Export of internal Abseil changes

--
9fc37c11b9e46287acef00ee06ed9adcba54dd13 by Greg Falcon <gfalcon@google.com>:

Rename absl::hash_internal::HashState to absl::hash_internal::MixingHashState.

Before this change, we had two classes named HashState: absl::HashState, the public API used for type erasure, and absl::hash_internal::HashState, the internal concrete implementation ordinarily used.

The internal class used to be named `CityHashState`, but we renamed it to `HashState` it when we changed underlying hash implementation to wyhash.  This inadvertent naming conflict made the code much harder to read, and this change intends to undo that.

PiperOrigin-RevId: 373481959

--
4aec55ffddebd085c239352a2e20721091f719a1 by Greg Falcon <gfalcon@google.com>:

Introduce absl::HashOf(), a convenience wrapper around absl::Hash that calculates hashes from the values of its arguments.

PiperOrigin-RevId: 373461406

--
86b5fd8db50bbc8bd0aa9258523527381fe0445d by Abseil Team <absl-team@google.com>:

Improve speed of BlockingCounter by making its most common path lock free.

With the new implementation, the fast path of BlockingCounter::DecrementCount()
is only a fetch_sub operation. This is most times much more efficient than the
previous implementation (full mutex lock/unlock). As a matter of fact, in most
actual usecases in practice, the waiter thread is already waiting on the
Wait() call when DecrementCount() is called, which makes Mutex::Unlock() take
the slow path as there's a waiter thread that it might need to wake up.

PiperOrigin-RevId: 373394164

--
65c876be5eac0cd32583ff8535ede4109d39cf3f by Martijn Vels <mvels@google.com>:

Move the 'sample copied cord' logic into MaybeTrackCord(),

This changes move the logic for selecting if a cord should remain being sampled from Cord to CordzInfo::MaybeTrackCord, and updates the documentation for the latter method.

PiperOrigin-RevId: 373363168

--
e84410bd0aada293a81dfb82656c952e209e21fb by Martijn Vels <mvels@google.com>:

Add check for the first call to cordz_should_profile() for each thread.

This prevents the first cord of a newly created thread to be always sampled, which is a 'bad' kind of determinism for sampling.

PiperOrigin-RevId: 373229768

--
bf09c589dc099ac8f4af780bf7e609c53c27574c by Samuel Benzaquen <sbenza@google.com>:

Refactor the Flags structure into an enum.
This gives us more control over the representation and allows for easier
merging during parsing.

PiperOrigin-RevId: 373163038

--
b947b0c51083b7b6508284b5d31819596c91729e by Derek Mauro <dmauro@google.com>:

Fixes warnings about shadowed variables
Fixes #956

PiperOrigin-RevId: 373158133
GitOrigin-RevId: 9fc37c11b9e46287acef00ee06ed9adcba54dd13
Change-Id: I91f35699f9bf439d1a870c6493946a310afe088c

26 files changed, 501 insertions, 320 deletions

diff --git a/absl/copts/GENERATED_AbseilCopts.cmake b/absl/copts/GENERATED_AbseilCopts.cmake
index 18a1e5c3..22a25eba 100644
--- a/absl/copts/GENERATED_AbseilCopts.cmake
+++ b/absl/copts/GENERATED_AbseilCopts.cmake
@@ -77,7 +77,7 @@ list(APPEND ABSL_LLVM_FLAGS
     "-Woverlength-strings"
     "-Wpointer-arith"
     "-Wself-assign"
-    "-Wshadow"
+    "-Wshadow-all"
     "-Wstring-conversion"
     "-Wtautological-overlap-compare"
     "-Wundef"
diff --git a/absl/copts/GENERATED_copts.bzl b/absl/copts/GENERATED_copts.bzl
index d2bd5608..0c7a91bd 100644
--- a/absl/copts/GENERATED_copts.bzl
+++ b/absl/copts/GENERATED_copts.bzl
@@ -78,7 +78,7 @@ ABSL_LLVM_FLAGS = [
     "-Woverlength-strings",
     "-Wpointer-arith",
     "-Wself-assign",
-    "-Wshadow",
+    "-Wshadow-all",
     "-Wstring-conversion",
     "-Wtautological-overlap-compare",
     "-Wundef",
diff --git a/absl/copts/copts.py b/absl/copts/copts.py
index ce30df89..7268c680 100644
--- a/absl/copts/copts.py
+++ b/absl/copts/copts.py
@@ -93,7 +93,7 @@ COPT_VARS = {
         "-Woverlength-strings",
         "-Wpointer-arith",
         "-Wself-assign",
-        "-Wshadow",
+        "-Wshadow-all",
         "-Wstring-conversion",
         "-Wtautological-overlap-compare",
         "-Wundef",
diff --git a/absl/hash/hash.h b/absl/hash/hash.h
index 5de132ca..69c44fde 100644
--- a/absl/hash/hash.h
+++ b/absl/hash/hash.h
@@ -73,6 +73,8 @@
 #ifndef ABSL_HASH_HASH_H_
 #define ABSL_HASH_HASH_H_
 
+#include <tuple>
+
 #include "absl/hash/internal/hash.h"
 
 namespace absl {
@@ -214,6 +216,26 @@ ABSL_NAMESPACE_BEGIN
 template <typename T>
 using Hash = absl::hash_internal::Hash<T>;
 
+// HashOf
+//
+// absl::HashOf() is a helper that generates a hash from the values of its
+// arguments.  It dispatches to absl::Hash directly, as follows:
+//  * HashOf(t) == absl::Hash<T>{}(t)
+//  * HashOf(a, b, c) == HashOf(std::make_tuple(a, b, c))
+//
+// HashOf(a1, a2, ...) == HashOf(b1, b2, ...) is guaranteed when
+//  * The argument lists have pairwise identical C++ types
+//  * a1 == b1 && a2 == b2 && ...
+//
+// The requirement that the arguments match in both type and value is critical.
+// It means that `a == b` does not necessarily imply `HashOf(a) == HashOf(b)` if
+// `a` and `b` have different types. For example, `HashOf(2) != HashOf(2.0)`.
+template <typename... Types>
+size_t HashOf(const Types&... values) {
+  auto tuple = std::tie(values...);
+  return absl::Hash<decltype(tuple)>{}(tuple);
+}
+
 // HashState
 //
 // A type erased version of the hash state concept, for use in user-defined
diff --git a/absl/hash/hash_test.cc b/absl/hash/hash_test.cc
index 1d2e6cf0..02b7733d 100644
--- a/absl/hash/hash_test.cc
+++ b/absl/hash/hash_test.cc
@@ -973,4 +973,26 @@ TEST(HashTest, DoesNotUseImplicitConversionsToBool) {
             absl::Hash<ValueWithBoolConversion>()(ValueWithBoolConversion{1}));
 }
 
+TEST(HashOf, MatchesHashForSingleArgument) {
+  std::string s = "forty two";
+  int i = 42;
+  double d = 42.0;
+  std::tuple<int, int> t{4, 2};
+
+  EXPECT_EQ(absl::HashOf(s), absl::Hash<std::string>{}(s));
+  EXPECT_EQ(absl::HashOf(i), absl::Hash<int>{}(i));
+  EXPECT_EQ(absl::HashOf(d), absl::Hash<double>{}(d));
+  EXPECT_EQ(absl::HashOf(t), (absl::Hash<std::tuple<int, int>>{}(t)));
+}
+
+TEST(HashOf, MatchesHashOfTupleForMultipleArguments) {
+  std::string hello = "hello";
+  std::string world = "world";
+
+  EXPECT_EQ(absl::HashOf(), absl::HashOf(std::make_tuple()));
+  EXPECT_EQ(absl::HashOf(hello), absl::HashOf(std::make_tuple(hello)));
+  EXPECT_EQ(absl::HashOf(hello, world),
+            absl::HashOf(std::make_tuple(hello, world)));
+}
+
 }  // namespace
diff --git a/absl/hash/internal/hash.cc b/absl/hash/internal/hash.cc
index 1433eb9d..06f53a59 100644
--- a/absl/hash/internal/hash.cc
+++ b/absl/hash/internal/hash.cc
@@ -18,9 +18,8 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
-uint64_t HashState::CombineLargeContiguousImpl32(uint64_t state,
-                                                 const unsigned char* first,
-                                                 size_t len) {
+uint64_t MixingHashState::CombineLargeContiguousImpl32(
+    uint64_t state, const unsigned char* first, size_t len) {
   while (len >= PiecewiseChunkSize()) {
     state =
         Mix(state, absl::hash_internal::CityHash32(reinterpret_cast<const char*>(first),
@@ -33,9 +32,8 @@ uint64_t HashState::CombineLargeContiguousImpl32(uint64_t state,
                                std::integral_constant<int, 4>{});
 }
 
