on multiple Mutexes if,
+// while waiting on one Mutex, the implementation calls a client callback
+// (such as a Condition function) that acquires another Mutex. We don't
+// strictly need to allow this, but programmers become confused if we do not
+// allow them to use functions such a LOG() within Condition functions.  The
+// PerThreadSynch struct points at the most recent SynchWaitParams struct when
+// the thread is on a Mutex's waiter queue.
+struct SynchWaitParams {
+  SynchWaitParams(Mutex::MuHow how_arg, const Condition *cond_arg,
+                  KernelTimeout timeout_arg, Mutex *cvmu_arg,
+                  PerThreadSynch *thread_arg,
+                  std::atomic<intptr_t> *cv_word_arg)
+      : how(how_arg),
+        cond(cond_arg),
+        timeout(timeout_arg),
+        cvmu(cvmu_arg),
+        thread(thread_arg),
+        cv_word(cv_word_arg),
+        contention_start_cycles(base_internal::CycleClock::Now()) {}
+
+  const Mutex::MuHow how;  // How this thread needs to wait.
+  const Condition *cond;  // The condition that this thread is waiting for.
+                          // In Mutex, this field is set to zero if a timeout
+                          // expires.
+  KernelTimeout timeout;  // timeout expiry---absolute time
+                          // In Mutex, this field is set to zero if a timeout
+                          // expires.
+  Mutex *const cvmu;      // used for transfer from cond var to mutex
+  PerThreadSynch *const thread;  // thread that is waiting
+
+  // If not null, thread should be enqueued on the CondVar whose state
+  // word is cv_word instead of queueing normally on the Mutex.
+  std::atomic<intptr_t> *cv_word;
+
+  int64_t contention_start_cycles;  // Time (in cycles) when this thread started
+                                  // to contend for the mutex.
+};
+
+struct SynchLocksHeld {
+  int n;              // number of valid entries in locks[]
+  bool overflow;      // true iff we overflowed the array at some point
+  struct {
+    Mutex *mu;        // lock acquired
+    int32_t count;      // times acquired
+    GraphId id;       // deadlock_graph id of acquired lock
+  } locks[40];
+  // If a thread overfills the array during deadlock detection, we
+  // continue, discarding information as needed.  If no overflow has
+  // taken place, we can provide more error checking, such as
+  // detecting when a thread releases a lock it does not hold.
+};
+
+// A sentinel value in lists that is not 0.
+// A 0 value is used to mean "not on a list".
+static PerThreadSynch *const kPerThreadSynchNull =
+  reinterpret_cast<PerThreadSynch *>(1);
+
+static SynchLocksHeld *LocksHeldAlloc() {
+  SynchLocksHeld *ret = reinterpret_cast<SynchLocksHeld *>(
+      base_internal::LowLevelAlloc::Alloc(sizeof(SynchLocksHeld)));
+  ret->n = 0;
+  ret->overflow = false;
+  return ret;
+}
+
+// Return the PerThreadSynch-struct for this thread.
+static PerThreadSynch *Synch_GetPerThread() {
+  ThreadIdentity *identity = GetOrCreateCurrentThreadIdentity();
+  return &identity->per_thread_synch;
+}
+
+static PerThreadSynch *Synch_GetPerThreadAnnotated(Mutex *mu) {
+  if (mu) {
+    ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
+  }
+  PerThreadSynch *w = Synch_GetPerThread();
+  if (mu) {
+    ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
+  }
+  return w;
+}
+
+static SynchLocksHeld *Synch_GetAllLocks() {
+  PerThreadSynch *s = Synch_GetPerThread();
+  if (s->all_locks == nullptr) {
+    s->all_locks = LocksHeldAlloc();  // Freed by ReclaimThreadIdentity.
+  }
+  return s->all_locks;
+}
+
+// Post on "w"'s associated PerThreadSem.
+inline void Mutex::IncrementSynchSem(Mutex *mu, PerThreadSynch *w) {
+  if (mu) {
+    ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
+  }
+  PerThreadSem::Post(w->thread_identity());
+  if (mu) {
+    ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
+  }
+}
+
+// Wait on "w"'s associated PerThreadSem; returns false if timeout expired.
+bool Mutex::DecrementSynchSem(Mutex *mu, PerThreadSynch *w, KernelTimeout t) {
+  if (mu) {
+    ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
+  }
+  assert(w == Synch_GetPerThread());
+  static_cast<void>(w);
+  bool res = PerThreadSem::Wait(t);
+  if (mu) {
+    ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
+  }
+  return res;
+}
+
+// We're in a fatal signal handler that hopes to use Mutex and to get
+// lucky by not deadlocking.  We try to improve its chances of success
+// by effectively disabling some of the consistency checks.  This will
+// prevent certain ABSL_RAW_CHECK() statements from being triggered when
+// re-rentry is detected.  The ABSL_RAW_CHECK() statements are those in the
+// Mutex code checking that the "waitp" field has not been reused.
+void Mutex::InternalAttemptToUseMutexInFatalSignalHandler() {
+  // Fix the per-thread state only if it exists.
+  ThreadIdentity *identity = CurrentThreadIdentityIfPresent();
+  if (identity != nullptr) {
+    identity->per_thread_synch.suppress_fatal_errors = true;
+  }
+  // Don't do deadlock detection when we are already failing.
+  synch_deadlock_detection.store(OnDeadlockCycle::kIgnore,
+                                 std::memory_order_release);
+}
+
+// --------------------------time support
+
+// Return the current time plus the timeout.  Use the same clock as
+// PerThreadSem::Wait() for consistency.  Unfortunately, we don't have
+// such a choice when a deadline is given directly.
+static absl::Time DeadlineFromTimeout(absl::Duration timeout) {
+#ifndef _WIN32
+  struct timeval tv;
+  gettimeofday(&tv, nullptr);
+  return absl::TimeFromTimeval(tv) + timeout;
+#else
+  return absl::Now() + timeout;
+#endif
+}
+
+// --------------------------Mutexes
+
+// In the layout below, the msb of the bottom byte is currently unused.  Also,
+// the following constraints were considered in choosing the layout:
+//  o Both the debug allocator's "uninitialized" and "freed" patterns (0xab and
+//    0xcd) are illegal: reader and writer lock both held.
+//  o kMuWriter and kMuEvent should exceed kMuDesig and kMuWait, to enable the
+//    bit-twiddling trick in Mutex::Unlock().
+//  o kMuWriter / kMuReader == kMuWrWait / kMuWait,
+//    to enable the bit-twiddling trick in CheckForMutexCorruption().
+static const intptr_t kMuReader      = 0x0001L;  // a reader holds the lock
+static const intptr_t kMuDesig       = 0x0002L;  // there's a designated waker
+static const intptr_t kMuWait        = 0x0004L;  // threads are waiting
+static const intptr_t kMuWriter      = 0x0008L;  // a writer holds the lock
+static const intptr_t kMuEvent       = 0x0010L;  // record this mutex's events
+// INVARIANT1:  there's a thread that was blocked on the mutex, is
+// no longer, yet has not yet acquired the mutex.  If there's a
+// designated waker, all threads can avoid taking the slow path in
+// unlock because the designated waker will subsequently acquire
+// the lock and wake someone.  To maintain INVARIANT1 the bit is
+// set when a thread is unblocked(INV1a), and threads that were
+// unblocked reset the bit when they either acquire or re-block
+// (INV1b).
+static const intptr_t kMuWrWait      = 0x0020L;  // runnable writer is waiting
+                                                 // for a reader
+static const intptr_t kMuSpin        = 0x0040L;  // spinlock protects wait list
+static const intptr_t kMuLow         = 0x00ffL;  // mask all mutex bits
+static const intptr_t kMuHigh        = ~kMuLow;  // mask pointer/reader count
+
+// Hack to make constant values available to gdb pretty printer
+enum {
+  kGdbMuSpin = kMuSpin,
+  kGdbMuEvent = kMuEvent,
+  kGdbMuWait = kMuWait,
+  kGdbMuWriter = kMuWriter,
+  kGdbMuDesig = kMuDesig,
+  kGdbMuWrWait = kMuWrWait,
+  kGdbMuReader = kMuReader,
+  kGdbMuLow = kMuLow,
+};
+
+// kMuWrWait implies kMuWait.
+// kMuReader and kMuWriter are mutually exclusive.
+// If kMuReader is zero, there are no readers.
+// Otherwise, if kMuWait is zero, the high order bits contain a count of the
+// number of readers.  Otherwise, the reader count is held in
+// PerThreadSynch::readers of the most recently queued waiter, again in the
+// bits above kMuLow.
+static const intptr_t kMuOne = 0x0100;  // a count of one reader
+
+// flags passed to Enqueue and LockSlow{,WithTimeout,Loop}
+static const int kMuHasBlocked = 0x01;  // already blocked (MUST == 1)
+static const int kMuIsCond = 0x02;      // conditional waiter (CV or Condition)
+
+static_assert(PerThreadSynch::kAlignment > kMuLow,
+              "PerThreadSynch::kAlignment must be greater than kMuLow");
+
+// This struct contains various bitmasks to be used in
+// acquiring and releasing a mutex in a particular mode.
+struct MuHowS {
+  // if all the bits in fast_need_zero are zero, the lock can be acquired by
+  // adding fast_add and oring fast_or.  The bit kMuDesig should be reset iff
+  // this is the designated waker.
+  intptr_t fast_need_zero;
+  intptr_t fast_or;
+  intptr_t fast_add;
+
+  intptr_t slow_need_zero;  // fast_need_zero with events (e.g. logging)
+
+  intptr_t slow_inc_need_zero;  // if all the bits in slow_inc_need_zero are
+                                // zero a reader can acquire a read share by
+                                // setting the reader bit and incrementing
+                                // the reader count (in last waiter since
+                                // we're now slow-path).  kMuWrWait be may
+                                // be ignored if we already waited once.
+};
+
+static const MuHowS kSharedS = {
+    // shared or read lock
+    kMuWriter | kMuWait | kMuEvent,   // fast_need_zero
+    kMuReader,                        // fast_or
+    kMuOne,                           // fast_add
+    kMuWriter | kMuWait,              // slow_need_zero
+    kMuSpin | kMuWriter | kMuWrWait,  // slow_inc_need_zero
+};
+static const MuHowS kExclusiveS = {
+    // exclusive or write lock
+    kMuWriter | kMuReader | kMuEvent,  // fast_need_zero
+    kMuWriter,                         // fast_or
+    0,                                 // fast_add
+    kMuWriter | kMuReader,             // slow_need_zero
+    ~static_cast<intptr_t>(0),         // slow_inc_need_zero
+};
+static const Mutex::MuHow kShared = &kSharedS;        // shared lock
+static const Mutex::MuHow kExclusive = &kExclusiveS;  // exclusive lock
+
+#ifdef NDEBUG
+static constexpr bool kDebugMode = false;
+#else
+static constexpr bool kDebugMode = true;
+#endif
+
+#ifdef THREAD_SANITIZER
+static unsigned TsanFlags(Mutex::MuHow how) {
+  return how == kShared ? __tsan_mutex_read_lock : 0;
+}
+#endif
+
+static bool DebugOnlyIsExiting() {
+  return false;
+}
+
+Mutex::~Mutex() {
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  if ((v & kMuEvent) != 0 && !DebugOnlyIsExiting()) {
+    ForgetSynchEvent(&this->mu_, kMuEvent, kMuSpin);
+  }
+  if (kDebugMode) {
+    this->ForgetDeadlockInfo();
+  }
+  ABSL_TSAN_MUTEX_DESTROY(this, __tsan_mutex_not_static);
+}
+
+void Mutex::EnableDebugLog(const char *name) {
+  SynchEvent *e = EnsureSynchEvent(&this->mu_, name, kMuEvent, kMuSpin);
+  e->log = true;
+  UnrefSynchEvent(e);
+}
+
+void EnableMutexInvariantDebugging(bool enabled) {
+  synch_check_invariants.store(enabled, std::memory_order_release);
+}
+
+void Mutex::EnableInvariantDebugging(void (*invariant)(void *),
+                                     void *arg) {
+  if (synch_check_invariants.load(std::memory_order_acquire) &&
+      invariant != nullptr) {
+    SynchEvent *e = EnsureSynchEvent(&this->mu_, nullptr, kMuEvent, kMuSpin);
+    e->invariant = invariant;
+    e->arg = arg;
+    UnrefSynchEvent(e);
+  }
+}
+
+void SetMutexDeadlockDetectionMode(OnDeadlockCycle mode) {
+  synch_deadlock_detection.store(mode, std::memory_order_release);
+}
+
+// Return true iff threads x and y are waiting on the same condition for the
+// same type of lock.  Requires that x and y be waiting on the same Mutex
+// queue.
