Initial Commit

1 files changed, 547 insertions, 0 deletions

diff --git a/absl/time/clock.cc b/absl/time/clock.cc
new file mode 100644
index 000000000000..e2bc01bdb6a3
--- /dev/null
+++ b/absl/time/clock.cc
@@ -0,0 +1,547 @@
+#include "absl/time/clock.h"
+
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
+#include <algorithm>
+#include <atomic>
+#include <cerrno>
+#include <cstdint>
+#include <ctime>
+#include <limits>
+
+#include "absl/base/internal/spinlock.h"
+#include "absl/base/internal/unscaledcycleclock.h"
+#include "absl/base/macros.h"
+#include "absl/base/port.h"
+#include "absl/base/thread_annotations.h"
+
+namespace absl {
+Time Now() {
+  // TODO(bww): Get a timespec instead so we don't have to divide.
+  int64_t n = absl::GetCurrentTimeNanos();
+  if (n >= 0) {
+    return time_internal::FromUnixDuration(
+        time_internal::MakeDuration(n / 1000000000, n % 1000000000 * 4));
+  }
+  return time_internal::FromUnixDuration(absl::Nanoseconds(n));
+}
+}  // namespace absl
+
+// Decide if we should use the fast GetCurrentTimeNanos() algorithm
+// based on the cyclecounter, otherwise just get the time directly
+// from the OS on every call. This can be chosen at compile-time via
+// -DABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS=[0|1]
+#ifndef ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
+#if ABSL_USE_UNSCALED_CYCLECLOCK
+#define ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS 1
+#else
+#define ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS 0
+#endif
+#endif
+
+#if defined(__APPLE__)
+#include "absl/time/internal/get_current_time_ios.inc"
+#elif defined(_WIN32)
+#include "absl/time/internal/get_current_time_windows.inc"
+#else
+#include "absl/time/internal/get_current_time_posix.inc"
+#endif
+
+// Allows override by test.
+#ifndef GET_CURRENT_TIME_NANOS_FROM_SYSTEM
+#define GET_CURRENT_TIME_NANOS_FROM_SYSTEM() \
+  ::absl::time_internal::GetCurrentTimeNanosFromSystem()
+#endif
+
+#if !ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
+namespace absl {
+int64_t GetCurrentTimeNanos() {
+  return GET_CURRENT_TIME_NANOS_FROM_SYSTEM();
+}
+}  // namespace absl
+#else  // Use the cyclecounter-based implementation below.
+
+// Allows override by test.
+#ifndef GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW
+#define GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW() \
+  ::absl::time_internal::UnscaledCycleClockWrapperForGetCurrentTime::Now()
+#endif
+
+// The following counters are used only by the test code.
+static int64_t stats_initializations;
+static int64_t stats_reinitializations;
+static int64_t stats_calibrations;
+static int64_t stats_slow_paths;
+static int64_t stats_fast_slow_paths;
+
+namespace absl {
+namespace time_internal {
+// This is a friend wrapper around UnscaledCycleClock::Now()
+// (needed to access UnscaledCycleClock).
+class UnscaledCycleClockWrapperForGetCurrentTime {
+ public:
+  static int64_t Now() { return base_internal::UnscaledCycleClock::Now(); }
+};
+}  // namespace time_internal
+
+// uint64_t is used in this module to provide an extra bit in multiplications
+
+// Return the time in ns as told by the kernel interface.  Place in *cycleclock
+// the value of the cycleclock at about the time of the syscall.
+// This call represents the time base that this module synchronizes to.
+// Ensures that *cycleclock does not step back by up to (1 << 16) from
+// last_cycleclock, to discard small backward counter steps.  (Larger steps are
+// assumed to be complete resyncs, which shouldn't happen.  If they do, a full
+// reinitialization of the outer algorithm should occur.)
+static int64_t GetCurrentTimeNanosFromKernel(uint64_t last_cycleclock,
+                                             uint64_t *cycleclock) {
+  // We try to read clock values at about the same time as the kernel clock.
+  // This value gets adjusted up or down as estimate of how long that should
+  // take, so we can reject attempts that take unusually long.