-uint64_t HashState::CombineLargeContiguousImpl64(uint64_t state,
-                                                 const unsigned char* first,
-                                                 size_t len) {
+uint64_t MixingHashState::CombineLargeContiguousImpl64(
+    uint64_t state, const unsigned char* first, size_t len) {
   while (len >= PiecewiseChunkSize()) {
     state = Mix(state, Hash64(first, PiecewiseChunkSize()));
     len -= PiecewiseChunkSize();
@@ -46,7 +44,7 @@ uint64_t HashState::CombineLargeContiguousImpl64(uint64_t state,
                                std::integral_constant<int, 8>{});
 }
 
-ABSL_CONST_INIT const void* const HashState::kSeed = &kSeed;
+ABSL_CONST_INIT const void* const MixingHashState::kSeed = &kSeed;
 
 // The salt array used by Wyhash. This array is NOT the mechanism used to make
 // absl::Hash non-deterministic between program invocations.  See `Seed()` for
@@ -61,7 +59,7 @@ constexpr uint64_t kWyhashSalt[5] = {
     uint64_t{0x452821E638D01377},
 };
 
-uint64_t HashState::WyhashImpl(const unsigned char* data, size_t len) {
+uint64_t MixingHashState::WyhashImpl(const unsigned char* data, size_t len) {
   return Wyhash(data, len, Seed(), kWyhashSalt);
 }
 
diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index 7fb0af0b..69dbbc6b 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -379,7 +379,7 @@ template <typename H, typename... Ts>
 // This SFINAE gets MSVC confused under some conditions. Let's just disable it
 // for now.
 H
-#else  // _MSC_VER
+#else   // _MSC_VER
 typename std::enable_if<absl::conjunction<is_hashable<Ts>...>::value, H>::type
 #endif  // _MSC_VER
 AbslHashValue(H hash_state, const std::tuple<Ts...>& t) {
@@ -714,8 +714,8 @@ template <typename T>
 struct is_hashable
     : std::integral_constant<bool, HashSelect::template Apply<T>::value> {};
 
-// HashState
-class ABSL_DLL HashState : public HashStateBase<HashState> {
+// MixingHashState
+class ABSL_DLL MixingHashState : public HashStateBase<MixingHashState> {
   // absl::uint128 is not an alias or a thin wrapper around the intrinsic.
   // We use the intrinsic when available to improve performance.
 #ifdef ABSL_HAVE_INTRINSIC_INT128
@@ -734,22 +734,23 @@ class ABSL_DLL HashState : public HashStateBase<HashState> {
 
  public:
   // Move only
-  HashState(HashState&&) = default;
-  HashState& operator=(HashState&&) = default;
+  MixingHashState(MixingHashState&&) = default;
+  MixingHashState& operator=(MixingHashState&&) = default;
 
-  // HashState::combine_contiguous()
+  // MixingHashState::combine_contiguous()
   //
   // Fundamental base case for hash recursion: mixes the given range of bytes
   // into the hash state.
-  static HashState combine_contiguous(HashState hash_state,
-                                      const unsigned char* first, size_t size) {
-    return HashState(
+  static MixingHashState combine_contiguous(MixingHashState hash_state,
+                                            const unsigned char* first,
+                                            size_t size) {
+    return MixingHashState(
         CombineContiguousImpl(hash_state.state_, first, size,
                               std::integral_constant<int, sizeof(size_t)>{}));
   }
-  using HashState::HashStateBase::combine_contiguous;
+  using MixingHashState::HashStateBase::combine_contiguous;
 
-  // HashState::hash()
+  // MixingHashState::hash()
   //
   // For performance reasons in non-opt mode, we specialize this for
   // integral types.
@@ -761,24 +762,24 @@ class ABSL_DLL HashState : public HashStateBase<HashState> {
     return static_cast<size_t>(Mix(Seed(), static_cast<uint64_t>(value)));
   }
 
-  // Overload of HashState::hash()
+  // Overload of MixingHashState::hash()
   template <typename T, absl::enable_if_t<!IntegralFastPath<T>::value, int> = 0>
   static size_t hash(const T& value) {
-    return static_cast<size_t>(combine(HashState{}, value).state_);
+    return static_cast<size_t>(combine(MixingHashState{}, value).state_);
   }
 
  private:
   // Invoked only once for a given argument; that plus the fact that this is
   // move-only ensures that there is only one non-moved-from object.
-  HashState() : state_(Seed()) {}
+  MixingHashState() : state_(Seed()) {}
 
   // Workaround for MSVC bug.
   // We make the type copyable to fix the calling convention, even though we
   // never actually copy it. Keep it private to not affect the public API of the
   // type.
-  HashState(const HashState&) = default;
+  MixingHashState(const MixingHashState&) = default;
 
-  explicit HashState(uint64_t state) : state_(state) {}
+  explicit MixingHashState(uint64_t state) : state_(state) {}
 
   // Implementation of the base case for combine_contiguous where we actually
   // mix the bytes into the state.
@@ -793,7 +794,6 @@ class ABSL_DLL HashState : public HashStateBase<HashState> {
                                         std::integral_constant<int, 8>
                                         /* sizeof_size_t */);
 
-
   // Slow dispatch path for calls to CombineContiguousImpl with a size argument
   // larger than PiecewiseChunkSize().  Has the same effect as calling
   // CombineContiguousImpl() repeatedly with the chunk stride size.
@@ -911,8 +911,8 @@ class ABSL_DLL HashState : public HashStateBase<HashState> {
   uint64_t state_;
 };
 
-// HashState::CombineContiguousImpl()
-inline uint64_t HashState::CombineContiguousImpl(
+// MixingHashState::CombineContiguousImpl()
+inline uint64_t MixingHashState::CombineContiguousImpl(
     uint64_t state, const unsigned char* first, size_t len,
     std::integral_constant<int, 4> /* sizeof_size_t */) {
   // For large values we use CityHash, for small ones we just use a
@@ -934,8 +934,8 @@ inline uint64_t HashState::CombineContiguousImpl(
   return Mix(state, v);
 }
 
-// Overload of HashState::CombineContiguousImpl()
-inline uint64_t HashState::CombineContiguousImpl(
+// Overload of MixingHashState::CombineContiguousImpl()
+inline uint64_t MixingHashState::CombineContiguousImpl(
     uint64_t state, const unsigned char* first, size_t len,
     std::integral_constant<int, 8> /* sizeof_size_t */) {
   // For large values we use Wyhash or CityHash depending on the platform, for
@@ -976,7 +976,9 @@ struct PoisonedHash : private AggregateBarrier {
 
 template <typename T>
 struct HashImpl {
-  size_t operator()(const T& value) const { return HashState::hash(value); }
+  size_t operator()(const T& value) const {
+    return MixingHashState::hash(value);
+  }
 };
 
 template <typename T>
diff --git a/absl/status/internal/status_internal.h b/absl/status/internal/status_internal.h
index ccafd702..ac12940a 100644
--- a/absl/status/internal/status_internal.h
+++ b/absl/status/internal/status_internal.h
@@ -47,12 +47,12 @@ using Payloads = absl::InlinedVector<Payload, 1>;
 
 // Reference-counted representation of Status data.
 struct StatusRep {
-  StatusRep(absl::StatusCode code, absl::string_view message,
-            std::unique_ptr<status_internal::Payloads> payloads)
+  StatusRep(absl::StatusCode code_arg, absl::string_view message_arg,
+            std::unique_ptr<status_internal::Payloads> payloads_arg)
       : ref(int32_t{1}),
-        code(code),
-        message(message),
-        payloads(std::move(payloads)) {}
+        code(code_arg),
+        message(message_arg),
+        payloads(std::move(payloads_arg)) {}
 
   std::atomic<int32_t> ref;
   absl::StatusCode code;
diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index 5dad781e..f5aa6e47 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc
@@ -536,24 +536,10 @@ void Cord::InlineRep::AssignSlow(const Cord::InlineRep& src) {
     return;
   }
 
-  // See b/187581164: unsample cord if already sampled
-  // TODO(b/117940323): continuously 'assigned to' cords would reach 100%
-  // sampling probability. Imagine a cord x in some cache:
-  //   cache.SetCord(const Cord& foo) {
-  //     x = foo;
-  //   }
-  // CordzInfo::MaybeTrackCord does:
-  // x.profiled = foo.profiled | x.profiled | random(cordz_mean_interval)
-  // Which means it will on the long run converge to 'always samples'
-  // The real fix is in CordzMaybeTrackCord, but the below is a low risk
-  // forward fix for b/187581164 and similar BT benchmark regressions.
-  if (ABSL_PREDICT_FALSE(is_profiled())) {
-    cordz_info()->Untrack();
-    clear_cordz_info();
-  }
-
   CordRep* tree = as_tree();
   if (CordRep* src_tree = src.tree()) {
+    // Leave any existing `cordz_info` in place, and let MaybeTrackCord()
+    // decide if this cord should be (or remains to be) sampled or not.
     data_.set_tree(CordRep::Ref(src_tree));
     CordzInfo::MaybeTrackCord(data_, src.data_, method);
   } else {
diff --git a/absl/strings/cordz_test.cc b/absl/strings/cordz_test.cc
index 0e11f5c8..2b7d30b0 100644
--- a/absl/strings/cordz_test.cc
+++ b/absl/strings/cordz_test.cc
@@ -69,6 +69,7 @@ absl::string_view MakeString(TestCordSize size) {
 
