Export of internal Abseil changes

--
51e9291d9bc385082b4061fe760b236ba59c79c3 by Abseil Team <absl-team@google.com>:

Cast to uint64_t using braces instead of parentheses.

PiperOrigin-RevId: 372475909

--
939fc409855da2639dcaf2d1d4971ca0e0568d03 by Martijn Vels <mvels@google.com>:

Expand # flat nodes into size detailed counters.

PiperOrigin-RevId: 372474608

--
54b158c99b32f8a14821ce74fed0f9f836525ce7 by Martijn Vels <mvels@google.com>:

Make copies of sampled cords sampled as well

This applies to the following methods:
- Cord(const Cord&)
- operator=(const Cord&)
- Subcord(...)

PiperOrigin-RevId: 372468160

--
876c2581ce008871464e8b471efbb967d150f83b by Andy Getzendanner <durandal@google.com>:

Document the type of an ABSL_FLAGS.OnUpdate() callback.

PiperOrigin-RevId: 372406390
GitOrigin-RevId: 51e9291d9bc385082b4061fe760b236ba59c79c3
Change-Id: Ifb75122cae56b66c28128aee90a63bbb28d93817

13 files changed, 293 insertions, 127 deletions

diff --git a/absl/strings/charconv.cc b/absl/strings/charconv.cc
index b8674c28..fefcfc90 100644
--- a/absl/strings/charconv.cc
+++ b/absl/strings/charconv.cc
@@ -111,7 +111,7 @@ struct FloatTraits<double> {
     return sign ? -ldexp(mantissa, exponent) : ldexp(mantissa, exponent);
 #else
     constexpr uint64_t kMantissaMask =
-        (uint64_t(1) << (kTargetMantissaBits - 1)) - 1;
+        (uint64_t{1} << (kTargetMantissaBits - 1)) - 1;
     uint64_t dbl = static_cast<uint64_t>(sign) << 63;
     if (mantissa > kMantissaMask) {
       // Normal value.
@@ -151,7 +151,7 @@ struct FloatTraits<float> {
     return sign ? -ldexpf(mantissa, exponent) : ldexpf(mantissa, exponent);
 #else
     constexpr uint32_t kMantissaMask =
-        (uint32_t(1) << (kTargetMantissaBits - 1)) - 1;
+        (uint32_t{1} << (kTargetMantissaBits - 1)) - 1;
     uint32_t flt = static_cast<uint32_t>(sign) << 31;
     if (mantissa > kMantissaMask) {
       // Normal value.
@@ -499,7 +499,7 @@ bool MustRoundUp(uint64_t guess_mantissa, int guess_exponent,
 template <typename FloatType>
 CalculatedFloat CalculatedFloatFromRawValues(uint64_t mantissa, int exponent) {
   CalculatedFloat result;
-  if (mantissa == uint64_t(1) << FloatTraits<FloatType>::kTargetMantissaBits) {
+  if (mantissa == uint64_t{1} << FloatTraits<FloatType>::kTargetMantissaBits) {
     mantissa >>= 1;
     exponent += 1;
   }
diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index 1c2ff9f2..238532f9 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc
@@ -531,18 +531,19 @@ void Cord::InlineRep::AssignSlow(const Cord::InlineRep& src) {
   assert(&src != this);
   assert(is_tree() || src.is_tree());
   auto constexpr method = CordzUpdateTracker::kAssignCord;
-  if (CordRep* tree = this->tree()) {
-    CordzUpdateScope scope(data_.cordz_info(), method);
-    CordRep::Unref(tree);
-    if (CordRep* src_tree = src.tree()) {
-      SetTree(CordRep::Ref(src_tree), scope);
-    } else {
-      scope.SetCordRep(nullptr);
-      data_ = src.data_;
-    }
+  if (ABSL_PREDICT_TRUE(!is_tree())) {
+    EmplaceTree(CordRep::Ref(src.as_tree()), src.data_, method);
+    return;
+  }
+  CordRep* tree = as_tree();
+  if (CordRep* src_tree = src.tree()) {
+    data_.set_tree(CordRep::Ref(src_tree));
+    CordzInfo::MaybeTrackCord(data_, src.data_, method);
   } else {
-    EmplaceTree(CordRep::Ref(src.as_tree()), method);
+    CordzInfo::MaybeUntrackCord(data_.cordz_info());
+    data_ = src.data_;
   }
+  CordRep::Unref(tree);
 }
 
 void Cord::InlineRep::UnrefTree() {
diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index 5c80a5cb..e758f1cd 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h
@@ -1000,12 +1000,12 @@ constexpr Cord::InlineRep::InlineRep(cord_internal::InlineData data)
     : data_(data) {}
 
