diff --git a/absl/cleanup/cleanup.h b/absl/cleanup/cleanup.h
index 61b53d5..960ccd0 100644
--- a/absl/cleanup/cleanup.h
+++ b/absl/cleanup/cleanup.h

@@ -86,25 +86,25 @@
                 "Callbacks that return values are not supported.");
 
  public:
-  Cleanup(Callback callback)  // NOLINT
-      : storage_(std::move(callback), /* is_callback_engaged = */ true) {}
+  Cleanup(Callback callback) : storage_(std::move(callback)) {}  // NOLINT
 
   Cleanup(Cleanup&& other) = default;
 
   void Cancel() && {
     ABSL_HARDENING_ASSERT(storage_.IsCallbackEngaged());
-    storage_.DisengageCallback();
+    storage_.DestroyCallback();
   }
 
   void Invoke() && {
     ABSL_HARDENING_ASSERT(storage_.IsCallbackEngaged());
-    storage_.DisengageCallback();
     storage_.InvokeCallback();
+    storage_.DestroyCallback();
   }
 
   ~Cleanup() {
     if (storage_.IsCallbackEngaged()) {
       storage_.InvokeCallback();
+      storage_.DestroyCallback();
     }
   }
 

diff --git a/absl/cleanup/cleanup_test.cc b/absl/cleanup/cleanup_test.cc
index 792595d..46b8858 100644
--- a/absl/cleanup/cleanup_test.cc
+++ b/absl/cleanup/cleanup_test.cc

@@ -264,4 +264,48 @@
   EXPECT_FALSE(called);  // Destructor shouldn't invoke the callback
 }
 
+int DestructionCount = 0;
+
+struct DestructionCounter {
+  void operator()() {}
+
+  ~DestructionCounter() { ++DestructionCount; }
+};
+
+TYPED_TEST(CleanupTest, DestructorDestroys) {
+  {
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback(DestructionCounter()));
+    DestructionCount = 0;
+  }
+
+  EXPECT_EQ(DestructionCount, 1);  // Engaged cleanup destroys
+}
+
+TYPED_TEST(CleanupTest, CancelDestroys) {
+  {
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback(DestructionCounter()));
+    DestructionCount = 0;
+
+    std::move(cleanup).Cancel();
+    EXPECT_EQ(DestructionCount, 1);  // Cancel destroys
+  }
+
+  EXPECT_EQ(DestructionCount, 1);  // Canceled cleanup does not double destroy
+}
+
+TYPED_TEST(CleanupTest, InvokeDestroys) {
+  {
+    auto cleanup =
+        absl::MakeCleanup(TypeParam::AsCallback(DestructionCounter()));
+    DestructionCount = 0;
+
+    std::move(cleanup).Invoke();
+    EXPECT_EQ(DestructionCount, 1);  // Invoke destroys
+  }
+
+  EXPECT_EQ(DestructionCount, 1);  // Invoked cleanup does not double destroy
+}
+
 }  // namespace

diff --git a/absl/cleanup/internal/cleanup.h b/absl/cleanup/internal/cleanup.h
index b4c4073..2783fcb 100644
--- a/absl/cleanup/internal/cleanup.h
+++ b/absl/cleanup/internal/cleanup.h

@@ -15,10 +15,12 @@
 #ifndef ABSL_CLEANUP_INTERNAL_CLEANUP_H_
 #define ABSL_CLEANUP_INTERNAL_CLEANUP_H_
 
+#include <new>
 #include <type_traits>
 #include <utility>
 
 #include "absl/base/internal/invoke.h"
+#include "absl/base/macros.h"
 #include "absl/base/thread_annotations.h"
 #include "absl/utility/utility.h"
 
@@ -45,14 +47,22 @@
  public:
   Storage() = delete;
 
-  Storage(Callback callback, bool is_callback_engaged)
-      : callback_(std::move(callback)),
-        is_callback_engaged_(is_callback_engaged) {}
+  explicit Storage(Callback callback) {
+    // Placement-new into a character buffer is used for eager destruction when
+    // the cleanup is invoked or cancelled. To ensure this optimizes well, the
+    // behavior is implemented locally instead of using an absl::optional.
+    ::new (GetCallbackBuffer()) Callback(std::move(callback));
+    is_callback_engaged_ = true;
+  }
 
-  Storage(Storage&& other)
-      : callback_(std::move(other.callback_)),
-        is_callback_engaged_(
-            absl::exchange(other.is_callback_engaged_, false)) {}
+  Storage(Storage&& other) {
+    ABSL_HARDENING_ASSERT(other.IsCallbackEngaged());
+
+    ::new (GetCallbackBuffer()) Callback(std::move(other.GetCallback()));
+    is_callback_engaged_ = true;
+
+    other.DestroyCallback();
+  }
 
   Storage(const Storage& other) = delete;
 
@@ -60,17 +70,26 @@
 
   Storage& operator=(const Storage& other) = delete;
 
+  void* GetCallbackBuffer() { return static_cast<void*>(+callback_buffer_); }
+
+  Callback& GetCallback() {
+    return *reinterpret_cast<Callback*>(GetCallbackBuffer());
+  }
+
   bool IsCallbackEngaged() const { return is_callback_engaged_; }
 
-  void DisengageCallback() { is_callback_engaged_ = false; }
+  void DestroyCallback() {
+    is_callback_engaged_ = false;
+    GetCallback().~Callback();
+  }
 
   void InvokeCallback() ABSL_NO_THREAD_SAFETY_ANALYSIS {
-    std::move(callback_)();
+    std::move(GetCallback())();
   }
 
  private:
-  Callback callback_;
   bool is_callback_engaged_;
+  alignas(Callback) char callback_buffer_[sizeof(Callback)];
 };
 
 }  // namespace cleanup_internal

diff --git a/absl/container/internal/raw_hash_set.cc b/absl/container/internal/raw_hash_set.cc
index a5dee6a..e219956 100644
--- a/absl/container/internal/raw_hash_set.cc
+++ b/absl/container/internal/raw_hash_set.cc

@@ -23,6 +23,12 @@
 ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
+ABSL_CONST_INIT ABSL_DLL alignas(16) const ctrl_t kEmptyGroup[16] = {
+    ctrl_t::kSentinel, ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
+    ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
+    ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
+    ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty};
+
 constexpr size_t Group::kWidth;
 
 // Returns "random" seed.

diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index d778326..bafafd4 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h

@@ -291,13 +291,9 @@
 
 // A single block of empty control bytes for tables without any slots allocated.
 // This enables removing a branch in the hot path of find().
+ABSL_DLL extern const ctrl_t kEmptyGroup[16];
 inline ctrl_t* EmptyGroup() {
-  alignas(16) static constexpr ctrl_t empty_group[] = {
-      ctrl_t::kSentinel, ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
-      ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
-      ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty,
-      ctrl_t::kEmpty,    ctrl_t::kEmpty, ctrl_t::kEmpty, ctrl_t::kEmpty};
-  return const_cast<ctrl_t*>(empty_group);
+  return const_cast<ctrl_t*>(kEmptyGroup);
 }
 
 // Mixes a randomly generated per-process seed with `hash` and `ctrl` to

diff --git a/absl/flags/internal/sequence_lock.h b/absl/flags/internal/sequence_lock.h
index 807b2a7..36318ab 100644
--- a/absl/flags/internal/sequence_lock.h
+++ b/absl/flags/internal/sequence_lock.h

@@ -49,7 +49,7 @@
 // The memory reads and writes protected by this lock must use the provided
 // `TryRead()` and `Write()` functions. These functions behave similarly to
 // `memcpy()`, with one oddity: the protected data must be an array of
-// `std::atomic<int64>`. This is to comply with the C++ standard, which
+// `std::atomic<uint64>`. This is to comply with the C++ standard, which
 // considers data races on non-atomic objects to be undefined behavior. See "Can
 // Seqlocks Get Along With Programming Language Memory Models?"[1] by Hans J.
 // Boehm for more details.

diff --git a/absl/flags/internal/usage.cc b/absl/flags/internal/usage.cc
index 949709e..a883567 100644
--- a/absl/flags/internal/usage.cc
+++ b/absl/flags/internal/usage.cc

@@ -17,6 +17,7 @@
 
 #include <stdint.h>
 
+#include <algorithm>
 #include <functional>
 #include <map>
 #include <ostream>
@@ -255,9 +256,6 @@
       matching_flags;
 
   flags_internal::ForEachFlag([&](absl::CommandLineFlag& flag) {
-    // Ignore retired flags.
-    if (flag.IsRetired()) return;
-
     // If the flag has been stripped, pretend that it doesn't exist.
     if (flag.Help() == flags_internal::kStrippedFlagHelp) return;
 
@@ -275,6 +273,14 @@
   absl::string_view file_separator;     // controls blank lines between files
   for (auto& package : matching_flags) {
     if (format == HelpFormat::kHumanReadable) {
+      // Hide packages with only retired flags
+      bool all_package_flags_are_retired = true;
+      for (const auto& flags_in_file : package.second) {
+        for (const auto* flag : flags_in_file.second) {
+          all_package_flags_are_retired &= flag->IsRetired();
+        }
+      }
+      if (all_package_flags_are_retired) continue;
       out << package_separator;
       package_separator = "\n\n";
     }
@@ -334,8 +340,11 @@
 // Produces the help message describing specific flag.
 void FlagHelp(std::ostream& out, const CommandLineFlag& flag,
               HelpFormat format) {
-  if (format == HelpFormat::kHumanReadable)
+  if (format == HelpFormat::kHumanReadable) {
+    // Ignore retired flags
+    if (flag.IsRetired()) return;
     flags_internal::FlagHelpHumanReadable(flag, out);
+  }
 }
 