+  static std::atomic<uint64_t> approx_syscall_time_in_cycles{10 * 1000};
+
+  uint64_t local_approx_syscall_time_in_cycles =  // local copy
+      approx_syscall_time_in_cycles.load(std::memory_order_relaxed);
+
+  int64_t current_time_nanos_from_system;
+  uint64_t before_cycles;
+  uint64_t after_cycles;
+  uint64_t elapsed_cycles;
+  int loops = 0;
+  do {
+    before_cycles = GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW();
+    current_time_nanos_from_system = GET_CURRENT_TIME_NANOS_FROM_SYSTEM();
+    after_cycles = GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW();
+    // elapsed_cycles is unsigned, so is large on overflow
+    elapsed_cycles = after_cycles - before_cycles;
+    if (elapsed_cycles >= local_approx_syscall_time_in_cycles &&
+        ++loops == 20) {  // clock changed frequencies?  Back off.
+      loops = 0;
+      if (local_approx_syscall_time_in_cycles < 1000 * 1000) {
+        local_approx_syscall_time_in_cycles =
+            (local_approx_syscall_time_in_cycles + 1) << 1;
+      }
+      approx_syscall_time_in_cycles.store(
+          local_approx_syscall_time_in_cycles,
+          std::memory_order_relaxed);
+    }
+  } while (elapsed_cycles >= local_approx_syscall_time_in_cycles ||
+           last_cycleclock - after_cycles < (static_cast<uint64_t>(1) << 16));
+
+  // Number of times in a row we've seen a kernel time call take substantially
+  // less than approx_syscall_time_in_cycles.
+  static std::atomic<uint32_t> seen_smaller{ 0 };
+
+  // Adjust approx_syscall_time_in_cycles to be within a factor of 2
+  // of the typical time to execute one iteration of the loop above.
+  if ((local_approx_syscall_time_in_cycles >> 1) < elapsed_cycles) {
+    // measured time is no smaller than half current approximation
+    seen_smaller.store(0, std::memory_order_relaxed);
+  } else if (seen_smaller.fetch_add(1, std::memory_order_relaxed) >= 3) {
+    // smaller delays several times in a row; reduce approximation by 12.5%
+    const uint64_t new_approximation =
+        local_approx_syscall_time_in_cycles -
+        (local_approx_syscall_time_in_cycles >> 3);
+    approx_syscall_time_in_cycles.store(new_approximation,
+                                        std::memory_order_relaxed);
+    seen_smaller.store(0, std::memory_order_relaxed);
+  }
+
+  *cycleclock = after_cycles;
+  return current_time_nanos_from_system;
+}
+
+
+// ---------------------------------------------------------------------
+// An implementation of reader-write locks that use no atomic ops in the read
+// case.  This is a generalization of Lamport's method for reading a multiword
+// clock.  Increment a word on each write acquisition, using the low-order bit
+// as a spinlock; the word is the high word of the "clock".  Readers read the
+// high word, then all other data, then the high word again, and repeat the
+// read if the reads of the high words yields different answers, or an odd
+// value (either case suggests possible interference from a writer).
+// Here we use a spinlock to ensure only one writer at a time, rather than
+// spinning on the bottom bit of the word to benefit from SpinLock
+// spin-delay tuning.
+
+// Acquire seqlock (*seq) and return the value to be written to unlock.
+static inline uint64_t SeqAcquire(std::atomic<uint64_t> *seq) {
+  uint64_t x = seq->fetch_add(1, std::memory_order_relaxed);
+
+  // We put a release fence between update to *seq and writes to shared data.
+  // Thus all stores to shared data are effectively release operations and
+  // update to *seq above cannot be re-ordered past any of them.  Note that
+  // this barrier is not for the fetch_add above.  A release barrier for the
+  // fetch_add would be before it, not after.
+  std::atomic_thread_fence(std::memory_order_release);
+
+  return x + 2;   // original word plus 2
+}
+
+// Release seqlock (*seq) by writing x to it---a value previously returned by
+// SeqAcquire.
+static inline void SeqRelease(std::atomic<uint64_t> *seq, uint64_t x) {
+  // The unlock store to *seq must have release ordering so that all
+  // updates to shared data must finish before this store.
+  seq->store(x, std::memory_order_release);  // release lock for readers
+}
+
+// ---------------------------------------------------------------------
+
+// "nsscaled" is unit of time equal to a (2**kScale)th of a nanosecond.
+enum { kScale = 30 };
+
+// The minimum interval between samples of the time base.
+// We pick enough time to amortize the cost of the sample,
+// to get a reasonably accurate cycle counter rate reading,
+// and not so much that calculations will overflow 64-bits.