 inline Cord::InlineRep::InlineRep(const Cord::InlineRep& src)
-    : data_(src.data_) {
-  if (is_tree()) {
-    data_.clear_cordz_info();
-    absl::cord_internal::CordRep::Ref(as_tree());
-    CordzInfo::MaybeTrackCord(data_, src.data_,
-                              CordzUpdateTracker::kConstructorCord);
+    : data_(InlineData::kDefaultInit) {
+  if (CordRep* tree = src.tree()) {
+    EmplaceTree(CordRep::Ref(tree), src.data_,
+                CordzUpdateTracker::kConstructorCord);
+  } else {
+    data_ = src.data_;
   }
 }
 
diff --git a/absl/strings/cordz_test.cc b/absl/strings/cordz_test.cc
index aeb3d13f..0e11f5c8 100644
--- a/absl/strings/cordz_test.cc
+++ b/absl/strings/cordz_test.cc
@@ -67,6 +67,13 @@ absl::string_view MakeString(TestCordSize size) {
   return MakeString(Length(size));
 }
 
+// Returns a cord with a sampled method of kAppendString.
+absl::Cord MakeAppendStringCord(TestCordSize size) {
+  absl::Cord cord;
+  cord.Append(MakeString(size));
+  return cord;
+}
+
 std::string TestParamToString(::testing::TestParamInfo<TestCordSize> size) {
   return absl::StrCat("On", ToString(size.param), "Cord");
 }
@@ -136,6 +143,16 @@ TEST(CordzTest, CopyConstruct) {
   EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorCord));
 }
 
+TEST(CordzTest, CopyConstructFromSampled) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src(MakeString(TestCordSize::kLarge));
+  Cord cord(src);
+  ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kConstructorString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(1));
+}
+
 TEST(CordzTest, MoveConstruct) {
   CordzSamplingIntervalHelper sample_every{1};
   Cord src(MakeString(TestCordSize::kLarge));
@@ -146,7 +163,43 @@ TEST(CordzTest, MoveConstruct) {
 TEST_P(CordzUpdateTest, AssignCord) {
   Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
   cord() = src;
-  EXPECT_THAT(cord(), HasValidCordzInfoOf(InitialOr(Method::kAssignCord)));
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(Method::kAssignCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord())->GetCordzStatistics();
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
+}
+
+TEST_P(CordzUpdateTest, AssignSampledCord) {
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  cord() = src;
+  ASSERT_THAT(cord(), HasValidCordzInfoOf(Method::kAssignCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord())->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
+}
+
+TEST(CordzUpdateTest, AssignSampledCordToUnsampledCord) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  Cord cord = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord = src;
+  ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kAssignCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
+}
+
+TEST(CordzUpdateTest, AssignSampledCordToSampledCord) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  Cord cord(MakeString(TestCordSize::kLarge));
+  cord = src;
+  ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kAssignCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
 }
 
 TEST(CordzTest, AssignInlinedCord) {
@@ -160,7 +213,7 @@ TEST(CordzTest, AssignInlinedCord) {
   EXPECT_FALSE(CordzInfoIsListed(info));
 }
 
-TEST(CordzTest, MoveAssignCord) {
+TEST(CordzUpdateTest, MoveAssignCord) {
   CordzSamplingIntervalHelper sample_every{1};
   Cord cord;
   Cord src(MakeString(TestCordSize::kLarge));
@@ -168,6 +221,11 @@ TEST(CordzTest, MoveAssignCord) {
   EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
 }
 
+TEST_P(CordzUpdateTest, AssignLargeArray) {
+  cord() = MakeString(TestCordSize::kSmall);
+  EXPECT_THAT(cord(), HasValidCordzInfoOf(Method::kAssignString));
+}
+
 TEST_P(CordzUpdateTest, AssignSmallArray) {
   cord() = MakeString(TestCordSize::kSmall);
   EXPECT_THAT(cord(), HasValidCordzInfoOf(Method::kAssignString));
@@ -333,16 +391,26 @@ TEST(CordzTest, RemoveSuffix) {
 
 TEST(CordzTest, SubCord) {
   CordzSamplingIntervalHelper sample_every{1};
-  Cord src(MakeString(TestCordSize::kLarge));
-
-  Cord cord1 = src.Subcord(10, src.size() / 2);
-  EXPECT_THAT(cord1, HasValidCordzInfoOf(Method::kSubCord));
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  Cord cord = src.Subcord(10, src.size() / 2);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kSubCord));
+}
 
-  Cord cord2 = src.Subcord(10, kMaxInline + 1);
-  EXPECT_THAT(cord2, HasValidCordzInfoOf(Method::kSubCord));
+TEST(CordzTest, SmallSubCord) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  Cord cord = src.Subcord(10, kMaxInline + 1);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kSubCord));
+}
 
