Export of internal Abseil changes

--
c68f1886f5e8fd90eb0c2d2e68feaf00a7cdacda by CJ Johnson <johnsoncj@google.com>:

Introduce absl::Cleanup to the OSS repo

PiperOrigin-RevId: 354583156

--
17030cf388e10f7eb959e3e566326d1072ce392e by Abseil Team <absl-team@google.com>:

Internal change only

PiperOrigin-RevId: 354574953

--
e979d7236d4f3252e79ddda6739b67a9a326bf6d by CJ Johnson <johnsoncj@google.com>:

Internal change

PiperOrigin-RevId: 354545297

--
7ea02b3783f7f49ef97d86a8f6580a19cc57df14 by Abseil Team <absl-team@google.com>:

Pre-allocate memory for vectors where the size is known.

PiperOrigin-RevId: 354344576

--
9246c7cb11f1d6444f79ebe25acc69a8a9b870e0 by Matt Kulukundis <kfm@google.com>:

Add support for Elbrus 2000 (e2k)

Import of https://github.com/abseil/abseil-cpp/pull/889

PiperOrigin-RevId: 354344013

--
0fc93d359cc1fb307552e917b37b7b2e7eed822f by Abseil Team <absl-team@google.com>:

Integrate CordRepRing logic into cord (but do not enable it)

PiperOrigin-RevId: 354312238

--
eda05622f7da71466723acb33403f783529df24b by Abseil Team <absl-team@google.com>:

Protect ignore diagnostic with "__has_warning".

PiperOrigin-RevId: 354112334

--
47716c5d8fb10efa4fdd801d28bac414c6f8ec32 by Abseil Team <absl-team@google.com>:

Rearrange InlinedVector copy constructor and destructor to treat
a few special cases inline and then tail-call a non-inlined routine
for the rest.  In particular, we optimize for empty vectors in both
cases.

Added a couple of benchmarks that copy either an InlVec<int64> or
an InlVec<InlVec<int64>>.

Speed difference:
```
BM_CopyTrivial/0                                    0.92ns +- 0%   0.47ns +- 0%  -48.91%  (p=0.000 n=11+12)
BM_CopyTrivial/1                                    0.92ns +- 0%   1.15ns +- 0%  +25.00%  (p=0.000 n=10+9)
BM_CopyTrivial/8                                    8.57ns +- 0%  10.72ns +- 1%  +25.16%  (p=0.000 n=10+12)
BM_CopyNonTrivial/0                                 3.21ns +- 0%   0.70ns +- 0%  -78.23%  (p=0.000 n=12+10)
BM_CopyNonTrivial/1                                 5.88ns +- 1%   5.51ns +- 0%   -6.28%  (p=0.000 n=10+8)
BM_CopyNonTrivial/8                                 21.5ns +- 1%   15.2ns +- 2%  -29.23%  (p=0.000 n=12+12)
```

Note: the slowdowns are a few cycles which is expected given the procedure
call added in that case. We decided this is a good tradeoff given the code
size reductions and the more significant speedups for empty vectors.

Size difference (as measured by nm):
```
BM_CopyTrivial     from 1048 bytes to 326 bytes.
BM_CopyNonTrivial  from  749 bytes to 470 bytes.
```

Code size for a large binary drops by ~500KB (from 349415719 to 348906015 348906191).

All of the benchmarks that showed a significant difference:

Ones that improve with this CL:
```
BM_CopyNonTrivial/0                                 3.21ns +- 0%   0.70ns +- 0%  -78.23%  (p=0.000 n=12+10)
BM_InlinedVectorFillString/0                        0.93ns +- 0%   0.24ns +- 0%  -74.19%  (p=0.000 n=12+10)
BM_InlinedVectorAssignments/1                       10.5ns +- 0%    4.1ns +- 0%  -60.64%  (p=0.000 n=11+10)
BM_InlinedVectorAssignments/2                       10.7ns +- 0%    4.4ns +- 0%  -59.08%  (p=0.000 n=11+11)
BM_CopyTrivial/0                                    0.92ns +- 0%   0.47ns +- 0%  -48.91%  (p=0.000 n=11+12)
BM_CopyNonTrivial/8                                 21.5ns +- 1%   15.2ns +- 2%  -29.23%  (p=0.000 n=12+12)
BM_StdVectorEmpty                                   0.47ns +- 1%   0.35ns +- 0%  -24.73%  (p=0.000 n=12+12)
BM_StdVectorSize                                    0.46ns +- 2%   0.35ns +- 0%  -24.32%  (p=0.000 n=12+12)
BM_SwapElements<LargeCopyableOnly>/0                3.44ns +- 0%   2.76ns +- 1%  -19.83%  (p=0.000 n=11+11)
BM_InlinedVectorFillRange/256                       20.7ns +- 1%   17.8ns +- 0%  -14.08%  (p=0.000 n=12+9)
BM_CopyNonTrivial/1                                 5.88ns +- 1%   5.51ns +- 0%   -6.28%  (p=0.000 n=10+8)
BM_SwapElements<LargeCopyableMovable>/1             4.19ns +- 0%   3.95ns +- 1%   -5.63%  (p=0.000 n=11+12)
BM_SwapElements<LargeCopyableMovableSwappable>/1    4.18ns +- 0%   3.99ns +- 0%   -4.70%  (p=0.000 n=9+11)
BM_SwapElements<LargeCopyableMovable>/0             2.41ns +- 0%   2.31ns +- 0%   -4.45%  (p=0.000 n=12+12)
BM_InlinedVectorFillRange/64                        8.25ns +- 0%   8.04ns +- 0%   -2.51%  (p=0.000 n=12+11)
BM_SwapElements<LargeCopyableOnly>/1                82.4ns +- 0%   81.5ns +- 0%   -1.06%  (p=0.000 n=12+12)
```

Ones that get worse with this CL:
```
BM_CopyTrivial/1                                    0.92ns +- 0%   1.15ns +- 0%  +25.00%  (p=0.000 n=10+9)
BM_CopyTrivial/8                                    8.57ns +- 0%  10.72ns +- 1%  +25.16%  (p=0.000 n=10+12)
BM_SwapElements<LargeCopyableMovableSwappable>/512  1.48ns +- 1%   1.66ns +- 1%  +11.88%  (p=0.000 n=12+12)
BM_InlinedVectorFillString/1                        11.5ns +- 0%   12.8ns +- 1%  +11.62%  (p=0.000 n=12+11)
BM_SwapElements<LargeCopyableMovableSwappable>/64   1.48ns +- 2%   1.66ns +- 1%  +11.66%  (p=0.000 n=12+11)
BM_SwapElements<LargeCopyableMovableSwappable>/1k   1.48ns +- 1%   1.65ns +- 2%  +11.32%  (p=0.000 n=12+12)
BM_SwapElements<LargeCopyableMovable>/512           1.48ns +- 2%   1.58ns +- 4%   +6.62%  (p=0.000 n=11+12)
BM_SwapElements<LargeCopyableMovable>/1k            1.49ns +- 2%   1.58ns +- 3%   +6.05%  (p=0.000 n=12+12)
BM_SwapElements<LargeCopyableMovable>/64            1.48ns +- 2%   1.57ns +- 4%   +6.04%  (p=0.000 n=11+12)
BM_InlinedVectorFillRange/1                         4.81ns +- 0%   5.05ns +- 0%   +4.83%  (p=0.000 n=11+11)
BM_InlinedVectorFillString/8                        79.4ns +- 1%   83.1ns +- 1%   +4.64%  (p=0.000 n=10+12)
BM_StdVectorFillString/1                            16.3ns +- 0%   16.6ns +- 0%   +2.13%  (p=0.000 n=11+8)
```