+static const uint64_t kMinNSBetweenSamples = 2000 << 20;
+
+// We require that kMinNSBetweenSamples shifted by kScale
+// have at least a bit left over for 64-bit calculations.
+static_assert(((kMinNSBetweenSamples << (kScale + 1)) >> (kScale + 1)) ==
+               kMinNSBetweenSamples,
+               "cannot represent kMaxBetweenSamplesNSScaled");
+
+// A reader-writer lock protecting the static locations below.
+// See SeqAcquire() and SeqRelease() above.
+static absl::base_internal::SpinLock lock(
+    absl::base_internal::kLinkerInitialized);
+static std::atomic<uint64_t> seq(0);
+
+// data from a sample of the kernel's time value
+struct TimeSampleAtomic {
+  std::atomic<uint64_t> raw_ns;              // raw kernel time
+  std::atomic<uint64_t> base_ns;             // our estimate of time
+  std::atomic<uint64_t> base_cycles;         // cycle counter reading
+  std::atomic<uint64_t> nsscaled_per_cycle;  // cycle period
+  // cycles before we'll sample again (a scaled reciprocal of the period,
+  // to avoid a division on the fast path).
+  std::atomic<uint64_t> min_cycles_per_sample;
+};
+// Same again, but with non-atomic types
+struct TimeSample {
+  uint64_t raw_ns;                 // raw kernel time
+  uint64_t base_ns;                // our estimate of time
+  uint64_t base_cycles;            // cycle counter reading
+  uint64_t nsscaled_per_cycle;     // cycle period
+  uint64_t min_cycles_per_sample;  // approx cycles before next sample
+};
+
+static struct TimeSampleAtomic last_sample;   // the last sample; under seq
+
+static int64_t GetCurrentTimeNanosSlowPath() ABSL_ATTRIBUTE_COLD;
+
+// Read the contents of *atomic into *sample.
+// Each field is read atomically, but to maintain atomicity between fields,
+// the access must be done under a lock.
+static void ReadTimeSampleAtomic(const struct TimeSampleAtomic *atomic,
+                                 struct TimeSample *sample) {
+  sample->base_ns = atomic->base_ns.load(std::memory_order_relaxed);
+  sample->base_cycles = atomic->base_cycles.load(std::memory_order_relaxed);
+  sample->nsscaled_per_cycle =
+      atomic->nsscaled_per_cycle.load(std::memory_order_relaxed);
+  sample->min_cycles_per_sample =
+      atomic->min_cycles_per_sample.load(std::memory_order_relaxed);
+  sample->raw_ns = atomic->raw_ns.load(std::memory_order_relaxed);
+}
+
+// Public routine.
+// Algorithm:  We wish to compute real time from a cycle counter.  In normal
+// operation, we construct a piecewise linear approximation to the kernel time
+// source, using the cycle counter value.  The start of each line segment is at
+// the same point as the end of the last, but may have a different slope (that
+// is, a different idea of the cycle counter frequency).  Every couple of
+// seconds, the kernel time source is sampled and compared with the current
+// approximation.  A new slope is chosen that, if followed for another couple
+// of seconds, will correct the error at the current position.  The information
+// for a sample is in the "last_sample" struct.  The linear approximation is
+//   estimated_time = last_sample.base_ns +
+//     last_sample.ns_per_cycle * (counter_reading - last_sample.base_cycles)
+// (ns_per_cycle is actually stored in different units and scaled, to avoid
+// overflow).  The base_ns of the next linear approximation is the
+// estimated_time using the last approximation; the base_cycles is the cycle
+// counter value at that time; the ns_per_cycle is the number of ns per cycle
+// measured since the last sample, but adjusted so that most of the difference
+// between the estimated_time and the kernel time will be corrected by the
+// estimated time to the next sample.  In normal operation, this algorithm
+// relies on:
+// - the cycle counter and kernel time rates not changing a lot in a few
+//   seconds.
+// - the client calling into the code often compared to a couple of seconds, so
+//   the time to the next correction can be estimated.
+// Any time ns_per_cycle is not known, a major error is detected, or the
+// assumption about frequent calls is violated, the implementation returns the
+// kernel time.  It records sufficient data that a linear approximation can
+// resume a little later.
+
+int64_t GetCurrentTimeNanos() {
+  // read the data from the "last_sample" struct (but don't need raw_ns yet)
+  // The reads of "seq" and test of the values emulate a reader lock.