-  Cord cord3 = src.Subcord(10, kMaxInline);
-  EXPECT_THAT(GetCordzInfoForTesting(cord3), Eq(nullptr));
+TEST(CordzTest, SubCordFromSampledCord) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord src(MakeString(TestCordSize::kLarge));
+  Cord cord = src.Subcord(10, src.size() / 2);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kSubCord));
+  CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
+  EXPECT_THAT(stats.parent_method, Eq(Method::kConstructorString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(1));
 }
 
 }  // namespace
diff --git a/absl/strings/internal/charconv_parse.cc b/absl/strings/internal/charconv_parse.cc
index 8b11868c..d29acaf4 100644
--- a/absl/strings/internal/charconv_parse.cc
+++ b/absl/strings/internal/charconv_parse.cc
@@ -52,7 +52,7 @@ static_assert(std::numeric_limits<double>::digits == 53, "IEEE double fact");
 
 // The lowest valued 19-digit decimal mantissa we can read still contains
 // sufficient information to reconstruct a binary mantissa.
-static_assert(1000000000000000000u > (uint64_t(1) << (53 + 3)), "(b) above");
+static_assert(1000000000000000000u > (uint64_t{1} << (53 + 3)), "(b) above");
 
 // ParseFloat<16> will read the first 15 significant digits of the mantissa.
 //
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index c7ae0104..813b3f35 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -366,6 +366,16 @@ class InlineData {
     return as_tree_.cordz_info != kNullCordzInfo;
   }
 
+  // Returns true if either of the provided instances hold a cordz_info value.
+  // This method is more efficient than the equivalent `data1.is_profiled() ||
+  // data2.is_profiled()`. Requires both arguments to hold a tree.
+  static bool is_either_profiled(const InlineData& data1,
+                                 const InlineData& data2) {
+    assert(data1.is_tree() && data2.is_tree());
+    return (data1.as_tree_.cordz_info | data2.as_tree_.cordz_info) !=
+           kNullCordzInfo;
+  }
+
   // Returns the cordz_info sampling instance for this instance, or nullptr
   // if the current instance is not sampled and does not have CordzInfo data.
   // Requires the current instance to hold a tree value.
diff --git a/absl/strings/internal/cordz_handle_test.cc b/absl/strings/internal/cordz_handle_test.cc
index 4eefd72c..fd68e06b 100644
--- a/absl/strings/internal/cordz_handle_test.cc
+++ b/absl/strings/internal/cordz_handle_test.cc
@@ -191,65 +191,72 @@ TEST(CordzHandleTest, MultiThreaded) {
   // manipulating a CordzHandle that might be operated upon in another thread.
   std::vector<std::atomic<CordzHandle*>> handles(kNumHandles);
 