PiperOrigin-RevId: 353906786

--
8e26518b3cec9c598e5e9573c46c3bd1b03a67ef by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 353737330

--
f206ae0983e58c9904ed8b8f05f9caf564a446be by Matt Kulukundis <kfm@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 353682256
GitOrigin-RevId: c68f1886f5e8fd90eb0c2d2e68feaf00a7cdacda
Change-Id: I5790c1036c4f543c701d1039848fabf7ae881ad8

22 files changed, 876 insertions, 32 deletions

diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
index b8c38d68..47707dfd 100644
--- a/CMake/AbseilDll.cmake
+++ b/CMake/AbseilDll.cmake
@@ -60,6 +60,8 @@ set(ABSL_INTERNAL_DLL_FILES
   "base/policy_checks.h"
   "base/port.h"
   "base/thread_annotations.h"
+  "cleanup/cleanup.h"
+  "cleanup/internal/cleanup.h"
   "container/btree_map.h"
   "container/btree_set.h"
   "container/fixed_array.h"
diff --git a/README.md b/README.md
index 76f1d1e1..264c4b3f 100644
--- a/README.md
+++ b/README.md
@@ -72,6 +72,9 @@ Abseil contains the following C++ library components:
 * [`algorithm`](absl/algorithm/)
   <br /> The `algorithm` library contains additions to the C++ `<algorithm>`
   library and container-based versions of such algorithms.
+* [`cleanup`](absl/cleanup/)
+  <br /> The `cleanup` library contains the control-flow-construct-like type
+  `absl::Cleanup` which is used for executing a callback on scope exit.
 * [`container`](absl/container/)
   <br /> The `container` library contains additional STL-style containers,
   including Abseil's unordered "Swiss table" containers.
diff --git a/absl/base/config.h b/absl/base/config.h
index f6ddf0c5..3d6aa178 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -722,4 +722,13 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #define ABSL_HAVE_ADDRESS_SANITIZER 1
 #endif
 
+// ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
+//
+// Class template argument deduction is a language feature added in C++17.
+#ifdef ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
+#error "ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION cannot be directly set."
+#elif defined(__cpp_deduction_guides)
+#define ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION 1
+#endif
+
 #endif  // ABSL_BASE_CONFIG_H_
diff --git a/absl/base/spinlock_test_common.cc b/absl/base/spinlock_test_common.cc
index dee266e4..2b572c5b 100644
--- a/absl/base/spinlock_test_common.cc
+++ b/absl/base/spinlock_test_common.cc
@@ -92,6 +92,7 @@ static void TestFunction(int thread_salt, SpinLock* spinlock) {
 