 // Returns a cord with a sampled method of kAppendString.
 absl::Cord MakeAppendStringCord(TestCordSize size) {
+  CordzSamplingIntervalHelper always(1);
   absl::Cord cord;
   cord.Append(MakeString(size));
   return cord;
@@ -136,21 +137,21 @@ TEST_P(CordzStringTest, ConstructString) {
   }
 }
 
-TEST(CordzTest, CopyConstruct) {
+TEST(CordzTest, CopyConstructFromUnsampled) {
   CordzSamplingIntervalHelper sample_every{1};
   Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
   Cord cord(src);
-  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorCord));
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
 }
 
 TEST(CordzTest, CopyConstructFromSampled) {
-  CordzSamplingIntervalHelper sample_every{1};
-  Cord src(MakeString(TestCordSize::kLarge));
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord src = MakeAppendStringCord(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));
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
 }
 
 TEST(CordzTest, MoveConstruct) {
@@ -160,12 +161,12 @@ TEST(CordzTest, MoveConstruct) {
   EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kConstructorString));
 }
 
-TEST_P(CordzUpdateTest, AssignCord) {
+TEST_P(CordzUpdateTest, AssignUnsampledCord) {
   Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  const CordzInfo* info = GetCordzInfoForTesting(cord());
   cord() = src;
-  EXPECT_THAT(cord(), HasValidCordzInfoOf(Method::kAssignCord));
-  CordzStatistics stats = GetCordzInfoForTesting(cord())->GetCordzStatistics();
-  EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
+  EXPECT_THAT(GetCordzInfoForTesting(cord()), Eq(nullptr));
+  EXPECT_FALSE(CordzInfoIsListed(info));
 }
 
 TEST_P(CordzUpdateTest, AssignSampledCord) {
@@ -178,10 +179,22 @@ TEST_P(CordzUpdateTest, AssignSampledCord) {
   EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
 }
 
-TEST(CordzUpdateTest, AssignSampledCordToUnsampledCord) {
-  CordzSamplingIntervalHelper sample_every{1};
+TEST(CordzUpdateTest, AssignSampledCordToInlined) {
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord cord;
   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_never{99999};
   Cord cord = UnsampledCord(MakeString(TestCordSize::kLarge));
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
   cord = src;
   ASSERT_THAT(cord, HasValidCordzInfoOf(Method::kAssignCord));
   CordzStatistics stats = GetCordzInfoForTesting(cord)->GetCordzStatistics();
@@ -190,6 +203,26 @@ TEST(CordzUpdateTest, AssignSampledCordToUnsampledCord) {
   EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
 }
 
+TEST(CordzUpdateTest, AssignUnsampledCordToSampledCordWithoutSampling) {
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord cord = MakeAppendStringCord(TestCordSize::kLarge);
+  const CordzInfo* info = GetCordzInfoForTesting(cord);
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord = src;
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+  EXPECT_FALSE(CordzInfoIsListed(info));
+}
+
+TEST(CordzUpdateTest, AssignUnsampledCordToSampledCordWithSampling) {
+  CordzSamplingIntervalHelper sample_every{1};
+  Cord cord = MakeAppendStringCord(TestCordSize::kLarge);
+  const CordzInfo* info = GetCordzInfoForTesting(cord);
+  Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
+  cord = src;
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
+  EXPECT_FALSE(CordzInfoIsListed(info));
+}
+
 TEST(CordzUpdateTest, AssignSampledCordToSampledCord) {
   CordzSamplingIntervalHelper sample_every{1};
   Cord src = MakeAppendStringCord(TestCordSize::kLarge);
@@ -202,7 +235,19 @@ TEST(CordzUpdateTest, AssignSampledCordToSampledCord) {
   EXPECT_THAT(stats.update_tracker.Value(Method::kConstructorString), Eq(0));
 }
 
-TEST(CordzTest, AssignInlinedCord) {
+TEST(CordzUpdateTest, AssignUnsampledCordToSampledCord) {
+  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, AssignInlinedCordToSampledCord) {
   CordzSampleToken token;
   CordzSamplingIntervalHelper sample_every{1};
   Cord cord(MakeString(TestCordSize::kLarge));
@@ -389,28 +434,28 @@ TEST(CordzTest, RemoveSuffix) {
   EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
 }
 
-TEST(CordzTest, SubCord) {
+TEST(CordzTest, SubCordFromUnsampledCord) {
   CordzSamplingIntervalHelper sample_every{1};
   Cord src = UnsampledCord(MakeString(TestCordSize::kLarge));
   Cord cord = src.Subcord(10, src.size() / 2);
-  EXPECT_THAT(cord, 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));
+  EXPECT_THAT(GetCordzInfoForTesting(cord), Eq(nullptr));
 }
 
 TEST(CordzTest, SubCordFromSampledCord) {
-  CordzSamplingIntervalHelper sample_every{1};
-  Cord src(MakeString(TestCordSize::kLarge));
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
   Cord cord = src.Subcord(10, src.size() / 2);
-  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kSubCord));
+  ASSERT_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));
+  EXPECT_THAT(stats.parent_method, Eq(Method::kAppendString));
+  EXPECT_THAT(stats.update_tracker.Value(Method::kAppendString), Eq(1));
+}
+
+TEST(CordzTest, SmallSubCord) {
+  CordzSamplingIntervalHelper sample_never{99999};
+  Cord src = MakeAppendStringCord(TestCordSize::kLarge);
+  Cord cord = src.Subcord(10, kMaxInline + 1);
+  EXPECT_THAT(cord, HasValidCordzInfoOf(Method::kSubCord));
 }
 
 }  // namespace
diff --git a/absl/strings/internal/cord_rep_ring.cc b/absl/strings/internal/cord_rep_ring.cc
index 09951290..f78c94e1 100644
--- a/absl/strings/internal/cord_rep_ring.cc
+++ b/absl/strings/internal/cord_rep_ring.cc
@@ -32,15 +32,6 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
-// See https://bugs.llvm.org/show_bug.cgi?id=48477
-#ifdef __clang__
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wshadow"
-#if __has_warning("-Wshadow-field")
-#pragma clang diagnostic ignored "-Wshadow-field"
-#endif
-#endif
-
 namespace {
 
 using index_type = CordRepRing::index_type;
@@ -450,12 +441,12 @@ Span<char> CordRepRing::GetPrependBuffer(size_t size) {
 }
 
 CordRepRing* CordRepRing::CreateFromLeaf(CordRep* child, size_t offset,
-                                         size_t length, size_t extra) {
+                                         size_t len, size_t extra) {
   CordRepRing* rep = CordRepRing::New(1, extra);
   rep->head_ = 0;
   rep->tail_ = rep->advance(0);
-  rep->length = length;
-  rep->entry_end_pos()[0] = length;
+  rep->length = len;
+  rep->entry_end_pos()[0] = len;
   rep->entry_child()[0] = child;
   rep->entry_data_offset()[0] = static_cast<offset_type>(offset);
   return Validate(rep);
@@ -463,16 +454,16 @@ CordRepRing* CordRepRing::CreateFromLeaf(CordRep* child, size_t offset,
 
 CordRepRing* CordRepRing::CreateSlow(CordRep* child, size_t extra) {
   CordRepRing* rep = nullptr;
-  Consume(child, [&](CordRep* child, size_t offset, size_t length) {
-    if (IsFlatOrExternal(child)) {
-      rep = rep ? AppendLeaf(rep, child, offset, length)
-                : CreateFromLeaf(child, offset, length, extra);
+  Consume(child, [&](CordRep* child_arg, size_t offset, size_t len) {
+    if (IsFlatOrExternal(child_arg)) {
+      rep = rep ? AppendLeaf(rep, child_arg, offset, len)
+                : CreateFromLeaf(child_arg, offset, len, extra);
     } else if (rep) {
-      rep = AddRing<AddMode::kAppend>(rep, child->ring(), offset, length);
-    } else if (offset == 0 && child->length == length) {
-      rep = Mutable(child->ring(), extra);
+      rep = AddRing<AddMode::kAppend>(rep, child_arg->ring(), offset, len);
+    } else if (offset == 0 && child_arg->length == len) {
+      rep = Mutable(child_arg->ring(), extra);
     } else {
-      rep = SubRing(child->ring(), offset, length, extra);
+      rep = SubRing(child_arg->ring(), offset, len, extra);
     }
   });
   return Validate(rep, nullptr, __LINE__);
@@ -491,18 +482,18 @@ CordRepRing* CordRepRing::Create(CordRep* child, size_t extra) {
 
 template <CordRepRing::AddMode mode>
 CordRepRing* CordRepRing::AddRing(CordRepRing* rep, CordRepRing* ring,
-                                  size_t offset, size_t length) {
+                                  size_t offset, size_t len) {
   assert(offset < ring->length);
   constexpr bool append = mode == AddMode::kAppend;
   Position head = ring->Find(offset);
-  Position tail = ring->FindTail(head.index, offset + length);
+  Position tail = ring->FindTail(head.index, offset + len);
   const index_type entries = ring->entries(head.index, tail.index);
 
   rep = Mutable(rep, entries);
 