+static bool MuSameCondition(PerThreadSynch *x, PerThreadSynch *y) {
+  return x->waitp->how == y->waitp->how &&
+         Condition::GuaranteedEqual(x->waitp->cond, y->waitp->cond);
+}
+
+// Given the contents of a mutex word containing a PerThreadSynch pointer,
+// return the pointer.
+static inline PerThreadSynch *GetPerThreadSynch(intptr_t v) {
+  return reinterpret_cast<PerThreadSynch *>(v & kMuHigh);
+}
+
+// The next several routines maintain the per-thread next and skip fields
+// used in the Mutex waiter queue.
+// The queue is a circular singly-linked list, of which the "head" is the
+// last element, and head->next if the first element.
+// The skip field has the invariant:
+//   For thread x, x->skip is one of:
+//     - invalid (iff x is not in a Mutex wait queue),
+//     - null, or
+//     - a pointer to a distinct thread waiting later in the same Mutex queue
+//       such that all threads in [x, x->skip] have the same condition and
+//       lock type (MuSameCondition() is true for all pairs in [x, x->skip]).
+// In addition, if x->skip is  valid, (x->may_skip || x->skip == null)
+//
+// By the spec of MuSameCondition(), it is not necessary when removing the
+// first runnable thread y from the front a Mutex queue to adjust the skip
+// field of another thread x because if x->skip==y, x->skip must (have) become
+// invalid before y is removed.  The function TryRemove can remove a specified
+// thread from an arbitrary position in the queue whether runnable or not, so
+// it fixes up skip fields that would otherwise be left dangling.
+// The statement
+//     if (x->may_skip && MuSameCondition(x, x->next)) { x->skip = x->next; }
+// maintains the invariant provided x is not the last waiter in a Mutex queue
+// The statement
+//          if (x->skip != null) { x->skip = x->skip->skip; }
+// maintains the invariant.
+
+// Returns the last thread y in a mutex waiter queue such that all threads in
+// [x, y] inclusive share the same condition.  Sets skip fields of some threads
+// in that range to optimize future evaluation of Skip() on x values in
+// the range.  Requires thread x is in a mutex waiter queue.
+// The locking is unusual.  Skip() is called under these conditions:
+//   - spinlock is held in call from Enqueue(), with maybe_unlocking == false
+//   - Mutex is held in call from UnlockSlow() by last unlocker, with
+//     maybe_unlocking == true
+//   - both Mutex and spinlock are held in call from DequeueAllWakeable() (from
+//     UnlockSlow()) and TryRemove()
+// These cases are mutually exclusive, so Skip() never runs concurrently
+// with itself on the same Mutex.   The skip chain is used in these other places
+// that cannot occur concurrently:
+//   - FixSkip() (from TryRemove()) - spinlock and Mutex are held)
+//   - Dequeue() (with spinlock and Mutex held)
+//   - UnlockSlow() (with spinlock and Mutex held)
+// A more complex case is Enqueue()
+//   - Enqueue() (with spinlock held and maybe_unlocking == false)
+//               This is the first case in which Skip is called, above.
+//   - Enqueue() (without spinlock held; but queue is empty and being freshly
+//                formed)
+//   - Enqueue() (with spinlock held and maybe_unlocking == true)
+// The first case has mutual exclusion, and the second isolation through
+// working on an otherwise unreachable data structure.
+// In the last case, Enqueue() is required to change no skip/next pointers
+// except those in the added node and the former "head" node.  This implies
+// that the new node is added after head, and so must be the new head or the
+// new front of the queue.
+static PerThreadSynch *Skip(PerThreadSynch *x) {
+  PerThreadSynch *x0 = nullptr;
+  PerThreadSynch *x1 = x;
+  PerThreadSynch *x2 = x->skip;
+  if (x2 != nullptr) {
+    // Each iteration attempts to advance sequence (x0,x1,x2) to next sequence
+    // such that   x1 == x0->skip && x2 == x1->skip
+    while ((x0 = x1, x1 = x2, x2 = x2->skip) != nullptr) {
+      x0->skip = x2;      // short-circuit skip from x0 to x2
+    }
+    x->skip = x1;         // short-circuit skip from x to result
+  }
+  return x1;
+}
+
+// "ancestor" appears before "to_be_removed" in the same Mutex waiter queue.
+// The latter is going to be removed out of order, because of a timeout.
+// Check whether "ancestor" has a skip field pointing to "to_be_removed",
+// and fix it if it does.
+static void FixSkip(PerThreadSynch *ancestor, PerThreadSynch *to_be_removed) {
+  if (ancestor->skip == to_be_removed) {  // ancestor->skip left dangling
+    if (to_be_removed->skip != nullptr) {
+      ancestor->skip = to_be_removed->skip;  // can skip past to_be_removed
+    } else if (ancestor->next != to_be_removed) {  // they are not adjacent
+      ancestor->skip = ancestor->next;             // can skip one past ancestor
+    } else {
+      ancestor->skip = nullptr;  // can't skip at all
+    }
+  }
+}
+
+static void CondVarEnqueue(SynchWaitParams *waitp);
+
+// Enqueue thread "waitp->thread" on a waiter queue.
+// Called with mutex spinlock held if head != nullptr
+// If head==nullptr and waitp->cv_word==nullptr, then Enqueue() is
+// idempotent; it alters no state associated with the existing (empty)
+// queue.
+//
+// If waitp->cv_word == nullptr, queue the thread at either the front or
+// the end (according to its priority) of the circular mutex waiter queue whose
+// head is "head", and return the new head.  mu is the previous mutex state,
+// which contains the reader count (perhaps adjusted for the operation in
+// progress) if the list was empty and a read lock held, and the holder hint if
+// the list was empty and a write lock held.  (flags & kMuIsCond) indicates
+// whether this thread was transferred from a CondVar or is waiting for a
+// non-trivial condition.  In this case, Enqueue() never returns nullptr
+//
+// If waitp->cv_word != nullptr, CondVarEnqueue() is called, and "head" is
+// returned. This mechanism is used by CondVar to queue a thread on the
+// condition variable queue instead of the mutex queue in implementing Wait().
+// In this case, Enqueue() can return nullptr (if head==nullptr).
+static PerThreadSynch *Enqueue(PerThreadSynch *head,
+                               SynchWaitParams *waitp, intptr_t mu, int flags) {
+  // If we have been given a cv_word, call CondVarEnqueue() and return
+  // the previous head of the Mutex waiter queue.
+  if (waitp->cv_word != nullptr) {
+    CondVarEnqueue(waitp);
+    return head;
+  }
+
+  PerThreadSynch *s = waitp->thread;
+  ABSL_RAW_CHECK(
+      s->waitp == nullptr ||    // normal case
+          s->waitp == waitp ||  // Fer()---transfer from condition variable
+          s->suppress_fatal_errors,
+      "detected illegal recursion into Mutex code");
+  s->waitp = waitp;
+  s->skip = nullptr;             // maintain skip invariant (see above)
+  s->may_skip = true;            // always true on entering queue
+  s->wake = false;               // not being woken
+  s->cond_waiter = ((flags & kMuIsCond) != 0);
+  if (head == nullptr) {         // s is the only waiter
+    s->next = s;                 // it's the only entry in the cycle
+    s->readers = mu;             // reader count is from mu word
+    s->maybe_unlocking = false;  // no one is searching an empty list
+    head = s;                    // s is new head
+  } else {
+    PerThreadSynch *enqueue_after = nullptr;  // we'll put s after this element
+#ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
+    int64_t now_cycles = base_internal::CycleClock::Now();
+    if (s->next_priority_read_cycles < now_cycles) {
+      // Every so often, update our idea of the thread's priority.
+      // pthread_getschedparam() is 5% of the block/wakeup time;
+      // base_internal::CycleClock::Now() is 0.5%.
+      int policy;
+      struct sched_param param;
+      const int err = pthread_getschedparam(pthread_self(), &policy, &param);
+      if (err != 0) {
+        ABSL_RAW_LOG(ERROR, "pthread_getschedparam failed: %d", err);
+      } else {
+        s->priority = param.sched_priority;
+        s->next_priority_read_cycles =
+            now_cycles +
+            static_cast<int64_t>(base_internal::CycleClock::Frequency());
+      }
+    }
+    if (s->priority > head->priority) {  // s's priority is above head's
+      // try to put s in priority-fifo order, or failing that at the front.
+      if (!head->maybe_unlocking) {
+        // No unlocker can be scanning the queue, so we can insert between
+        // skip-chains, and within a skip-chain if it has the same condition as
+        // s.  We insert in priority-fifo order, examining the end of every
+        // skip-chain, plus every element with the same condition as s.
+        PerThreadSynch *advance_to = head;    // next value of enqueue_after
+        PerThreadSynch *cur;                  // successor of enqueue_after
+        do {
+          enqueue_after = advance_to;
+          cur = enqueue_after->next;  // this advance ensures progress
+          advance_to = Skip(cur);   // normally, advance to end of skip chain
+                                    // (side-effect: optimizes skip chain)
+          if (advance_to != cur && s->priority > advance_to->priority &&
+              MuSameCondition(s, cur)) {
+            // but this skip chain is not a singleton, s has higher priority
+            // than its tail and has the same condition as the chain,
+            // so we can insert within the skip-chain
+            advance_to = cur;         // advance by just one
+          }
+        } while (s->priority <= advance_to->priority);
+              // termination guaranteed because s->priority > head->priority
+              // and head is the end of a skip chain
+      } else if (waitp->how == kExclusive &&
+                 Condition::GuaranteedEqual(waitp->cond, nullptr)) {
+        // An unlocker could be scanning the queue, but we know it will recheck
+        // the queue front for writers that have no condition, which is what s
+        // is, so an insert at front is safe.
+        enqueue_after = head;       // add after head, at front
+      }
+    }
+#endif
+    if (enqueue_after != nullptr) {
+      s->next = enqueue_after->next;
+      enqueue_after->next = s;
+
+      // enqueue_after can be: head, Skip(...), or cur.
+      // The first two imply enqueue_after->skip == nullptr, and
+      // the last is used only if MuSameCondition(s, cur).
+      // We require this because clearing enqueue_after->skip
+      // is impossible; enqueue_after's predecessors might also
+      // incorrectly skip over s if we were to allow other
+      // insertion points.
+      ABSL_RAW_CHECK(
+          enqueue_after->skip == nullptr || MuSameCondition(enqueue_after, s),
+          "Mutex Enqueue failure");
+
+      if (enqueue_after != head && enqueue_after->may_skip &&
+          MuSameCondition(enqueue_after, enqueue_after->next)) {
+        // enqueue_after can skip to its new successor, s
+        enqueue_after->skip = enqueue_after->next;
+      }
+      if (MuSameCondition(s, s->next)) {  // s->may_skip is known to be true
+        s->skip = s->next;                // s may skip to its successor
+      }
+    } else {   // enqueue not done any other way, so
+               // we're inserting s at the back
+      // s will become new head; copy data from head into it
+      s->next = head->next;        // add s after head
+      head->next = s;
+      s->readers = head->readers;  // reader count is from previous head
+      s->maybe_unlocking = head->maybe_unlocking;  // same for unlock hint
+      if (head->may_skip && MuSameCondition(head, s)) {
+        // head now has successor; may skip
+        head->skip = s;
+      }
+      head = s;  // s is new head
+    }
+  }
+  s->state.store(PerThreadSynch::kQueued, std::memory_order_relaxed);
+  return head;
+}
+
+// Dequeue the successor pw->next of thread pw from the Mutex waiter queue
+// whose last element is head.  The new head element is returned, or null
+// if the list is made empty.
+// Dequeue is called with both spinlock and Mutex held.
+static PerThreadSynch *Dequeue(PerThreadSynch *head, PerThreadSynch *pw) {
+  PerThreadSynch *w = pw->next;
+  pw->next = w->next;         // snip w out of list
+  if (head == w) {            // we removed the head
+    head = (pw == w) ? nullptr : pw;  // either emptied list, or pw is new head
+  } else if (pw != head && MuSameCondition(pw, pw->next)) {
+    // pw can skip to its new successor
+    if (pw->next->skip !=
+        nullptr) {  // either skip to its successors skip target
+      pw->skip = pw->next->skip;
+    } else {                   // or to pw's successor
+      pw->skip = pw->next;
+    }
+  }
+  return head;
+}
+
+// Traverse the elements [ pw->next, h] of the circular list whose last element
+// is head.
+// Remove all elements with wake==true and place them in the
+// singly-linked list wake_list in the order found.   Assumes that
+// there is only one such element if the element has how == kExclusive.
+// Return the new head.
+static PerThreadSynch *DequeueAllWakeable(PerThreadSynch *head,
+                                          PerThreadSynch *pw,
+                                          PerThreadSynch **wake_tail) {
+  PerThreadSynch *orig_h = head;
+  PerThreadSynch *w = pw->next;
+  bool skipped = false;
+  do {
+    if (w->wake) {                    // remove this element
+      ABSL_RAW_CHECK(pw->skip == nullptr, "bad skip in DequeueAllWakeable");
+      // we're removing pw's successor so either pw->skip is zero or we should
+      // already have removed pw since if pw->skip!=null, pw has the same
+      // condition as w.