 // --------------------------------------------------------------------

diff --git a/absl/flags/internal/usage_test.cc b/absl/flags/internal/usage_test.cc
index 044d71c..5055640 100644
--- a/absl/flags/internal/usage_test.cc
+++ b/absl/flags/internal/usage_test.cc

@@ -61,6 +61,9 @@
     "Even more long long long long long long long long long long long long "
     "help message.");
 
+ABSL_RETIRED_FLAG(int64_t, usage_reporting_test_flag_07, 1,
+                  "usage_reporting_test_flag_07 help message");
+
 namespace {
 
 namespace flags = absl::flags_internal;

diff --git a/absl/numeric/int128.cc b/absl/numeric/int128.cc
index 5160df7..17d8874 100644
--- a/absl/numeric/int128.cc
+++ b/absl/numeric/int128.cc

@@ -138,28 +138,21 @@
 uint128::uint128(double v) : uint128(MakeUint128FromFloat(v)) {}
 uint128::uint128(long double v) : uint128(MakeUint128FromFloat(v)) {}
 
+#if !defined(ABSL_HAVE_INTRINSIC_INT128)
 uint128 operator/(uint128 lhs, uint128 rhs) {
-#if defined(ABSL_HAVE_INTRINSIC_INT128)
-  return static_cast<unsigned __int128>(lhs) /
-         static_cast<unsigned __int128>(rhs);
-#else  // ABSL_HAVE_INTRINSIC_INT128
   uint128 quotient = 0;
   uint128 remainder = 0;
   DivModImpl(lhs, rhs, &quotient, &remainder);
   return quotient;
-#endif  // ABSL_HAVE_INTRINSIC_INT128
 }
+
 uint128 operator%(uint128 lhs, uint128 rhs) {
-#if defined(ABSL_HAVE_INTRINSIC_INT128)
-  return static_cast<unsigned __int128>(lhs) %
-         static_cast<unsigned __int128>(rhs);
-#else  // ABSL_HAVE_INTRINSIC_INT128
   uint128 quotient = 0;
   uint128 remainder = 0;
   DivModImpl(lhs, rhs, &quotient, &remainder);
   return remainder;
-#endif  // ABSL_HAVE_INTRINSIC_INT128
 }
+#endif  // !defined(ABSL_HAVE_INTRINSIC_INT128)
 
 namespace {
 

diff --git a/absl/numeric/int128.h b/absl/numeric/int128.h
index 198aa19..a9cbe48 100644
--- a/absl/numeric/int128.h
+++ b/absl/numeric/int128.h

@@ -18,6 +18,10 @@
 // -----------------------------------------------------------------------------
 //
 // This header file defines 128-bit integer types, `uint128` and `int128`.
+//
+// TODO(absl-team): This module is inconsistent as many inline `uint128` methods
+// are defined in this file, while many inline `int128` methods are defined in
+// the `int128_*_intrinsic.inc` files.
 
 #ifndef ABSL_NUMERIC_INT128_H_
 #define ABSL_NUMERIC_INT128_H_
@@ -783,8 +787,13 @@
 // Comparison operators.
 
 inline bool operator==(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) ==
+         static_cast<unsigned __int128>(rhs);
+#else
   return (Uint128Low64(lhs) == Uint128Low64(rhs) &&
           Uint128High64(lhs) == Uint128High64(rhs));
+#endif
 }
 
 inline bool operator!=(uint128 lhs, uint128 rhs) {
@@ -819,52 +828,76 @@
 }
 
 inline uint128 operator-(uint128 val) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return -static_cast<unsigned __int128>(val);
+#else
   uint64_t hi = ~Uint128High64(val);
   uint64_t lo = ~Uint128Low64(val) + 1;
   if (lo == 0) ++hi;  // carry
   return MakeUint128(hi, lo);
+#endif
 }
 
 constexpr inline bool operator!(uint128 val) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return !static_cast<unsigned __int128>(val);
+#else
   return !Uint128High64(val) && !Uint128Low64(val);
+#endif
 }
 
 // Logical operators.
 
 constexpr inline uint128 operator~(uint128 val) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return ~static_cast<unsigned __int128>(val);
+#else
   return MakeUint128(~Uint128High64(val), ~Uint128Low64(val));
+#endif
 }
 
 constexpr inline uint128 operator|(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) |
+         static_cast<unsigned __int128>(rhs);
+#else
   return MakeUint128(Uint128High64(lhs) | Uint128High64(rhs),
-                           Uint128Low64(lhs) | Uint128Low64(rhs));
+                     Uint128Low64(lhs) | Uint128Low64(rhs));
+#endif
 }
 
 constexpr inline uint128 operator&(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) &
+         static_cast<unsigned __int128>(rhs);
+#else
   return MakeUint128(Uint128High64(lhs) & Uint128High64(rhs),
-                           Uint128Low64(lhs) & Uint128Low64(rhs));
+                     Uint128Low64(lhs) & Uint128Low64(rhs));
+#endif
 }
 
 constexpr inline uint128 operator^(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) ^
+         static_cast<unsigned __int128>(rhs);
+#else
   return MakeUint128(Uint128High64(lhs) ^ Uint128High64(rhs),
-                           Uint128Low64(lhs) ^ Uint128Low64(rhs));
+                     Uint128Low64(lhs) ^ Uint128Low64(rhs));
+#endif
 }
 
 inline uint128& uint128::operator|=(uint128 other) {
-  hi_ |= other.hi_;
-  lo_ |= other.lo_;
+  *this = *this | other;
   return *this;
 }
 
 inline uint128& uint128::operator&=(uint128 other) {
-  hi_ &= other.hi_;
-  lo_ &= other.lo_;
+  *this = *this & other;
   return *this;
 }
 
 inline uint128& uint128::operator^=(uint128 other) {
-  hi_ ^= other.hi_;
-  lo_ ^= other.lo_;
+  *this = *this ^ other;
   return *this;
 }
 
@@ -907,21 +940,31 @@
 }
 
 inline uint128 operator+(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) +
+         static_cast<unsigned __int128>(rhs);
+#else
   uint128 result = MakeUint128(Uint128High64(lhs) + Uint128High64(rhs),
                                Uint128Low64(lhs) + Uint128Low64(rhs));
   if (Uint128Low64(result) < Uint128Low64(lhs)) {  // check for carry
     return MakeUint128(Uint128High64(result) + 1, Uint128Low64(result));
   }
   return result;
+#endif
 }
 
 inline uint128 operator-(uint128 lhs, uint128 rhs) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return static_cast<unsigned __int128>(lhs) -
+         static_cast<unsigned __int128>(rhs);
+#else
   uint128 result = MakeUint128(Uint128High64(lhs) - Uint128High64(rhs),
                                Uint128Low64(lhs) - Uint128Low64(rhs));
   if (Uint128Low64(lhs) < Uint128Low64(rhs)) {  // check for carry
     return MakeUint128(Uint128High64(result) - 1, Uint128Low64(result));
   }
   return result;
+#endif
 }
 
 inline uint128 operator*(uint128 lhs, uint128 rhs) {
@@ -951,6 +994,18 @@
 #endif  // ABSL_HAVE_INTRINSIC128
 }
 
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+inline uint128 operator/(uint128 lhs, uint128 rhs) {
+  return static_cast<unsigned __int128>(lhs) /
+         static_cast<unsigned __int128>(rhs);
+}
+
+inline uint128 operator%(uint128 lhs, uint128 rhs) {
+  return static_cast<unsigned __int128>(lhs) %
+         static_cast<unsigned __int128>(rhs);
+}
+#endif
+
 // Increment/decrement operators.
 
 inline uint128 uint128::operator++(int) {

diff --git a/absl/random/internal/generate_real.h b/absl/random/internal/generate_real.h
index 4f62873..d5fbb44 100644
--- a/absl/random/internal/generate_real.h
+++ b/absl/random/internal/generate_real.h

@@ -127,10 +127,8 @@
 
   // Construct the 32-bit or 64-bit IEEE 754 floating-point value from
   // the individual fields: sign, exp, mantissa(bits).
-  uint_type val =
-      (std::is_same<SignedTag, GeneratePositiveTag>::value ? 0u : sign) |
-      (static_cast<uint_type>(exp) << kExp) |
-      (static_cast<uint_type>(bits) & kMask);
+  uint_type val = sign | (static_cast<uint_type>(exp) << kExp) |
+                  (static_cast<uint_type>(bits) & kMask);
 
   // bit_cast to the output-type
   real_type result;

diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 9720b68..f9735b4 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel

@@ -318,6 +318,7 @@
         ":strings",
         "//absl/base:config",
         "//absl/base:raw_logging_internal",
+        "//absl/cleanup",
         "@com_google_googletest//:gtest_main",
     ],
 )

diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index 88f076a..8ad5f9c 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt

@@ -952,6 +952,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::base
+    absl::cleanup
     absl::config
     absl::cord_internal
     absl::cord_rep_test_util

diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index f5aa6e4..e9d72fa 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc

@@ -36,8 +36,8 @@
 #include "absl/container/inlined_vector.h"
 #include "absl/strings/escaping.h"
 #include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
 #include "absl/strings/internal/cord_rep_flat.h"
-#include "absl/strings/internal/cord_rep_ring.h"
 #include "absl/strings/internal/cordz_statistics.h"
 #include "absl/strings/internal/cordz_update_scope.h"
 #include "absl/strings/internal/cordz_update_tracker.h"
@@ -51,22 +51,16 @@
 ABSL_NAMESPACE_BEGIN
 
 using ::absl::cord_internal::CordRep;
+using ::absl::cord_internal::CordRepBtree;
 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::CordzUpdateTracker;
 using ::absl::cord_internal::InlineData;
 using ::absl::cord_internal::kMaxFlatLength;
 using ::absl::cord_internal::kMinFlatLength;
 
-using ::absl::cord_internal::CONCAT;
-using ::absl::cord_internal::EXTERNAL;
-using ::absl::cord_internal::FLAT;
-using ::absl::cord_internal::RING;
-using ::absl::cord_internal::SUBSTRING;
-
 using ::absl::cord_internal::kInlinedVectorSize;
 using ::absl::cord_internal::kMaxBytesToCopy;
 
@@ -100,13 +94,13 @@
 
 static const int kMinLengthSize = ABSL_ARRAYSIZE(min_length);
 
-static inline bool cord_ring_enabled() {
-  return cord_internal::cord_ring_buffer_enabled.load(
+static inline bool btree_enabled() {
+  return cord_internal::cord_btree_enabled.load(
       std::memory_order_relaxed);
 }
 
 static inline bool IsRootBalanced(CordRep* node) {
-  if (node->tag != CONCAT) {
+  if (!node->IsConcat()) {
     return true;
   } else if (node->concat()->depth() <= 15) {
     return true;
@@ -143,7 +137,7 @@
 
 // Return the depth of a node
 static int Depth(const CordRep* rep) {
-  if (rep->tag == CONCAT) {
+  if (rep->IsConcat()) {
     return rep->concat()->depth();
   } else {
     return 0;
@@ -176,7 +170,7 @@
   }
 
   CordRepConcat* rep = new CordRepConcat();
-  rep->tag = CONCAT;
+  rep->tag = cord_internal::CONCAT;
   SetConcatChildren(rep, left, right);
 
   return rep;
@@ -218,27 +212,25 @@
   return flat;
 }
 
-// Creates a new flat or ringbuffer out of the specified array.
+// Creates a new flat or Btree 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) {
+static CordRep* NewBtree(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);
+  auto* root = CordRepBtree::Create(flat);
+  return CordRepBtree::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);
+  if (btree_enabled()) {
+    return NewBtree(data, length, alloc_hint);
   }
   absl::FixedArray<CordRep*> reps((length - 1) / kMaxFlatLength + 1);
   size_t n = 0;
@@ -275,7 +267,7 @@
     CordRepSubstring* rep = new CordRepSubstring();
     assert((offset + length) <= child->length);
     rep->length = length;
-    rep->tag = SUBSTRING;
+    rep->tag = cord_internal::SUBSTRING;
     rep->start = offset;
     rep->child = child;
     return VerifyTree(rep);
@@ -346,10 +338,10 @@
   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);
+// Returns `rep` converted into a CordRepBtree.
+// Directly returns `rep` if `rep` is already a CordRepBtree.
+static CordRepBtree* ForceBtree(CordRep* rep) {
+  return rep->IsBtree() ? rep->btree() : CordRepBtree::Create(rep);
 }
 
 void Cord::InlineRep::AppendTreeToInlined(CordRep* tree,
@@ -357,8 +349,8 @@
   assert(!is_tree());
   if (!data_.is_empty()) {
     CordRepFlat* flat = MakeFlatWithExtraCapacity(0);
-    if (cord_ring_enabled()) {
-      tree = CordRepRing::Append(CordRepRing::Create(flat, 1), tree);
+    if (btree_enabled()) {
+      tree = CordRepBtree::Append(CordRepBtree::Create(flat), tree);
     } else {
       tree = Concat(flat, tree);
     }
@@ -369,8 +361,8 @@
 void Cord::InlineRep::AppendTreeToTree(CordRep* tree, MethodIdentifier method) {
   assert(is_tree());
   const CordzUpdateScope scope(data_.cordz_info(), method);
-  if (cord_ring_enabled()) {
-    tree = CordRepRing::Append(ForceRing(data_.as_tree(), 1), tree);
+  if (btree_enabled()) {
+    tree = CordRepBtree::Append(ForceBtree(data_.as_tree()), tree);
   } else {
     tree = Concat(data_.as_tree(), tree);
   }
@@ -391,8 +383,8 @@
   assert(!is_tree());
   if (!data_.is_empty()) {
     CordRepFlat* flat = MakeFlatWithExtraCapacity(0);
-    if (cord_ring_enabled()) {
-      tree = CordRepRing::Prepend(CordRepRing::Create(flat, 1), tree);
+    if (btree_enabled()) {
+      tree = CordRepBtree::Prepend(CordRepBtree::Create(flat), tree);
     } else {
       tree = Concat(tree, flat);
     }
@@ -404,8 +396,8 @@
                                         MethodIdentifier method) {
   assert(is_tree());
   const CordzUpdateScope scope(data_.cordz_info(), method);
-  if (cord_ring_enabled()) {
-    tree = CordRepRing::Prepend(ForceRing(data_.as_tree(), 1), tree);
+  if (btree_enabled()) {
+    tree = CordRepBtree::Prepend(ForceBtree(data_.as_tree()), tree);
   } else {
     tree = Concat(tree, data_.as_tree());
   }
@@ -427,8 +419,8 @@
 // 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 (root->IsBtree() && root->refcount.IsOne()) {
+    Span<char> span = root->btree()->GetAppendBuffer(max_length);
     if (!span.empty()) {
       *region = span.data();
       *size = span.size();
@@ -438,11 +430,11 @@
 
   // Search down the right-hand path for a non-full FLAT node.
   CordRep* dst = root;
-  while (dst->tag == CONCAT && dst->refcount.IsOne()) {
+  while (dst->IsConcat() && dst->refcount.IsOne()) {
     dst = dst->concat()->right;
   }
 
-  if (dst->tag < FLAT || !dst->refcount.IsOne()) {
+  if (!dst->IsFlat() || !dst->refcount.IsOne()) {
     *region = nullptr;
     *size = 0;
     return false;
@@ -500,22 +492,48 @@
   *region = new_node->Data();
   *size = new_node->length;
 
-  if (cord_ring_enabled()) {
-    rep = CordRepRing::Append(ForceRing(rep, 1), new_node);
+  if (btree_enabled()) {
+    rep = CordRepBtree::Append(ForceBtree(rep), new_node);
   } else {
     rep = Concat(rep, new_node);
   }
   CommitTree(root, rep, scope, method);
 }
 
+// Computes the memory side of the provided edge which must be a valid data edge
+// for a btrtee, i.e., a FLAT, EXTERNAL or SUBSTRING of a FLAT or EXTERNAL node.
+static bool RepMemoryUsageDataEdge(const CordRep* rep,
+                                   size_t* total_mem_usage) {
+  size_t maybe_sub_size = 0;
+  if (ABSL_PREDICT_FALSE(rep->IsSubstring())) {
+    maybe_sub_size = sizeof(cord_internal::CordRepSubstring);
+    rep = rep->substring()->child;
+  }
+  if (rep->IsFlat()) {
+    *total_mem_usage += maybe_sub_size + rep->flat()->AllocatedSize();
+    return true;
+  }
+  if (rep->IsExternal()) {
+    // We don't know anything about the embedded / bound data, but we can safely
+    // assume it is 'at least' a word / pointer to data. In the future we may
+    // choose to use the 'data' byte as a tag to identify the types of some
+    // well-known externals, such as a std::string instance.
+    *total_mem_usage += maybe_sub_size +
+                        sizeof(cord_internal::CordRepExternalImpl<intptr_t>) +
+                        rep->length;
+    return true;
+  }
+  return false;
+}
+
 // If the rep is a leaf, this will increment the value at total_mem_usage and
 // will return true.
 static bool RepMemoryUsageLeaf(const CordRep* rep, size_t* total_mem_usage) {
-  if (rep->tag >= FLAT) {
+  if (rep->IsFlat()) {
     *total_mem_usage += rep->flat()->AllocatedSize();
     return true;
   }
-  if (rep->tag == EXTERNAL) {
+  if (rep->IsExternal()) {
     // We don't know anything about the embedded / bound data, but we can safely
     // assume it is 'at least' a word / pointer to data. In the future we may
     // choose to use the 'data' byte as a tag to identify the types of some
@@ -630,7 +648,7 @@
   }
   if (tree != nullptr) {
     CordzUpdateScope scope(contents_.cordz_info(), method);
-    if (tree->tag >= FLAT && tree->flat()->Capacity() >= length &&
+    if (tree->IsFlat() && tree->flat()->Capacity() >= length &&
         tree->refcount.IsOne()) {
       // Copy in place if the existing FLAT node is reusable.
       memmove(tree->flat()->Data(), data, length);
@@ -690,9 +708,11 @@
     return;
   }
 