 static void ThreadedTest(SpinLock* spinlock) {
   std::vector<std::thread> threads;
+  threads.reserve(kNumThreads);
   for (int i = 0; i < kNumThreads; ++i) {
     threads.push_back(std::thread(TestFunction, i, spinlock));
   }
diff --git a/absl/cleanup/BUILD.bazel b/absl/cleanup/BUILD.bazel
new file mode 100644
index 00000000..5cca898f
--- /dev/null
+++ b/absl/cleanup/BUILD.bazel
@@ -0,0 +1,66 @@
+# Copyright 2021 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])
+
+cc_library(
+    name = "cleanup_internal",
+    hdrs = ["internal/cleanup.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:base_internal",
+        "//absl/base:core_headers",
+        "//absl/utility",
+    ],
+)
+
+cc_library(
+    name = "cleanup",
+    hdrs = [
+        "cleanup.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":cleanup_internal",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
+)
+
+cc_test(
+    name = "cleanup_test",
+    size = "small",
+    srcs = [
+        "cleanup_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":cleanup",
+        "//absl/base:config",
+        "//absl/utility",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
diff --git a/absl/cleanup/CMakeLists.txt b/absl/cleanup/CMakeLists.txt
new file mode 100644
index 00000000..a2dd78a8
--- /dev/null
+++ b/absl/cleanup/CMakeLists.txt
@@ -0,0 +1,55 @@
+# Copyright 2021 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+absl_cc_library(
+  NAME
+    cleanup_internal
+  HDRS
+    "internal/cleanup.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base_internal
+    absl::core_headers
+    absl::utility
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    cleanup
+  HDRS
+    "cleanup.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::cleanup_internal
+    absl::config
+    absl::core_headers
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    cleanup_test
+  SRCS
+    "cleanup_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::cleanup
+    absl::config
+    absl::utility
+    gmock_main
+)
diff --git a/absl/cleanup/cleanup.h b/absl/cleanup/cleanup.h
new file mode 100644
index 00000000..eba04b33
--- /dev/null
+++ b/absl/cleanup/cleanup.h
@@ -0,0 +1,129 @@
+// Copyright 2021 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: cleanup.h
+// -----------------------------------------------------------------------------
+//
+// `absl::Cleanup` implements the scope guard idiom, invoking `operator()() &&`
+// on the callback it was constructed with, on scope exit.
+//
+// Example:
+//
+// ```
+//   void CopyGoodData(const char* input_path, const char* output_path) {
+//     FILE* in_file = fopen(input_path, "r");
+//     FILE* out_file = fopen(output_path, "w");
+//     if (in_file == nullptr || out_file == nullptr) return;
+//
+//     // C++17 style using class template argument deduction
+//     absl::Cleanup in_closer = [&in_file] { fclose(in_file); };
+//
+//     // C++11 style using the factory function
+//     auto out_closer = absl::MakeCleanup([&out_file] { fclose(out_file); });
+//
+//     // `fclose` will be called on all exit paths by the cleanup instances
+//
+//     Data data;
+//     while (ReadData(in_file, &data)) {
+//       if (data.IsBad()) {
+//         LOG(ERROR) << "Found bad data.";
+//         return;  // `in_closer` and `out_closer` will call their callbacks
+//       }
+//       SaveData(out_file, &data);
+//     }
+//     return;  // `in_closer` and `out_closer` will call their callbacks
+//   }
+// ```
+//
+// `std::move(cleanup).Invoke()` will execute the callback early, before
+// destruction, and prevent the callback from executing in the destructor.
+//
+// Alternatively, `std::move(cleanup).Cancel()` will prevent the callback from
+// ever executing at all.
+//
+// Once a cleanup object has been `std::move(...)`-ed, it may not be used again.
+
+#ifndef ABSL_CLEANUP_CLEANUP_H_
+#define ABSL_CLEANUP_CLEANUP_H_
+
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+#include "absl/cleanup/internal/cleanup.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+template <typename Arg, typename Callback = void()>
+class ABSL_MUST_USE_RESULT Cleanup {
+  static_assert(cleanup_internal::WasDeduced<Arg>(),
+                "Explicit template parameters are not supported.");
+
+  static_assert(cleanup_internal::ReturnsVoid<Callback>(),
+                "Callbacks that return values are not supported.");
+
+ public:
+  Cleanup(Callback callback) : storage_(std::move(callback)) {}  // NOLINT
+
+  Cleanup(Cleanup&& other) : storage_(std::move(other.storage_)) {}
+
+  void Cancel() && {
+    ABSL_HARDENING_ASSERT(storage_.IsCallbackEngaged());
+    storage_.DisengageCallback();
+  }
+
+  void Invoke() && {
+    ABSL_HARDENING_ASSERT(storage_.IsCallbackEngaged());
+    storage_.DisengageCallback();
+    storage_.InvokeCallback();
+  }
+
+  ~Cleanup() {
+    if (storage_.IsCallbackEngaged()) {
+      storage_.InvokeCallback();
+    }
+  }
+
+ private:
+  cleanup_internal::Storage<Callback> storage_;
+};
+
+// `auto c = absl::MakeCleanup(/* callback */);`
+//
+// C++11 type deduction API for creating an instance of `absl::Cleanup`.
+template <typename... Args, typename Callback>
+absl::Cleanup<cleanup_internal::Tag, Callback> MakeCleanup(Callback callback) {
+  static_assert(cleanup_internal::WasDeduced<cleanup_internal::Tag, Args...>(),
+                "Explicit template parameters are not supported.");
+
+  static_assert(cleanup_internal::ReturnsVoid<Callback>(),
+                "Callbacks that return values are not supported.");
+
+  return {std::move(callback)};
+}
+
+// `absl::Cleanup c = /* callback */;`
+//
+// C++17 type deduction API for creating an instance of `absl::Cleanup`.
+#if defined(ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION)
+template <typename Callback>
+Cleanup(Callback callback) -> Cleanup<cleanup_internal::Tag, Callback>;
+#endif  // defined(ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION)
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CLEANUP_CLEANUP_H_
diff --git a/absl/cleanup/cleanup_test.cc b/absl/cleanup/cleanup_test.cc
new file mode 100644
index 00000000..34e3bfad
--- /dev/null
+++ b/absl/cleanup/cleanup_test.cc
@@ -0,0 +1,240 @@
+// Copyright 2021 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/cleanup/cleanup.h"
+
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/utility/utility.h"
+
+namespace {
+
+using Tag = absl::cleanup_internal::Tag;
+
+template <typename Type1, typename Type2>
+void AssertSameType() {
+  static_assert(std::is_same<Type1, Type2>::value, "");
+}
+
+struct IdentityFactory {
+  template <typename Callback>
+  static Callback AsCallback(Callback callback) {
+    return Callback(std::move(callback));
+  }
+};
+
+// `FunctorClass` is a type used for testing `absl::Cleanup`. It is intended to
+// represent users that make their own move-only callback types outside of
+// `std::function` and lambda literals.
+class FunctorClass {
+  using Callback = std::function<void()>;
+
+ public:
+  explicit FunctorClass(Callback callback) : callback_(std::move(callback)) {}
+
+  FunctorClass(FunctorClass&& other)
+      : callback_(absl::exchange(other.callback_, Callback())) {}
+
+  FunctorClass(const FunctorClass&) = delete;
+
+  FunctorClass& operator=(const FunctorClass&) = delete;
+
+  FunctorClass& operator=(FunctorClass&&) = delete;
+
+  void operator()() const& = delete;
+
+  void operator()() && {
+    ASSERT_TRUE(callback_);
+    callback_();
+    callback_ = nullptr;
+  }
+
+ private:
+  Callback callback_;
+};
+
+struct FunctorClassFactory {
+  template <typename Callback>
+  static FunctorClass AsCallback(Callback callback) {
+    return FunctorClass(std::move(callback));
+  }
+};
+
+struct StdFunctionFactory {
+  template <typename Callback>
+  static std::function<void()> AsCallback(Callback callback) {
+    return std::function<void()>(std::move(callback));
+  }
+};
+
+using CleanupTestParams =
+    ::testing::Types<IdentityFactory, FunctorClassFactory, StdFunctionFactory>;
+template <typename>
+struct CleanupTest : public ::testing::Test {};
+TYPED_TEST_SUITE(CleanupTest, CleanupTestParams);
+
+bool function_pointer_called = false;
+void FunctionPointerFunction() { function_pointer_called = true; }
+
+TYPED_TEST(CleanupTest, FactoryProducesCorrectType) {
+  {
+    auto callback = TypeParam::AsCallback([] {});
+    auto cleanup = absl::MakeCleanup(std::move(callback));
+
+    AssertSameType<absl::Cleanup<Tag, decltype(callback)>, decltype(cleanup)>();
+  }
+
+  {
+    auto cleanup = absl::MakeCleanup(&FunctionPointerFunction);
+
+    AssertSameType<absl::Cleanup<Tag, void (*)()>, decltype(cleanup)>();
+  }
+
+  {
+    auto cleanup = absl::MakeCleanup(FunctionPointerFunction);
+
+    AssertSameType<absl::Cleanup<Tag, void (*)()>, decltype(cleanup)>();
+  }
+}
+
+#if defined(ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION)
+TYPED_TEST(CleanupTest, CTADProducesCorrectType) {
+  {
+    auto callback = TypeParam::AsCallback([] {});
+    absl::Cleanup cleanup = std::move(callback);
+
+    AssertSameType<absl::Cleanup<Tag, decltype(callback)>, decltype(cleanup)>();
+  }
+
+  {
+    absl::Cleanup cleanup = &FunctionPointerFunction;
+
+    AssertSameType<absl::Cleanup<Tag, void (*)()>, decltype(cleanup)>();
+  }
+
+  {
+    absl::Cleanup cleanup = FunctionPointerFunction;
+
+    AssertSameType<absl::Cleanup<Tag, void (*)()>, decltype(cleanup)>();
+  }
+}
+
+TYPED_TEST(CleanupTest, FactoryAndCTADProduceSameType) {
+  {
+    auto callback = IdentityFactory::AsCallback([] {});
+    auto factory_cleanup = absl::MakeCleanup(callback);
+    absl::Cleanup deduction_cleanup = callback;
+
+    AssertSameType<decltype(factory_cleanup), decltype(deduction_cleanup)>();
+  }
+
+  {
+    auto factory_cleanup =
+        absl::MakeCleanup(FunctorClassFactory::AsCallback([] {}));
+    absl::Cleanup deduction_cleanup = FunctorClassFactory::AsCallback([] {});
+
+    AssertSameType<decltype(factory_cleanup), decltype(deduction_cleanup)>();
+  }
+
+  {
+    auto factory_cleanup =
+        absl::MakeCleanup(StdFunctionFactory::AsCallback([] {}));
+    absl::Cleanup deduction_cleanup = StdFunctionFactory::AsCallback([] {});
+
+    AssertSameType<decltype(factory_cleanup), decltype(deduction_cleanup)>();
+  }
+
+  {
+    auto factory_cleanup = absl::MakeCleanup(&FunctionPointerFunction);
+    absl::Cleanup deduction_cleanup = &FunctionPointerFunction;
+
+    AssertSameType<decltype(factory_cleanup), decltype(deduction_cleanup)>();
+  }
+
+  {
+    auto factory_cleanup = absl::MakeCleanup(FunctionPointerFunction);
+    absl::Cleanup deduction_cleanup = FunctionPointerFunction;
+
+    AssertSameType<decltype(factory_cleanup), decltype(deduction_cleanup)>();
+  }
+}
+#endif  // defined(ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION)
+
+TYPED_TEST(CleanupTest, BasicUsage) {
+  bool called = false;
+
+  {
+    EXPECT_FALSE(called);
+
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback([&called] { called = true; }));
+
+    EXPECT_FALSE(called);
+  }
+
+  EXPECT_TRUE(called);
+}
+
+TYPED_TEST(CleanupTest, BasicUsageWithFunctionPointer) {
+  function_pointer_called = false;
+
+  {
+    EXPECT_FALSE(function_pointer_called);
+
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback(&FunctionPointerFunction));
+
+    EXPECT_FALSE(function_pointer_called);
+  }
+
+  EXPECT_TRUE(function_pointer_called);
+}
+
+TYPED_TEST(CleanupTest, Cancel) {
+  bool called = false;
+
+  {
+    EXPECT_FALSE(called);
+
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback([&called] { called = true; }));
+    std::move(cleanup).Cancel();
+
+    EXPECT_FALSE(called);
+  }
+
+  EXPECT_FALSE(called);
+}
+
+TYPED_TEST(CleanupTest, Invoke) {
+  bool called = false;
+
+  {
+    EXPECT_FALSE(called);
+
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback([&called] { called = true; }));
+    std::move(cleanup).Invoke();
+
+    EXPECT_TRUE(called);
+  }
+
+  EXPECT_TRUE(called);
+}
+
+}  // namespace
diff --git a/absl/cleanup/internal/cleanup.h b/absl/cleanup/internal/cleanup.h
new file mode 100644
index 00000000..a126ff30
--- /dev/null
+++ b/absl/cleanup/internal/cleanup.h
@@ -0,0 +1,77 @@
+// Copyright 2021 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CLEANUP_INTERNAL_CLEANUP_H_
+#define ABSL_CLEANUP_INTERNAL_CLEANUP_H_
+
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/internal/invoke.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace cleanup_internal {
+
+struct Tag {};
+
+template <typename Arg, typename... Args>
+constexpr bool WasDeduced() {
+  return (std::is_same<cleanup_internal::Tag, Arg>::value) &&
+         (sizeof...(Args) == 0);
+}
+
+template <typename Callback>
+constexpr bool ReturnsVoid() {
+  return (std::is_same<base_internal::invoke_result_t<Callback>, void>::value);
+}
+
+template <typename Callback>
+class Storage {
+ public:
+  explicit Storage(Callback callback)
+      : engaged_(true), callback_(std::move(callback)) {}
+
+  Storage(Storage&& other)
+      : engaged_(absl::exchange(other.engaged_, false)),
+        callback_(std::move(other.callback_)) {}
+
+  Storage(const Storage& other) = delete;
+
+  Storage& operator=(Storage&& other) = delete;
+
+  Storage& operator=(const Storage& other) = delete;
+
+  bool IsCallbackEngaged() const { return engaged_; }
+
+  void DisengageCallback() { engaged_ = false; }
+
+  void InvokeCallback() ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    std::move(callback_)();
+  }
+
+ private:
+  bool engaged_;
+  Callback callback_;
+};
+
+}  // namespace cleanup_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CLEANUP_INTERNAL_CLEANUP_H_
diff --git a/absl/container/inlined_vector.h b/absl/container/inlined_vector.h
index 90bb96e8..7c182342 100644
--- a/absl/container/inlined_vector.h
+++ b/absl/container/inlined_vector.h
@@ -167,11 +167,13 @@ class InlinedVector {
   // Creates an inlined vector by copying the contents of `other` using `alloc`.
   InlinedVector(const InlinedVector& other, const allocator_type& alloc)
       : storage_(alloc) {
-    if (IsMemcpyOk::value && !other.storage_.GetIsAllocated()) {
+    if (other.empty()) {
+      // Empty; nothing to do.
+    } else if (IsMemcpyOk::value && !other.storage_.GetIsAllocated()) {
+      // Memcpy-able and do not need allocation.
       storage_.MemcpyFrom(other.storage_);
     } else {
-      storage_.Initialize(IteratorValueAdapter<const_pointer>(other.data()),
-                          other.size());
+      storage_.InitFrom(other.storage_);
     }
   }
 