   // The delta for making ring[head].end_pos into 'len - offset'
   const pos_type delta_length =
-      (append ? rep->begin_pos_ + rep->length : rep->begin_pos_ - length) -
+      (append ? rep->begin_pos_ + rep->length : rep->begin_pos_ - len) -
       ring->entry_begin_pos(head.index) - head.offset;
 
   // Start filling at `tail`, or `entries` before `head`
@@ -543,36 +534,36 @@ CordRepRing* CordRepRing::AddRing(CordRepRing* rep, CordRepRing* ring,
   }
 
   // Commit changes
-  rep->length += length;
+  rep->length += len;
   if (append) {
     rep->tail_ = filler.pos();
   } else {
     rep->head_ = filler.head();
-    rep->begin_pos_ -= length;
+    rep->begin_pos_ -= len;
   }
 
   return Validate(rep);
 }
 
 CordRepRing* CordRepRing::AppendSlow(CordRepRing* rep, CordRep* child) {
-  Consume(child, [&rep](CordRep* child, size_t offset, size_t length) {
-    if (child->tag == RING) {
-      rep = AddRing<AddMode::kAppend>(rep, child->ring(), offset, length);
+  Consume(child, [&rep](CordRep* child_arg, size_t offset, size_t len) {
+    if (child_arg->tag == RING) {
+      rep = AddRing<AddMode::kAppend>(rep, child_arg->ring(), offset, len);
     } else {
-      rep = AppendLeaf(rep, child, offset, length);
+      rep = AppendLeaf(rep, child_arg, offset, len);
     }
   });
   return rep;
 }
 
 CordRepRing* CordRepRing::AppendLeaf(CordRepRing* rep, CordRep* child,
-                                     size_t offset, size_t length) {
+                                     size_t offset, size_t len) {
   rep = Mutable(rep, 1);
   index_type back = rep->tail_;
   const pos_type begin_pos = rep->begin_pos_ + rep->length;
   rep->tail_ = rep->advance(rep->tail_);
-  rep->length += length;
-  rep->entry_end_pos()[back] = begin_pos + length;
+  rep->length += len;
+  rep->entry_end_pos()[back] = begin_pos + len;
   rep->entry_child()[back] = child;
   rep->entry_data_offset()[back] = static_cast<offset_type>(offset);
   return Validate(rep, nullptr, __LINE__);
@@ -590,24 +581,24 @@ CordRepRing* CordRepRing::Append(CordRepRing* rep, CordRep* child) {
 }
 
 CordRepRing* CordRepRing::PrependSlow(CordRepRing* rep, CordRep* child) {
-  RConsume(child, [&](CordRep* child, size_t offset, size_t length) {
-    if (IsFlatOrExternal(child)) {
-      rep = PrependLeaf(rep, child, offset, length);
+  RConsume(child, [&](CordRep* child_arg, size_t offset, size_t len) {
+    if (IsFlatOrExternal(child_arg)) {
+      rep = PrependLeaf(rep, child_arg, offset, len);
     } else {
-      rep = AddRing<AddMode::kPrepend>(rep, child->ring(), offset, length);
+      rep = AddRing<AddMode::kPrepend>(rep, child_arg->ring(), offset, len);
     }
   });
   return Validate(rep);
 }
 
 CordRepRing* CordRepRing::PrependLeaf(CordRepRing* rep, CordRep* child,
-                                      size_t offset, size_t length) {
+                                      size_t offset, size_t len) {
   rep = Mutable(rep, 1);
   index_type head = rep->retreat(rep->head_);
   pos_type end_pos = rep->begin_pos_;
   rep->head_ = head;
-  rep->length += length;
-  rep->begin_pos_ -= length;
+  rep->length += len;
+  rep->begin_pos_ -= len;
   rep->entry_end_pos()[head] = end_pos;
   rep->entry_child()[head] = child;
   rep->entry_data_offset()[head] = static_cast<offset_type>(offset);
@@ -787,18 +778,18 @@ char CordRepRing::GetCharacter(size_t offset) const {
 }
 
 CordRepRing* CordRepRing::SubRing(CordRepRing* rep, size_t offset,
-                                  size_t length, size_t extra) {
+                                  size_t len, size_t extra) {
   assert(offset <= rep->length);
-  assert(offset <= rep->length - length);
+  assert(offset <= rep->length - len);
 
-  if (length == 0) {
+  if (len == 0) {
     CordRep::Unref(rep);
     return nullptr;
   }
 
   // Find position of first byte
   Position head = rep->Find(offset);
-  Position tail = rep->FindTail(head.index, offset + length);
+  Position tail = rep->FindTail(head.index, offset + len);
   const size_t new_entries = rep->entries(head.index, tail.index);
 
   if (rep->refcount.IsOne() && extra <= (rep->capacity() - new_entries)) {
@@ -815,7 +806,7 @@ CordRepRing* CordRepRing::SubRing(CordRepRing* rep, size_t offset,
   }
 
   // Adjust begin_pos and length
-  rep->length = length;
+  rep->length = len;
   rep->begin_pos_ += offset;
 
   // Adjust head and tail blocks
@@ -889,10 +880,6 @@ CordRepRing* CordRepRing::RemoveSuffix(CordRepRing* rep, size_t len,
   return Validate(rep);
 }
 
-#ifdef __clang__
-#pragma clang diagnostic pop
-#endif
-
 }  // namespace cord_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/cord_rep_ring.h b/absl/strings/internal/cord_rep_ring.h
index 830f2b2a..2082a565 100644
--- a/absl/strings/internal/cord_rep_ring.h
+++ b/absl/strings/internal/cord_rep_ring.h
@@ -30,15 +30,6 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
-// See https://bugs.llvm.org/show_bug.cgi?id=48477
-#ifdef __clang__
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wshadow"
-#if __has_warning("-Wshadow-field")
-#pragma clang diagnostic ignored "-Wshadow-field"
-#endif
-#endif
-
 // All operations modifying a ring buffer are implemented as static methods
 // requiring a CordRepRing instance with a reference adopted by the method.
 //
@@ -210,23 +201,23 @@ class CordRepRing : public CordRep {
   // referencing up to `size` capacity directly before the existing data.
   Span<char> GetPrependBuffer(size_t size);
 
-  // Returns a cord ring buffer containing `length` bytes of data starting at
+  // Returns a cord ring buffer containing `len` bytes of data starting at
   // `offset`. If the input is not shared, this function will remove all head
   // and tail child nodes outside of the requested range, and adjust the new
   // head and tail nodes as required. If the input is shared, this function
   // returns a new instance sharing some or all of the nodes from the input.
-  static CordRepRing* SubRing(CordRepRing* r, size_t offset, size_t length,
+  static CordRepRing* SubRing(CordRepRing* r, size_t offset, size_t len,
                               size_t extra = 0);
 
-  // Returns a cord ring buffer with the first `length` bytes removed.
+  // Returns a cord ring buffer with the first `len` bytes removed.
   // If the input is not shared, this function will remove all head child nodes
   // fully inside the first `length` bytes, and adjust the new head as required.
   // If the input is shared, this function returns a new instance sharing some
   // or all of the nodes from the input.
-  static CordRepRing* RemoveSuffix(CordRepRing* r, size_t length,
+  static CordRepRing* RemoveSuffix(CordRepRing* r, size_t len,
                                    size_t extra = 0);
 
-  // Returns a cord ring buffer with the last `length` bytes removed.
+  // Returns a cord ring buffer with the last `len` bytes removed.
   // If the input is not shared, this function will remove all head child nodes
   // fully inside the first `length` bytes, and adjust the new head as required.
   // If the input is shared, this function returns a new instance sharing some
@@ -242,12 +233,12 @@ class CordRepRing : public CordRep {
   // function returns false, and `fragment` is left unchanged.
   bool IsFlat(absl::string_view* fragment) const;
 
-  // Returns true if the data starting at `offset` with length `length` is
+  // Returns true if the data starting at `offset` with length `len` is
   // managed by this instance inside a single contiguous buffer, in which case
   // the (optional) output parameter `fragment` is set to the contiguous memory
   // starting at offset `offset` with length `length`. Otherwise, the function
   // returns false, and `fragment` is left unchanged.
-  bool IsFlat(size_t offset, size_t length, absl::string_view* fragment) const;
+  bool IsFlat(size_t offset, size_t len, absl::string_view* fragment) const;
 
   // Testing only: set capacity to requested capacity.
   void SetCapacityForTesting(size_t capacity);
@@ -473,10 +464,10 @@ class CordRepRing : public CordRep {
                                      size_t length, size_t extra);
 
   // Appends or prepends (depending on AddMode) the ring buffer in `ring' to
-  // `rep` starting at `offset` with length `length`.
+  // `rep` starting at `offset` with length `len`.
   template <AddMode mode>
   static CordRepRing* AddRing(CordRepRing* rep, CordRepRing* ring,
-                              size_t offset, size_t length);
+                              size_t offset, size_t len);
 