-  if (cord_ring_enabled()) {
-    rep = ForceRing(rep, (src.size() - 1) / kMaxFlatLength + 1);
-    rep = CordRepRing::Append(rep->ring(), src);
+  if (btree_enabled()) {
+    // TODO(b/192061034): keep legacy 10% growth rate: consider other rates.
+    rep = ForceBtree(rep);
+    const size_t alloc_hint = (std::min)(kMaxFlatLength, rep->length / 10);
+    rep = CordRepBtree::Append(rep->btree(), src, alloc_hint);
   } else {
     // 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
@@ -747,7 +767,7 @@
       contents_.AppendArray({src.contents_.data(), src_size}, method);
       return;
     }
-    if (src_tree->tag >= FLAT) {
+    if (src_tree->IsFlat()) {
       // src tree just has one flat node.
       contents_.AppendArray({src_tree->flat()->Data(), src_size}, method);
       return;
@@ -769,9 +789,13 @@
   contents_.AppendTree(rep, CordzUpdateTracker::kAppendCord);
 }
 
-void Cord::Append(const Cord& src) { AppendImpl(src); }
+void Cord::Append(const Cord& src) {
+  AppendImpl(src);
+}
 
-void Cord::Append(Cord&& src) { AppendImpl(std::move(src)); }
+void Cord::Append(Cord&& src) {
+  AppendImpl(std::move(src));
+}
 
 template <typename T, Cord::EnableIfString<T>>
 void Cord::Append(T&& src) {
@@ -833,7 +857,7 @@
   if (n == 0) return CordRep::Ref(node);
   absl::InlinedVector<CordRep*, kInlinedVectorSize> rhs_stack;
 
-  while (node->tag == CONCAT) {
+  while (node->IsConcat()) {
     assert(n <= node->length);
     if (n < node->concat()->left->length) {
       // Push right to stack, descend left.
@@ -852,7 +876,7 @@
   } else {
     size_t start = n;
     size_t len = node->length - n;
-    if (node->tag == SUBSTRING) {
+    if (node->IsSubstring()) {
       // Consider in-place update of node, similar to in RemoveSuffixFrom().
       start += node->substring()->start;
       node = node->substring()->child;
@@ -875,7 +899,7 @@
   absl::InlinedVector<CordRep*, kInlinedVectorSize> lhs_stack;
   bool inplace_ok = node->refcount.IsOne();
 
-  while (node->tag == CONCAT) {
+  while (node->IsConcat()) {
     assert(n <= node->length);
     if (n < node->concat()->right->length) {
       // Push left to stack, descend right.
@@ -892,7 +916,7 @@
 
   if (n == 0) {
     CordRep::Ref(node);
-  } else if (inplace_ok && node->tag != EXTERNAL) {
+  } else if (inplace_ok && !node->IsExternal()) {
     // Consider making a new buffer if the current node capacity is much
     // larger than the new length.
     CordRep::Ref(node);
@@ -900,7 +924,7 @@
   } else {
     size_t start = 0;
     size_t len = node->length - n;
-    if (node->tag == SUBSTRING) {
+    if (node->IsSubstring()) {
       start = node->substring()->start;
       node = node->substring()->child;
     }
@@ -923,8 +947,10 @@
   } else {
     auto constexpr method = CordzUpdateTracker::kRemovePrefix;
     CordzUpdateScope scope(contents_.cordz_info(), method);
-    if (tree->tag == RING) {
-      tree = CordRepRing::RemovePrefix(tree->ring(), n);
+    if (tree->IsBtree()) {
+      CordRep* old = tree;
+      tree = tree->btree()->SubTree(n, tree->length - n);
+      CordRep::Unref(old);
     } else {
       CordRep* newrep = RemovePrefixFrom(tree, n);
       CordRep::Unref(tree);
@@ -944,8 +970,10 @@
   } else {
     auto constexpr method = CordzUpdateTracker::kRemoveSuffix;
     CordzUpdateScope scope(contents_.cordz_info(), method);
-    if (tree->tag == RING) {
-      tree = CordRepRing::RemoveSuffix(tree->ring(), n);
+    if (tree->IsBtree()) {
+      CordRep* old = tree;
+      tree = tree->btree()->SubTree(0, tree->length - n);
+      CordRep::Unref(old);
     } else {
       CordRep* newrep = RemoveSuffixFrom(tree, n);
       CordRep::Unref(tree);
@@ -984,8 +1012,8 @@
       results.push_back(Concat(left, right));
     } else if (pos == 0 && n == node->length) {
       results.push_back(CordRep::Ref(node));
-    } else if (node->tag != CONCAT) {
-      if (node->tag == SUBSTRING) {
+    } else if (!node->IsConcat()) {
+      if (node->IsSubstring()) {
         pos += node->substring()->start;
         node = node->substring()->child;
       }
@@ -1037,9 +1065,8 @@
     return sub_cord;
   }
 
-  if (tree->tag == RING) {
-    CordRepRing* ring = CordRep::Ref(tree)->ring();
-    tree = CordRepRing::SubRing(ring, pos, new_size);
+  if (tree->IsBtree()) {
+    tree = tree->btree()->SubTree(pos, new_size);
   } else {
     tree = NewSubRange(tree, pos, new_size);
   }
@@ -1063,7 +1090,7 @@
       CordRep* node = pending.back();
       pending.pop_back();
       CheckNode(node);
-      if (ABSL_PREDICT_FALSE(node->tag != CONCAT)) {
+      if (ABSL_PREDICT_FALSE(!node->IsConcat())) {
         AddNode(node);
         continue;
       }
@@ -1157,7 +1184,7 @@
 
   static void CheckNode(CordRep* node) {
     ABSL_INTERNAL_CHECK(node->length != 0u, "");
-    if (node->tag == CONCAT) {
+    if (node->IsConcat()) {
       ABSL_INTERNAL_CHECK(node->concat()->left != nullptr, "");
       ABSL_INTERNAL_CHECK(node->concat()->right != nullptr, "");
       ABSL_INTERNAL_CHECK(node->length == (node->concat()->left->length +
@@ -1177,7 +1204,7 @@
 
 static CordRep* Rebalance(CordRep* node) {
   VerifyTree(node);
-  assert(node->tag == CONCAT);
+  assert(node->IsConcat());
 
   if (node->length == 0) {
     return nullptr;
@@ -1227,28 +1254,33 @@
 
 }  // namespace
 
-// Helper routine. Locates the first flat chunk of the Cord without
-// initializing the iterator.
+// Helper routine. Locates the first flat or external chunk of the Cord without
+// initializing the iterator, and returns a string_view referencing the data.
 inline absl::string_view Cord::InlineRep::FindFlatStartPiece() const {
   if (!is_tree()) {
     return absl::string_view(data_.as_chars(), data_.inline_size());
   }
 
   CordRep* node = tree();
-  if (node->tag >= FLAT) {
+  if (node->IsFlat()) {
     return absl::string_view(node->flat()->Data(), node->length);
   }
 
-  if (node->tag == EXTERNAL) {
+  if (node->IsExternal()) {
     return absl::string_view(node->external()->base, node->length);
   }
 
-  if (node->tag == RING) {
-    return node->ring()->entry_data(node->ring()->head());
+  if (node->IsBtree()) {
+    CordRepBtree* tree = node->btree();
+    int height = tree->height();
+    while (--height >= 0) {
+      tree = tree->Edge(CordRepBtree::kFront)->btree();
+    }
+    return tree->Data(tree->begin());
   }
 
   // Walk down the left branches until we hit a non-CONCAT node.
-  while (node->tag == CONCAT) {
+  while (node->IsConcat()) {
     node = node->concat()->left;
   }
 
@@ -1257,16 +1289,16 @@
   size_t length = node->length;
   assert(length != 0);
 
-  if (node->tag == SUBSTRING) {
+  if (node->IsSubstring()) {
     offset = node->substring()->start;
     node = node->substring()->child;
   }
 
-  if (node->tag >= FLAT) {
+  if (node->IsFlat()) {
     return absl::string_view(node->flat()->Data() + offset, length);
   }
 
-  assert((node->tag == EXTERNAL) && "Expect FLAT or EXTERNAL node here");
+  assert(node->IsExternal() && "Expect FLAT or EXTERNAL node here");
 
   return absl::string_view(node->external()->base + offset, length);
 }
@@ -1460,7 +1492,7 @@
 
   // Walk down the left branches until we hit a non-CONCAT node. Save the
   // right children to the stack for subsequent traversal.
-  while (node->tag == CONCAT) {
+  while (node->IsConcat()) {
     stack_of_right_children.push_back(node->concat()->right);
     node = node->concat()->left;
   }
@@ -1468,15 +1500,15 @@
   // Get the child node if we encounter a SUBSTRING.
   size_t offset = 0;
   size_t length = node->length;
-  if (node->tag == SUBSTRING) {
+  if (node->IsSubstring()) {
     offset = node->substring()->start;
     node = node->substring()->child;
   }
 
-  assert(node->tag == EXTERNAL || node->tag >= FLAT);
+  assert(node->IsExternal() || node->IsFlat());
   assert(length != 0);
   const char* data =
-      node->tag == EXTERNAL ? node->external()->base : node->flat()->Data();
+      node->IsExternal() ? node->external()->base : node->flat()->Data();
   current_chunk_ = absl::string_view(data + offset, length);
   current_leaf_ = node;
   return *this;
@@ -1506,22 +1538,21 @@
     return subcord;
   }
 