+      head = Dequeue(head, pw);
+      w->next = *wake_tail;           // keep list terminated
+      *wake_tail = w;                 // add w to wake_list;
+      wake_tail = &w->next;           // next addition to end
+      if (w->waitp->how == kExclusive) {  // wake at most 1 writer
+        break;
+      }
+    } else {                // not waking this one; skip
+      pw = Skip(w);       // skip as much as possible
+      skipped = true;
+    }
+    w = pw->next;
+    // We want to stop processing after we've considered the original head,
+    // orig_h.  We can't test for w==orig_h in the loop because w may skip over
+    // it; we are guaranteed only that w's predecessor will not skip over
+    // orig_h.  When we've considered orig_h, either we've processed it and
+    // removed it (so orig_h != head), or we considered it and skipped it (so
+    // skipped==true && pw == head because skipping from head always skips by
+    // just one, leaving pw pointing at head).  So we want to
+    // continue the loop with the negation of that expression.
+  } while (orig_h == head && (pw != head || !skipped));
+  return head;
+}
+
+// Try to remove thread s from the list of waiters on this mutex.
+// Does nothing if s is not on the waiter list.
+void Mutex::TryRemove(PerThreadSynch *s) {
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  // acquire spinlock & lock
+  if ((v & (kMuWait | kMuSpin | kMuWriter | kMuReader)) == kMuWait &&
+      mu_.compare_exchange_strong(v, v | kMuSpin | kMuWriter,
+                                  std::memory_order_acquire,
+                                  std::memory_order_relaxed)) {
+    PerThreadSynch *h = GetPerThreadSynch(v);
+    if (h != nullptr) {
+      PerThreadSynch *pw = h;   // pw is w's predecessor
+      PerThreadSynch *w;
+      if ((w = pw->next) != s) {  // search for thread,
+        do {                      // processing at least one element
+          if (!MuSameCondition(s, w)) {  // seeking different condition
+            pw = Skip(w);                // so skip all that won't match
+            // we don't have to worry about dangling skip fields
+            // in the threads we skipped; none can point to s
+            // because their condition differs from s
+          } else {          // seeking same condition
+            FixSkip(w, s);  // fix up any skip pointer from w to s
+            pw = w;
+          }
+          // don't search further if we found the thread, or we're about to
+          // process the first thread again.
+        } while ((w = pw->next) != s && pw != h);
+      }
+      if (w == s) {                 // found thread; remove it
+        // pw->skip may be non-zero here; the loop above ensured that
+        // no ancestor of s can skip to s, so removal is safe anyway.
+        h = Dequeue(h, pw);
+        s->next = nullptr;
+        s->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
+      }
+    }
+    intptr_t nv;
+    do {                        // release spinlock and lock
+      v = mu_.load(std::memory_order_relaxed);
+      nv = v & (kMuDesig | kMuEvent);
+      if (h != nullptr) {
+        nv |= kMuWait | reinterpret_cast<intptr_t>(h);
+        h->readers = 0;            // we hold writer lock
+        h->maybe_unlocking = false;  // finished unlocking
+      }
+    } while (!mu_.compare_exchange_weak(v, nv,
+                                        std::memory_order_release,
+                                        std::memory_order_relaxed));
+  }
+}
+
+// Wait until thread "s", which must be the current thread, is removed from the
+// this mutex's waiter queue.  If "s->waitp->timeout" has a timeout, wake up
+// if the wait extends past the absolute time specified, even if "s" is still
+// on the mutex queue.  In this case, remove "s" from the queue and return
+// true, otherwise return false.
+ABSL_XRAY_LOG_ARGS(1) void Mutex::Block(PerThreadSynch *s) {
+  while (s->state.load(std::memory_order_acquire) == PerThreadSynch::kQueued) {
+    if (!DecrementSynchSem(this, s, s->waitp->timeout)) {
+      // After a timeout, we go into a spin loop until we remove ourselves
+      // from the queue, or someone else removes us.  We can't be sure to be
+      // able to remove ourselves in a single lock acquisition because this
+      // mutex may be held, and the holder has the right to read the centre
+      // of the waiter queue without holding the spinlock.
+      this->TryRemove(s);
+      int c = 0;
+      while (s->next != nullptr) {
+        c = Delay(c, GENTLE);
+        this->TryRemove(s);
+      }
+      if (kDebugMode) {
+        // This ensures that we test the case that TryRemove() is called when s
+        // is not on the queue.
+        this->TryRemove(s);
+      }
+      s->waitp->timeout = KernelTimeout::Never();      // timeout is satisfied
+      s->waitp->cond = nullptr;  // condition no longer relevant for wakeups
+    }
+  }
+  ABSL_RAW_CHECK(s->waitp != nullptr || s->suppress_fatal_errors,
+                 "detected illegal recursion in Mutex code");
+  s->waitp = nullptr;
+}
+
+// Wake thread w, and return the next thread in the list.
+PerThreadSynch *Mutex::Wakeup(PerThreadSynch *w) {
+  PerThreadSynch *next = w->next;
+  w->next = nullptr;
+  w->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
+  IncrementSynchSem(this, w);
+
+  return next;
+}
+
+static GraphId GetGraphIdLocked(Mutex *mu)
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(deadlock_graph_mu) {
+  if (!deadlock_graph) {  // (re)create the deadlock graph.
+    deadlock_graph =
+        new (base_internal::LowLevelAlloc::Alloc(sizeof(*deadlock_graph)))
+            GraphCycles;
+  }
+  return deadlock_graph->GetId(mu);
+}
+
+static GraphId GetGraphId(Mutex *mu) ABSL_LOCKS_EXCLUDED(deadlock_graph_mu) {
+  deadlock_graph_mu.Lock();
+  GraphId id = GetGraphIdLocked(mu);
+  deadlock_graph_mu.Unlock();
+  return id;
+}
+
+// Record a lock acquisition.  This is used in debug mode for deadlock
+// detection.  The held_locks pointer points to the relevant data
+// structure for each case.
+static void LockEnter(Mutex* mu, GraphId id, SynchLocksHeld *held_locks) {
+  int n = held_locks->n;
+  int i = 0;
+  while (i != n && held_locks->locks[i].id != id) {
+    i++;
+  }
+  if (i == n) {
+    if (n == ABSL_ARRAYSIZE(held_locks->locks)) {
+      held_locks->overflow = true;  // lost some data
+    } else {                        // we have room for lock
+      held_locks->locks[i].mu = mu;
+      held_locks->locks[i].count = 1;
+      held_locks->locks[i].id = id;
+      held_locks->n = n + 1;
+    }
+  } else {
+    held_locks->locks[i].count++;
+  }
+}
+
+// Record a lock release.  Each call to LockEnter(mu, id, x) should be
+// eventually followed by a call to LockLeave(mu, id, x) by the same thread.
+// It does not process the event if is not needed when deadlock detection is
+// disabled.
+static void LockLeave(Mutex* mu, GraphId id, SynchLocksHeld *held_locks) {
+  int n = held_locks->n;
+  int i = 0;
+  while (i != n && held_locks->locks[i].id != id) {
+    i++;
+  }
+  if (i == n) {
+    if (!held_locks->overflow) {
+      // The deadlock id may have been reassigned after ForgetDeadlockInfo,
+      // but in that case mu should still be present.
+      i = 0;
+      while (i != n && held_locks->locks[i].mu != mu) {
+        i++;
+      }
+      if (i == n) {  // mu missing means releasing unheld lock
+        SynchEvent *mu_events = GetSynchEvent(mu);
+        ABSL_RAW_LOG(FATAL,
+                     "thread releasing lock it does not hold: %p %s; "
+                     ,
+                     static_cast<void *>(mu),
+                     mu_events == nullptr ? "" : mu_events->name);
+      }
+    }
+  } else if (held_locks->locks[i].count == 1) {
+    held_locks->n = n - 1;
+    held_locks->locks[i] = held_locks->locks[n - 1];
+    held_locks->locks[n - 1].id = InvalidGraphId();
+    held_locks->locks[n - 1].mu =
+        nullptr;  // clear mu to please the leak detector.
+  } else {
+    assert(held_locks->locks[i].count > 0);
+    held_locks->locks[i].count--;
+  }
+}
+
+// Call LockEnter() if in debug mode and deadlock detection is enabled.
+static inline void DebugOnlyLockEnter(Mutex *mu) {
+  if (kDebugMode) {
+    if (synch_deadlock_detection.load(std::memory_order_acquire) !=
+        OnDeadlockCycle::kIgnore) {
+      LockEnter(mu, GetGraphId(mu), Synch_GetAllLocks());
+    }
+  }
+}
+
+// Call LockEnter() if in debug mode and deadlock detection is enabled.
+static inline void DebugOnlyLockEnter(Mutex *mu, GraphId id) {
+  if (kDebugMode) {
+    if (synch_deadlock_detection.load(std::memory_order_acquire) !=
+        OnDeadlockCycle::kIgnore) {
+      LockEnter(mu, id, Synch_GetAllLocks());
+    }
+  }
+}
+
+// Call LockLeave() if in debug mode and deadlock detection is enabled.
+static inline void DebugOnlyLockLeave(Mutex *mu) {
+  if (kDebugMode) {
+    if (synch_deadlock_detection.load(std::memory_order_acquire) !=
+        OnDeadlockCycle::kIgnore) {
+      LockLeave(mu, GetGraphId(mu), Synch_GetAllLocks());
+    }
+  }
+}
+
+static char *StackString(void **pcs, int n, char *buf, int maxlen,
+                         bool symbolize) {
+  static const int kSymLen = 200;
+  char sym[kSymLen];
+  int len = 0;
+  for (int i = 0; i != n; i++) {
+    if (symbolize) {
+      if (!symbolizer(pcs[i], sym, kSymLen)) {
+        sym[0] = '\0';
+      }
+      snprintf(buf + len, maxlen - len, "%s\t@ %p %s\n",
+               (i == 0 ? "\n" : ""),
+               pcs[i], sym);
+    } else {
+      snprintf(buf + len, maxlen - len, " %p", pcs[i]);
+    }
+    len += strlen(&buf[len]);
+  }
+  return buf;
+}
+
+static char *CurrentStackString(char *buf, int maxlen, bool symbolize) {
+  void *pcs[40];
+  return StackString(pcs, absl::GetStackTrace(pcs, ABSL_ARRAYSIZE(pcs), 2), buf,
+                     maxlen, symbolize);
+}
+
+namespace {
+enum { kMaxDeadlockPathLen = 10 };  // maximum length of a deadlock cycle;
+                                    // a path this long would be remarkable
+// Buffers required to report a deadlock.
+// We do not allocate them on stack to avoid large stack frame.
+struct DeadlockReportBuffers {
+  char buf[6100];
+  GraphId path[kMaxDeadlockPathLen];
+};
+
+struct ScopedDeadlockReportBuffers {
+  ScopedDeadlockReportBuffers() {
+    b = reinterpret_cast<DeadlockReportBuffers *>(
+        base_internal::LowLevelAlloc::Alloc(sizeof(*b)));
+  }
+  ~ScopedDeadlockReportBuffers() { base_internal::LowLevelAlloc::Free(b); }
+  DeadlockReportBuffers *b;
+};
+
+// Helper to pass to GraphCycles::UpdateStackTrace.
+int GetStack(void** stack, int max_depth) {
+  return absl::GetStackTrace(stack, max_depth, 3);
+}
+}  // anonymous namespace
+
+// Called in debug mode when a thread is about to acquire a lock in a way that
+// may block.
+static GraphId DeadlockCheck(Mutex *mu) {
+  if (synch_deadlock_detection.load(std::memory_order_acquire) ==
+      OnDeadlockCycle::kIgnore) {
+    return InvalidGraphId();
+  }
+
+  SynchLocksHeld *all_locks = Synch_GetAllLocks();
+
+  absl::base_internal::SpinLockHolder lock(&deadlock_graph_mu);
+  const GraphId mu_id = GetGraphIdLocked(mu);
+
+  if (all_locks->n == 0) {
+    // There are no other locks held. Return now so that we don't need to
+    // call GetSynchEvent(). This way we do not record the stack trace
+    // for this Mutex. It's ok, since if this Mutex is involved in a deadlock,
+    // it can't always be the first lock acquired by a thread.
+    return mu_id;
+  }
+
+  // We prefer to keep stack traces that show a thread holding and acquiring
+  // as many locks as possible.  This increases the chances that a given edge
+  // in the acquires-before graph will be represented in the stack traces
+  // recorded for the locks.