-  absl::synchronization_internal::ThreadPool pool(kNumThreads);
-
-  for (int i = 0; i < kNumThreads; ++i) {
-    pool.Schedule([&stop, &handles]() {
-      std::minstd_rand gen;
-      std::uniform_int_distribution<int> dist_type(0, 2);
-      std::uniform_int_distribution<int> dist_handle(0, kNumHandles - 1);
-      size_t max_safe_to_inspect = 0;
-      while (!stop.HasBeenNotified()) {
-        CordzHandle* handle;
-        switch (dist_type(gen)) {
-          case 0:
-            handle = new CordzHandle();
-            break;
-          case 1:
-            handle = new CordzSnapshot();
-            break;
-          default:
-            handle = nullptr;
-            break;
-        }
-        CordzHandle* old_handle = handles[dist_handle(gen)].exchange(handle);
-        if (old_handle != nullptr) {
-          std::vector<const CordzHandle*> safe_to_inspect =
-              old_handle->DiagnosticsGetSafeToInspectDeletedHandles();
-          for (const CordzHandle* handle : safe_to_inspect) {
-            // We're in a tight loop, so don't generate too many error messages.
-            ASSERT_FALSE(handle->is_snapshot());
+  // global bool which is set when any thread did get some 'safe to inspect'
+  // handles. On some platforms and OSS tests, we might risk that some pool
+  // threads are starved, stalled, or just got a few unlikely random 'handle'
+  // coin tosses, so we satisfy this test with simply observing 'some' thread
+  // did something meaningful, which should minimize the potential for flakes.
+  std::atomic<bool> found_safe_to_inspect(false);
+
+  {
+    absl::synchronization_internal::ThreadPool pool(kNumThreads);
+    for (int i = 0; i < kNumThreads; ++i) {
+      pool.Schedule([&stop, &handles, &found_safe_to_inspect]() {
+        std::minstd_rand gen;
+        std::uniform_int_distribution<int> dist_type(0, 2);
+        std::uniform_int_distribution<int> dist_handle(0, kNumHandles - 1);
+
+        while (!stop.HasBeenNotified()) {
+          CordzHandle* handle;
+          switch (dist_type(gen)) {
+            case 0:
+              handle = new CordzHandle();
+              break;
+            case 1:
+              handle = new CordzSnapshot();
+              break;
+            default:
+              handle = nullptr;
+              break;
           }
-          if (safe_to_inspect.size() > max_safe_to_inspect) {
-            max_safe_to_inspect = safe_to_inspect.size();
+          CordzHandle* old_handle = handles[dist_handle(gen)].exchange(handle);
+          if (old_handle != nullptr) {
+            std::vector<const CordzHandle*> safe_to_inspect =
+                old_handle->DiagnosticsGetSafeToInspectDeletedHandles();
+            for (const CordzHandle* handle : safe_to_inspect) {
+              // We're in a tight loop, so don't generate too many error
+              // messages.
+              ASSERT_FALSE(handle->is_snapshot());
+            }
+            if (!safe_to_inspect.empty()) {
+              found_safe_to_inspect.store(true);
+            }
+            CordzHandle::Delete(old_handle);
           }
-          CordzHandle::Delete(old_handle);
         }
-      }
-
-      // Confirm that the test did *something*. This check will be satisfied as
-      // long as this thread has delete a CordzSnapshot object and a
-      // non-snapshot CordzHandle was deleted after the CordzSnapshot was
-      // created. This max_safe_to_inspect count will often reach around 30
-      // (assuming 4 threads and 10 slots for live handles). Don't use a strict
-      // bound to avoid making this test flaky.
-      EXPECT_THAT(max_safe_to_inspect, Gt(0));
-
-      // Have each thread attempt to clean up everything. Some thread will be
-      // the last to reach this cleanup code, and it will be guaranteed to clean
-      // up everything because nothing remains to create new handles.
-      for (auto& h : handles) {
-        if (CordzHandle* handle = h.exchange(nullptr)) {
-          CordzHandle::Delete(handle);
+
+        // Have each thread attempt to clean up everything. Some thread will be
+        // the last to reach this cleanup code, and it will be guaranteed to
+        // clean up everything because nothing remains to create new handles.
+        for (auto& h : handles) {
+          if (CordzHandle* handle = h.exchange(nullptr)) {
+            CordzHandle::Delete(handle);
+          }
         }
-      }
-  });
+      });
+    }
+
+    // The threads will hammer away.  Give it a little bit of time for tsan to
+    // spot errors.
+    absl::SleepFor(absl::Seconds(3));
+    stop.Notify();
   }
 
-  // The threads will hammer away.  Give it a little bit of time for tsan to
-  // spot errors.
-  absl::SleepFor(absl::Seconds(3));
-  stop.Notify();
+  // Confirm that the test did *something*. This check will be satisfied as
+  // long as any thread has deleted a CordzSnapshot object and a non-snapshot
+  // CordzHandle was deleted after the CordzSnapshot was created.
+  // See also comments on `found_safe_to_inspect`
+  EXPECT_TRUE(found_safe_to_inspect.load());
 }
 
 }  // namespace
diff --git a/absl/strings/internal/cordz_info.cc b/absl/strings/internal/cordz_info.cc
index e1fa6b6b..a6b045ff 100644
--- a/absl/strings/internal/cordz_info.cc
+++ b/absl/strings/internal/cordz_info.cc
@@ -135,6 +135,23 @@ class CordRepAnalyzer {
     return (rep != nullptr && rep->tag == CONCAT) ? repref : RepRef{nullptr, 0};
   }
 
+  // Counts a flat of the provide allocated size
+  void CountFlat(size_t size) {
+    statistics_.node_count++;
+    statistics_.node_counts.flat++;
+    if (size <= 64) {
+      statistics_.node_counts.flat_64++;
+    } else if (size <= 128) {
+      statistics_.node_counts.flat_128++;
+    } else if (size <= 256) {
+      statistics_.node_counts.flat_256++;
+    } else if (size <= 512) {
+      statistics_.node_counts.flat_512++;
+    } else if (size <= 1024) {
+      statistics_.node_counts.flat_1k++;
+    }
+  }
+
   // Processes 'linear' reps (substring, flat, external) not requiring iteration
   // or recursion. Returns RefRep{null} if all reps were processed, else returns
   // the top-most non-linear concat or ring cordrep.
@@ -152,9 +169,9 @@ class CordRepAnalyzer {
 