-  if (ring_reader_) {
+  if (btree_reader_) {
     size_t chunk_size = current_chunk_.size();
     if (n <= chunk_size && n <= kMaxBytesToCopy) {
       subcord = Cord(current_chunk_.substr(0, n), method);
+      if (n < chunk_size) {
+        current_chunk_.remove_prefix(n);
+      } else {
+        current_chunk_ = btree_reader_.Next();
+      }
     } else {
-      auto* ring = CordRep::Ref(ring_reader_.ring())->ring();
-      size_t offset = ring_reader_.length() - bytes_remaining_;
-      CordRep* rep = CordRepRing::SubRing(ring, offset, n);
+      CordRep* rep;
+      current_chunk_ = btree_reader_.Read(n, chunk_size, rep);
       subcord.contents_.EmplaceTree(rep, method);
     }
-    if (n < chunk_size) {
-      bytes_remaining_ -= n;
-      current_chunk_.remove_prefix(n);
-    } else {
-      AdvanceBytesRing(n);
-    }
+    bytes_remaining_ -= n;
     return subcord;
   }
 
@@ -1530,8 +1561,8 @@
     // Range to read is a proper subrange of the current chunk.
     assert(current_leaf_ != nullptr);
     CordRep* subnode = CordRep::Ref(current_leaf_);
-    const char* data = subnode->tag == EXTERNAL ? subnode->external()->base
-                                                : subnode->flat()->Data();
+    const char* data = subnode->IsExternal() ? subnode->external()->base
+                                             : subnode->flat()->Data();
     subnode = NewSubstring(subnode, current_chunk_.data() - data, n);
     subcord.contents_.EmplaceTree(VerifyTree(subnode), method);
     RemoveChunkPrefix(n);
@@ -1543,8 +1574,8 @@
   assert(current_leaf_ != nullptr);
   CordRep* subnode = CordRep::Ref(current_leaf_);
   if (current_chunk_.size() < subnode->length) {
-    const char* data = subnode->tag == EXTERNAL ? subnode->external()->base
-                                                : subnode->flat()->Data();
+    const char* data = subnode->IsExternal() ? subnode->external()->base
+                                             : subnode->flat()->Data();
     subnode = NewSubstring(subnode, current_chunk_.data() - data,
                            current_chunk_.size());
   }
@@ -1582,7 +1613,7 @@
 
   // Walk down the appropriate branches until we hit a non-CONCAT node. Save the
   // right children to the stack for subsequent traversal.
-  while (node->tag == CONCAT) {
+  while (node->IsConcat()) {
     if (node->concat()->left->length > n) {
       // Push right, descend left.
       stack_of_right_children.push_back(node->concat()->right);
@@ -1599,20 +1630,20 @@
   // Get the child node if we encounter a SUBSTRING.
   size_t offset = 0;
   size_t length = node->length;
-  if (node->tag == SUBSTRING) {
+  if (node->IsSubstring()) {
     offset = node->substring()->start;
     node = node->substring()->child;
   }
 
   // Range to read ends with a proper (possibly empty) subrange of the current
   // chunk.
-  assert(node->tag == EXTERNAL || node->tag >= FLAT);
+  assert(node->IsExternal() || node->IsFlat());
   assert(length > n);
   if (n > 0) {
     subnode = Concat(subnode, NewSubstring(CordRep::Ref(node), offset, n));
   }
   const char* data =
-      node->tag == EXTERNAL ? node->external()->base : node->flat()->Data();
+      node->IsExternal() ? node->external()->base : node->flat()->Data();
   current_chunk_ = absl::string_view(data + offset + n, length - n);
   current_leaf_ = node;
   bytes_remaining_ -= n;
@@ -1655,7 +1686,7 @@
 
   // Walk down the appropriate branches until we hit a non-CONCAT node. Save the
   // right children to the stack for subsequent traversal.
-  while (node->tag == CONCAT) {
+  while (node->IsConcat()) {
     if (node->concat()->left->length > n) {
       // Push right, descend left.
       stack_of_right_children.push_back(node->concat()->right);
@@ -1671,15 +1702,15 @@
   // Get the child node if we encounter a SUBSTRING.
   size_t offset = 0;
   size_t length = node->length;
-  if (node->tag == SUBSTRING) {
+  if (node->IsSubstring()) {
     offset = node->substring()->start;
     node = node->substring()->child;
   }
 
-  assert(node->tag == EXTERNAL || node->tag >= FLAT);
+  assert(node->IsExternal() || node->IsFlat());
   assert(length > n);
   const char* data =
-      node->tag == EXTERNAL ? node->external()->base : node->flat()->Data();
+      node->IsExternal() ? node->external()->base : node->flat()->Data();
   current_chunk_ = absl::string_view(data + offset + n, length - n);
   current_leaf_ = node;
   bytes_remaining_ -= n;
@@ -1695,15 +1726,15 @@
   while (true) {
     assert(rep != nullptr);
     assert(offset < rep->length);
-    if (rep->tag >= FLAT) {
+    if (rep->IsFlat()) {
       // 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) {
+    } else if (rep->IsBtree()) {
+      return rep->btree()->GetCharacter(offset);
+    } else if (rep->IsExternal()) {
       // Get the "i"th character from the external array.
       return rep->external()->base[offset];
-    } else if (rep->tag == CONCAT) {
+    } else if (rep->IsConcat()) {
       // Recursively branch to the side of the concatenation that the "i"th
       // character is on.
       size_t left_length = rep->concat()->left->length;
@@ -1715,7 +1746,7 @@
       }
     } else {
       // This must be a substring a node, so bypass it to get to the child.
-      assert(rep->tag == SUBSTRING);
+      assert(rep->IsSubstring());
       offset += rep->substring()->start;
       rep = rep->substring()->child;
     }
@@ -1752,27 +1783,27 @@
 
 /* static */ bool Cord::GetFlatAux(CordRep* rep, absl::string_view* fragment) {
   assert(rep != nullptr);
-  if (rep->tag >= FLAT) {
+  if (rep->IsFlat()) {
     *fragment = absl::string_view(rep->flat()->Data(), rep->length);
     return true;
-  } else if (rep->tag == EXTERNAL) {
+  } else if (rep->IsExternal()) {
     *fragment = absl::string_view(rep->external()->base, rep->length);
     return true;
-  } else if (rep->tag == RING) {
-    return rep->ring()->IsFlat(fragment);
-  } else if (rep->tag == SUBSTRING) {
+  } else if (rep->IsBtree()) {
+    return rep->btree()->IsFlat(fragment);
+  } else if (rep->IsSubstring()) {
     CordRep* child = rep->substring()->child;
-    if (child->tag >= FLAT) {
+    if (child->IsFlat()) {
       *fragment = absl::string_view(
           child->flat()->Data() + rep->substring()->start, rep->length);
       return true;
-    } else if (child->tag == EXTERNAL) {
+    } else if (child->IsExternal()) {
       *fragment = absl::string_view(
           child->external()->base + rep->substring()->start, rep->length);
       return true;
-    } else if (child->tag == RING) {
-      return child->ring()->IsFlat(rep->substring()->start, rep->length,
-                                   fragment);
+    } else if (child->IsBtree()) {
+      return child->btree()->IsFlat(rep->substring()->start, rep->length,
+                                    fragment);
     }
   }
   return false;
@@ -1781,7 +1812,7 @@
 /* static */ void Cord::ForEachChunkAux(
     absl::cord_internal::CordRep* rep,
     absl::FunctionRef<void(absl::string_view)> callback) {
-  if (rep->tag == RING) {
+  if (rep->IsBtree()) {
     ChunkIterator it(rep), end;
     while (it != end) {
       callback(*it);
@@ -1797,7 +1828,7 @@
   absl::cord_internal::CordRep* stack[stack_max];
   absl::cord_internal::CordRep* current_node = rep;
   while (true) {
-    if (current_node->tag == CONCAT) {
+    if (current_node->IsConcat()) {
       if (stack_pos == stack_max) {
         // There's no more room on our stack array to add another right branch,
         // and the idea is to avoid allocations, so call this function
@@ -1844,37 +1875,29 @@
     *os << "]";
     *os << " " << (IsRootBalanced(rep) ? 'b' : 'u');
     *os << " " << std::setw(indent) << "";
-    if (rep->tag == CONCAT) {
+    if (rep->IsConcat()) {
       *os << "CONCAT depth=" << Depth(rep) << "\n";
       indent += kIndentStep;
       indents.push_back(indent);
       stack.push_back(rep->concat()->right);
       rep = rep->concat()->left;
-    } else if (rep->tag == SUBSTRING) {
+    } else if (rep->IsSubstring()) {
       *os << "SUBSTRING @ " << rep->substring()->start << "\n";
       indent += kIndentStep;
       rep = rep->substring()->child;
     } else {  // Leaf or ring
-      if (rep->tag == EXTERNAL) {
+      if (rep->IsExternal()) {
         *os << "EXTERNAL [";
         if (include_data)
           *os << absl::CEscape(std::string(rep->external()->base, rep->length));
         *os << "]\n";
-      } else if (rep->tag >= FLAT) {
+      } else if (rep->IsFlat()) {
         *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());
+        CordRepBtree::Dump(rep, /*label=*/ "", include_data, *os);
       }
       if (stack.empty()) break;
       rep = stack.back();
@@ -1906,7 +1929,7 @@
       ABSL_INTERNAL_CHECK(node->length != 0, ReportError(root, node));
     }
 