+  deadlock_graph->UpdateStackTrace(mu_id, all_locks->n + 1, GetStack);
+
+  // For each other mutex already held by this thread:
+  for (int i = 0; i != all_locks->n; i++) {
+    const GraphId other_node_id = all_locks->locks[i].id;
+    const Mutex *other =
+        static_cast<const Mutex *>(deadlock_graph->Ptr(other_node_id));
+    if (other == nullptr) {
+      // Ignore stale lock
+      continue;
+    }
+
+    // Add the acquired-before edge to the graph.
+    if (!deadlock_graph->InsertEdge(other_node_id, mu_id)) {
+      ScopedDeadlockReportBuffers scoped_buffers;
+      DeadlockReportBuffers *b = scoped_buffers.b;
+      static int number_of_reported_deadlocks = 0;
+      number_of_reported_deadlocks++;
+      // Symbolize only 2 first deadlock report to avoid huge slowdowns.
+      bool symbolize = number_of_reported_deadlocks <= 2;
+      ABSL_RAW_LOG(ERROR, "Potential Mutex deadlock: %s",
+                   CurrentStackString(b->buf, sizeof (b->buf), symbolize));
+      int len = 0;
+      for (int j = 0; j != all_locks->n; j++) {
+        void* pr = deadlock_graph->Ptr(all_locks->locks[j].id);
+        if (pr != nullptr) {
+          snprintf(b->buf + len, sizeof (b->buf) - len, " %p", pr);
+          len += static_cast<int>(strlen(&b->buf[len]));
+        }
+      }
+      ABSL_RAW_LOG(ERROR, "Acquiring %p    Mutexes held: %s",
+                   static_cast<void *>(mu), b->buf);
+      ABSL_RAW_LOG(ERROR, "Cycle: ");
+      int path_len = deadlock_graph->FindPath(
+          mu_id, other_node_id, ABSL_ARRAYSIZE(b->path), b->path);
+      for (int j = 0; j != path_len; j++) {
+        GraphId id = b->path[j];
+        Mutex *path_mu = static_cast<Mutex *>(deadlock_graph->Ptr(id));
+        if (path_mu == nullptr) continue;
+        void** stack;
+        int depth = deadlock_graph->GetStackTrace(id, &stack);
+        snprintf(b->buf, sizeof(b->buf),
+                 "mutex@%p stack: ", static_cast<void *>(path_mu));
+        StackString(stack, depth, b->buf + strlen(b->buf),
+                    static_cast<int>(sizeof(b->buf) - strlen(b->buf)),
+                    symbolize);
+        ABSL_RAW_LOG(ERROR, "%s", b->buf);
+      }
+      if (synch_deadlock_detection.load(std::memory_order_acquire) ==
+          OnDeadlockCycle::kAbort) {
+        deadlock_graph_mu.Unlock();  // avoid deadlock in fatal sighandler
+        ABSL_RAW_LOG(FATAL, "dying due to potential deadlock");
+        return mu_id;
+      }
+      break;   // report at most one potential deadlock per acquisition
+    }
+  }
+
+  return mu_id;
+}
+
+// Invoke DeadlockCheck() iff we're in debug mode and
+// deadlock checking has been enabled.
+static inline GraphId DebugOnlyDeadlockCheck(Mutex *mu) {
+  if (kDebugMode && synch_deadlock_detection.load(std::memory_order_acquire) !=
+                        OnDeadlockCycle::kIgnore) {
+    return DeadlockCheck(mu);
+  } else {
+    return InvalidGraphId();
+  }
+}
+
+void Mutex::ForgetDeadlockInfo() {
+  if (kDebugMode && synch_deadlock_detection.load(std::memory_order_acquire) !=
+                        OnDeadlockCycle::kIgnore) {
+    deadlock_graph_mu.Lock();
+    if (deadlock_graph != nullptr) {
+      deadlock_graph->RemoveNode(this);
+    }
+    deadlock_graph_mu.Unlock();
+  }
+}
+
+void Mutex::AssertNotHeld() const {
+  // We have the data to allow this check only if in debug mode and deadlock
+  // detection is enabled.
+  if (kDebugMode &&
+      (mu_.load(std::memory_order_relaxed) & (kMuWriter | kMuReader)) != 0 &&
+      synch_deadlock_detection.load(std::memory_order_acquire) !=
+          OnDeadlockCycle::kIgnore) {
+    GraphId id = GetGraphId(const_cast<Mutex *>(this));
+    SynchLocksHeld *locks = Synch_GetAllLocks();
+    for (int i = 0; i != locks->n; i++) {
+      if (locks->locks[i].id == id) {
+        SynchEvent *mu_events = GetSynchEvent(this);
+        ABSL_RAW_LOG(FATAL, "thread should not hold mutex %p %s",
+                     static_cast<const void *>(this),
+                     (mu_events == nullptr ? "" : mu_events->name));
+      }
+    }
+  }
+}
+
+// Attempt to acquire *mu, and return whether successful.  The implementation
+// may spin for a short while if the lock cannot be acquired immediately.
+static bool TryAcquireWithSpinning(std::atomic<intptr_t>* mu) {
+  int c = mutex_globals.spinloop_iterations;
+  int result = -1;  // result of operation:  0=false, 1=true, -1=unknown
+
+  do {  // do/while somewhat faster on AMD
+    intptr_t v = mu->load(std::memory_order_relaxed);
+    if ((v & (kMuReader|kMuEvent)) != 0) {  // a reader or tracing -> give up
+      result = 0;
+    } else if (((v & kMuWriter) == 0) &&  // no holder -> try to acquire
+               mu->compare_exchange_strong(v, kMuWriter | v,
+                                           std::memory_order_acquire,
+                                           std::memory_order_relaxed)) {
+      result = 1;
+    }
+  } while (result == -1 && --c > 0);
+  return result == 1;
+}
+
+ABSL_XRAY_LOG_ARGS(1) void Mutex::Lock() {
+  ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
+  GraphId id = DebugOnlyDeadlockCheck(this);
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  // try fast acquire, then spin loop
+  if ((v & (kMuWriter | kMuReader | kMuEvent)) != 0 ||
+      !mu_.compare_exchange_strong(v, kMuWriter | v,
+                                   std::memory_order_acquire,
+                                   std::memory_order_relaxed)) {
+    // try spin acquire, then slow loop
+    if (!TryAcquireWithSpinning(&this->mu_)) {
+      this->LockSlow(kExclusive, nullptr, 0);
+    }
+  }
+  DebugOnlyLockEnter(this, id);
+  ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);
+}
+
+ABSL_XRAY_LOG_ARGS(1) void Mutex::ReaderLock() {
+  ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_read_lock);
+  GraphId id = DebugOnlyDeadlockCheck(this);
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  // try fast acquire, then slow loop
+  if ((v & (kMuWriter | kMuWait | kMuEvent)) != 0 ||
+      !mu_.compare_exchange_strong(v, (kMuReader | v) + kMuOne,
+                                   std::memory_order_acquire,
+                                   std::memory_order_relaxed)) {
+    this->LockSlow(kShared, nullptr, 0);
+  }
+  DebugOnlyLockEnter(this, id);
+  ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_read_lock, 0);
+}
+
+void Mutex::LockWhen(const Condition &cond) {
+  ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
+  GraphId id = DebugOnlyDeadlockCheck(this);
+  this->LockSlow(kExclusive, &cond, 0);
+  DebugOnlyLockEnter(this, id);
+  ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);
+}
+
+bool Mutex::LockWhenWithTimeout(const Condition &cond, absl::Duration timeout) {
+  return LockWhenWithDeadline(cond, DeadlineFromTimeout(timeout));
+}
+
+bool Mutex::LockWhenWithDeadline(const Condition &cond, absl::Time deadline) {
+  ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
+  GraphId id = DebugOnlyDeadlockCheck(this);
+  bool res = LockSlowWithDeadline(kExclusive, &cond,
+                                  KernelTimeout(deadline), 0);
+  DebugOnlyLockEnter(this, id);
+  ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);
+  return res;
+}
+
+void Mutex::ReaderLockWhen(const Condition &cond) {
+  ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_read_lock);
+  GraphId id = DebugOnlyDeadlockCheck(this);
+  this->LockSlow(kShared, &cond, 0);
+  DebugOnlyLockEnter(this, id);
+  ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_read_lock, 0);
+}
+
+bool Mutex::ReaderLockWhenWithTimeout(const Condition &cond,
+                                      absl::Duration timeout) {
+  return ReaderLockWhenWithDeadline(cond, DeadlineFromTimeout(timeout));
+}
+
+bool Mutex::ReaderLockWhenWithDeadline(const Condition &cond,
+                                       absl::Time deadline) {
+  ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_read_lock);
+  GraphId id = DebugOnlyDeadlockCheck(this);
+  bool res = LockSlowWithDeadline(kShared, &cond, KernelTimeout(deadline), 0);
+  DebugOnlyLockEnter(this, id);
+  ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_read_lock, 0);
+  return res;
+}
+
+void Mutex::Await(const Condition &cond) {
+  if (cond.Eval()) {    // condition already true; nothing to do
+    if (kDebugMode) {
+      this->AssertReaderHeld();
+    }
+  } else {              // normal case
+    ABSL_RAW_CHECK(this->AwaitCommon(cond, KernelTimeout::Never()),
+                   "condition untrue on return from Await");
+  }
+}
+
+bool Mutex::AwaitWithTimeout(const Condition &cond, absl::Duration timeout) {
+  return AwaitWithDeadline(cond, DeadlineFromTimeout(timeout));
+}
+
+bool Mutex::AwaitWithDeadline(const Condition &cond, absl::Time deadline) {
+  if (cond.Eval()) {      // condition already true; nothing to do
+    if (kDebugMode) {
+      this->AssertReaderHeld();
+    }
+    return true;
+  }
+
+  KernelTimeout t{deadline};
+  bool res = this->AwaitCommon(cond, t);
+  ABSL_RAW_CHECK(res || t.has_timeout(),
+                 "condition untrue on return from Await");
+  return res;
+}
+
+bool Mutex::AwaitCommon(const Condition &cond, KernelTimeout t) {
+  this->AssertReaderHeld();
+  MuHow how =
+      (mu_.load(std::memory_order_relaxed) & kMuWriter) ? kExclusive : kShared;
+  ABSL_TSAN_MUTEX_PRE_UNLOCK(this, TsanFlags(how));
+  SynchWaitParams waitp(
+      how, &cond, t, nullptr /*no cvmu*/, Synch_GetPerThreadAnnotated(this),
+      nullptr /*no cv_word*/);
+  int flags = kMuHasBlocked;
+  if (!Condition::GuaranteedEqual(&cond, nullptr)) {
+    flags |= kMuIsCond;
+  }
+  this->UnlockSlow(&waitp);
+  this->Block(waitp.thread);
+  ABSL_TSAN_MUTEX_POST_UNLOCK(this, TsanFlags(how));
+  ABSL_TSAN_MUTEX_PRE_LOCK(this, TsanFlags(how));
+  this->LockSlowLoop(&waitp, flags);
+  bool res = waitp.cond != nullptr ||  // => cond known true from LockSlowLoop
+             EvalConditionAnnotated(&cond, this, true, false, how == kShared);
+  ABSL_TSAN_MUTEX_POST_LOCK(this, TsanFlags(how), 0);
+  return res;
+}
+
+ABSL_XRAY_LOG_ARGS(1) bool Mutex::TryLock() {
+  ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_try_lock);
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  if ((v & (kMuWriter | kMuReader | kMuEvent)) == 0 &&  // try fast acquire
+      mu_.compare_exchange_strong(v, kMuWriter | v,
+                                  std::memory_order_acquire,
+                                  std::memory_order_relaxed)) {
+    DebugOnlyLockEnter(this);
+    ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_try_lock, 0);
+    return true;
+  }
+  if ((v & kMuEvent) != 0) {              // we're recording events
+    if ((v & kExclusive->slow_need_zero) == 0 &&  // try fast acquire
+        mu_.compare_exchange_strong(
+            v, (kExclusive->fast_or | v) + kExclusive->fast_add,
+            std::memory_order_acquire, std::memory_order_relaxed)) {
+      DebugOnlyLockEnter(this);
+      PostSynchEvent(this, SYNCH_EV_TRYLOCK_SUCCESS);
+      ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_try_lock, 0);
+      return true;
+    } else {
+      PostSynchEvent(this, SYNCH_EV_TRYLOCK_FAILED);
+    }
+  }
+  ABSL_TSAN_MUTEX_POST_LOCK(
+      this, __tsan_mutex_try_lock | __tsan_mutex_try_lock_failed, 0);
+  return false;
+}
+
+ABSL_XRAY_LOG_ARGS(1) bool Mutex::ReaderTryLock() {
+  ABSL_TSAN_MUTEX_PRE_LOCK(this,
+                           __tsan_mutex_read_lock | __tsan_mutex_try_lock);
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  // The while-loops (here and below) iterate only if the mutex word keeps
+  // changing (typically because the reader count changes) under the CAS.  We
+  // limit the number of attempts to avoid having to think about livelock.