   // Increases the data offset for entry `index` by `n`.
   void AddDataOffset(index_type index, size_t n);
@@ -596,12 +587,12 @@ inline bool CordRepRing::IsFlat(absl::string_view* fragment) const {
   return false;
 }
 
-inline bool CordRepRing::IsFlat(size_t offset, size_t length,
+inline bool CordRepRing::IsFlat(size_t offset, size_t len,
                                 absl::string_view* fragment) const {
   const Position pos = Find(offset);
   const absl::string_view data = entry_data(pos.index);
-  if (data.length() >= length && data.length() - length >= pos.offset) {
-    if (fragment) *fragment = data.substr(pos.offset, length);
+  if (data.length() >= len && data.length() - len >= pos.offset) {
+    if (fragment) *fragment = data.substr(pos.offset, len);
     return true;
   }
   return false;
@@ -609,10 +600,6 @@ inline bool CordRepRing::IsFlat(size_t offset, size_t length,
 
 std::ostream& operator<<(std::ostream& s, const CordRepRing& rep);
 
-#ifdef __clang__
-#pragma clang diagnostic pop
-#endif
-
 }  // namespace cord_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/cordz_functions.cc b/absl/strings/internal/cordz_functions.cc
index 6ad864f1..f30080f8 100644
--- a/absl/strings/internal/cordz_functions.cc
+++ b/absl/strings/internal/cordz_functions.cc
@@ -44,7 +44,10 @@ std::atomic<int> g_cordz_mean_interval(50000);
 
 #ifdef ABSL_INTERNAL_CORDZ_ENABLED
 
-ABSL_CONST_INIT thread_local int64_t cordz_next_sample = 0;
+// Special negative 'not initialized' per thread value for cordz_next_sample.
+static constexpr int64_t kInitCordzNextSample = -1;
+
+ABSL_CONST_INIT thread_local int64_t cordz_next_sample = kInitCordzNextSample;
 
 // kIntervalIfDisabled is the number of profile-eligible events need to occur
 // before the code will confirm that cordz is still disabled.
@@ -77,8 +80,11 @@ ABSL_ATTRIBUTE_NOINLINE bool cordz_should_profile_slow() {
   }
 
   if (cordz_next_sample <= 0) {
+    // If first check on current thread, check cordz_should_profile()
+    // again using the created (initial) stride in cordz_next_sample.
+    const bool initialized = cordz_next_sample != kInitCordzNextSample;
     cordz_next_sample = exponential_biased_generator.GetStride(mean_interval);
-    return true;
+    return initialized || cordz_should_profile();
   }
 
   --cordz_next_sample;
diff --git a/absl/strings/internal/cordz_functions_test.cc b/absl/strings/internal/cordz_functions_test.cc
index f2cefae3..350623c1 100644
--- a/absl/strings/internal/cordz_functions_test.cc
+++ b/absl/strings/internal/cordz_functions_test.cc
@@ -14,6 +14,8 @@
 
 #include "absl/strings/internal/cordz_functions.h"
 
+#include <thread>  // NOLINT we need real clean new threads
+
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
@@ -63,6 +65,22 @@ TEST(CordzFunctionsTest, ShouldProfileAlways) {
   set_cordz_mean_interval(orig_sample_rate);
 }
 
+TEST(CordzFunctionsTest, DoesNotAlwaysSampleFirstCord) {
+  // Set large enough interval such that the chance of 'tons' of threads
+  // randomly sampling the first call is infinitely small.
+  set_cordz_mean_interval(10000);
+  int tries = 0;
+  bool sampled = false;
+  do {
+    ++tries;
+    ASSERT_THAT(tries, Le(1000));
+    std::thread thread([&sampled] {
+      sampled = cordz_should_profile();
+    });
+    thread.join();
+  } while (sampled);
+}
+
 TEST(CordzFunctionsTest, ShouldProfileRate) {
   static constexpr int kDesiredMeanInterval = 1000;
   static constexpr int kSamples = 10000;
diff --git a/absl/strings/internal/cordz_info.cc b/absl/strings/internal/cordz_info.cc
index a6b045ff..a3a0b9c0 100644
--- a/absl/strings/internal/cordz_info.cc
+++ b/absl/strings/internal/cordz_info.cc
@@ -293,6 +293,16 @@ void CordzInfo::TrackCord(InlineData& cord, const InlineData& src,
   cordz_info->Track();
 }
 
+void CordzInfo::MaybeTrackCordImpl(InlineData& cord, const InlineData& src,
+                                   MethodIdentifier method) {
+  if (src.is_profiled()) {
+    TrackCord(cord, src, method);
+  } else if (cord.is_profiled()) {
+    cord.cordz_info()->Untrack();
+    cord.clear_cordz_info();
+  }
+}
+
 CordzInfo::MethodIdentifier CordzInfo::GetParentMethod(const CordzInfo* src) {
   if (src == nullptr) return MethodIdentifier::kUnknown;
   return src->parent_method_ != MethodIdentifier::kUnknown ? src->parent_method_
diff --git a/absl/strings/internal/cordz_info.h b/absl/strings/internal/cordz_info.h
index 29237930..13aaee17 100644
--- a/absl/strings/internal/cordz_info.h
+++ b/absl/strings/internal/cordz_info.h
@@ -77,13 +77,31 @@ class ABSL_LOCKABLE CordzInfo : public CordzHandle {
   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.
+  // `src` identifies a 'parent' cord which is assigned to `cord`, typically the
+  // input cord for a copy constructor, or an assign method such as `operator=`
+  // `cord` will be sampled if (and only if) `src` is sampled.
+  // If `cord` is currently being sampled and `src` is not being sampled, then
+  // this function will stop sampling the cord and reset the cord's cordz_info.
+  //
+  // Previously this function defined that `cord` will be sampled if either
+  // `src` is sampled, or if `cord` is randomly picked for sampling. However,
+  // this can cause issues, as there may be paths where some cord is assigned an
+  // indirect copy of it's own value. As such a 'string of copies' would then
+  // remain sampled (`src.is_profiled`), then assigning such a cord back to
+  // 'itself' creates a cycle where the cord will converge to 'always sampled`.
+  //
+  // For example:
+  //
+  //   Cord x;
+  //   for (...) {
+  //     // Copy ctor --> y.is_profiled := x.is_profiled | random(...)
+  //     Cord y = x;
+  //     ...
+  //     // Assign x = y --> x.is_profiled = y.is_profiled | random(...)
+  //     //              ==> x.is_profiled |= random(...)
+  //     //              ==> x converges to 'always profiled'
+  //     x = y;
+  //   }
   static void MaybeTrackCord(InlineData& cord, const InlineData& src,
                              MethodIdentifier method);
 
@@ -201,6 +219,12 @@ class ABSL_LOCKABLE CordzInfo : public CordzHandle {
 #endif
   }
 
+  // Non-inlined implementation of `MaybeTrackCord`, which is executed if
+  // either `src` is sampled or `cord` is sampled, and either untracks or
+  // tracks `cord` as documented per `MaybeTrackCord`.
+  static void MaybeTrackCordImpl(InlineData& cord, const InlineData& src,
+                                 MethodIdentifier method);
+
   ABSL_CONST_INIT static List global_list_;
   List* const list_ = &global_list_;
 
@@ -232,9 +256,8 @@ 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(InlineData::is_either_profiled(cord, src)) ||
-      ABSL_PREDICT_FALSE(cordz_should_profile())) {
-    TrackCord(cord, src, method);
+  if (ABSL_PREDICT_FALSE(InlineData::is_either_profiled(cord, src))) {
+    MaybeTrackCordImpl(cord, src, method);
   }
 }
 
diff --git a/absl/strings/internal/cordz_info_test.cc b/absl/strings/internal/cordz_info_test.cc
index 59a8c525..b98343ae 100644
--- a/absl/strings/internal/cordz_info_test.cc
+++ b/absl/strings/internal/cordz_info_test.cc
@@ -74,19 +74,49 @@ 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, MaybeTrackChildCordWithoutSampling) {
+  CordzSamplingIntervalHelper sample_none(99999);
+  TestCordData parent, child;
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  EXPECT_THAT(child.data.cordz_info(), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, MaybeTrackChildCordWithSampling) {
+  CordzSamplingIntervalHelper sample_all(1);
+  TestCordData parent, child;
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  EXPECT_THAT(child.data.cordz_info(), Eq(nullptr));
 }
 