-    if (node->tag == CONCAT) {
+    if (node->IsConcat()) {
       ABSL_INTERNAL_CHECK(node->concat()->left != nullptr,
                           ReportError(root, node));
       ABSL_INTERNAL_CHECK(node->concat()->right != nullptr,
@@ -1918,13 +1941,13 @@
         worklist.push_back(node->concat()->right);
         worklist.push_back(node->concat()->left);
       }
-    } else if (node->tag >= FLAT) {
+    } else if (node->IsFlat()) {
       ABSL_INTERNAL_CHECK(node->length <= node->flat()->Capacity(),
                           ReportError(root, node));
-    } else if (node->tag == EXTERNAL) {
+    } else if (node->IsExternal()) {
       ABSL_INTERNAL_CHECK(node->external()->base != nullptr,
                           ReportError(root, node));
-    } else if (node->tag == SUBSTRING) {
+    } else if (node->IsSubstring()) {
       ABSL_INTERNAL_CHECK(
           node->substring()->start < node->substring()->child->length,
           ReportError(root, node));
@@ -1953,7 +1976,7 @@
   while (true) {
     const CordRep* next_node = nullptr;
 
-    if (cur_node->tag == CONCAT) {
+    if (cur_node->IsConcat()) {
       total_mem_usage += sizeof(CordRepConcat);
       const CordRep* left = cur_node->concat()->left;
       if (!RepMemoryUsageLeaf(left, &total_mem_usage)) {
@@ -1967,18 +1990,21 @@
         }
         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 if (cur_node->IsBtree()) {
+      total_mem_usage += sizeof(CordRepBtree);
+      const CordRepBtree* node = cur_node->btree();
+      if (node->height() == 0) {
+        for (const CordRep* edge : node->Edges()) {
+          RepMemoryUsageDataEdge(edge, &total_mem_usage);
+        }
+      } else {
+        for (const CordRep* edge : node->Edges()) {
+          tree_stack.push_back(edge);
+        }
+      }
     } else {
       // Since cur_node is not a leaf or a concat node it must be a substring.
-      assert(cur_node->tag == SUBSTRING);
+      assert(cur_node->IsSubstring());
       total_mem_usage += sizeof(CordRepSubstring);
       next_node = cur_node->substring()->child;
       if (RepMemoryUsageLeaf(next_node, &total_mem_usage)) {

diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index e758f1c..ac1832f 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h

@@ -79,8 +79,9 @@
 #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_btree.h"
+#include "absl/strings/internal/cord_rep_btree_reader.h"
 #include "absl/strings/internal/cord_rep_ring.h"
-#include "absl/strings/internal/cord_rep_ring_reader.h"
 #include "absl/strings/internal/cordz_functions.h"
 #include "absl/strings/internal/cordz_info.h"
 #include "absl/strings/internal/cordz_statistics.h"
@@ -370,8 +371,8 @@
 
    private:
     using CordRep = absl::cord_internal::CordRep;
-    using CordRepRing = absl::cord_internal::CordRepRing;
-    using CordRepRingReader = absl::cord_internal::CordRepRingReader;
+    using CordRepBtree = absl::cord_internal::CordRepBtree;
+    using CordRepBtreeReader = absl::cord_internal::CordRepBtreeReader;
 
     // Stack of right children of concat nodes that we have to visit.
     // Keep this at the end of the structure to avoid cache-thrashing.
@@ -397,9 +398,9 @@
     // Stack specific operator++
     ChunkIterator& AdvanceStack();
 
-    // Ring buffer specific operator++
-    ChunkIterator& AdvanceRing();
-    void AdvanceBytesRing(size_t n);
+    // Btree specific operator++
+    ChunkIterator& AdvanceBtree();
+    void AdvanceBytesBtree(size_t n);
 
     // Iterates `n` bytes, where `n` is expected to be greater than or equal to
     // `current_chunk_.size()`.
@@ -415,8 +416,8 @@
     // 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_;
+    // Cord reader for cord btrees. Empty if not traversing a btree.
+    CordRepBtreeReader btree_reader_;
 
     // See 'Stack' alias definition.
     Stack stack_of_right_children_;
@@ -1247,8 +1248,8 @@
 }
 
 inline void Cord::ChunkIterator::InitTree(cord_internal::CordRep* tree) {
-  if (tree->tag == cord_internal::RING) {
-    current_chunk_ = ring_reader_.Reset(tree->ring());
+  if (tree->tag == cord_internal::BTREE) {
+    current_chunk_ = btree_reader_.Init(tree->btree());
     return;
   }
 
@@ -1271,20 +1272,20 @@
   }
 }
 
-inline Cord::ChunkIterator& Cord::ChunkIterator::AdvanceRing() {
-  current_chunk_ = ring_reader_.Next();
+inline Cord::ChunkIterator& Cord::ChunkIterator::AdvanceBtree() {
+  current_chunk_ = btree_reader_.Next();
   return *this;
 }
 
-inline void Cord::ChunkIterator::AdvanceBytesRing(size_t n) {
+inline void Cord::ChunkIterator::AdvanceBytesBtree(size_t n) {
   assert(n >= current_chunk_.size());
   bytes_remaining_ -= n;
   if (bytes_remaining_) {
     if (n == current_chunk_.size()) {
-      current_chunk_ = ring_reader_.Next();
+      current_chunk_ = btree_reader_.Next();
     } else {
-      size_t offset = ring_reader_.length() - bytes_remaining_;
-      current_chunk_ = ring_reader_.Seek(offset);
+      size_t offset = btree_reader_.length() - bytes_remaining_;
+      current_chunk_ = btree_reader_.Seek(offset);
     }
   } else {
     current_chunk_ = {};
@@ -1297,7 +1298,7 @@
   assert(bytes_remaining_ >= current_chunk_.size());
   bytes_remaining_ -= current_chunk_.size();
   if (bytes_remaining_ > 0) {
-    return ring_reader_ ? AdvanceRing() : AdvanceStack();
+    return btree_reader_ ? AdvanceBtree() : AdvanceStack();
   } else {
     current_chunk_ = {};
   }
@@ -1339,7 +1340,7 @@
   if (ABSL_PREDICT_TRUE(n < current_chunk_.size())) {
     RemoveChunkPrefix(n);
   } else if (n != 0) {
-    ring_reader_ ? AdvanceBytesRing(n) : AdvanceBytesSlowPath(n);
+    btree_reader_ ? AdvanceBytesBtree(n) : AdvanceBytesSlowPath(n);
   }
 }
 

diff --git a/absl/strings/cord_test.cc b/absl/strings/cord_test.cc
index 597378c..d029633 100644
--- a/absl/strings/cord_test.cc
+++ b/absl/strings/cord_test.cc

@@ -206,7 +206,32 @@
 ABSL_NAMESPACE_END
 }  // namespace absl
 
-TEST(Cord, AllFlatSizes) {
+// The CordTest fixture runs all tests with and without Cord Btree enabled.
+class CordTest : public testing::TestWithParam<bool> {
+ public:
+  CordTest() : was_btree_(absl::cord_internal::cord_btree_enabled.load()) {
+    absl::cord_internal::cord_btree_enabled.store(UseBtree());
+  }
+  ~CordTest() override {
+    absl::cord_internal::cord_btree_enabled.store(was_btree_);
+  }
+
+  // Returns true if test is running with btree enabled.
+  bool UseBtree() const { return GetParam(); }
+
+  // Returns human readable string representation of the test parameter.
+  static std::string ToString(testing::TestParamInfo<bool> param) {
+    return param.param ? "Btree" : "Concat";
+  }
+
+ private:
+  const bool was_btree_;
+};
+
+INSTANTIATE_TEST_SUITE_P(WithParam, CordTest, testing::Bool(),
+                         CordTest::ToString);
+
+TEST_P(CordTest, AllFlatSizes) {
   using absl::strings_internal::CordTestAccess;
 
   for (size_t s = 0; s < CordTestAccess::MaxFlatLength(); s++) {
@@ -224,7 +249,7 @@
 // We create a Cord at least 128GB in size using the fact that Cords can
 // internally reference-count; thus the Cord is enormous without actually
 // consuming very much memory.
-TEST(GigabyteCord, FromExternal) {
+TEST_P(CordTest, GigabyteCordFromExternal) {
   const size_t one_gig = 1024U * 1024U * 1024U;
   size_t max_size = 2 * one_gig;
   if (sizeof(max_size) > 4) max_size = 128 * one_gig;
@@ -273,7 +298,7 @@
 extern bool my_unique_true_boolean;
 bool my_unique_true_boolean = true;
 
-TEST(Cord, Assignment) {
+TEST_P(CordTest, Assignment) {
   absl::Cord x(absl::string_view("hi there"));
   absl::Cord y(x);
   ASSERT_EQ(std::string(x), "hi there");
@@ -327,7 +352,7 @@
   }
 }
 
-TEST(Cord, StartsEndsWith) {
+TEST_P(CordTest, StartsEndsWith) {
   absl::Cord x(absl::string_view("abcde"));
   absl::Cord empty("");
 