     // Consume possible FLAT
     if (rep.rep->tag >= FLAT) {
-      statistics_.node_count++;
-      statistics_.node_counts.flat++;
-      memory_usage.Add(rep.rep->flat()->AllocatedSize(), rep.refcount);
+      size_t size = rep.rep->flat()->AllocatedSize();
+      CountFlat(size);
+      memory_usage.Add(size, rep.refcount);
       return RepRef{nullptr, 0};
     }
 
@@ -263,9 +280,15 @@ void CordzInfo::TrackCord(InlineData& cord, MethodIdentifier method) {
 void CordzInfo::TrackCord(InlineData& cord, const InlineData& src,
                           MethodIdentifier method) {
   assert(cord.is_tree());
-  assert(!cord.is_profiled());
-  auto* info = src.is_tree() && src.is_profiled() ? src.cordz_info() : nullptr;
-  CordzInfo* cordz_info = new CordzInfo(cord.as_tree(), info, method);
+  assert(src.is_tree());
+
+  // Unsample current as we the current cord is being replaced with 'src',
+  // so any method history is no longer relevant.
+  CordzInfo* cordz_info = cord.cordz_info();
+  if (cordz_info != nullptr) cordz_info->Untrack();
+
+  // Start new cord sample
+  cordz_info = new CordzInfo(cord.as_tree(), src.cordz_info(), method);
   cord.set_cordz_info(cordz_info);
   cordz_info->Track();
 }
@@ -297,6 +320,10 @@ CordzInfo::CordzInfo(CordRep* rep, const CordzInfo* src,
       parent_method_(GetParentMethod(src)),
       create_time_(absl::Now()) {
   update_tracker_.LossyAdd(method);
+  if (src) {
+    // Copy parent counters.
+    update_tracker_.LossyAdd(src->update_tracker_);
+  }
 }
 
 CordzInfo::~CordzInfo() {
diff --git a/absl/strings/internal/cordz_info.h b/absl/strings/internal/cordz_info.h
index e84006c5..29237930 100644
--- a/absl/strings/internal/cordz_info.h
+++ b/absl/strings/internal/cordz_info.h
@@ -65,7 +65,9 @@ class ABSL_LOCKABLE CordzInfo : public CordzHandle {
   // Identical to TrackCord(), except that this function fills the
   // `parent_stack` and `parent_method` properties of the returned CordzInfo
   // instance from the provided `src` instance if `src` is sampled.
-  // This function should be used for sampling 'copy constructed' cords.
+  // This function should be used for sampling 'copy constructed' and 'copy
+  // assigned' cords. This function allows 'cord` to be already sampled, in
+  // which case the CordzInfo will be newly created from `src`.
   static void TrackCord(InlineData& cord, const InlineData& src,
                         MethodIdentifier method);
 
@@ -73,6 +75,15 @@ class ABSL_LOCKABLE CordzInfo : public CordzHandle {
   // Uses `cordz_should_profile` to randomly pick cords to be sampled, and if
   // so, invokes `TrackCord` to start sampling `cord`.
   static void MaybeTrackCord(InlineData& cord, MethodIdentifier method);
+
+  // Maybe sample the cord identified by 'cord' for method 'method'.
+  // `src` identifies a 'parent' cord which content is copied into the current
+  // cord, typically the input cord for an assign emthod or copy constructor.
+  // Invokes the corresponding `TrackCord` method if either cord is sampled, or
+  // if `cord` is randomly picked for sampling. Possible scenarios:
+  //   * `src` is sampled: `cord` will be set to sampled if not already sampled.
+  //     Parent stack and update stats of `src` are copied into `cord`
+  //   * `src` is not sampled: `cord` may be randomly picked for sampling.
   static void MaybeTrackCord(InlineData& cord, const InlineData& src,
                              MethodIdentifier method);
 
@@ -221,7 +232,8 @@ inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeTrackCord(
 
 inline ABSL_ATTRIBUTE_ALWAYS_INLINE void CordzInfo::MaybeTrackCord(
     InlineData& cord, const InlineData& src, MethodIdentifier method) {
-  if (ABSL_PREDICT_FALSE(cordz_should_profile())) {
+  if (ABSL_PREDICT_FALSE(InlineData::is_either_profiled(cord, src)) ||
+      ABSL_PREDICT_FALSE(cordz_should_profile())) {
     TrackCord(cord, src, method);
   }
 }
diff --git a/absl/strings/internal/cordz_info_statistics_test.cc b/absl/strings/internal/cordz_info_statistics_test.cc
index d46d6d62..9f2842d9 100644
--- a/absl/strings/internal/cordz_info_statistics_test.cc
+++ b/absl/strings/internal/cordz_info_statistics_test.cc
@@ -42,10 +42,18 @@ inline void PrintTo(const CordzStatistics& stats, std::ostream* s) {
 