+  uint64_t base_ns;
+  uint64_t base_cycles;
+  uint64_t nsscaled_per_cycle;
+  uint64_t min_cycles_per_sample;
+  uint64_t seq_read0;
+  uint64_t seq_read1;
+
+  // If we have enough information to interpolate, the value returned will be
+  // derived from this cycleclock-derived time estimate.  On some platforms
+  // (POWER) the function to retrieve this value has enough complexity to
+  // contribute to register pressure - reading it early before initializing
+  // the other pieces of the calculation minimizes spill/restore instructions,
+  // minimizing icache cost.
+  uint64_t now_cycles = GET_CURRENT_TIME_NANOS_CYCLECLOCK_NOW();
+
+  // Acquire pairs with the barrier in SeqRelease - if this load sees that
+  // store, the shared-data reads necessarily see that SeqRelease's updates
+  // to the same shared data.
+  seq_read0 = seq.load(std::memory_order_acquire);
+
+  base_ns = last_sample.base_ns.load(std::memory_order_relaxed);
+  base_cycles = last_sample.base_cycles.load(std::memory_order_relaxed);
+  nsscaled_per_cycle =
+      last_sample.nsscaled_per_cycle.load(std::memory_order_relaxed);
+  min_cycles_per_sample =
+      last_sample.min_cycles_per_sample.load(std::memory_order_relaxed);
+
+  // This acquire fence pairs with the release fence in SeqAcquire.  Since it
+  // is sequenced between reads of shared data and seq_read1, the reads of
+  // shared data are effectively acquiring.
+  std::atomic_thread_fence(std::memory_order_acquire);
+
+  // The shared-data reads are effectively acquire ordered, and the
+  // shared-data writes are effectively release ordered. Therefore if our
+  // shared-data reads see any of a particular update's shared-data writes,
+  // seq_read1 is guaranteed to see that update's SeqAcquire.
+  seq_read1 = seq.load(std::memory_order_relaxed);
+
+  // Fast path.  Return if min_cycles_per_sample has not yet elapsed since the
+  // last sample, and we read a consistent sample.  The fast path activates
+  // only when min_cycles_per_sample is non-zero, which happens when we get an
+  // estimate for the cycle time.  The predicate will fail if now_cycles <
+  // base_cycles, or if some other thread is in the slow path.
+  //
+  // Since we now read now_cycles before base_ns, it is possible for now_cycles
+  // to be less than base_cycles (if we were interrupted between those loads and
+  // last_sample was updated). This is harmless, because delta_cycles will wrap
+  // and report a time much much bigger than min_cycles_per_sample. In that case
+  // we will take the slow path.
+  uint64_t delta_cycles = now_cycles - base_cycles;
+  if (seq_read0 == seq_read1 && (seq_read0 & 1) == 0 &&
+      delta_cycles < min_cycles_per_sample) {
+    return base_ns + ((delta_cycles * nsscaled_per_cycle) >> kScale);
+  }
+  return GetCurrentTimeNanosSlowPath();
+}
+
+// Return (a << kScale)/b.
+// Zero is returned if b==0.   Scaling is performed internally to
+// preserve precision without overflow.
+static uint64_t SafeDivideAndScale(uint64_t a, uint64_t b) {
+  // Find maximum safe_shift so that
+  //  0 <= safe_shift <= kScale  and  (a << safe_shift) does not overflow.
+  int safe_shift = kScale;
+  while (((a << safe_shift) >> safe_shift) != a) {
+    safe_shift--;
+  }
+  uint64_t scaled_b = b >> (kScale - safe_shift);
+  uint64_t quotient = 0;
+  if (scaled_b != 0) {
+    quotient = (a << safe_shift) / scaled_b;
+  }
+  return quotient;
+}
+
+static uint64_t UpdateLastSample(
+    uint64_t now_cycles, uint64_t now_ns, uint64_t delta_cycles,
+    const struct TimeSample *sample) ABSL_ATTRIBUTE_COLD;
+
+// The slow path of GetCurrentTimeNanos().  This is taken while gathering
+// initial samples, when enough time has elapsed since the last sample, and if
+// any other thread is writing to last_sample.
+//
+// Manually mark this 'noinline' to minimize stack frame size of the fast
+// path.  Without this, sometimes a compiler may inline this big block of code
+// into the fast past.  That causes lots of register spills and reloads that
+// are unnecessary unless the slow path is taken.