+  int loop_limit = 5;
+  while ((v & (kMuWriter|kMuWait|kMuEvent)) == 0 && loop_limit != 0) {
+    if (mu_.compare_exchange_strong(v, (kMuReader | v) + kMuOne,
+                                    std::memory_order_acquire,
+                                    std::memory_order_relaxed)) {
+      DebugOnlyLockEnter(this);
+      ABSL_TSAN_MUTEX_POST_LOCK(
+          this, __tsan_mutex_read_lock | __tsan_mutex_try_lock, 0);
+      return true;
+    }
+    loop_limit--;
+    v = mu_.load(std::memory_order_relaxed);
+  }
+  if ((v & kMuEvent) != 0) {   // we're recording events
+    loop_limit = 5;
+    while ((v & kShared->slow_need_zero) == 0 && loop_limit != 0) {
+      if (mu_.compare_exchange_strong(v, (kMuReader | v) + kMuOne,
+                                      std::memory_order_acquire,
+                                      std::memory_order_relaxed)) {
+        DebugOnlyLockEnter(this);
+        PostSynchEvent(this, SYNCH_EV_READERTRYLOCK_SUCCESS);
+        ABSL_TSAN_MUTEX_POST_LOCK(
+            this, __tsan_mutex_read_lock | __tsan_mutex_try_lock, 0);
+        return true;
+      }
+      loop_limit--;
+      v = mu_.load(std::memory_order_relaxed);
+    }
+    if ((v & kMuEvent) != 0) {
+      PostSynchEvent(this, SYNCH_EV_READERTRYLOCK_FAILED);
+    }
+  }
+  ABSL_TSAN_MUTEX_POST_LOCK(this,
+                            __tsan_mutex_read_lock | __tsan_mutex_try_lock |
+                                __tsan_mutex_try_lock_failed,
+                            0);
+  return false;
+}
+
+ABSL_XRAY_LOG_ARGS(1) void Mutex::Unlock() {
+  ABSL_TSAN_MUTEX_PRE_UNLOCK(this, 0);
+  DebugOnlyLockLeave(this);
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+
+  if (kDebugMode && ((v & (kMuWriter | kMuReader)) != kMuWriter)) {
+    ABSL_RAW_LOG(FATAL, "Mutex unlocked when destroyed or not locked: v=0x%x",
+                 static_cast<unsigned>(v));
+  }
+
+  // should_try_cas is whether we'll try a compare-and-swap immediately.
+  // NOTE: optimized out when kDebugMode is false.
+  bool should_try_cas = ((v & (kMuEvent | kMuWriter)) == kMuWriter &&
+                          (v & (kMuWait | kMuDesig)) != kMuWait);
+  // But, we can use an alternate computation of it, that compilers
+  // currently don't find on their own.  When that changes, this function
+  // can be simplified.
+  intptr_t x = (v ^ (kMuWriter | kMuWait)) & (kMuWriter | kMuEvent);
+  intptr_t y = (v ^ (kMuWriter | kMuWait)) & (kMuWait | kMuDesig);
+  // Claim: "x == 0 && y > 0" is equal to should_try_cas.
+  // Also, because kMuWriter and kMuEvent exceed kMuDesig and kMuWait,
+  // all possible non-zero values for x exceed all possible values for y.
+  // Therefore, (x == 0 && y > 0) == (x < y).
+  if (kDebugMode && should_try_cas != (x < y)) {
+    // We would usually use PRIdPTR here, but is not correctly implemented
+    // within the android toolchain.
+    ABSL_RAW_LOG(FATAL, "internal logic error %llx %llx %llx\n",
+                 static_cast<long long>(v), static_cast<long long>(x),
+                 static_cast<long long>(y));
+  }
+  if (x < y &&
+      mu_.compare_exchange_strong(v, v & ~(kMuWrWait | kMuWriter),
+                                  std::memory_order_release,
+                                  std::memory_order_relaxed)) {
+    // fast writer release (writer with no waiters or with designated waker)
+  } else {
+    this->UnlockSlow(nullptr /*no waitp*/);  // take slow path
+  }
+  ABSL_TSAN_MUTEX_POST_UNLOCK(this, 0);
+}
+
+// Requires v to represent a reader-locked state.
+static bool ExactlyOneReader(intptr_t v) {
+  assert((v & (kMuWriter|kMuReader)) == kMuReader);
+  assert((v & kMuHigh) != 0);
+  // The more straightforward "(v & kMuHigh) == kMuOne" also works, but
+  // on some architectures the following generates slightly smaller code.
+  // It may be faster too.
+  constexpr intptr_t kMuMultipleWaitersMask = kMuHigh ^ kMuOne;
+  return (v & kMuMultipleWaitersMask) == 0;
+}
+
+ABSL_XRAY_LOG_ARGS(1) void Mutex::ReaderUnlock() {
+  ABSL_TSAN_MUTEX_PRE_UNLOCK(this, __tsan_mutex_read_lock);
+  DebugOnlyLockLeave(this);
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  assert((v & (kMuWriter|kMuReader)) == kMuReader);
+  if ((v & (kMuReader|kMuWait|kMuEvent)) == kMuReader) {
+    // fast reader release (reader with no waiters)
+    intptr_t clear = ExactlyOneReader(v) ? kMuReader|kMuOne : kMuOne;
+    if (mu_.compare_exchange_strong(v, v - clear,
+                                    std::memory_order_release,
+                                    std::memory_order_relaxed)) {
+      ABSL_TSAN_MUTEX_POST_UNLOCK(this, __tsan_mutex_read_lock);
+      return;
+    }
+  }
+  this->UnlockSlow(nullptr /*no waitp*/);  // take slow path
+  ABSL_TSAN_MUTEX_POST_UNLOCK(this, __tsan_mutex_read_lock);
+}
+
+// The zap_desig_waker bitmask is used to clear the designated waker flag in
+// the mutex if this thread has blocked, and therefore may be the designated
+// waker.
+static const intptr_t zap_desig_waker[] = {
+    ~static_cast<intptr_t>(0),  // not blocked
+    ~static_cast<intptr_t>(
+        kMuDesig)  // blocked; turn off the designated waker bit
+};
+
+// The ignore_waiting_writers bitmask is used to ignore the existence
+// of waiting writers if a reader that has already blocked once
+// wakes up.
+static const intptr_t ignore_waiting_writers[] = {
+    ~static_cast<intptr_t>(0),  // not blocked
+    ~static_cast<intptr_t>(
+        kMuWrWait)  // blocked; pretend there are no waiting writers
+};
+
+// Internal version of LockWhen().  See LockSlowWithDeadline()
+void Mutex::LockSlow(MuHow how, const Condition *cond, int flags) {
+  ABSL_RAW_CHECK(
+      this->LockSlowWithDeadline(how, cond, KernelTimeout::Never(), flags),
+      "condition untrue on return from LockSlow");
+}
+
+// Compute cond->Eval() and tell race detectors that we do it under mutex mu.
+static inline bool EvalConditionAnnotated(const Condition *cond, Mutex *mu,
+                                          bool locking, bool trylock,
+                                          bool read_lock) {
+  // Delicate annotation dance.
+  // We are currently inside of read/write lock/unlock operation.
+  // All memory accesses are ignored inside of mutex operations + for unlock
+  // operation tsan considers that we've already released the mutex.
+  bool res = false;
+#ifdef THREAD_SANITIZER
+  const int flags = read_lock ? __tsan_mutex_read_lock : 0;
+  const int tryflags = flags | (trylock ? __tsan_mutex_try_lock : 0);
+#endif
+  if (locking) {
+    // For lock we pretend that we have finished the operation,
+    // evaluate the predicate, then unlock the mutex and start locking it again
+    // to match the annotation at the end of outer lock operation.
+    // Note: we can't simply do POST_LOCK, Eval, PRE_LOCK, because then tsan
+    // will think the lock acquisition is recursive which will trigger
+    // deadlock detector.
+    ABSL_TSAN_MUTEX_POST_LOCK(mu, tryflags, 0);
+    res = cond->Eval();
+    // There is no "try" version of Unlock, so use flags instead of tryflags.
+    ABSL_TSAN_MUTEX_PRE_UNLOCK(mu, flags);
+    ABSL_TSAN_MUTEX_POST_UNLOCK(mu, flags);
+    ABSL_TSAN_MUTEX_PRE_LOCK(mu, tryflags);
+  } else {
+    // Similarly, for unlock we pretend that we have unlocked the mutex,
+    // lock the mutex, evaluate the predicate, and start unlocking it again
+    // to match the annotation at the end of outer unlock operation.
+    ABSL_TSAN_MUTEX_POST_UNLOCK(mu, flags);
+    ABSL_TSAN_MUTEX_PRE_LOCK(mu, flags);
+    ABSL_TSAN_MUTEX_POST_LOCK(mu, flags, 0);
+    res = cond->Eval();
+    ABSL_TSAN_MUTEX_PRE_UNLOCK(mu, flags);
+  }
+  // Prevent unused param warnings in non-TSAN builds.
+  static_cast<void>(mu);
+  static_cast<void>(trylock);
+  static_cast<void>(read_lock);
+  return res;
+}
+
+// Compute cond->Eval() hiding it from race detectors.
+// We are hiding it because inside of UnlockSlow we can evaluate a predicate
+// that was just added by a concurrent Lock operation; Lock adds the predicate
+// to the internal Mutex list without actually acquiring the Mutex
+// (it only acquires the internal spinlock, which is rightfully invisible for
+// tsan). As the result there is no tsan-visible synchronization between the
+// addition and this thread. So if we would enable race detection here,
+// it would race with the predicate initialization.
+static inline bool EvalConditionIgnored(Mutex *mu, const Condition *cond) {
+  // Memory accesses are already ignored inside of lock/unlock operations,
+  // but synchronization operations are also ignored. When we evaluate the
+  // predicate we must ignore only memory accesses but not synchronization,
+  // because missed synchronization can lead to false reports later.
+  // So we "divert" (which un-ignores both memory accesses and synchronization)
+  // and then separately turn on ignores of memory accesses.
+  ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
+  ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN();
+  bool res = cond->Eval();
+  ANNOTATE_IGNORE_READS_AND_WRITES_END();
+  ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
+  static_cast<void>(mu);  // Prevent unused param warning in non-TSAN builds.
+  return res;
+}
+
+// Internal equivalent of *LockWhenWithDeadline(), where
+//   "t" represents the absolute timeout; !t.has_timeout() means "forever".
+//   "how" is "kShared" (for ReaderLockWhen) or "kExclusive" (for LockWhen)
+// In flags, bits are ored together:
+// - kMuHasBlocked indicates that the client has already blocked on the call so
+//   the designated waker bit must be cleared and waiting writers should not
+//   obstruct this call
+// - kMuIsCond indicates that this is a conditional acquire (condition variable,
+//   Await,  LockWhen) so contention profiling should be suppressed.
+bool Mutex::LockSlowWithDeadline(MuHow how, const Condition *cond,
+                                 KernelTimeout t, int flags) {
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  bool unlock = false;
+  if ((v & how->fast_need_zero) == 0 &&  // try fast acquire
+      mu_.compare_exchange_strong(
+          v, (how->fast_or | (v & zap_desig_waker[flags & kMuHasBlocked])) +
+                 how->fast_add,
+          std::memory_order_acquire, std::memory_order_relaxed)) {
+    if (cond == nullptr ||
+        EvalConditionAnnotated(cond, this, true, false, how == kShared)) {
+      return true;
+    }
+    unlock = true;
+  }
+  SynchWaitParams waitp(
+      how, cond, t, nullptr /*no cvmu*/, Synch_GetPerThreadAnnotated(this),
+      nullptr /*no cv_word*/);
+  if (!Condition::GuaranteedEqual(cond, nullptr)) {
+    flags |= kMuIsCond;
+  }
+  if (unlock) {
+    this->UnlockSlow(&waitp);
+    this->Block(waitp.thread);
+    flags |= kMuHasBlocked;
+  }
+  this->LockSlowLoop(&waitp, flags);
+  return waitp.cond != nullptr ||  // => cond known true from LockSlowLoop
+         cond == nullptr ||
+         EvalConditionAnnotated(cond, this, true, false, how == kShared);
+}
+
+// RAW_CHECK_FMT() takes a condition, a printf-style format string, and
+// the printf-style argument list.   The format string must be a literal.
+// Arguments after the first are not evaluated unless the condition is true.
+#define RAW_CHECK_FMT(cond, ...)                                   \
+  do {                                                             \
+    if (ABSL_PREDICT_FALSE(!(cond))) {                             \
+      ABSL_RAW_LOG(FATAL, "Check " #cond " failed: " __VA_ARGS__); \
+    }                                                              \
+  } while (0)
+
+static void CheckForMutexCorruption(intptr_t v, const char* label) {
+  // Test for either of two situations that should not occur in v:
+  //   kMuWriter and kMuReader
+  //   kMuWrWait and !kMuWait
+  const uintptr_t w = v ^ kMuWait;
+  // By flipping that bit, we can now test for:
+  //   kMuWriter and kMuReader in w
+  //   kMuWrWait and kMuWait in w
+  // We've chosen these two pairs of values to be so that they will overlap,
+  // respectively, when the word is left shifted by three.  This allows us to
+  // save a branch in the common (correct) case of them not being coincident.