@@ -360,13 +385,13 @@
   ASSERT_TRUE(!empty.EndsWith("xyz"));
 }
 
-TEST(Cord, Subcord) {
+TEST_P(CordTest, Subcord) {
   RandomEngine rng(GTEST_FLAG_GET(random_seed));
   const std::string s = RandomLowercaseString(&rng, 1024);
 
   absl::Cord a;
   AppendWithFragments(s, &rng, &a);
-  ASSERT_EQ(s.size(), a.size());
+  ASSERT_EQ(s, std::string(a));
 
   // Check subcords of a, from a variety of interesting points.
   std::set<size_t> positions;
@@ -421,7 +446,7 @@
   EXPECT_TRUE(sa.empty());
 }
 
-TEST(Cord, Swap) {
+TEST_P(CordTest, Swap) {
   absl::string_view a("Dexter");
   absl::string_view b("Mandark");
   absl::Cord x(a);
@@ -453,7 +478,7 @@
   }
 }
 
-TEST(Cord, CopyToString) {
+TEST_P(CordTest, CopyToString) {
   VerifyCopyToString(absl::Cord());
   VerifyCopyToString(absl::Cord("small cord"));
   VerifyCopyToString(
@@ -461,45 +486,45 @@
                                 "copying ", "to ", "a ", "string."}));
 }
 
-TEST(TryFlat, Empty) {
+TEST_P(CordTest, TryFlatEmpty) {
   absl::Cord c;
   EXPECT_EQ(c.TryFlat(), "");
 }
 
-TEST(TryFlat, Flat) {
+TEST_P(CordTest, TryFlatFlat) {
   absl::Cord c("hello");
   EXPECT_EQ(c.TryFlat(), "hello");
 }
 
-TEST(TryFlat, SubstrInlined) {
+TEST_P(CordTest, TryFlatSubstrInlined) {
   absl::Cord c("hello");
   c.RemovePrefix(1);
   EXPECT_EQ(c.TryFlat(), "ello");
 }
 
-TEST(TryFlat, SubstrFlat) {
+TEST_P(CordTest, TryFlatSubstrFlat) {
   absl::Cord c("longer than 15 bytes");
   absl::Cord sub = absl::CordTestPeer::MakeSubstring(c, 1, c.size() - 1);
   EXPECT_EQ(sub.TryFlat(), "onger than 15 bytes");
 }
 
-TEST(TryFlat, Concat) {
+TEST_P(CordTest, TryFlatConcat) {
   absl::Cord c = absl::MakeFragmentedCord({"hel", "lo"});
   EXPECT_EQ(c.TryFlat(), absl::nullopt);
 }
 
-TEST(TryFlat, External) {
+TEST_P(CordTest, TryFlatExternal) {
   absl::Cord c = absl::MakeCordFromExternal("hell", [](absl::string_view) {});
   EXPECT_EQ(c.TryFlat(), "hell");
 }
 
-TEST(TryFlat, SubstrExternal) {
+TEST_P(CordTest, TryFlatSubstrExternal) {
   absl::Cord c = absl::MakeCordFromExternal("hell", [](absl::string_view) {});
   absl::Cord sub = absl::CordTestPeer::MakeSubstring(c, 1, c.size() - 1);
   EXPECT_EQ(sub.TryFlat(), "ell");
 }
 
-TEST(TryFlat, SubstrConcat) {
+TEST_P(CordTest, TryFlatSubstrConcat) {
   absl::Cord c = absl::MakeFragmentedCord({"hello", " world"});
   absl::Cord sub = absl::CordTestPeer::MakeSubstring(c, 1, c.size() - 1);
   EXPECT_EQ(sub.TryFlat(), absl::nullopt);
@@ -507,7 +532,7 @@
   EXPECT_EQ(c.TryFlat(), absl::nullopt);
 }
 
-TEST(TryFlat, CommonlyAssumedInvariants) {
+TEST_P(CordTest, TryFlatCommonlyAssumedInvariants) {
   // The behavior tested below is not part of the API contract of Cord, but it's
   // something we intend to be true in our current implementation.  This test
   // exists to detect and prevent accidental breakage of the implementation.
@@ -563,7 +588,7 @@
   EXPECT_TRUE(IsFlat(c));
 }
 
-TEST(Cord, Flatten) {
+TEST_P(CordTest, Flatten) {
   VerifyFlatten(absl::Cord());
   VerifyFlatten(absl::Cord("small cord"));
   VerifyFlatten(absl::Cord("larger than small buffer optimization"));
@@ -617,7 +642,7 @@
 };
 }  // namespace
 
-TEST(Cord, MultipleLengths) {
+TEST_P(CordTest, MultipleLengths) {
   TestData d;
   for (size_t i = 0; i < d.size(); i++) {
     std::string a = d.data(i);
@@ -693,7 +718,7 @@
 
 namespace {
 
-TEST(Cord, RemoveSuffixWithExternalOrSubstring) {
+TEST_P(CordTest, RemoveSuffixWithExternalOrSubstring) {
   absl::Cord cord = absl::MakeCordFromExternal(
       "foo bar baz", [](absl::string_view s) { DoNothing(s, nullptr); });
 
@@ -708,7 +733,7 @@
   EXPECT_EQ("foo", std::string(cord));
 }
 
-TEST(Cord, RemoveSuffixMakesZeroLengthNode) {
+TEST_P(CordTest, RemoveSuffixMakesZeroLengthNode) {
   absl::Cord c;
   c.Append(absl::Cord(std::string(100, 'x')));
   absl::Cord other_ref = c;  // Prevent inplace appends
@@ -735,7 +760,7 @@
 }
 
 // Establish that ZedBlock does what we think it does.
-TEST(CordSpliceTest, ZedBlock) {
+TEST_P(CordTest, CordSpliceTestZedBlock) {
   absl::Cord blob = CordWithZedBlock(10);
   EXPECT_EQ(10, blob.size());
   std::string s;
@@ -743,7 +768,7 @@
   EXPECT_EQ("zzzzzzzzzz", s);
 }
 
-TEST(CordSpliceTest, ZedBlock0) {
+TEST_P(CordTest, CordSpliceTestZedBlock0) {
   absl::Cord blob = CordWithZedBlock(0);
   EXPECT_EQ(0, blob.size());
   std::string s;
@@ -751,7 +776,7 @@
   EXPECT_EQ("", s);
 }
 
-TEST(CordSpliceTest, ZedBlockSuffix1) {
+TEST_P(CordTest, CordSpliceTestZedBlockSuffix1) {
   absl::Cord blob = CordWithZedBlock(10);
   EXPECT_EQ(10, blob.size());
   absl::Cord suffix(blob);
@@ -763,7 +788,7 @@
 }
 
 // Remove all of a prefix block
-TEST(CordSpliceTest, ZedBlockSuffix0) {
+TEST_P(CordTest, CordSpliceTestZedBlockSuffix0) {
   absl::Cord blob = CordWithZedBlock(10);
   EXPECT_EQ(10, blob.size());
   absl::Cord suffix(blob);
@@ -795,7 +820,7 @@
 }
 
 // Taking an empty suffix of a block breaks appending.
-TEST(CordSpliceTest, RemoveEntireBlock1) {
+TEST_P(CordTest, CordSpliceTestRemoveEntireBlock1) {
   absl::Cord zero = CordWithZedBlock(10);
   absl::Cord suffix(zero);
   suffix.RemovePrefix(10);
@@ -803,7 +828,7 @@
   result.Append(suffix);
 }
 
-TEST(CordSpliceTest, RemoveEntireBlock2) {
+TEST_P(CordTest, CordSpliceTestRemoveEntireBlock2) {
   absl::Cord zero = CordWithZedBlock(10);
   absl::Cord prefix(zero);
   prefix.RemoveSuffix(10);
@@ -813,7 +838,7 @@
   result.Append(suffix);
 }
 
-TEST(CordSpliceTest, RemoveEntireBlock3) {
+TEST_P(CordTest, CordSpliceTestRemoveEntireBlock3) {
   absl::Cord blob = CordWithZedBlock(10);
   absl::Cord block = BigCord(10, 'b');
   blob = SpliceCord(blob, 0, block);
@@ -844,7 +869,7 @@
       << "LHS=" << rhs_string << "; RHS=" << lhs_string;
 }
 
-TEST(Cord, Compare) {
+TEST_P(CordTest, Compare) {
   absl::Cord subcord("aaaaaBBBBBcccccDDDDD");
   subcord = subcord.Subcord(3, 10);
 
@@ -907,7 +932,7 @@
   }
 }
 
-TEST(Cord, CompareAfterAssign) {
+TEST_P(CordTest, CompareAfterAssign) {
   absl::Cord a("aaaaaa1111111");
   absl::Cord b("aaaaaa2222222");
   a = "cccccc";
@@ -936,7 +961,7 @@
   EXPECT_EQ(expected, sign(c.Compare(d))) << c << ", " << d;
 }
 
-TEST(Compare, ComparisonIsUnsigned) {
+TEST_P(CordTest, CompareComparisonIsUnsigned) {
   RandomEngine rng(GTEST_FLAG_GET(random_seed));
   std::uniform_int_distribution<uint32_t> uniform_uint8(0, 255);
   char x = static_cast<char>(uniform_uint8(rng));
@@ -945,7 +970,7 @@
       absl::Cord(std::string(GetUniformRandomUpTo(&rng, 100), x ^ 0x80)), &rng);
 }
 