diff --git a/absl/container/inlined_vector_benchmark.cc b/absl/container/inlined_vector_benchmark.cc
index b8dafe93..e256fad6 100644
--- a/absl/container/inlined_vector_benchmark.cc
+++ b/absl/container/inlined_vector_benchmark.cc
@@ -534,6 +534,28 @@ void BM_ConstructFromMove(benchmark::State& state) {
 ABSL_INTERNAL_BENCHMARK_ONE_SIZE(BM_ConstructFromMove, TrivialType);
 ABSL_INTERNAL_BENCHMARK_ONE_SIZE(BM_ConstructFromMove, NontrivialType);
 
+// Measure cost of copy-constructor+destructor.
+void BM_CopyTrivial(benchmark::State& state) {
+  const int n = state.range(0);
+  InlVec<int64_t> src(n);
+  for (auto s : state) {
+    InlVec<int64_t> copy(src);
+    benchmark::DoNotOptimize(copy);
+  }
+}
+BENCHMARK(BM_CopyTrivial)->Arg(0)->Arg(1)->Arg(kLargeSize);
+
+// Measure cost of copy-constructor+destructor.
+void BM_CopyNonTrivial(benchmark::State& state) {
+  const int n = state.range(0);
+  InlVec<InlVec<int64_t>> src(n);
+  for (auto s : state) {
+    InlVec<InlVec<int64_t>> copy(src);
+    benchmark::DoNotOptimize(copy);
+  }
+}
+BENCHMARK(BM_CopyNonTrivial)->Arg(0)->Arg(1)->Arg(kLargeSize);
+
 template <typename T, size_t FromSize, size_t ToSize>
 void BM_AssignSizeRef(benchmark::State& state) {
   auto size = ToSize;
diff --git a/absl/container/internal/inlined_vector.h b/absl/container/internal/inlined_vector.h
index 120849cd..b8aec45b 100644
--- a/absl/container/internal/inlined_vector.h
+++ b/absl/container/internal/inlined_vector.h
@@ -81,6 +81,23 @@ void DestroyElements(AllocatorType* alloc_ptr, Pointer destroy_first,
   }
 }
 
+// If kUseMemcpy is true, memcpy(dst, src, n); else do nothing.
+// Useful to avoid compiler warnings when memcpy() is used for T values
+// that are not trivially copyable in non-reachable code.
+template <bool kUseMemcpy>
+inline void MemcpyIfAllowed(void* dst, const void* src, size_t n);
+
+// memcpy when allowed.
+template <>
+inline void MemcpyIfAllowed<true>(void* dst, const void* src, size_t n) {
+  memcpy(dst, src, n);
+}
+
+// Do nothing for types that are not memcpy-able. This function is only
+// called from non-reachable branches.
+template <>
+inline void MemcpyIfAllowed<false>(void*, const void*, size_t) {}
+
 template <typename AllocatorType, typename Pointer, typename ValueAdapter,
           typename SizeType>
 void ConstructElements(AllocatorType* alloc_ptr, Pointer construct_first,
@@ -310,9 +327,14 @@ class Storage {
       : metadata_(alloc, /* size and is_allocated */ 0) {}
 