+//
+// TODO(b/36012148) Remove this attribute when our compiler is smart enough
+// to do the right thing.
+ABSL_ATTRIBUTE_NOINLINE
+static int64_t GetCurrentTimeNanosSlowPath() LOCKS_EXCLUDED(lock) {
+  // Serialize access to slow-path.  Fast-path readers are not blocked yet, and
+  // code below must not modify last_sample until the seqlock is acquired.
+  lock.Lock();
+
+  // Sample the kernel time base.  This is the definition of
+  // "now" if we take the slow path.
+  static uint64_t last_now_cycles;  // protected by lock
+  uint64_t now_cycles;
+  uint64_t now_ns = GetCurrentTimeNanosFromKernel(last_now_cycles, &now_cycles);
+  last_now_cycles = now_cycles;
+
+  uint64_t estimated_base_ns;
+
+  // ----------
+  // Read the "last_sample" values again; this time holding the write lock.
+  struct TimeSample sample;
+  ReadTimeSampleAtomic(&last_sample, &sample);
+
+  // ----------
+  // Try running the fast path again; another thread may have updated the
+  // sample between our run of the fast path and the sample we just read.
+  uint64_t delta_cycles = now_cycles - sample.base_cycles;
+  if (delta_cycles < sample.min_cycles_per_sample) {
+    // Another thread updated the sample.  This path does not take the seqlock
+    // so that blocked readers can make progress without blocking new readers.
+    estimated_base_ns = sample.base_ns +
+        ((delta_cycles * sample.nsscaled_per_cycle) >> kScale);
+    stats_fast_slow_paths++;
+  } else {
+    estimated_base_ns =
+        UpdateLastSample(now_cycles, now_ns, delta_cycles, &sample);
+  }
+
+  lock.Unlock();
+
+  return estimated_base_ns;
+}
+
+// Main part of the algorithm.  Locks out readers, updates the approximation
+// using the new sample from the kernel, and stores the result in last_sample
+// for readers.  Returns the new estimated time.
+static uint64_t UpdateLastSample(uint64_t now_cycles, uint64_t now_ns,
+                                 uint64_t delta_cycles,
+                                 const struct TimeSample *sample)
+    EXCLUSIVE_LOCKS_REQUIRED(lock) {
+  uint64_t estimated_base_ns = now_ns;
+  uint64_t lock_value = SeqAcquire(&seq);  // acquire seqlock to block readers
+
+  // The 5s in the next if-statement limits the time for which we will trust
+  // the cycle counter and our last sample to give a reasonable result.
+  // Errors in the rate of the source clock can be multiplied by the ratio
+  // between this limit and kMinNSBetweenSamples.
+  if (sample->raw_ns == 0 ||  // no recent sample, or clock went backwards
+      sample->raw_ns + static_cast<uint64_t>(5) * 1000 * 1000 * 1000 < now_ns ||
+      now_ns < sample->raw_ns || now_cycles < sample->base_cycles) {
+    // record this sample, and forget any previously known slope.
+    last_sample.raw_ns.store(now_ns, std::memory_order_relaxed);
+    last_sample.base_ns.store(estimated_base_ns, std::memory_order_relaxed);
+    last_sample.base_cycles.store(now_cycles, std::memory_order_relaxed);
+    last_sample.nsscaled_per_cycle.store(0, std::memory_order_relaxed);
+    last_sample.min_cycles_per_sample.store(0, std::memory_order_relaxed);
+    stats_initializations++;
+  } else if (sample->raw_ns + 500 * 1000 * 1000 < now_ns &&
+             sample->base_cycles + 100 < now_cycles) {
+    // Enough time has passed to compute the cycle time.
+    if (sample->nsscaled_per_cycle != 0) {  // Have a cycle time estimate.
+      // Compute time from counter reading, but avoiding overflow
+      // delta_cycles may be larger than on the fast path.
+      uint64_t estimated_scaled_ns;
+      int s = -1;
+      do {
+        s++;
+        estimated_scaled_ns = (delta_cycles >> s) * sample->nsscaled_per_cycle;
+      } while (estimated_scaled_ns / sample->nsscaled_per_cycle !=
+               (delta_cycles >> s));
+      estimated_base_ns = sample->base_ns +
+                          (estimated_scaled_ns >> (kScale - s));
+    }
+
+    // Compute the assumed cycle time kMinNSBetweenSamples ns into the future
+    // assuming the cycle counter rate stays the same as the last interval.