+TEST(CordzInfoTest, MaybeTrackChildCordWithoutSamplingParentSampled) {
+  CordzSamplingIntervalHelper sample_none(99999);
+  TestCordData parent, child;
+  CordzInfo::TrackCord(parent.data, kTrackCordMethod);
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  CordzInfo* parent_info = parent.data.cordz_info();
+  CordzInfo* child_info = child.data.cordz_info();
+  ASSERT_THAT(child_info, Ne(nullptr));
+  EXPECT_THAT(child_info->GetCordRepForTesting(), Eq(child.rep.rep));
+  EXPECT_THAT(child_info->GetParentStack(), parent_info->GetStack());
+  parent_info->Untrack();
+  child_info->Untrack();
+}
+
+TEST(CordzInfoTest, MaybeTrackChildCordWithoutSamplingChildSampled) {
+  CordzSamplingIntervalHelper sample_none(99999);
+  TestCordData parent, child;
+  CordzInfo::TrackCord(child.data, kTrackCordMethod);
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  EXPECT_THAT(child.data.cordz_info(), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, MaybeTrackChildCordWithSamplingChildSampled) {
+  CordzSamplingIntervalHelper sample_all(1);
+  TestCordData parent, child;
+  CordzInfo::TrackCord(child.data, kTrackCordMethod);
+  CordzInfo::MaybeTrackCord(child.data, parent.data, kTrackCordMethod);
+  EXPECT_THAT(child.data.cordz_info(), Eq(nullptr));
+}
 
 TEST(CordzInfoTest, UntrackCord) {
   TestCordData data;
diff --git a/absl/strings/internal/str_format/bind.cc b/absl/strings/internal/str_format/bind.cc
index 4e68b90b..c988ba8f 100644
--- a/absl/strings/internal/str_format/bind.cc
+++ b/absl/strings/internal/str_format/bind.cc
@@ -58,7 +58,7 @@ inline bool ArgContext::Bind(const UnboundConversion* unbound,
   if (static_cast<size_t>(arg_position - 1) >= pack_.size()) return false;
   arg = &pack_[arg_position - 1];  // 1-based
 
-  if (!unbound->flags.basic) {
+  if (unbound->flags != Flags::kBasic) {
     int width = unbound->width.value();
     bool force_left = false;
     if (unbound->width.is_from_arg()) {
@@ -84,9 +84,8 @@ inline bool ArgContext::Bind(const UnboundConversion* unbound,
     FormatConversionSpecImplFriend::SetPrecision(precision, bound);
 
     if (force_left) {
-      Flags flags = unbound->flags;
-      flags.left = true;
-      FormatConversionSpecImplFriend::SetFlags(flags, bound);
+      FormatConversionSpecImplFriend::SetFlags(unbound->flags | Flags::kLeft,
+                                               bound);
     } else {
       FormatConversionSpecImplFriend::SetFlags(unbound->flags, bound);
     }
diff --git a/absl/strings/internal/str_format/extension.cc b/absl/strings/internal/str_format/extension.cc
index bb0d96cf..484f6ebf 100644
--- a/absl/strings/internal/str_format/extension.cc
+++ b/absl/strings/internal/str_format/extension.cc
@@ -23,13 +23,13 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-std::string Flags::ToString() const {
+std::string FlagsToString(Flags v) {
   std::string s;
-  s.append(left     ? "-" : "");
-  s.append(show_pos ? "+" : "");
-  s.append(sign_col ? " " : "");
-  s.append(alt      ? "#" : "");
-  s.append(zero     ? "0" : "");
+  s.append(FlagsContains(v, Flags::kLeft) ? "-" : "");
+  s.append(FlagsContains(v, Flags::kShowPos) ? "+" : "");
+  s.append(FlagsContains(v, Flags::kSignCol) ? " " : "");
+  s.append(FlagsContains(v, Flags::kAlt) ? "#" : "");
+  s.append(FlagsContains(v, Flags::kZero) ? "0" : "");
   return s;
 }
 
diff --git a/absl/strings/internal/str_format/extension.h b/absl/strings/internal/str_format/extension.h
index a9b9e137..55cbb56d 100644
--- a/absl/strings/internal/str_format/extension.h
+++ b/absl/strings/internal/str_format/extension.h
@@ -128,19 +128,33 @@ class FormatSinkImpl {
   char buf_[1024];
 };
 
-struct Flags {
-  bool basic : 1;     // fastest conversion: no flags, width, or precision
-  bool left : 1;      // "-"
-  bool show_pos : 1;  // "+"
-  bool sign_col : 1;  // " "
-  bool alt : 1;       // "#"
-  bool zero : 1;      // "0"
-  std::string ToString() const;
-  friend std::ostream& operator<<(std::ostream& os, const Flags& v) {
-    return os << v.ToString();
-  }
+enum class Flags : uint8_t {
+  kBasic = 0,
+  kLeft = 1 << 0,
+  kShowPos = 1 << 1,
+  kSignCol = 1 << 2,
+  kAlt = 1 << 3,
+  kZero = 1 << 4,
+  // This is not a real flag. It just exists to turn off kBasic when no other
+  // flags are set. This is for when width/precision are specified.
+  kNonBasic = 1 << 5,
 };
 
+constexpr Flags operator|(Flags a, Flags b) {
+  return static_cast<Flags>(static_cast<uint8_t>(a) | static_cast<uint8_t>(b));
+}
+
+constexpr bool FlagsContains(Flags haystack, Flags needle) {
+  return (static_cast<uint8_t>(haystack) & static_cast<uint8_t>(needle)) ==
+         static_cast<uint8_t>(needle);
+}
+
+std::string FlagsToString(Flags v);
+
+inline std::ostream& operator<<(std::ostream& os, Flags v) {
+  return os << FlagsToString(v);
+}
+
 // clang-format off
 #define ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(X_VAL, X_SEP) \
   /* text */ \
@@ -257,12 +271,16 @@ struct FormatConversionSpecImplFriend;
 class FormatConversionSpecImpl {
  public:
   // Width and precison are not specified, no flags are set.
-  bool is_basic() const { return flags_.basic; }
-  bool has_left_flag() const { return flags_.left; }
-  bool has_show_pos_flag() const { return flags_.show_pos; }
-  bool has_sign_col_flag() const { return flags_.sign_col; }
-  bool has_alt_flag() const { return flags_.alt; }
-  bool has_zero_flag() const { return flags_.zero; }
+  bool is_basic() const { return flags_ == Flags::kBasic; }
+  bool has_left_flag() const { return FlagsContains(flags_, Flags::kLeft); }
+  bool has_show_pos_flag() const {
+    return FlagsContains(flags_, Flags::kShowPos);
+  }
+  bool has_sign_col_flag() const {
+    return FlagsContains(flags_, Flags::kSignCol);
+  }
+  bool has_alt_flag() const { return FlagsContains(flags_, Flags::kAlt); }
+  bool has_zero_flag() const { return FlagsContains(flags_, Flags::kZero); }
 
   FormatConversionChar conversion_char() const {
     // Keep this field first in the struct . It generates better code when
@@ -306,7 +324,7 @@ struct FormatConversionSpecImplFriend final {
     conv->precision_ = p;
   }
   static std::string FlagsToString(const FormatConversionSpecImpl& spec) {
-    return spec.flags_.ToString();
+    return str_format_internal::FlagsToString(spec.flags_);
   }
 };
 
diff --git a/absl/strings/internal/str_format/parser.cc b/absl/strings/internal/str_format/parser.cc
index f308d023..2c9c07da 100644
--- a/absl/strings/internal/str_format/parser.cc
+++ b/absl/strings/internal/str_format/parser.cc
@@ -34,60 +34,67 @@ namespace str_format_internal {
 using CC = FormatConversionCharInternal;
 using LM = LengthMod;
 