   ~Storage() {
-    pointer data = GetIsAllocated() ? GetAllocatedData() : GetInlinedData();
-    inlined_vector_internal::DestroyElements(GetAllocPtr(), data, GetSize());
-    DeallocateIfAllocated();
+    if (GetSizeAndIsAllocated() == 0) {
+      // Empty and not allocated; nothing to do.
+    } else if (IsMemcpyOk::value) {
+      // No destructors need to be run; just deallocate if necessary.
+      DeallocateIfAllocated();
+    } else {
+      DestroyContents();
+    }
   }
 
   // ---------------------------------------------------------------------------
@@ -370,6 +392,8 @@ class Storage {
   // Storage Member Mutators
   // ---------------------------------------------------------------------------
 
+  ABSL_ATTRIBUTE_NOINLINE void InitFrom(const Storage& other);
+
   template <typename ValueAdapter>
   void Initialize(ValueAdapter values, size_type new_size);
 
@@ -452,6 +476,8 @@ class Storage {
   }
 
  private:
+  ABSL_ATTRIBUTE_NOINLINE void DestroyContents();
+
   using Metadata =
       container_internal::CompressedTuple<allocator_type, size_type>;
 
@@ -477,6 +503,40 @@ class Storage {
 };
 
 template <typename T, size_t N, typename A>
+void Storage<T, N, A>::DestroyContents() {
+  pointer data = GetIsAllocated() ? GetAllocatedData() : GetInlinedData();
+  inlined_vector_internal::DestroyElements(GetAllocPtr(), data, GetSize());
+  DeallocateIfAllocated();
+}
+
+template <typename T, size_t N, typename A>
+void Storage<T, N, A>::InitFrom(const Storage& other) {
+  const auto n = other.GetSize();
+  assert(n > 0);  // Empty sources handled handled in caller.
+  const_pointer src;
+  pointer dst;
+  if (!other.GetIsAllocated()) {
+    dst = GetInlinedData();
+    src = other.GetInlinedData();
+  } else {
+    // Because this is only called from the `InlinedVector` constructors, it's
+    // safe to take on the allocation with size `0`. If `ConstructElements(...)`
+    // throws, deallocation will be automatically handled by `~Storage()`.
+    size_type new_capacity = ComputeCapacity(GetInlinedCapacity(), n);
+    dst = AllocatorTraits::allocate(*GetAllocPtr(), new_capacity);
+    SetAllocatedData(dst, new_capacity);
+    src = other.GetAllocatedData();
+  }
+  if (IsMemcpyOk::value) {
+    MemcpyIfAllowed<IsMemcpyOk::value>(dst, src, sizeof(dst[0]) * n);
+  } else {
+    auto values = IteratorValueAdapter<const_pointer>(src);
+    inlined_vector_internal::ConstructElements(GetAllocPtr(), dst, &values, n);
+  }
+  GetSizeAndIsAllocated() = other.GetSizeAndIsAllocated();
+}
+
+template <typename T, size_t N, typename A>
 template <typename ValueAdapter>
 auto Storage<T, N, A>::Initialize(ValueAdapter values, size_type new_size)
     -> void {
diff --git a/absl/status/status.cc b/absl/status/status.cc
index c71de846..7962bb9e 100644
--- a/absl/status/status.cc
+++ b/absl/status/status.cc
@@ -207,10 +207,12 @@ void Status::UnrefNonInlined(uintptr_t rep) {
   }
 }
 
-uintptr_t Status::NewRep(absl::StatusCode code, absl::string_view msg,
-                         std::unique_ptr<status_internal::Payloads> payloads) {
+uintptr_t Status::NewRep(
+    absl::StatusCode code, absl::string_view msg,
+    std::unique_ptr<status_internal::Payloads> payloads) {
   status_internal::StatusRep* rep = new status_internal::StatusRep(
-      code, std::string(msg.data(), msg.size()), std::move(payloads));
+      code, std::string(msg.data(), msg.size()),
+      std::move(payloads));
   return PointerToRep(rep);
 }
 
@@ -236,8 +238,9 @@ absl::StatusCode Status::code() const {
 void Status::PrepareToModify() {
   ABSL_RAW_CHECK(!ok(), "PrepareToModify shouldn't be called on OK status.");
   if (IsInlined(rep_)) {
-    rep_ = NewRep(static_cast<absl::StatusCode>(raw_code()),
-                  absl::string_view(), nullptr);
+    rep_ =
+        NewRep(static_cast<absl::StatusCode>(raw_code()), absl::string_view(),
+               nullptr);
     return;
   }
 
@@ -248,7 +251,8 @@ void Status::PrepareToModify() {
     if (rep->payloads) {
       payloads = absl::make_unique<status_internal::Payloads>(*rep->payloads);
     }
-    rep_ = NewRep(rep->code, message(), std::move(payloads));
+    rep_ = NewRep(rep->code, message(),
+                  std::move(payloads));
     UnrefNonInlined(rep_i);
   }
 }
diff --git a/absl/status/status.h b/absl/status/status.h
index 08d3e806..118f64fb 100644
--- a/absl/status/status.h
+++ b/absl/status/status.h
@@ -371,10 +371,10 @@ class ABSL_MUST_USE_RESULT Status final {
   Status();
 
   // Creates a status in the canonical error space with the specified
-  // `absl::StatusCode` and error message.  If `code == absl::StatusCode::kOk`,
+  // `absl::StatusCode` and error message.  If `code == absl::StatusCode::kOk`,  // NOLINT
   // `msg` is ignored and an object identical to an OK status is constructed.
   //
-  // The `msg` string must be in UTF-8. The implementation may complain (e.g.,
+  // The `msg` string must be in UTF-8. The implementation may complain (e.g.,  // NOLINT
   // by printing a warning) if it is not.
   Status(absl::StatusCode code, absl::string_view msg);
 
@@ -551,8 +551,9 @@ class ABSL_MUST_USE_RESULT Status final {
   status_internal::Payloads* GetPayloads();
 
   // Takes ownership of payload.
-  static uintptr_t NewRep(absl::StatusCode code, absl::string_view msg,
-                          std::unique_ptr<status_internal::Payloads> payload);
+  static uintptr_t NewRep(
+      absl::StatusCode code, absl::string_view msg,
+      std::unique_ptr<status_internal::Payloads> payload);
   static bool EqualsSlow(const absl::Status& a, const absl::Status& b);
 
   // MSVC 14.0 limitation requires the const.
diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index df8c26d4..39191ef5 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc
@@ -37,6 +37,7 @@
 #include "absl/strings/escaping.h"
 #include "absl/strings/internal/cord_internal.h"
 #include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cord_rep_ring.h"
 #include "absl/strings/internal/resize_uninitialized.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
@@ -50,8 +51,8 @@ using ::absl::cord_internal::CordRep;
 using ::absl::cord_internal::CordRepConcat;
 using ::absl::cord_internal::CordRepExternal;
 using ::absl::cord_internal::CordRepFlat;
+using ::absl::cord_internal::CordRepRing;
 using ::absl::cord_internal::CordRepSubstring;
-
 using ::absl::cord_internal::kMinFlatLength;
 using ::absl::cord_internal::kMaxFlatLength;
 