 namespace {
 
-// Creates a flat of the specified length
-CordRepFlat* Flat(int length = 512) {
-  auto* flat = CordRepFlat::New(length);
-  flat->length = length;
+// Creates a flat of the specified allocated size
+CordRepFlat* Flat(size_t size) {
+  // Round up to a tag size, as we are going to poke an exact tag size back into
+  // the allocated flat. 'size returning allocators' could grant us more than we
+  // wanted, but we are ok to poke the 'requested' size in the tag, even in the
+  // presence of sized deletes, so we need to make sure the size rounds
+  // perfectly to a tag value.
+  assert(size >= kMinFlatSize);
+  size = RoundUpForTag(size);
+  CordRepFlat* flat = CordRepFlat::New(size - kFlatOverhead);
+  flat->tag = AllocatedSizeToTag(size);
+  flat->length = size - kFlatOverhead;
   return flat;
 }
 
@@ -174,6 +182,11 @@ MATCHER_P(EqStatistics, stats, "Statistics equal expected values") {
   STATS_MATCHER_EXPECT_EQ(size);
   STATS_MATCHER_EXPECT_EQ(node_count);
   STATS_MATCHER_EXPECT_EQ(node_counts.flat);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_64);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_128);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_256);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_512);
+  STATS_MATCHER_EXPECT_EQ(node_counts.flat_1k);
   STATS_MATCHER_EXPECT_EQ(node_counts.external);
   STATS_MATCHER_EXPECT_EQ(node_counts.concat);
   STATS_MATCHER_EXPECT_EQ(node_counts.substring);
@@ -188,7 +201,7 @@ MATCHER_P(EqStatistics, stats, "Statistics equal expected values") {
 
 TEST(CordzInfoStatisticsTest, Flat) {
   RefHelper ref;
-  auto* flat = ref.NeedsUnref(Flat());
+  auto* flat = ref.NeedsUnref(Flat(512));
 
   CordzStatistics expected;
   expected.size = flat->length;
@@ -196,13 +209,14 @@ TEST(CordzInfoStatisticsTest, Flat) {
   expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
   expected.node_count = 1;
   expected.node_counts.flat = 1;
+  expected.node_counts.flat_512 = 1;
 
   EXPECT_THAT(SampleCord(flat), EqStatistics(expected));
 }
 
 TEST(CordzInfoStatisticsTest, SharedFlat) {
   RefHelper ref;
-  auto* flat = ref.Ref(ref.NeedsUnref(Flat()));
+  auto* flat = ref.Ref(ref.NeedsUnref(Flat(64)));
 
   CordzStatistics expected;
   expected.size = flat->length;
@@ -210,6 +224,7 @@ TEST(CordzInfoStatisticsTest, SharedFlat) {
   expected.estimated_fair_share_memory_usage = SizeOf(flat) / 2;
   expected.node_count = 1;
   expected.node_counts.flat = 1;
+  expected.node_counts.flat_64 = 1;
 
   EXPECT_THAT(SampleCord(flat), EqStatistics(expected));
 }
@@ -244,7 +259,7 @@ TEST(CordzInfoStatisticsTest, SharedExternal) {
 
 TEST(CordzInfoStatisticsTest, Substring) {
   RefHelper ref;
-  auto* flat = Flat();
+  auto* flat = Flat(1024);
   auto* substring = ref.NeedsUnref(Substring(flat));
 
   CordzStatistics expected;
@@ -253,6 +268,7 @@ TEST(CordzInfoStatisticsTest, Substring) {
   expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
   expected.node_count = 2;
   expected.node_counts.flat = 1;
+  expected.node_counts.flat_1k = 1;
   expected.node_counts.substring = 1;
 
   EXPECT_THAT(SampleCord(substring), EqStatistics(expected));
@@ -260,7 +276,7 @@ TEST(CordzInfoStatisticsTest, Substring) {
 
 TEST(CordzInfoStatisticsTest, SharedSubstring) {
   RefHelper ref;
-  auto* flat = ref.Ref(Flat(), 2);
+  auto* flat = ref.Ref(Flat(511), 2);
   auto* substring = ref.Ref(ref.NeedsUnref(Substring(flat)));
 
   CordzStatistics expected;
@@ -270,6 +286,7 @@ TEST(CordzInfoStatisticsTest, SharedSubstring) {
       SizeOf(substring) / 2 + SizeOf(flat) / 6;
   expected.node_count = 2;
   expected.node_counts.flat = 1;
+  expected.node_counts.flat_512 = 1;
   expected.node_counts.substring = 1;
 
   EXPECT_THAT(SampleCord(substring), EqStatistics(expected));
@@ -277,7 +294,7 @@ TEST(CordzInfoStatisticsTest, SharedSubstring) {
 