+  static_assert(kMuReader << 3 == kMuWriter, "must match");
+  static_assert(kMuWait << 3 == kMuWrWait, "must match");
+  if (ABSL_PREDICT_TRUE((w & (w << 3) & (kMuWriter | kMuWrWait)) == 0)) return;
+  RAW_CHECK_FMT((v & (kMuWriter | kMuReader)) != (kMuWriter | kMuReader),
+                "%s: Mutex corrupt: both reader and writer lock held: %p",
+                label, reinterpret_cast<void *>(v));
+  RAW_CHECK_FMT((v & (kMuWait | kMuWrWait)) != kMuWrWait,
+                "%s: Mutex corrupt: waiting writer with no waiters: %p",
+                label, reinterpret_cast<void *>(v));
+  assert(false);
+}
+
+void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
+  int c = 0;
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  if ((v & kMuEvent) != 0) {
+    PostSynchEvent(this,
+         waitp->how == kExclusive?  SYNCH_EV_LOCK: SYNCH_EV_READERLOCK);
+  }
+  ABSL_RAW_CHECK(
+      waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
+      "detected illegal recursion into Mutex code");
+  for (;;) {
+    v = mu_.load(std::memory_order_relaxed);
+    CheckForMutexCorruption(v, "Lock");
+    if ((v & waitp->how->slow_need_zero) == 0) {
+      if (mu_.compare_exchange_strong(
+              v, (waitp->how->fast_or |
+                  (v & zap_desig_waker[flags & kMuHasBlocked])) +
+                     waitp->how->fast_add,
+              std::memory_order_acquire, std::memory_order_relaxed)) {
+        if (waitp->cond == nullptr ||
+            EvalConditionAnnotated(waitp->cond, this, true, false,
+                                   waitp->how == kShared)) {
+          break;  // we timed out, or condition true, so return
+        }
+        this->UnlockSlow(waitp);  // got lock but condition false
+        this->Block(waitp->thread);
+        flags |= kMuHasBlocked;
+        c = 0;
+      }
+    } else {                      // need to access waiter list
+      bool dowait = false;
+      if ((v & (kMuSpin|kMuWait)) == 0) {   // no waiters
+        // This thread tries to become the one and only waiter.
+        PerThreadSynch *new_h = Enqueue(nullptr, waitp, v, flags);
+        intptr_t nv = (v & zap_desig_waker[flags & kMuHasBlocked] & kMuLow) |
+                      kMuWait;
+        ABSL_RAW_CHECK(new_h != nullptr, "Enqueue to empty list failed");
+        if (waitp->how == kExclusive && (v & kMuReader) != 0) {
+          nv |= kMuWrWait;
+        }
+        if (mu_.compare_exchange_strong(
+                v, reinterpret_cast<intptr_t>(new_h) | nv,
+                std::memory_order_release, std::memory_order_relaxed)) {
+          dowait = true;
+        } else {            // attempted Enqueue() failed
+          // zero out the waitp field set by Enqueue()
+          waitp->thread->waitp = nullptr;
+        }
+      } else if ((v & waitp->how->slow_inc_need_zero &
+                  ignore_waiting_writers[flags & kMuHasBlocked]) == 0) {
+        // This is a reader that needs to increment the reader count,
+        // but the count is currently held in the last waiter.
+        if (mu_.compare_exchange_strong(
+                v, (v & zap_desig_waker[flags & kMuHasBlocked]) | kMuSpin |
+                       kMuReader,
+                std::memory_order_acquire, std::memory_order_relaxed)) {
+          PerThreadSynch *h = GetPerThreadSynch(v);
+          h->readers += kMuOne;       // inc reader count in waiter
+          do {                        // release spinlock
+            v = mu_.load(std::memory_order_relaxed);
+          } while (!mu_.compare_exchange_weak(v, (v & ~kMuSpin) | kMuReader,
+                                              std::memory_order_release,
+                                              std::memory_order_relaxed));
+          if (waitp->cond == nullptr ||
+              EvalConditionAnnotated(waitp->cond, this, true, false,
+                                     waitp->how == kShared)) {
+            break;  // we timed out, or condition true, so return
+          }
+          this->UnlockSlow(waitp);           // got lock but condition false
+          this->Block(waitp->thread);
+          flags |= kMuHasBlocked;
+          c = 0;
+        }
+      } else if ((v & kMuSpin) == 0 &&  // attempt to queue ourselves
+                 mu_.compare_exchange_strong(
+                     v, (v & zap_desig_waker[flags & kMuHasBlocked]) | kMuSpin |
+                            kMuWait,
+                     std::memory_order_acquire, std::memory_order_relaxed)) {
+        PerThreadSynch *h = GetPerThreadSynch(v);
+        PerThreadSynch *new_h = Enqueue(h, waitp, v, flags);
+        intptr_t wr_wait = 0;
+        ABSL_RAW_CHECK(new_h != nullptr, "Enqueue to list failed");
+        if (waitp->how == kExclusive && (v & kMuReader) != 0) {
+          wr_wait = kMuWrWait;      // give priority to a waiting writer
+        }
+        do {                        // release spinlock
+          v = mu_.load(std::memory_order_relaxed);
+        } while (!mu_.compare_exchange_weak(
+            v, (v & (kMuLow & ~kMuSpin)) | kMuWait | wr_wait |
+            reinterpret_cast<intptr_t>(new_h),
+            std::memory_order_release, std::memory_order_relaxed));
+        dowait = true;
+      }
+      if (dowait) {
+        this->Block(waitp->thread);  // wait until removed from list or timeout
+        flags |= kMuHasBlocked;
+        c = 0;
+      }
+    }
+    ABSL_RAW_CHECK(
+        waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
+        "detected illegal recursion into Mutex code");
+    c = Delay(c, GENTLE);          // delay, then try again
+  }
+  ABSL_RAW_CHECK(
+      waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
+      "detected illegal recursion into Mutex code");
+  if ((v & kMuEvent) != 0) {
+    PostSynchEvent(this,
+                   waitp->how == kExclusive? SYNCH_EV_LOCK_RETURNING :
+                                      SYNCH_EV_READERLOCK_RETURNING);
+  }
+}
+
+// Unlock this mutex, which is held by the current thread.
+// If waitp is non-zero, it must be the wait parameters for the current thread
+// which holds the lock but is not runnable because its condition is false
+// or it is in the process of blocking on a condition variable; it must requeue
+// itself on the mutex/condvar to wait for its condition to become true.
+void Mutex::UnlockSlow(SynchWaitParams *waitp) {
+  intptr_t v = mu_.load(std::memory_order_relaxed);
+  this->AssertReaderHeld();
+  CheckForMutexCorruption(v, "Unlock");
+  if ((v & kMuEvent) != 0) {
+    PostSynchEvent(this,
+                (v & kMuWriter) != 0? SYNCH_EV_UNLOCK: SYNCH_EV_READERUNLOCK);
+  }
+  int c = 0;
+  // the waiter under consideration to wake, or zero
+  PerThreadSynch *w = nullptr;
+  // the predecessor to w or zero
+  PerThreadSynch *pw = nullptr;
+  // head of the list searched previously, or zero
+  PerThreadSynch *old_h = nullptr;
+  // a condition that's known to be false.
+  const Condition *known_false = nullptr;
+  PerThreadSynch *wake_list = kPerThreadSynchNull;   // list of threads to wake
+  intptr_t wr_wait = 0;        // set to kMuWrWait if we wake a reader and a
+                               // later writer could have acquired the lock
+                               // (starvation avoidance)
+  ABSL_RAW_CHECK(waitp == nullptr || waitp->thread->waitp == nullptr ||
+                     waitp->thread->suppress_fatal_errors,
+                 "detected illegal recursion into Mutex code");
+  // This loop finds threads wake_list to wakeup if any, and removes them from
+  // the list of waiters.  In addition, it places waitp.thread on the queue of
+  // waiters if waitp is non-zero.
+  for (;;) {
+    v = mu_.load(std::memory_order_relaxed);
+    if ((v & kMuWriter) != 0 && (v & (kMuWait | kMuDesig)) != kMuWait &&
+        waitp == nullptr) {
+      // fast writer release (writer with no waiters or with designated waker)
+      if (mu_.compare_exchange_strong(v, v & ~(kMuWrWait | kMuWriter),
+                                      std::memory_order_release,
+                                      std::memory_order_relaxed)) {
+        return;
+      }
+    } else if ((v & (kMuReader | kMuWait)) == kMuReader && waitp == nullptr) {
+      // fast reader release (reader with no waiters)
+      intptr_t clear = ExactlyOneReader(v) ? kMuReader | kMuOne : kMuOne;
+      if (mu_.compare_exchange_strong(v, v - clear,
+                                      std::memory_order_release,
+                                      std::memory_order_relaxed)) {
+        return;
+      }
+    } else if ((v & kMuSpin) == 0 &&  // attempt to get spinlock
+               mu_.compare_exchange_strong(v, v | kMuSpin,
+                                           std::memory_order_acquire,
+                                           std::memory_order_relaxed)) {
+      if ((v & kMuWait) == 0) {       // no one to wake
+        intptr_t nv;
+        bool do_enqueue = true;  // always Enqueue() the first time
+        ABSL_RAW_CHECK(waitp != nullptr,
+                       "UnlockSlow is confused");  // about to sleep
+        do {    // must loop to release spinlock as reader count may change
+          v = mu_.load(std::memory_order_relaxed);
+          // decrement reader count if there are readers
+          intptr_t new_readers = (v >= kMuOne)?  v - kMuOne : v;
+          PerThreadSynch *new_h = nullptr;
+          if (do_enqueue) {
+            // If we are enqueuing on a CondVar (waitp->cv_word != nullptr) then
+            // we must not retry here.  The initial attempt will always have
+            // succeeded, further attempts would enqueue us against *this due to
+            // Fer() handling.
+            do_enqueue = (waitp->cv_word == nullptr);
+            new_h = Enqueue(nullptr, waitp, new_readers, kMuIsCond);
+          }
+          intptr_t clear = kMuWrWait | kMuWriter;  // by default clear write bit
+          if ((v & kMuWriter) == 0 && ExactlyOneReader(v)) {  // last reader
+            clear = kMuWrWait | kMuReader;                    // clear read bit
+          }
+          nv = (v & kMuLow & ~clear & ~kMuSpin);
+          if (new_h != nullptr) {
+            nv |= kMuWait | reinterpret_cast<intptr_t>(new_h);
+          } else {  // new_h could be nullptr if we queued ourselves on a
+                    // CondVar
+            // In that case, we must place the reader count back in the mutex
+            // word, as Enqueue() did not store it in the new waiter.
+            nv |= new_readers & kMuHigh;
+          }
+          // release spinlock & our lock; retry if reader-count changed
+          // (writer count cannot change since we hold lock)
+        } while (!mu_.compare_exchange_weak(v, nv,
+                                            std::memory_order_release,
+                                            std::memory_order_relaxed));
+        break;
+      }
+
+      // There are waiters.
+      // Set h to the head of the circular waiter list.
+      PerThreadSynch *h = GetPerThreadSynch(v);
+      if ((v & kMuReader) != 0 && (h->readers & kMuHigh) > kMuOne) {
+        // a reader but not the last
+        h->readers -= kMuOne;  // release our lock
+        intptr_t nv = v;       // normally just release spinlock
+        if (waitp != nullptr) {  // but waitp!=nullptr => must queue ourselves
+          PerThreadSynch *new_h = Enqueue(h, waitp, v, kMuIsCond);
+          ABSL_RAW_CHECK(new_h != nullptr,
+                         "waiters disappeared during Enqueue()!");
+          nv &= kMuLow;
+          nv |= kMuWait | reinterpret_cast<intptr_t>(new_h);
+        }
+        mu_.store(nv, std::memory_order_release);  // release spinlock
+        // can release with a store because there were waiters
+        break;
+      }
+
+      // Either we didn't search before, or we marked the queue
+      // as "maybe_unlocking" and no one else should have changed it.