+// Abbreviations to fit in the table below.
+constexpr auto f_sign = Flags::kSignCol;
+constexpr auto f_alt = Flags::kAlt;
+constexpr auto f_pos = Flags::kShowPos;
+constexpr auto f_left = Flags::kLeft;
+constexpr auto f_zero = Flags::kZero;
+
 ABSL_CONST_INIT const ConvTag kTags[256] = {
-    {},    {},    {},    {},    {},    {},    {},    {},     // 00-07
-    {},    {},    {},    {},    {},    {},    {},    {},     // 08-0f
-    {},    {},    {},    {},    {},    {},    {},    {},     // 10-17
-    {},    {},    {},    {},    {},    {},    {},    {},     // 18-1f
-    {},    {},    {},    {},    {},    {},    {},    {},     // 20-27
-    {},    {},    {},    {},    {},    {},    {},    {},     // 28-2f
-    {},    {},    {},    {},    {},    {},    {},    {},     // 30-37
-    {},    {},    {},    {},    {},    {},    {},    {},     // 38-3f
-    {},    CC::A, {},    {},    {},    CC::E, CC::F, CC::G,  // @ABCDEFG
-    {},    {},    {},    {},    LM::L, {},    {},    {},     // HIJKLMNO
-    {},    {},    {},    {},    {},    {},    {},    {},     // PQRSTUVW
-    CC::X, {},    {},    {},    {},    {},    {},    {},     // XYZ[\]^_
-    {},    CC::a, {},    CC::c, CC::d, CC::e, CC::f, CC::g,  // `abcdefg
-    LM::h, CC::i, LM::j, {},    LM::l, {},    CC::n, CC::o,  // hijklmno
-    CC::p, LM::q, {},    CC::s, LM::t, CC::u, {},    {},     // pqrstuvw
-    CC::x, {},    LM::z, {},    {},    {},    {},    {},     // xyz{|}!
-    {},    {},    {},    {},    {},    {},    {},    {},     // 80-87
-    {},    {},    {},    {},    {},    {},    {},    {},     // 88-8f
-    {},    {},    {},    {},    {},    {},    {},    {},     // 90-97
-    {},    {},    {},    {},    {},    {},    {},    {},     // 98-9f
-    {},    {},    {},    {},    {},    {},    {},    {},     // a0-a7
-    {},    {},    {},    {},    {},    {},    {},    {},     // a8-af
-    {},    {},    {},    {},    {},    {},    {},    {},     // b0-b7
-    {},    {},    {},    {},    {},    {},    {},    {},     // b8-bf
-    {},    {},    {},    {},    {},    {},    {},    {},     // c0-c7
-    {},    {},    {},    {},    {},    {},    {},    {},     // c8-cf
-    {},    {},    {},    {},    {},    {},    {},    {},     // d0-d7
-    {},    {},    {},    {},    {},    {},    {},    {},     // d8-df
-    {},    {},    {},    {},    {},    {},    {},    {},     // e0-e7
-    {},    {},    {},    {},    {},    {},    {},    {},     // e8-ef
-    {},    {},    {},    {},    {},    {},    {},    {},     // f0-f7
-    {},    {},    {},    {},    {},    {},    {},    {},     // f8-ff
+    {},     {},    {},    {},    {},    {},     {},    {},     // 00-07
+    {},     {},    {},    {},    {},    {},     {},    {},     // 08-0f
+    {},     {},    {},    {},    {},    {},     {},    {},     // 10-17
+    {},     {},    {},    {},    {},    {},     {},    {},     // 18-1f
+    f_sign, {},    {},    f_alt, {},    {},     {},    {},     //  !"#$%&'
+    {},     {},    {},    f_pos, {},    f_left, {},    {},     // ()*+,-./
+    f_zero, {},    {},    {},    {},    {},     {},    {},     // 01234567
+    {},     {},    {},    {},    {},    {},     {},    {},     // 89:;<=>?
+    {},     CC::A, {},    {},    {},    CC::E,  CC::F, CC::G,  // @ABCDEFG
+    {},     {},    {},    {},    LM::L, {},     {},    {},     // HIJKLMNO
+    {},     {},    {},    {},    {},    {},     {},    {},     // PQRSTUVW
+    CC::X,  {},    {},    {},    {},    {},     {},    {},     // XYZ[\]^_
+    {},     CC::a, {},    CC::c, CC::d, CC::e,  CC::f, CC::g,  // `abcdefg
+    LM::h,  CC::i, LM::j, {},    LM::l, {},     CC::n, CC::o,  // hijklmno
+    CC::p,  LM::q, {},    CC::s, LM::t, CC::u,  {},    {},     // pqrstuvw
+    CC::x,  {},    LM::z, {},    {},    {},     {},    {},     // xyz{|}!
+    {},     {},    {},    {},    {},    {},     {},    {},     // 80-87
+    {},     {},    {},    {},    {},    {},     {},    {},     // 88-8f
+    {},     {},    {},    {},    {},    {},     {},    {},     // 90-97
+    {},     {},    {},    {},    {},    {},     {},    {},     // 98-9f
+    {},     {},    {},    {},    {},    {},     {},    {},     // a0-a7
+    {},     {},    {},    {},    {},    {},     {},    {},     // a8-af
+    {},     {},    {},    {},    {},    {},     {},    {},     // b0-b7
+    {},     {},    {},    {},    {},    {},     {},    {},     // b8-bf
+    {},     {},    {},    {},    {},    {},     {},    {},     // c0-c7
+    {},     {},    {},    {},    {},    {},     {},    {},     // c8-cf
+    {},     {},    {},    {},    {},    {},     {},    {},     // d0-d7
+    {},     {},    {},    {},    {},    {},     {},    {},     // d8-df
+    {},     {},    {},    {},    {},    {},     {},    {},     // e0-e7
+    {},     {},    {},    {},    {},    {},     {},    {},     // e8-ef
+    {},     {},    {},    {},    {},    {},     {},    {},     // f0-f7
+    {},     {},    {},    {},    {},    {},     {},    {},     // f8-ff
 };
 
 namespace {
 
 bool CheckFastPathSetting(const UnboundConversion& conv) {
-  bool should_be_basic = !conv.flags.left &&      //
-                         !conv.flags.show_pos &&  //
-                         !conv.flags.sign_col &&  //
-                         !conv.flags.alt &&       //
-                         !conv.flags.zero &&      //
-                         (conv.width.value() == -1) &&
-                         (conv.precision.value() == -1);
-  if (should_be_basic != conv.flags.basic) {
+  bool width_precision_needed =
+      conv.width.value() >= 0 || conv.precision.value() >= 0;
+  if (width_precision_needed && conv.flags == Flags::kBasic) {
     fprintf(stderr,
             "basic=%d left=%d show_pos=%d sign_col=%d alt=%d zero=%d "
             "width=%d precision=%d\n",
-            conv.flags.basic, conv.flags.left, conv.flags.show_pos,
-            conv.flags.sign_col, conv.flags.alt, conv.flags.zero,
-            conv.width.value(), conv.precision.value());
+            conv.flags == Flags::kBasic ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kLeft) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kShowPos) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kSignCol) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kAlt) ? 1 : 0,
+            FlagsContains(conv.flags, Flags::kZero) ? 1 : 0, conv.width.value(),
+            conv.precision.value());
+    return false;
   }
-  return should_be_basic == conv.flags.basic;
+  return true;
 }
 
 template <bool is_positional>
@@ -131,40 +138,21 @@ const char *ConsumeConversion(const char *pos, const char *const end,
   ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
 
   // We should start with the basic flag on.
-  assert(conv->flags.basic);
+  assert(conv->flags == Flags::kBasic);
 
   // Any non alpha character makes this conversion not basic.
   // This includes flags (-+ #0), width (1-9, *) or precision (.).
   // All conversion characters and length modifiers are alpha characters.
   if (c < 'A') {
-    conv->flags.basic = false;
-
-    for (; c <= '0';) {
-      // FIXME: We might be able to speed this up reusing the lookup table from
-      // above. It might require changing Flags to be a plain integer where we
-      // can |= a value.
-      switch (c) {
-        case '-':
-          conv->flags.left = true;
-          break;
-        case '+':
-          conv->flags.show_pos = true;
-          break;
-        case ' ':
-          conv->flags.sign_col = true;
-          break;
-        case '#':
-          conv->flags.alt = true;
-          break;
-        case '0':
-          conv->flags.zero = true;
-          break;
-        default:
-          goto flags_done;
+    while (c <= '0') {
+      auto tag = GetTagForChar(c);
+      if (tag.is_flags()) {
+        conv->flags = conv->flags | tag.as_flags();
+        ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
+      } else {
+        break;
       }
-      ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
     }
-flags_done:
 
     if (c <= '9') {
       if (c >= '0') {
@@ -173,12 +161,12 @@ flags_done:
           if (ABSL_PREDICT_FALSE(*next_arg != 0)) return nullptr;
           // Positional conversion.
           *next_arg = -1;
-          conv->flags = Flags();
-          conv->flags.basic = true;
           return ConsumeConversion<true>(original_pos, end, conv, next_arg);
         }
+        conv->flags = conv->flags | Flags::kNonBasic;
         conv->width.set_value(maybe_width);
       } else if (c == '*') {
+        conv->flags = conv->flags | Flags::kNonBasic;
         ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
         if (is_positional) {
           if (ABSL_PREDICT_FALSE(c < '1' || c > '9')) return nullptr;
@@ -192,6 +180,7 @@ flags_done:
     }
 
     if (c == '.') {
+      conv->flags = conv->flags | Flags::kNonBasic;
       ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
       if (std::isdigit(c)) {
         conv->precision.set_value(parse_digits());
diff --git a/absl/strings/internal/str_format/parser.h b/absl/strings/internal/str_format/parser.h
index 6504dd3d..ad8646ed 100644
--- a/absl/strings/internal/str_format/parser.h
+++ b/absl/strings/internal/str_format/parser.h
@@ -41,10 +41,7 @@ std::string LengthModToString(LengthMod v);
 
 // The analyzed properties of a single specified conversion.
 struct UnboundConversion {
-  UnboundConversion()
-      : flags() /* This is required to zero all the fields of flags. */ {
-    flags.basic = true;
-  }
+  UnboundConversion() {}
 
   class InputValue {
    public:
@@ -79,7 +76,7 @@ struct UnboundConversion {
   InputValue width;
   InputValue precision;
 
-  Flags flags;
+  Flags flags = Flags::kBasic;
   LengthMod length_mod = LengthMod::none;
   FormatConversionChar conv = FormatConversionCharInternal::kNone;
 };
@@ -93,32 +90,43 @@ const char* ConsumeUnboundConversion(const char* p, const char* end,
                                      UnboundConversion* conv, int* next_arg);
 