 TEST(CordzInfoStatisticsTest, Concat) {
   RefHelper ref;
-  auto* flat1 = Flat(20);
+  auto* flat1 = Flat(300);
   auto* flat2 = Flat(2000);
   auto* concat = ref.NeedsUnref(Concat(flat1, flat2));
 
@@ -288,6 +305,7 @@ TEST(CordzInfoStatisticsTest, Concat) {
   expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
   expected.node_count = 3;
   expected.node_counts.flat = 2;
+  expected.node_counts.flat_512 = 1;
   expected.node_counts.concat = 1;
 
   EXPECT_THAT(SampleCord(concat), EqStatistics(expected));
@@ -295,9 +313,9 @@ TEST(CordzInfoStatisticsTest, Concat) {
 
 TEST(CordzInfoStatisticsTest, DeepConcat) {
   RefHelper ref;
-  auto* flat1 = Flat(20);
+  auto* flat1 = Flat(300);
   auto* flat2 = Flat(2000);
-  auto* flat3 = Flat(30);
+  auto* flat3 = Flat(400);
   auto* external = External(3000);
   auto* substring = Substring(external);
   auto* concat1 = Concat(flat1, flat2);
@@ -313,6 +331,7 @@ TEST(CordzInfoStatisticsTest, DeepConcat) {
 
   expected.node_count = 8;
   expected.node_counts.flat = 3;
+  expected.node_counts.flat_512 = 2;
   expected.node_counts.external = 1;
   expected.node_counts.concat = 3;
   expected.node_counts.substring = 1;
@@ -322,9 +341,9 @@ TEST(CordzInfoStatisticsTest, DeepConcat) {
 
 TEST(CordzInfoStatisticsTest, DeepSharedConcat) {
   RefHelper ref;
-  auto* flat1 = Flat(20);
+  auto* flat1 = Flat(40);
   auto* flat2 = ref.Ref(Flat(2000), 4);
-  auto* flat3 = Flat(30);
+  auto* flat3 = Flat(70);
   auto* external = ref.Ref(External(3000));
   auto* substring = ref.Ref(Substring(external), 3);
   auto* concat1 = Concat(flat1, flat2);
@@ -339,6 +358,8 @@ TEST(CordzInfoStatisticsTest, DeepSharedConcat) {
   expected.estimated_fair_share_memory_usage = FairShare(concat);
   expected.node_count = 8;
   expected.node_counts.flat = 3;
+  expected.node_counts.flat_64 = 1;
+  expected.node_counts.flat_128 = 1;
   expected.node_counts.external = 1;
   expected.node_counts.concat = 3;
   expected.node_counts.substring = 1;
@@ -348,9 +369,9 @@ TEST(CordzInfoStatisticsTest, DeepSharedConcat) {
 
 TEST(CordzInfoStatisticsTest, Ring) {
   RefHelper ref;
-  auto* flat1 = Flat(20);
+  auto* flat1 = Flat(240);
   auto* flat2 = Flat(2000);
-  auto* flat3 = Flat(30);
+  auto* flat3 = Flat(70);
   auto* external = External(3000);
   CordRepRing* ring = CordRepRing::Create(flat1);
   ring = CordRepRing::Append(ring, flat2);
@@ -365,6 +386,8 @@ TEST(CordzInfoStatisticsTest, Ring) {
   expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
   expected.node_count = 5;
   expected.node_counts.flat = 3;
+  expected.node_counts.flat_128 = 1;
+  expected.node_counts.flat_256 = 1;
   expected.node_counts.external = 1;
   expected.node_counts.ring = 1;
 
@@ -373,9 +396,9 @@ TEST(CordzInfoStatisticsTest, Ring) {
 
 TEST(CordzInfoStatisticsTest, SharedSubstringRing) {
   RefHelper ref;
-  auto* flat1 = ref.Ref(Flat(20));
+  auto* flat1 = ref.Ref(Flat(240));
   auto* flat2 = Flat(200);
-  auto* flat3 = Flat(30);
+  auto* flat3 = Flat(70);
   auto* external = ref.Ref(External(3000), 5);
   CordRepRing* ring = CordRepRing::Create(flat1);
   ring = CordRepRing::Append(ring, flat2);
@@ -392,6 +415,8 @@ TEST(CordzInfoStatisticsTest, SharedSubstringRing) {
   expected.estimated_fair_share_memory_usage = FairShare(substring);
   expected.node_count = 6;
   expected.node_counts.flat = 3;
+  expected.node_counts.flat_128 = 1;
+  expected.node_counts.flat_256 = 2;
   expected.node_counts.external = 1;
   expected.node_counts.ring = 1;
   expected.node_counts.substring = 1;
@@ -447,7 +472,7 @@ TEST(CordzInfoStatisticsTest, ThreadSafety) {
                 cord.set_inline_size(0);
               } else {
                 // 50/50 Ring or Flat coin toss
-                CordRep* rep = Flat();
+                CordRep* rep = Flat(256);
                 rep = (coin_toss(gen) != 0) ? CordRepRing::Create(rep) : rep;
                 cord.make_tree(rep);
 