+      ABSL_RAW_CHECK(old_h == nullptr || h->maybe_unlocking,
+                     "Mutex queue changed beneath us");
+
+      // The lock is becoming free, and there's a waiter
+      if (old_h != nullptr &&
+          !old_h->may_skip) {                  // we used old_h as a terminator
+        old_h->may_skip = true;                // allow old_h to skip once more
+        ABSL_RAW_CHECK(old_h->skip == nullptr, "illegal skip from head");
+        if (h != old_h && MuSameCondition(old_h, old_h->next)) {
+          old_h->skip = old_h->next;  // old_h not head & can skip to successor
+        }
+      }
+      if (h->next->waitp->how == kExclusive &&
+          Condition::GuaranteedEqual(h->next->waitp->cond, nullptr)) {
+        // easy case: writer with no condition; no need to search
+        pw = h;                       // wake w, the successor of h (=pw)
+        w = h->next;
+        w->wake = true;
+        // We are waking up a writer.  This writer may be racing against
+        // an already awake reader for the lock.  We want the
+        // writer to usually win this race,
+        // because if it doesn't, we can potentially keep taking a reader
+        // perpetually and writers will starve.  Worse than
+        // that, this can also starve other readers if kMuWrWait gets set
+        // later.
+        wr_wait = kMuWrWait;
+      } else if (w != nullptr && (w->waitp->how == kExclusive || h == old_h)) {
+        // we found a waiter w to wake on a previous iteration and either it's
+        // a writer, or we've searched the entire list so we have all the
+        // readers.
+        if (pw == nullptr) {  // if w's predecessor is unknown, it must be h
+          pw = h;
+        }
+      } else {
+        // At this point we don't know all the waiters to wake, and the first
+        // waiter has a condition or is a reader.  We avoid searching over
+        // waiters we've searched on previous iterations by starting at
+        // old_h if it's set.  If old_h==h, there's no one to wakeup at all.
+        if (old_h == h) {      // we've searched before, and nothing's new
+                               // so there's no one to wake.
+          intptr_t nv = (v & ~(kMuReader|kMuWriter|kMuWrWait));
+          h->readers = 0;
+          h->maybe_unlocking = false;   // finished unlocking
+          if (waitp != nullptr) {       // we must queue ourselves and sleep
+            PerThreadSynch *new_h = Enqueue(h, waitp, v, kMuIsCond);
+            nv &= kMuLow;
+            if (new_h != nullptr) {
+              nv |= kMuWait | reinterpret_cast<intptr_t>(new_h);
+            }  // else new_h could be nullptr if we queued ourselves on a
+               // CondVar
+          }
+          // release spinlock & lock
+          // can release with a store because there were waiters
+          mu_.store(nv, std::memory_order_release);
+          break;
+        }
+
+        // set up to walk the list
+        PerThreadSynch *w_walk;   // current waiter during list walk
+        PerThreadSynch *pw_walk;  // previous waiter during list walk
+        if (old_h != nullptr) {  // we've searched up to old_h before
+          pw_walk = old_h;
+          w_walk = old_h->next;
+        } else {            // no prior search, start at beginning
+          pw_walk =
+              nullptr;  // h->next's predecessor may change; don't record it
+          w_walk = h->next;
+        }
+
+        h->may_skip = false;  // ensure we never skip past h in future searches
+                              // even if other waiters are queued after it.
+        ABSL_RAW_CHECK(h->skip == nullptr, "illegal skip from head");
+
+        h->maybe_unlocking = true;  // we're about to scan the waiter list
+                                    // without the spinlock held.
+                                    // Enqueue must be conservative about
+                                    // priority queuing.
+
+        // We must release the spinlock to evaluate the conditions.
+        mu_.store(v, std::memory_order_release);  // release just spinlock
+        // can release with a store because there were waiters
+
+        // h is the last waiter queued, and w_walk the first unsearched waiter.
+        // Without the spinlock, the locations mu_ and h->next may now change
+        // underneath us, but since we hold the lock itself, the only legal
+        // change is to add waiters between h and w_walk.  Therefore, it's safe
+        // to walk the path from w_walk to h inclusive. (TryRemove() can remove
+        // a waiter anywhere, but it acquires both the spinlock and the Mutex)
+
+        old_h = h;        // remember we searched to here
+
+        // Walk the path upto and including h looking for waiters we can wake.
+        while (pw_walk != h) {
+          w_walk->wake = false;
+          if (w_walk->waitp->cond ==
+                  nullptr ||  // no condition => vacuously true OR
+              (w_walk->waitp->cond != known_false &&
+               // this thread's condition is not known false, AND
+               //  is in fact true
+               EvalConditionIgnored(this, w_walk->waitp->cond))) {
+            if (w == nullptr) {
+              w_walk->wake = true;    // can wake this waiter
+              w = w_walk;
+              pw = pw_walk;
+              if (w_walk->waitp->how == kExclusive) {
+                wr_wait = kMuWrWait;
+                break;                // bail if waking this writer
+              }
+            } else if (w_walk->waitp->how == kShared) {  // wake if a reader
+              w_walk->wake = true;
+            } else {   // writer with true condition
+              wr_wait = kMuWrWait;
+            }
+          } else {                  // can't wake; condition false
+            known_false = w_walk->waitp->cond;  // remember last false condition
+          }
+          if (w_walk->wake) {   // we're waking reader w_walk
+            pw_walk = w_walk;   // don't skip similar waiters
+          } else {              // not waking; skip as much as possible
+            pw_walk = Skip(w_walk);
+          }
+          // If pw_walk == h, then load of pw_walk->next can race with
+          // concurrent write in Enqueue(). However, at the same time
+          // we do not need to do the load, because we will bail out
+          // from the loop anyway.
+          if (pw_walk != h) {
+            w_walk = pw_walk->next;
+          }
+        }
+
+        continue;  // restart for(;;)-loop to wakeup w or to find more waiters
+      }
+      ABSL_RAW_CHECK(pw->next == w, "pw not w's predecessor");
+      // The first (and perhaps only) waiter we've chosen to wake is w, whose
+      // predecessor is pw.  If w is a reader, we must wake all the other
+      // waiters with wake==true as well.  We may also need to queue
+      // ourselves if waitp != null.  The spinlock and the lock are still
+      // held.
+
+      // This traverses the list in [ pw->next, h ], where h is the head,
+      // removing all elements with wake==true and placing them in the
+      // singly-linked list wake_list.  Returns the new head.
+      h = DequeueAllWakeable(h, pw, &wake_list);
+
+      intptr_t nv = (v & kMuEvent) | kMuDesig;
+                                             // assume no waiters left,
+                                             // set kMuDesig for INV1a
+
+      if (waitp != nullptr) {  // we must queue ourselves and sleep
+        h = Enqueue(h, waitp, v, kMuIsCond);
+        // h is new last waiter; could be null if we queued ourselves on a
+        // CondVar
+      }
+
+      ABSL_RAW_CHECK(wake_list != kPerThreadSynchNull,
+                     "unexpected empty wake list");
+
+      if (h != nullptr) {  // there are waiters left
+        h->readers = 0;
+        h->maybe_unlocking = false;     // finished unlocking
+        nv |= wr_wait | kMuWait | reinterpret_cast<intptr_t>(h);
+      }
+
+      // release both spinlock & lock
+      // can release with a store because there were waiters
+      mu_.store(nv, std::memory_order_release);
+      break;  // out of for(;;)-loop
+    }
+    c = Delay(c, AGGRESSIVE);  // aggressive here; no one can proceed till we do
+  }                            // end of for(;;)-loop
+
+  if (wake_list != kPerThreadSynchNull) {
+    int64_t enqueue_timestamp = wake_list->waitp->contention_start_cycles;
+    bool cond_waiter = wake_list->cond_waiter;
+    do {
+      wake_list = Wakeup(wake_list);              // wake waiters
+    } while (wake_list != kPerThreadSynchNull);
+    if (!cond_waiter) {
+      // Sample lock contention events only if the (first) waiter was trying to
+      // acquire the lock, not waiting on a condition variable or Condition.
+      int64_t wait_cycles = base_internal::CycleClock::Now() - enqueue_timestamp;
+      mutex_tracer("slow release", this, wait_cycles);
+      ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
+      submit_profile_data(enqueue_timestamp);
+      ABSL_TSAN_MUTEX_POST_DIVERT(this, 0);
+    }
+  }
+}
+
+// Used by CondVar implementation to reacquire mutex after waking from
+// condition variable.  This routine is used instead of Lock() because the
+// waiting thread may have been moved from the condition variable queue to the
+// mutex queue without a wakeup, by Trans().  In that case, when the thread is
+// finally woken, the woken thread will believe it has been woken from the
+// condition variable (i.e. its PC will be in when in the CondVar code), when
+// in fact it has just been woken from the mutex.  Thus, it must enter the slow
+// path of the mutex in the same state as if it had just woken from the mutex.
+// That is, it must ensure to clear kMuDesig (INV1b).
+void Mutex::Trans(MuHow how) {
+  this->LockSlow(how, nullptr, kMuHasBlocked | kMuIsCond);
+}
+
+// Used by CondVar implementation to effectively wake thread w from the
+// condition variable.  If this mutex is free, we simply wake the thread.
+// It will later acquire the mutex with high probability.  Otherwise, we
+// enqueue thread w on this mutex.
+void Mutex::Fer(PerThreadSynch *w) {
+  int c = 0;
+  ABSL_RAW_CHECK(w->waitp->cond == nullptr,
+                 "Mutex::Fer while waiting on Condition");
+  ABSL_RAW_CHECK(!w->waitp->timeout.has_timeout(),
+                 "Mutex::Fer while in timed wait");
+  ABSL_RAW_CHECK(w->waitp->cv_word == nullptr,
+                 "Mutex::Fer with pending CondVar queueing");
+  for (;;) {
+    intptr_t v = mu_.load(std::memory_order_relaxed);
+    // Note: must not queue if the mutex is unlocked (nobody will wake it).
+    // For example, we can have only kMuWait (conditional) or maybe
+    // kMuWait|kMuWrWait.
+    // conflicting != 0 implies that the waking thread cannot currently take
+    // the mutex, which in turn implies that someone else has it and can wake
+    // us if we queue.
+    const intptr_t conflicting =
+        kMuWriter | (w->waitp->how == kShared ? 0 : kMuReader);
+    if ((v & conflicting) == 0) {
+      w->next = nullptr;
+      w->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
+      IncrementSynchSem(this, w);
+      return;
+    } else {
+      if ((v & (kMuSpin|kMuWait)) == 0) {       // no waiters
+        // This thread tries to become the one and only waiter.
+        PerThreadSynch *new_h = Enqueue(nullptr, w->waitp, v, kMuIsCond);
+        ABSL_RAW_CHECK(new_h != nullptr,
+                       "Enqueue failed");  // we must queue ourselves
+        if (mu_.compare_exchange_strong(
+                v, reinterpret_cast<intptr_t>(new_h) | (v & kMuLow) | kMuWait,
+                std::memory_order_release, std::memory_order_relaxed)) {
+          return;
+        }
+      } else if ((v & kMuSpin) == 0 &&
+                 mu_.compare_exchange_strong(v, v | kMuSpin | kMuWait)) {
+        PerThreadSynch *h = GetPerThreadSynch(v);
+        PerThreadSynch *new_h = Enqueue(h, w->waitp, v, kMuIsCond);
+        ABSL_RAW_CHECK(new_h != nullptr,
+                       "Enqueue failed");  // we must queue ourselves
+        do {
+          v = mu_.load(std::memory_order_relaxed);
+        } while (!mu_.compare_exchange_weak(
+            v,
+            (v & kMuLow & ~kMuSpin) | kMuWait |
+                reinterpret_cast<intptr_t>(new_h),
+            std::memory_order_release, std::memory_order_relaxed));
+        return;
+      }
+    }
+    c = Delay(c, GENTLE);
+  }
+}
+
+void Mutex::AssertHeld() const {
+  if ((mu_.load(std::memory_order_relaxed) & kMuWriter) == 0) {
+    SynchEvent *e = GetSynchEvent(this);
+    ABSL_RAW_LOG(FATAL, "thread should hold write lock on Mutex %p %s",
+                 static_cast<const void *>(this),
+                 (e == nullptr ? "" : e->name));
+  }
+}
+
+void Mutex::AssertReaderHeld() const {
+  if ((mu_.load(std::memory_order_relaxed) & (kMuReader | kMuWriter)) == 0) {
+    SynchEvent *e = GetSynchEvent(this);
+    ABSL_RAW_LOG(
+        FATAL, "thread should hold at least a read lock on Mutex %p %s",
+        static_cast<const void *>(this), (e == nullptr ? "" : e->name));
+  }
+}
+
+// -------------------------------- condition variables
+static const intptr_t kCvSpin = 0x0001L;   // spinlock protects waiter list
+static const intptr_t kCvEvent = 0x0002L;  // record events
+
+static const intptr_t kCvLow = 0x0003L;  // low order bits of CV
+
+// Hack to make constant values available to gdb pretty printer
+enum { kGdbCvSpin = kCvSpin, kGdbCvEvent = kCvEvent, kGdbCvLow = kCvLow, };
+
+static_assert(PerThreadSynch::kAlignment > kCvLow,
+              "PerThreadSynch::kAlignment must be greater than kCvLow");
+
+void CondVar::EnableDebugLog(const char *name) {
+  SynchEvent *e = EnsureSynchEvent(&this->cv_, name, kCvEvent, kCvSpin);
+  e->log = true;
+  UnrefSynchEvent(e);
+}
+
+CondVar::~CondVar() {
+  if ((cv_.load(std::memory_order_relaxed) & kCvEvent) != 0) {
+    ForgetSynchEvent(&this->cv_, kCvEvent, kCvSpin);
+  }
+}
+
+
+// Remove thread s from the list of waiters on this condition variable.