 // Helper tag class for the table below.
-// It allows fast `char -> ConversionChar/LengthMod` checking and
+// It allows fast `char -> ConversionChar/LengthMod/Flags` checking and
 // conversions.
 class ConvTag {
  public:
   constexpr ConvTag(FormatConversionChar conversion_char)  // NOLINT
-      : tag_(static_cast<int8_t>(conversion_char)) {}
-  // We invert the length modifiers to make them negative so that we can easily
-  // test for them.
+      : tag_(static_cast<uint8_t>(conversion_char)) {}
   constexpr ConvTag(LengthMod length_mod)  // NOLINT
-      : tag_(~static_cast<std::int8_t>(length_mod)) {}
-  // Everything else is -128, which is negative to make is_conv() simpler.
-  constexpr ConvTag() : tag_(-128) {}
+      : tag_(0x80 | static_cast<uint8_t>(length_mod)) {}
+  constexpr ConvTag(Flags flags)  // NOLINT
+      : tag_(0xc0 | static_cast<uint8_t>(flags)) {}
+  constexpr ConvTag() : tag_(0xFF) {}
+
+  bool is_conv() const { return (tag_ & 0x80) == 0; }
+  bool is_length() const { return (tag_ & 0xC0) == 0x80; }
+  bool is_flags() const { return (tag_ & 0xE0) == 0xC0; }
 
-  bool is_conv() const { return tag_ >= 0; }
-  bool is_length() const { return tag_ < 0 && tag_ != -128; }
   FormatConversionChar as_conv() const {
     assert(is_conv());
+    assert(!is_length());
+    assert(!is_flags());
     return static_cast<FormatConversionChar>(tag_);
   }
   LengthMod as_length() const {
+    assert(!is_conv());
     assert(is_length());
-    return static_cast<LengthMod>(~tag_);
+    assert(!is_flags());
+    return static_cast<LengthMod>(tag_ & 0x3F);
+  }
+  Flags as_flags() const {
+    assert(!is_conv());
+    assert(!is_length());
+    assert(is_flags());
+    return static_cast<Flags>(tag_ & 0x1F);
   }
 
  private:
-  std::int8_t tag_;
+  uint8_t tag_;
 };
 
 extern const ConvTag kTags[256];
diff --git a/absl/strings/internal/str_format/parser_test.cc b/absl/strings/internal/str_format/parser_test.cc
index a5fa1c79..fe0d2963 100644
--- a/absl/strings/internal/str_format/parser_test.cc
+++ b/absl/strings/internal/str_format/parser_test.cc
@@ -270,15 +270,22 @@ TEST_F(ConsumeUnboundConversionTest, Flags) {
       for (int k = 0; k < kNumFlags; ++k)
         if ((i >> k) & 1) fmt += kAllFlags[k];
       // flag order shouldn't matter
-      if (rev == 1) { std::reverse(fmt.begin(), fmt.end()); }
+      if (rev == 1) {
+        std::reverse(fmt.begin(), fmt.end());
+      }
       fmt += 'd';
       SCOPED_TRACE(fmt);
       EXPECT_TRUE(Run(fmt.c_str()));
-      EXPECT_EQ(fmt.find('-') == std::string::npos, !o.flags.left);
-      EXPECT_EQ(fmt.find('+') == std::string::npos, !o.flags.show_pos);
-      EXPECT_EQ(fmt.find(' ') == std::string::npos, !o.flags.sign_col);
-      EXPECT_EQ(fmt.find('#') == std::string::npos, !o.flags.alt);
-      EXPECT_EQ(fmt.find('0') == std::string::npos, !o.flags.zero);
+      EXPECT_EQ(fmt.find('-') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kLeft));
+      EXPECT_EQ(fmt.find('+') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kShowPos));
+      EXPECT_EQ(fmt.find(' ') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kSignCol));
+      EXPECT_EQ(fmt.find('#') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kAlt));
+      EXPECT_EQ(fmt.find('0') == std::string::npos,
+                !FlagsContains(o.flags, Flags::kZero));
     }
   }
 }
@@ -288,14 +295,14 @@ TEST_F(ConsumeUnboundConversionTest, BasicFlag) {
   for (const char* fmt : {"d", "llx", "G", "1$X"}) {
     SCOPED_TRACE(fmt);
     EXPECT_TRUE(Run(fmt));
-    EXPECT_TRUE(o.flags.basic);
+    EXPECT_EQ(o.flags, Flags::kBasic);
   }
 
   // Flag is off
   for (const char* fmt : {"3d", ".llx", "-G", "1$#X"}) {
     SCOPED_TRACE(fmt);
     EXPECT_TRUE(Run(fmt));
-    EXPECT_FALSE(o.flags.basic);
+    EXPECT_NE(o.flags, Flags::kBasic);
   }
 }
 
diff --git a/absl/synchronization/blocking_counter.cc b/absl/synchronization/blocking_counter.cc
index 3cea7aed..d2f82da3 100644
--- a/absl/synchronization/blocking_counter.cc
+++ b/absl/synchronization/blocking_counter.cc
@@ -14,41 +14,51 @@
 
 #include "absl/synchronization/blocking_counter.h"
 
+#include <atomic>
+
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
-// Return whether int *arg is zero.
-static bool IsZero(void *arg) {
-  return 0 == *reinterpret_cast<int *>(arg);
+namespace {
+
+// Return whether int *arg is true.
+bool IsDone(void *arg) { return *reinterpret_cast<bool *>(arg); }
+
+}  // namespace
+
+BlockingCounter::BlockingCounter(int initial_count)
+    : count_(initial_count),
+      num_waiting_(0),
+      done_{initial_count == 0 ? true : false} {
+  ABSL_RAW_CHECK(initial_count >= 0, "BlockingCounter initial_count negative");
 }
 
 bool BlockingCounter::DecrementCount() {
-  MutexLock l(&lock_);
-  count_--;
-  if (count_ < 0) {
-    ABSL_RAW_LOG(
-        FATAL,
-        "BlockingCounter::DecrementCount() called too many times.  count=%d",
-        count_);
+  int count = count_.fetch_sub(1, std::memory_order_acq_rel) - 1;
+  ABSL_RAW_CHECK(count >= 0,
+                 "BlockingCounter::DecrementCount() called too many times");
+  if (count == 0) {
+    MutexLock l(&lock_);
+    done_ = true;
+    return true;
   }
-  return count_ == 0;
+  return false;
 }
 
 void BlockingCounter::Wait() {
   MutexLock l(&this->lock_);
-  ABSL_RAW_CHECK(count_ >= 0, "BlockingCounter underflow");
 
   // only one thread may call Wait(). To support more than one thread,
   // implement a counter num_to_exit, like in the Barrier class.
   ABSL_RAW_CHECK(num_waiting_ == 0, "multiple threads called Wait()");
   num_waiting_++;
 
-  this->lock_.Await(Condition(IsZero, &this->count_));
+  this->lock_.Await(Condition(IsDone, &this->done_));
 
-  // At this point, We know that all threads executing DecrementCount have
-  // released the lock, and so will not touch this object again.
+  // At this point, we know that all threads executing DecrementCount
+  // will not touch this object again.
   // Therefore, the thread calling this method is free to delete the object
   // after we return from this method.
 }
diff --git a/absl/synchronization/blocking_counter.h b/absl/synchronization/blocking_counter.h
index 1f53f9f2..1908fdb1 100644
--- a/absl/synchronization/blocking_counter.h
+++ b/absl/synchronization/blocking_counter.h
@@ -20,6 +20,8 @@
 #ifndef ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
 #define ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
 
+#include <atomic>
+
 #include "absl/base/thread_annotations.h"
 #include "absl/synchronization/mutex.h"
 
@@ -60,8 +62,7 @@ ABSL_NAMESPACE_BEGIN
 //
 class BlockingCounter {
  public:
-  explicit BlockingCounter(int initial_count)
-      : count_(initial_count), num_waiting_(0) {}
+  explicit BlockingCounter(int initial_count);
 
   BlockingCounter(const BlockingCounter&) = delete;
   BlockingCounter& operator=(const BlockingCounter&) = delete;
@@ -89,8 +90,9 @@ class BlockingCounter {
 
  private:
   Mutex lock_;
-  int count_ ABSL_GUARDED_BY(lock_);
+  std::atomic<int> count_;
   int num_waiting_ ABSL_GUARDED_BY(lock_);
+  bool done_ ABSL_GUARDED_BY(lock_);
 };
 
 ABSL_NAMESPACE_END
diff --git a/absl/synchronization/blocking_counter_test.cc b/absl/synchronization/blocking_counter_test.cc
index 2926224a..06885f57 100644
--- a/absl/synchronization/blocking_counter_test.cc
+++ b/absl/synchronization/blocking_counter_test.cc
@@ -63,6 +63,18 @@ TEST(BlockingCounterTest, BasicFunctionality) {
   }
 }
 
+TEST(BlockingCounterTest, WaitZeroInitialCount) {
+  BlockingCounter counter(0);
+  counter.Wait();
+}
+
+#if GTEST_HAS_DEATH_TEST
+TEST(BlockingCounterTest, WaitNegativeInitialCount) {
+  EXPECT_DEATH(BlockingCounter counter(-1),
+               "BlockingCounter initial_count negative");
+}
+#endif
+
 }  // namespace
 ABSL_NAMESPACE_END
 }  // namespace absl