@@ -94,6 +95,11 @@ static constexpr uint64_t min_length[] = {
 
 static const int kMinLengthSize = ABSL_ARRAYSIZE(min_length);
 
+static inline bool cord_ring_enabled() {
+  return cord_internal::cord_ring_buffer_enabled.load(
+      std::memory_order_relaxed);
+}
+
 static inline bool IsRootBalanced(CordRep* node) {
   if (node->tag != CONCAT) {
     return true;
@@ -109,7 +115,8 @@ static inline bool IsRootBalanced(CordRep* node) {
 }
 
 static CordRep* Rebalance(CordRep* node);
-static void DumpNode(CordRep* rep, bool include_data, std::ostream* os);
+static void DumpNode(CordRep* rep, bool include_data, std::ostream* os,
+                     int indent = 0);
 static bool VerifyNode(CordRep* root, CordRep* start_node,
                        bool full_validation);
 
@@ -198,12 +205,38 @@ static CordRep* MakeBalancedTree(CordRep** reps, size_t n) {
   return reps[0];
 }
 
+static CordRepFlat* CreateFlat(const char* data, size_t length,
+                            size_t alloc_hint) {
+  CordRepFlat* flat = CordRepFlat::New(length + alloc_hint);
+  flat->length = length;
+  memcpy(flat->Data(), data, length);
+  return flat;
+}
+
+// Creates a new flat or ringbuffer out of the specified array.
+// The returned node has a refcount of 1.
+static CordRep* RingNewTree(const char* data, size_t length,
+                            size_t alloc_hint) {
+  if (length <= kMaxFlatLength) {
+    return CreateFlat(data, length, alloc_hint);
+  }
+  CordRepFlat* flat = CreateFlat(data, kMaxFlatLength, 0);
+  data += kMaxFlatLength;
+  length -= kMaxFlatLength;
+  size_t extra = (length - 1) / kMaxFlatLength + 1;
+  auto* root = CordRepRing::Create(flat, extra);
+  return CordRepRing::Append(root, {data, length}, alloc_hint);
+}
+
 // Create a new tree out of the specified array.
 // The returned node has a refcount of 1.
 static CordRep* NewTree(const char* data,
                         size_t length,
                         size_t alloc_hint) {
   if (length == 0) return nullptr;
+  if (cord_ring_enabled()) {
+    return RingNewTree(data, length, alloc_hint);
+  }
   absl::FixedArray<CordRep*> reps((length - 1) / kMaxFlatLength + 1);
   size_t n = 0;
   do {
@@ -295,10 +328,18 @@ inline void Cord::InlineRep::remove_prefix(size_t n) {
   reduce_size(n);
 }
 
+// Returns `rep` converted into a CordRepRing.
+// Directly returns `rep` if `rep` is already a CordRepRing.
+static CordRepRing* ForceRing(CordRep* rep, size_t extra) {
+  return (rep->tag == RING) ? rep->ring() : CordRepRing::Create(rep, extra);
+}
+
 void Cord::InlineRep::AppendTree(CordRep* tree) {
   if (tree == nullptr) return;
   if (data_.is_empty()) {
     set_tree(tree);
+  } else if (cord_ring_enabled()) {
+    set_tree(CordRepRing::Append(ForceRing(force_tree(0), 1), tree));
   } else {
     set_tree(Concat(force_tree(0), tree));
   }
@@ -308,6 +349,8 @@ void Cord::InlineRep::PrependTree(CordRep* tree) {
   assert(tree != nullptr);
   if (data_.is_empty()) {
     set_tree(tree);
+  } else if (cord_ring_enabled()) {
+    set_tree(CordRepRing::Prepend(ForceRing(force_tree(0), 1), tree));
   } else {
     set_tree(Concat(tree, force_tree(0)));
   }
@@ -319,6 +362,15 @@ void Cord::InlineRep::PrependTree(CordRep* tree) {
 // written to region and the actual size increase will be written to size.
 static inline bool PrepareAppendRegion(CordRep* root, char** region,
                                        size_t* size, size_t max_length) {
+  if (root->tag == RING && root->refcount.IsOne()) {
+    Span<char> span = root->ring()->GetAppendBuffer(max_length);
+    if (!span.empty()) {
+      *region = span.data();
+      *size = span.size();
+      return true;
+    }
+  }
+
   // Search down the right-hand path for a non-full FLAT node.
   CordRep* dst = root;
   while (dst->tag == CONCAT && dst->refcount.IsOne()) {
@@ -383,6 +435,11 @@ void Cord::InlineRep::GetAppendRegion(char** region, size_t* size,
   new_node->length = std::min(new_node->Capacity(), max_length);
   *region = new_node->Data();
   *size = new_node->length;
+
+  if (cord_ring_enabled()) {
+    replace_tree(CordRepRing::Append(ForceRing(root, 1), new_node));
+    return;
+  }
   replace_tree(Concat(root, new_node));
 }
 
@@ -411,6 +468,11 @@ void Cord::InlineRep::GetAppendRegion(char** region, size_t* size) {
   new_node->length = new_node->Capacity();
   *region = new_node->Data();
   *size = new_node->length;
+
+  if (cord_ring_enabled()) {
+    replace_tree(CordRepRing::Append(ForceRing(root, 1), new_node));
+    return;
+  }
   replace_tree(Concat(root, new_node));
 }
 
@@ -593,6 +655,13 @@ void Cord::InlineRep::AppendArray(const char* src_data, size_t src_size) {
     return;
   }
 
+  if (cord_ring_enabled()) {
+    absl::string_view data(src_data, src_size);
+    root = ForceRing(root, (data.size() - 1) / kMaxFlatLength + 1);
+    replace_tree(CordRepRing::Append(root->ring(), data));
+    return;
+  }
+
   // Use new block(s) for any remaining bytes that were not handled above.
   // Alloc extra memory only if the right child of the root of the new tree is
   // going to be a FLAT node, which will permit further inplace appends.
@@ -805,6 +874,8 @@ void Cord::RemovePrefix(size_t n) {
   CordRep* tree = contents_.tree();
   if (tree == nullptr) {
     contents_.remove_prefix(n);
+  } else if (tree->tag == RING) {
+    contents_.replace_tree(CordRepRing::RemovePrefix(tree->ring(), n));
   } else {
     CordRep* newrep = RemovePrefixFrom(tree, n);
     CordRep::Unref(tree);
@@ -819,6 +890,8 @@ void Cord::RemoveSuffix(size_t n) {
   CordRep* tree = contents_.tree();
   if (tree == nullptr) {
     contents_.reduce_size(n);
+  } else if (tree->tag == RING) {
+    contents_.replace_tree(CordRepRing::RemoveSuffix(tree->ring(), n));
   } else {
     CordRep* newrep = RemoveSuffixFrom(tree, n);
     CordRep::Unref(tree);
@@ -902,6 +975,9 @@ Cord Cord::Subcord(size_t pos, size_t new_size) const {
     }
     cord_internal::SmallMemmove(dest, it->data(), remaining_size);
     sub_cord.contents_.set_inline_size(new_size);
+  } else if (tree->tag == RING) {
+    tree = CordRepRing::SubRing(CordRep::Ref(tree)->ring(), pos, new_size);
+    sub_cord.contents_.set_tree(tree);
   } else {
     sub_cord.contents_.set_tree(NewSubRange(tree, pos, new_size));
   }
@@ -1103,6 +1179,10 @@ inline absl::string_view Cord::InlineRep::FindFlatStartPiece() const {
     return absl::string_view(node->external()->base, node->length);
   }
 