+    uint64_t ns = now_ns - sample->raw_ns;
+    uint64_t measured_nsscaled_per_cycle = SafeDivideAndScale(ns, delta_cycles);
+
+    uint64_t assumed_next_sample_delta_cycles =
+        SafeDivideAndScale(kMinNSBetweenSamples, measured_nsscaled_per_cycle);
+
+    int64_t diff_ns = now_ns - estimated_base_ns;  // estimate low by this much
+
+    // We want to set nsscaled_per_cycle so that our estimate of the ns time
+    // at the assumed cycle time is the assumed ns time.
+    // That is, we want to set nsscaled_per_cycle so:
+    //  kMinNSBetweenSamples + diff_ns  ==
+    //  (assumed_next_sample_delta_cycles * nsscaled_per_cycle) >> kScale
+    // But we wish to damp oscillations, so instead correct only most
+    // of our current error, by solving:
+    //  kMinNSBetweenSamples + diff_ns - (diff_ns / 16) ==
+    //  (assumed_next_sample_delta_cycles * nsscaled_per_cycle) >> kScale
+    ns = kMinNSBetweenSamples + diff_ns - (diff_ns / 16);
+    uint64_t new_nsscaled_per_cycle =
+        SafeDivideAndScale(ns, assumed_next_sample_delta_cycles);
+    if (new_nsscaled_per_cycle != 0 &&
+        diff_ns < 100 * 1000 * 1000 && -diff_ns < 100 * 1000 * 1000) {
+      // record the cycle time measurement
+      last_sample.nsscaled_per_cycle.store(
+          new_nsscaled_per_cycle, std::memory_order_relaxed);
+      uint64_t new_min_cycles_per_sample =
+          SafeDivideAndScale(kMinNSBetweenSamples, new_nsscaled_per_cycle);
+      last_sample.min_cycles_per_sample.store(
+          new_min_cycles_per_sample, std::memory_order_relaxed);
+      stats_calibrations++;
+    } else {  // something went wrong; forget the slope
+      last_sample.nsscaled_per_cycle.store(0, std::memory_order_relaxed);
+      last_sample.min_cycles_per_sample.store(0, std::memory_order_relaxed);
+      estimated_base_ns = now_ns;
+      stats_reinitializations++;
+    }
+    last_sample.raw_ns.store(now_ns, std::memory_order_relaxed);
+    last_sample.base_ns.store(estimated_base_ns, std::memory_order_relaxed);
+    last_sample.base_cycles.store(now_cycles, std::memory_order_relaxed);
+  } else {
+    // have a sample, but no slope; waiting for enough time for a calibration
+    stats_slow_paths++;
+  }
+
+  SeqRelease(&seq, lock_value);  // release the readers
+
+  return estimated_base_ns;
+}
+}  // namespace absl
+#endif  // ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
+
+namespace absl {
+namespace {
+
+// Returns the maximum duration that SleepOnce() can sleep for.
+constexpr absl::Duration MaxSleep() {
+#ifdef _WIN32
+  // Windows _sleep() takes unsigned long argument in milliseconds.
+  return absl::Milliseconds(
+      std::numeric_limits<unsigned long>::max());  // NOLINT(runtime/int)
+#else
+  return absl::Seconds(std::numeric_limits<time_t>::max());
+#endif
+}
+
+// Sleeps for the given duration.
+// REQUIRES: to_sleep <= MaxSleep().
+void SleepOnce(absl::Duration to_sleep) {
+#ifdef _WIN32
+  _sleep(to_sleep / absl::Milliseconds(1));
+#else
+  struct timespec sleep_time = absl::ToTimespec(to_sleep);
+  while (nanosleep(&sleep_time, &sleep_time) != 0 && errno == EINTR) {
+    // Ignore signals and wait for the full interval to elapse.
+  }
+#endif
+}
+
+}  // namespace
+}  // namespace absl
+
+extern "C" {
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSleepFor(absl::Duration duration) {
+  while (duration > absl::ZeroDuration()) {
+    absl::Duration to_sleep = std::min(duration, absl::MaxSleep());
+    absl::SleepOnce(to_sleep);
+    duration -= to_sleep;
+  }
+}
+
+}  // extern "C"