diff --git a/absl/strings/internal/cordz_info_test.cc b/absl/strings/internal/cordz_info_test.cc
index b7eb910d..59a8c525 100644
--- a/absl/strings/internal/cordz_info_test.cc
+++ b/absl/strings/internal/cordz_info_test.cc
@@ -74,6 +74,20 @@ TEST(CordzInfoTest, TrackCord) {
   info->Untrack();
 }
 
+TEST(CordzInfoTest, MaybeTrackCordOnSampledCord) {
+  TestCordData data1;
+  CordzInfo::TrackCord(data1.data, kTrackCordMethod);
+  CordzInfo* info1 = data1.data.cordz_info();
+  TestCordData data2;
+  CordzInfo::MaybeTrackCord(data2.data, data1.data, kTrackCordMethod);
+  CordzInfo* info2 = data2.data.cordz_info();
+  ASSERT_THAT(info2, Ne(nullptr));
+  EXPECT_THAT(info2->GetCordRepForTesting(), Eq(data2.rep.rep));
+  info2->Untrack();
+  info1->Untrack();
+}
+
+
 TEST(CordzInfoTest, UntrackCord) {
   TestCordData data;
   CordzInfo::TrackCord(data.data, kTrackCordMethod);
@@ -291,19 +305,6 @@ TEST(CordzInfoTest, FromParent) {
   info_child->Untrack();
 }
 
-TEST(CordzInfoTest, FromParentInlined) {
-  InlineData parent;
-  TestCordData child;
-  CordzInfo* info = TrackChildCord(child.data, parent);
-  EXPECT_TRUE(info->GetParentStack().empty());
-  CordzStatistics statistics = info->GetCordzStatistics();
-  EXPECT_THAT(statistics.size, Eq(child.rep.rep->length));
-  EXPECT_THAT(statistics.method, Eq(kChildMethod));
-  EXPECT_THAT(statistics.parent_method, Eq(kUnknownMethod));
-  EXPECT_THAT(statistics.update_tracker.Value(kChildMethod), Eq(1));
-  info->Untrack();
-}
-
 }  // namespace
 }  // namespace cord_internal
 ABSL_NAMESPACE_END
diff --git a/absl/strings/internal/cordz_statistics.h b/absl/strings/internal/cordz_statistics.h
index 1c93dfd0..e03c651e 100644
--- a/absl/strings/internal/cordz_statistics.h
+++ b/absl/strings/internal/cordz_statistics.h
@@ -30,11 +30,16 @@ struct CordzStatistics {
 
   // Node counts information
   struct NodeCounts {
-    size_t flat = 0;
-    size_t external = 0;
-    size_t substring = 0;
-    size_t concat = 0;
-    size_t ring = 0;
+    size_t flat = 0;       // #flats
+    size_t flat_64 = 0;    // #flats up to 64 bytes
+    size_t flat_128 = 0;   // #flats up to 128 bytes
+    size_t flat_256 = 0;   // #flats up to 256 bytes
+    size_t flat_512 = 0;   // #flats up to 512 bytes
+    size_t flat_1k = 0;    // #flats up to 1K bytes
+    size_t external = 0;   // #external reps
+    size_t substring = 0;  // #substring reps
+    size_t concat = 0;     // #concat reps
+    size_t ring = 0;       // #ring buffer reps
   };
 
   // The size of the cord in bytes. This matches the result of Cord::size().
diff --git a/absl/strings/internal/cordz_update_tracker.h b/absl/strings/internal/cordz_update_tracker.h
index d5b14067..02efcc3a 100644
--- a/absl/strings/internal/cordz_update_tracker.h
+++ b/absl/strings/internal/cordz_update_tracker.h
@@ -91,6 +91,16 @@ class CordzUpdateTracker {
                 std::memory_order_relaxed);
   }
 
+  // Adds all the values from `src` to this instance
+  void LossyAdd(const CordzUpdateTracker& src) {
+    for (int i = 0; i < kNumMethods; ++i) {
+      MethodIdentifier method = static_cast<MethodIdentifier>(i);
+      if (int64_t value = src.Value(method)) {
+        LossyAdd(method, value);
+      }
+    }
+  }
+
  private:
   // Until C++20 std::atomic is not constexpr default-constructible, so we need
   // a wrapper for this class to be constexpr constructible.