+  if (node->tag == RING) {
+    return node->ring()->entry_data(node->ring()->head());
+  }
+
   // Walk down the left branches until we hit a non-CONCAT node.
   while (node->tag == CONCAT) {
     node = node->concat()->left;
@@ -1360,6 +1440,25 @@ Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
     }
     return subcord;
   }
+
+  if (ring_reader_) {
+    size_t chunk_size = current_chunk_.size();
+    if (n <= chunk_size && n <= kMaxBytesToCopy) {
+      subcord = Cord(current_chunk_.substr(0, n));
+    } else {
+      auto* ring = CordRep::Ref(ring_reader_.ring())->ring();
+      size_t offset = ring_reader_.length() - bytes_remaining_;
+      subcord.contents_.set_tree(CordRepRing::SubRing(ring, offset, n));
+    }
+    if (n < chunk_size) {
+      bytes_remaining_ -= n;
+      current_chunk_.remove_prefix(n);
+    } else {
+      AdvanceBytesRing(n);
+    }
+    return subcord;
+  }
+
   auto& stack_of_right_children = stack_of_right_children_;
   if (n < current_chunk_.size()) {
     // Range to read is a proper subrange of the current chunk.
@@ -1533,6 +1632,8 @@ char Cord::operator[](size_t i) const {
     if (rep->tag >= FLAT) {
       // Get the "i"th character directly from the flat array.
       return rep->flat()->Data()[offset];
+    } else if (rep->tag == RING) {
+      return rep->ring()->GetCharacter(offset);
     } else if (rep->tag == EXTERNAL) {
       // Get the "i"th character from the external array.
       return rep->external()->base[offset];
@@ -1609,6 +1710,15 @@ absl::string_view Cord::FlattenSlowPath() {
 /* static */ void Cord::ForEachChunkAux(
     absl::cord_internal::CordRep* rep,
     absl::FunctionRef<void(absl::string_view)> callback) {
+  if (rep->tag == RING) {
+    ChunkIterator it(rep), end;
+    while (it != end) {
+      callback(*it);
+      ++it;
+    }
+    return;
+  }
+
   assert(rep != nullptr);
   int stack_pos = 0;
   constexpr int stack_max = 128;
@@ -1650,9 +1760,9 @@ absl::string_view Cord::FlattenSlowPath() {
   }
 }
 
-static void DumpNode(CordRep* rep, bool include_data, std::ostream* os) {
+static void DumpNode(CordRep* rep, bool include_data, std::ostream* os,
+                     int indent) {
   const int kIndentStep = 1;
-  int indent = 0;
   absl::InlinedVector<CordRep*, kInlinedVectorSize> stack;
   absl::InlinedVector<int, kInlinedVectorSize> indents;
   for (;;) {
@@ -1673,18 +1783,28 @@ static void DumpNode(CordRep* rep, bool include_data, std::ostream* os) {
       *os << "SUBSTRING @ " << rep->substring()->start << "\n";
       indent += kIndentStep;
       rep = rep->substring()->child;
-    } else {  // Leaf
+    } else {  // Leaf or ring
       if (rep->tag == EXTERNAL) {
         *os << "EXTERNAL [";
         if (include_data)
           *os << absl::CEscape(std::string(rep->external()->base, rep->length));
         *os << "]\n";
-      } else {
+      } else if (rep->tag >= FLAT) {
         *os << "FLAT cap=" << rep->flat()->Capacity()
             << " [";
         if (include_data)
           *os << absl::CEscape(std::string(rep->flat()->Data(), rep->length));
         *os << "]\n";
+      } else {
+        assert(rep->tag == RING);
+        auto* ring = rep->ring();
+        *os << "RING, entries = " << ring->entries() << "\n";
+        CordRepRing::index_type head = ring->head();
+        do {
+          DumpNode(ring->entry_child(head), include_data, os,
+                   indent + kIndentStep);
+          head = ring->advance(head);;
+        } while (head != ring->tail());
       }
       if (stack.empty()) break;
       rep = stack.back();
@@ -1778,6 +1898,15 @@ static bool VerifyNode(CordRep* root, CordRep* start_node,
         }
         next_node = right;
       }
+    } else if (cur_node->tag == RING) {
+      total_mem_usage += CordRepRing::AllocSize(cur_node->ring()->capacity());
+      const CordRepRing* ring = cur_node->ring();
+      CordRepRing::index_type pos = ring->head(), tail = ring->tail();
+      do {
+        CordRep* node = ring->entry_child(pos);
+        assert(node->tag >= FLAT || node->tag == EXTERNAL);
+        RepMemoryUsageLeaf(node, &total_mem_usage);
+      } while ((pos = ring->advance(pos)) != tail);
     } else {
       // Since cur_node is not a leaf or a concat node it must be a substring.
       assert(cur_node->tag == SUBSTRING);
diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index abef98fe..17341bda 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h
@@ -78,6 +78,8 @@
 #include "absl/functional/function_ref.h"
 #include "absl/meta/type_traits.h"
 #include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_ring.h"
+#include "absl/strings/internal/cord_rep_ring_reader.h"
 #include "absl/strings/internal/resize_uninitialized.h"
 #include "absl/strings/internal/string_constant.h"
 #include "absl/strings/string_view.h"
@@ -361,6 +363,10 @@ class Cord {
     friend class CharIterator;
 
    private:
+    using CordRep = absl::cord_internal::CordRep;
+    using CordRepRing = absl::cord_internal::CordRepRing;
+    using CordRepRingReader = absl::cord_internal::CordRepRingReader;
+
     // Stack of right children of concat nodes that we have to visit.
     // Keep this at the end of the structure to avoid cache-thrashing.
     // TODO(jgm): Benchmark to see if there's a more optimal value than 47 for
@@ -385,6 +391,10 @@ class Cord {
     // Stack specific operator++
     ChunkIterator& AdvanceStack();
 