+void CondVar::Remove(PerThreadSynch *s) {
+  intptr_t v;
+  int c = 0;
+  for (v = cv_.load(std::memory_order_relaxed);;
+       v = cv_.load(std::memory_order_relaxed)) {
+    if ((v & kCvSpin) == 0 &&  // attempt to acquire spinlock
+        cv_.compare_exchange_strong(v, v | kCvSpin,
+                                    std::memory_order_acquire,
+                                    std::memory_order_relaxed)) {
+      PerThreadSynch *h = reinterpret_cast<PerThreadSynch *>(v & ~kCvLow);
+      if (h != nullptr) {
+        PerThreadSynch *w = h;
+        while (w->next != s && w->next != h) {  // search for thread
+          w = w->next;
+        }
+        if (w->next == s) {           // found thread; remove it
+          w->next = s->next;
+          if (h == s) {
+            h = (w == s) ? nullptr : w;
+          }
+          s->next = nullptr;
+          s->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
+        }
+      }
+                                      // release spinlock
+      cv_.store((v & kCvEvent) | reinterpret_cast<intptr_t>(h),
+                std::memory_order_release);
+      return;
+    } else {
+      c = Delay(c, GENTLE);            // try again after a delay
+    }
+  }
+}
+
+// Queue thread waitp->thread on condition variable word cv_word using
+// wait parameters waitp.
+// We split this into a separate routine, rather than simply doing it as part
+// of WaitCommon().  If we were to queue ourselves on the condition variable
+// before calling Mutex::UnlockSlow(), the Mutex code might be re-entered (via
+// the logging code, or via a Condition function) and might potentially attempt
+// to block this thread.  That would be a problem if the thread were already on
+// a the condition variable waiter queue.  Thus, we use the waitp->cv_word
+// to tell the unlock code to call CondVarEnqueue() to queue the thread on the
+// condition variable queue just before the mutex is to be unlocked, and (most
+// importantly) after any call to an external routine that might re-enter the
+// mutex code.
+static void CondVarEnqueue(SynchWaitParams *waitp) {
+  // This thread might be transferred to the Mutex queue by Fer() when
+  // we are woken.  To make sure that is what happens, Enqueue() doesn't
+  // call CondVarEnqueue() again but instead uses its normal code.  We
+  // must do this before we queue ourselves so that cv_word will be null
+  // when seen by the dequeuer, who may wish immediately to requeue
+  // this thread on another queue.
+  std::atomic<intptr_t> *cv_word = waitp->cv_word;
+  waitp->cv_word = nullptr;
+
+  intptr_t v = cv_word->load(std::memory_order_relaxed);
+  int c = 0;
+  while ((v & kCvSpin) != 0 ||  // acquire spinlock
+         !cv_word->compare_exchange_weak(v, v | kCvSpin,
+                                         std::memory_order_acquire,
+                                         std::memory_order_relaxed)) {
+    c = Delay(c, GENTLE);
+    v = cv_word->load(std::memory_order_relaxed);
+  }
+  ABSL_RAW_CHECK(waitp->thread->waitp == nullptr, "waiting when shouldn't be");
+  waitp->thread->waitp = waitp;      // prepare ourselves for waiting
+  PerThreadSynch *h = reinterpret_cast<PerThreadSynch *>(v & ~kCvLow);
+  if (h == nullptr) {  // add this thread to waiter list
+    waitp->thread->next = waitp->thread;
+  } else {
+    waitp->thread->next = h->next;
+    h->next = waitp->thread;
+  }
+  waitp->thread->state.store(PerThreadSynch::kQueued,
+                             std::memory_order_relaxed);
+  cv_word->store((v & kCvEvent) | reinterpret_cast<intptr_t>(waitp->thread),
+                 std::memory_order_release);
+}
+
+bool CondVar::WaitCommon(Mutex *mutex, KernelTimeout t) {
+  bool rc = false;          // return value; true iff we timed-out
+
+  intptr_t mutex_v = mutex->mu_.load(std::memory_order_relaxed);
+  Mutex::MuHow mutex_how = ((mutex_v & kMuWriter) != 0) ? kExclusive : kShared;
+  ABSL_TSAN_MUTEX_PRE_UNLOCK(mutex, TsanFlags(mutex_how));
+
+  // maybe trace this call
+  intptr_t v = cv_.load(std::memory_order_relaxed);
+  cond_var_tracer("Wait", this);
+  if ((v & kCvEvent) != 0) {
+    PostSynchEvent(this, SYNCH_EV_WAIT);
+  }
+
+  // Release mu and wait on condition variable.
+  SynchWaitParams waitp(mutex_how, nullptr, t, mutex,
+                        Synch_GetPerThreadAnnotated(mutex), &cv_);
+  // UnlockSlow() will call CondVarEnqueue() just before releasing the
+  // Mutex, thus queuing this thread on the condition variable.  See
+  // CondVarEnqueue() for the reasons.
+  mutex->UnlockSlow(&waitp);
+
+  // wait for signal
+  while (waitp.thread->state.load(std::memory_order_acquire) ==
+         PerThreadSynch::kQueued) {
+    if (!Mutex::DecrementSynchSem(mutex, waitp.thread, t)) {
+      this->Remove(waitp.thread);
+      rc = true;
+    }
+  }
+
+  ABSL_RAW_CHECK(waitp.thread->waitp != nullptr, "not waiting when should be");
+  waitp.thread->waitp = nullptr;  // cleanup
+
+  // maybe trace this call
+  cond_var_tracer("Unwait", this);
+  if ((v & kCvEvent) != 0) {
+    PostSynchEvent(this, SYNCH_EV_WAIT_RETURNING);
+  }
+
+  // From synchronization point of view Wait is unlock of the mutex followed
+  // by lock of the mutex. We've annotated start of unlock in the beginning
+  // of the function. Now, finish unlock and annotate lock of the mutex.
+  // (Trans is effectively lock).
+  ABSL_TSAN_MUTEX_POST_UNLOCK(mutex, TsanFlags(mutex_how));
+  ABSL_TSAN_MUTEX_PRE_LOCK(mutex, TsanFlags(mutex_how));
+  mutex->Trans(mutex_how);  // Reacquire mutex
+  ABSL_TSAN_MUTEX_POST_LOCK(mutex, TsanFlags(mutex_how), 0);
+  return rc;
+}
+
+bool CondVar::WaitWithTimeout(Mutex *mu, absl::Duration timeout) {
+  return WaitWithDeadline(mu, DeadlineFromTimeout(timeout));
+}
+
+bool CondVar::WaitWithDeadline(Mutex *mu, absl::Time deadline) {
+  return WaitCommon(mu, KernelTimeout(deadline));
+}
+
+void CondVar::Wait(Mutex *mu) {
+  WaitCommon(mu, KernelTimeout::Never());
+}
+
+// Wake thread w
+// If it was a timed wait, w will be waiting on w->cv
+// Otherwise, if it was not a Mutex mutex, w will be waiting on w->sem
+// Otherwise, w is transferred to the Mutex mutex via Mutex::Fer().
+void CondVar::Wakeup(PerThreadSynch *w) {
+  if (w->waitp->timeout.has_timeout() || w->waitp->cvmu == nullptr) {
+    // The waiting thread only needs to observe "w->state == kAvailable" to be
+    // released, we must cache "cvmu" before clearing "next".
+    Mutex *mu = w->waitp->cvmu;
+    w->next = nullptr;
+    w->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
+    Mutex::IncrementSynchSem(mu, w);
+  } else {
+    w->waitp->cvmu->Fer(w);
+  }
+}
+
+void CondVar::Signal() {
+  ABSL_TSAN_MUTEX_PRE_SIGNAL(nullptr, 0);
+  intptr_t v;
+  int c = 0;
+  for (v = cv_.load(std::memory_order_relaxed); v != 0;
+       v = cv_.load(std::memory_order_relaxed)) {
+    if ((v & kCvSpin) == 0 &&  // attempt to acquire spinlock
+        cv_.compare_exchange_strong(v, v | kCvSpin,
+                                    std::memory_order_acquire,
+                                    std::memory_order_relaxed)) {
+      PerThreadSynch *h = reinterpret_cast<PerThreadSynch *>(v & ~kCvLow);
+      PerThreadSynch *w = nullptr;
+      if (h != nullptr) {  // remove first waiter
+        w = h->next;
+        if (w == h) {
+          h = nullptr;
+        } else {
+          h->next = w->next;
+        }
+      }
+                                      // release spinlock
+      cv_.store((v & kCvEvent) | reinterpret_cast<intptr_t>(h),
+                std::memory_order_release);
+      if (w != nullptr) {
+        CondVar::Wakeup(w);                // wake waiter, if there was one
+        cond_var_tracer("Signal wakeup", this);
+      }
+      if ((v & kCvEvent) != 0) {
+        PostSynchEvent(this, SYNCH_EV_SIGNAL);
+      }
+      ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
+      return;
+    } else {
+      c = Delay(c, GENTLE);
+    }
+  }
+  ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
+}
+
+void CondVar::SignalAll () {
+  ABSL_TSAN_MUTEX_PRE_SIGNAL(nullptr, 0);
+  intptr_t v;
+  int c = 0;
+  for (v = cv_.load(std::memory_order_relaxed); v != 0;
+       v = cv_.load(std::memory_order_relaxed)) {
+    // empty the list if spinlock free
+    // We do this by simply setting the list to empty using
+    // compare and swap.   We then have the entire list in our hands,
+    // which cannot be changing since we grabbed it while no one
+    // held the lock.
+    if ((v & kCvSpin) == 0 &&
+        cv_.compare_exchange_strong(v, v & kCvEvent, std::memory_order_acquire,
+                                    std::memory_order_relaxed)) {
+      PerThreadSynch *h = reinterpret_cast<PerThreadSynch *>(v & ~kCvLow);
+      if (h != nullptr) {
+        PerThreadSynch *w;
+        PerThreadSynch *n = h->next;
+        do {                          // for every thread, wake it up
+          w = n;
+          n = n->next;
+          CondVar::Wakeup(w);
+        } while (w != h);
+        cond_var_tracer("SignalAll wakeup", this);
+      }
+      if ((v & kCvEvent) != 0) {
+        PostSynchEvent(this, SYNCH_EV_SIGNALALL);
+      }
+      ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
+      return;
+    } else {
+      c = Delay(c, GENTLE);           // try again after a delay
+    }
+  }
+  ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
+}
+
+void ReleasableMutexLock::Release() {
+  ABSL_RAW_CHECK(this->mu_ != nullptr,
+                 "ReleasableMutexLock::Release may only be called once");
+  this->mu_->Unlock();
+  this->mu_ = nullptr;
+}
+
+#ifdef THREAD_SANITIZER
+extern "C" void __tsan_read1(void *addr);
+#else
+#define __tsan_read1(addr)  // do nothing if TSan not enabled
+#endif
+
+// A function that just returns its argument, dereferenced
+static bool Dereference(void *arg) {
+  // ThreadSanitizer does not instrument this file for memory accesses.
+  // This function dereferences a user variable that can participate
+  // in a data race, so we need to manually tell TSan about this memory access.
+  __tsan_read1(arg);
+  return *(static_cast<bool *>(arg));
+}
+
+Condition::Condition() {}   // null constructor, used for kTrue only
+const Condition Condition::kTrue;
+
+Condition::Condition(bool (*func)(void *), void *arg)
+    : eval_(&CallVoidPtrFunction),
+      function_(func),
+      method_(nullptr),
+      arg_(arg) {}
+
+bool Condition::CallVoidPtrFunction(const Condition *c) {
+  return (*c->function_)(c->arg_);
+}
+
+Condition::Condition(const bool *cond)
+    : eval_(CallVoidPtrFunction),
+      function_(Dereference),
+      method_(nullptr),
+      // const_cast is safe since Dereference does not modify arg
+      arg_(const_cast<bool *>(cond)) {}
+
+bool Condition::Eval() const {
+  // eval_ == null for kTrue
+  return (this->eval_ == nullptr) || (*this->eval_)(this);
+}
+
+bool Condition::GuaranteedEqual(const Condition *a, const Condition *b) {
+  if (a == nullptr) {
+    return b == nullptr || b->eval_ == nullptr;
+  }
+  if (b == nullptr || b->eval_ == nullptr) {
+    return a->eval_ == nullptr;
+  }
+  return a->eval_ == b->eval_ && a->function_ == b->function_ &&
+         a->arg_ == b->arg_ && a->method_ == b->method_;
+}
+
+}  // namespace absl