+    // Ring buffer specific operator++
+    ChunkIterator& AdvanceRing();
+    void AdvanceBytesRing(size_t n);
+
     // Iterates `n` bytes, where `n` is expected to be greater than or equal to
     // `current_chunk_.size()`.
     void AdvanceBytesSlowPath(size_t n);
@@ -398,6 +408,10 @@ class Cord {
     absl::cord_internal::CordRep* current_leaf_ = nullptr;
     // The number of bytes left in the `Cord` over which we are iterating.
     size_t bytes_remaining_ = 0;
+
+    // Cord reader for ring buffers. Empty if not traversing a ring buffer.
+    CordRepRingReader ring_reader_;
+
     // See 'Stack' alias definition.
     Stack stack_of_right_children_;
   };
@@ -1107,6 +1121,11 @@ inline bool Cord::StartsWith(absl::string_view rhs) const {
 }
 
 inline void Cord::ChunkIterator::InitTree(cord_internal::CordRep* tree) {
+  if (tree->tag == cord_internal::RING) {
+    current_chunk_ = ring_reader_.Reset(tree->ring());
+    return;
+  }
+
   stack_of_right_children_.push_back(tree);
   operator++();
 }
@@ -1126,13 +1145,33 @@ inline Cord::ChunkIterator::ChunkIterator(const Cord* cord)
   }
 }
 
+inline Cord::ChunkIterator& Cord::ChunkIterator::AdvanceRing() {
+  current_chunk_ = ring_reader_.Next();
+  return *this;
+}
+
+inline void Cord::ChunkIterator::AdvanceBytesRing(size_t n) {
+  assert(n >= current_chunk_.size());
+  bytes_remaining_ -= n;
+  if (bytes_remaining_) {
+    if (n == current_chunk_.size()) {
+      current_chunk_ = ring_reader_.Next();
+    } else {
+      size_t offset = ring_reader_.length() - bytes_remaining_;
+      current_chunk_ = ring_reader_.Seek(offset);
+    }
+  } else {
+    current_chunk_ = {};
+  }
+}
+
 inline Cord::ChunkIterator& Cord::ChunkIterator::operator++() {
   ABSL_HARDENING_ASSERT(bytes_remaining_ > 0 &&
                         "Attempted to iterate past `end()`");
   assert(bytes_remaining_ >= current_chunk_.size());
   bytes_remaining_ -= current_chunk_.size();
   if (bytes_remaining_ > 0) {
-    return AdvanceStack();
+    return ring_reader_ ? AdvanceRing() : AdvanceStack();
   } else {
     current_chunk_ = {};
   }
@@ -1174,7 +1213,7 @@ inline void Cord::ChunkIterator::AdvanceBytes(size_t n) {
   if (ABSL_PREDICT_TRUE(n < current_chunk_.size())) {
     RemoveChunkPrefix(n);
   } else if (n != 0) {
-    AdvanceBytesSlowPath(n);
+    ring_reader_ ? AdvanceBytesRing(n) : AdvanceBytesSlowPath(n);
   }
 }
 
diff --git a/absl/strings/cord_test.cc b/absl/strings/cord_test.cc
index 7942bfc0..bf7a6820 100644
--- a/absl/strings/cord_test.cc
+++ b/absl/strings/cord_test.cc
@@ -367,7 +367,7 @@ TEST(Cord, Subcord) {
     for (size_t end_pos : positions) {
       if (end_pos < pos || end_pos > a.size()) continue;
       absl::Cord sa = a.Subcord(pos, end_pos - pos);
-      EXPECT_EQ(absl::string_view(s).substr(pos, end_pos - pos),
+      ASSERT_EQ(absl::string_view(s).substr(pos, end_pos - pos),
                 std::string(sa))
           << a;
     }
@@ -379,7 +379,7 @@ TEST(Cord, Subcord) {
   for (size_t pos = 0; pos <= sh.size(); ++pos) {
     for (size_t n = 0; n <= sh.size() - pos; ++n) {
       absl::Cord sc = c.Subcord(pos, n);
-      EXPECT_EQ(sh.substr(pos, n), std::string(sc)) << c;
+      ASSERT_EQ(sh.substr(pos, n), std::string(sc)) << c;
     }
   }
 
@@ -389,7 +389,7 @@ TEST(Cord, Subcord) {
   while (sa.size() > 1) {
     sa = sa.Subcord(1, sa.size() - 2);
     ss = ss.substr(1, ss.size() - 2);
-    EXPECT_EQ(ss, std::string(sa)) << a;
+    ASSERT_EQ(ss, std::string(sa)) << a;
     if (HasFailure()) break;  // halt cascade
   }
 
diff --git a/absl/strings/internal/cord_rep_flat.h b/absl/strings/internal/cord_rep_flat.h
index 55418153..a98aa9df 100644
--- a/absl/strings/internal/cord_rep_flat.h
+++ b/absl/strings/internal/cord_rep_flat.h
@@ -43,8 +43,9 @@ static constexpr size_t kMaxFlatSize = 4096;
 static constexpr size_t kMaxFlatLength = kMaxFlatSize - kFlatOverhead;
 static constexpr size_t kMinFlatLength = kMinFlatSize - kFlatOverhead;
 
-constexpr size_t AllocatedSizeToTagUnchecked(size_t size) {
-  return (size <= 1024) ? size / 8 : 128 + size / 32 - 1024 / 32;
+constexpr uint8_t AllocatedSizeToTagUnchecked(size_t size) {
+  return static_cast<uint8_t>((size <= 1024) ? size / 8
+                                             : 128 + size / 32 - 1024 / 32);
 }
 
 static_assert(kMinFlatSize / 8 >= FLAT, "");
@@ -65,7 +66,7 @@ inline size_t RoundUpForTag(size_t size) {
 // undefined if the size exceeds the maximum size that can be encoded in
 // a tag, i.e., if size is larger than TagToAllocatedSize(<max tag>).
 inline uint8_t AllocatedSizeToTag(size_t size) {
-  const size_t tag = AllocatedSizeToTagUnchecked(size);
+  const uint8_t tag = AllocatedSizeToTagUnchecked(size);
   assert(tag <= MAX_FLAT_TAG);
   return tag;
 }
diff --git a/absl/strings/internal/cord_rep_ring.cc b/absl/strings/internal/cord_rep_ring.cc
index 358b0d92..4d31d1d9 100644
--- a/absl/strings/internal/cord_rep_ring.cc
+++ b/absl/strings/internal/cord_rep_ring.cc
@@ -36,8 +36,10 @@ namespace cord_internal {
 #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 {
 
diff --git a/absl/strings/internal/cord_rep_ring.h b/absl/strings/internal/cord_rep_ring.h
index 55cba8b4..c74d3353 100644
--- a/absl/strings/internal/cord_rep_ring.h
+++ b/absl/strings/internal/cord_rep_ring.h
@@ -34,8 +34,10 @@ namespace cord_internal {
 #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.
@@ -81,7 +83,7 @@ class CordRepRing : public CordRep {
   // `end_pos` which is the `end_pos` of the previous node (or `begin_pos`) plus
   // this node's length. The purpose is to allow for a binary search on this
   // position, while allowing O(1) prepend and append operations.
-  using pos_type = uint64_t;
+  using pos_type = size_t;
 
   // `index_type` is the type for the `head`, `tail` and `capacity` indexes.
   // Ring buffers are limited to having no more than four billion entries.
diff --git a/absl/time/internal/cctz/testdata/version b/absl/time/internal/cctz/testdata/version
index a481df8c..1d590958 100644
--- a/absl/time/internal/cctz/testdata/version
+++ b/absl/time/internal/cctz/testdata/version
@@ -1 +1 @@
-2020f
+2021a
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Juba b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Juba
index 36b05220..0aba9ffd 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Africa/Juba
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Africa/Juba