-TEST(Compare, RandomComparisons) {
+TEST_P(CordTest, CompareRandomComparisons) {
   const int kIters = 5000;
   RandomEngine rng(GTEST_FLAG_GET(random_seed));
 
@@ -1003,43 +1028,43 @@
   EXPECT_FALSE(b <= a);
 }
 
-TEST(ComparisonOperators, Cord_Cord) {
+TEST_P(CordTest, ComparisonOperators_Cord_Cord) {
   CompareOperators<absl::Cord, absl::Cord>();
 }
 
-TEST(ComparisonOperators, Cord_StringPiece) {
+TEST_P(CordTest, ComparisonOperators_Cord_StringPiece) {
   CompareOperators<absl::Cord, absl::string_view>();
 }
 
-TEST(ComparisonOperators, StringPiece_Cord) {
+TEST_P(CordTest, ComparisonOperators_StringPiece_Cord) {
   CompareOperators<absl::string_view, absl::Cord>();
 }
 
-TEST(ComparisonOperators, Cord_string) {
+TEST_P(CordTest, ComparisonOperators_Cord_string) {
   CompareOperators<absl::Cord, std::string>();
 }
 
-TEST(ComparisonOperators, string_Cord) {
+TEST_P(CordTest, ComparisonOperators_string_Cord) {
   CompareOperators<std::string, absl::Cord>();
 }
 
-TEST(ComparisonOperators, stdstring_Cord) {
+TEST_P(CordTest, ComparisonOperators_stdstring_Cord) {
   CompareOperators<std::string, absl::Cord>();
 }
 
-TEST(ComparisonOperators, Cord_stdstring) {
+TEST_P(CordTest, ComparisonOperators_Cord_stdstring) {
   CompareOperators<absl::Cord, std::string>();
 }
 
-TEST(ComparisonOperators, charstar_Cord) {
+TEST_P(CordTest, ComparisonOperators_charstar_Cord) {
   CompareOperators<const char*, absl::Cord>();
 }
 
-TEST(ComparisonOperators, Cord_charstar) {
+TEST_P(CordTest, ComparisonOperators_Cord_charstar) {
   CompareOperators<absl::Cord, const char*>();
 }
 
-TEST(ConstructFromExternal, ReleaserInvoked) {
+TEST_P(CordTest, ConstructFromExternalReleaserInvoked) {
   // Empty external memory means the releaser should be called immediately.
   {
     bool invoked = false;
@@ -1081,7 +1106,7 @@
   }
 }
 
-TEST(ConstructFromExternal, CompareContents) {
+TEST_P(CordTest, ConstructFromExternalCompareContents) {
   RandomEngine rng(GTEST_FLAG_GET(random_seed));
 
   for (int length = 1; length <= 2048; length *= 2) {
@@ -1097,7 +1122,7 @@
   }
 }
 
-TEST(ConstructFromExternal, LargeReleaser) {
+TEST_P(CordTest, ConstructFromExternalLargeReleaser) {
   RandomEngine rng(GTEST_FLAG_GET(random_seed));
   constexpr size_t kLength = 256;
   std::string data = RandomLowercaseString(&rng, kLength);
@@ -1112,7 +1137,7 @@
   EXPECT_TRUE(invoked);
 }
 
-TEST(ConstructFromExternal, FunctionPointerReleaser) {
+TEST_P(CordTest, ConstructFromExternalFunctionPointerReleaser) {
   static absl::string_view data("hello world");
   static bool invoked;
   auto* releaser =
@@ -1129,7 +1154,7 @@
   EXPECT_TRUE(invoked);
 }
 
-TEST(ConstructFromExternal, MoveOnlyReleaser) {
+TEST_P(CordTest, ConstructFromExternalMoveOnlyReleaser) {
   struct Releaser {
     explicit Releaser(bool* invoked) : invoked(invoked) {}
     Releaser(Releaser&& other) noexcept : invoked(other.invoked) {}
@@ -1143,20 +1168,20 @@
   EXPECT_TRUE(invoked);
 }
 
-TEST(ConstructFromExternal, NoArgLambda) {
+TEST_P(CordTest, ConstructFromExternalNoArgLambda) {
   bool invoked = false;
   (void)absl::MakeCordFromExternal("dummy", [&invoked]() { invoked = true; });
   EXPECT_TRUE(invoked);
 }
 
-TEST(ConstructFromExternal, StringViewArgLambda) {
+TEST_P(CordTest, ConstructFromExternalStringViewArgLambda) {
   bool invoked = false;
   (void)absl::MakeCordFromExternal(
       "dummy", [&invoked](absl::string_view) { invoked = true; });
   EXPECT_TRUE(invoked);
 }
 
-TEST(ConstructFromExternal, NonTrivialReleaserDestructor) {
+TEST_P(CordTest, ConstructFromExternalNonTrivialReleaserDestructor) {
   struct Releaser {
     explicit Releaser(bool* destroyed) : destroyed(destroyed) {}
     ~Releaser() { *destroyed = true; }
@@ -1171,7 +1196,7 @@
   EXPECT_TRUE(destroyed);
 }
 
-TEST(ConstructFromExternal, ReferenceQualifierOverloads) {
+TEST_P(CordTest, ConstructFromExternalReferenceQualifierOverloads) {
   struct Releaser {
     void operator()(absl::string_view) & { *lvalue_invoked = true; }
     void operator()(absl::string_view) && { *rvalue_invoked = true; }
@@ -1199,7 +1224,7 @@
   EXPECT_TRUE(rvalue_invoked);
 }
 
-TEST(ExternalMemory, BasicUsage) {
+TEST_P(CordTest, ExternalMemoryBasicUsage) {
   static const char* strings[] = {"", "hello", "there"};
   for (const char* str : strings) {
     absl::Cord dst("(prefix)");
@@ -1210,7 +1235,7 @@
   }
 }
 
-TEST(ExternalMemory, RemovePrefixSuffix) {
+TEST_P(CordTest, ExternalMemoryRemovePrefixSuffix) {
   // Exhaustively try all sub-strings.
   absl::Cord cord = MakeComposite();
   std::string s = std::string(cord);
@@ -1225,7 +1250,7 @@
   }
 }
 
-TEST(ExternalMemory, Get) {
+TEST_P(CordTest, ExternalMemoryGet) {
   absl::Cord cord("hello");
   AddExternalMemory(" world!", &cord);
   AddExternalMemory(" how are ", &cord);
@@ -1244,16 +1269,16 @@
 // Additionally we have some whiteboxed expectations based on our knowledge of
 // the layout and size of empty and inlined cords, and flat nodes.
 
-TEST(CordMemoryUsage, Empty) {
+TEST_P(CordTest, CordMemoryUsageEmpty) {
   EXPECT_EQ(sizeof(absl::Cord), absl::Cord().EstimatedMemoryUsage());
 }
 
-TEST(CordMemoryUsage, Embedded) {
+TEST_P(CordTest, CordMemoryUsageEmbedded) {
   absl::Cord a("hello");
   EXPECT_EQ(a.EstimatedMemoryUsage(), sizeof(absl::Cord));
 }
 
-TEST(CordMemoryUsage, EmbeddedAppend) {
+TEST_P(CordTest, CordMemoryUsageEmbeddedAppend) {
   absl::Cord a("a");
   absl::Cord b("bcd");
   EXPECT_EQ(b.EstimatedMemoryUsage(), sizeof(absl::Cord));
@@ -1261,7 +1286,7 @@
   EXPECT_EQ(a.EstimatedMemoryUsage(), sizeof(absl::Cord));
 }
 
-TEST(CordMemoryUsage, ExternalMemory) {
+TEST_P(CordTest, CordMemoryUsageExternalMemory) {
   static const int kLength = 1000;
   absl::Cord cord;
   AddExternalMemory(std::string(kLength, 'x'), &cord);
@@ -1269,14 +1294,14 @@
   EXPECT_LE(cord.EstimatedMemoryUsage(), kLength * 1.5);
 }
 
-TEST(CordMemoryUsage, Flat) {
+TEST_P(CordTest, CordMemoryUsageFlat) {
   static const int kLength = 125;
   absl::Cord a(std::string(kLength, 'a'));
   EXPECT_GT(a.EstimatedMemoryUsage(), kLength);
   EXPECT_LE(a.EstimatedMemoryUsage(), kLength * 1.5);
 }
 
-TEST(CordMemoryUsage, AppendFlat) {
+TEST_P(CordTest, CordMemoryUsageAppendFlat) {
   using absl::strings_internal::CordTestAccess;
   absl::Cord a(std::string(CordTestAccess::MaxFlatLength(), 'a'));
   size_t length = a.EstimatedMemoryUsage();
@@ -1286,9 +1311,32 @@
   EXPECT_LE(delta, CordTestAccess::MaxFlatLength() * 1.5);
 }
 
+TEST_P(CordTest, CordMemoryUsageAppendExternal) {
+  static const int kLength = 1000;
+  using absl::strings_internal::CordTestAccess;
+  absl::Cord a(std::string(CordTestAccess::MaxFlatLength(), 'a'));
+  size_t length = a.EstimatedMemoryUsage();
+  AddExternalMemory(std::string(kLength, 'b'), &a);
+  size_t delta = a.EstimatedMemoryUsage() - length;
+  EXPECT_GT(delta, kLength);
+  EXPECT_LE(delta, kLength * 1.5);
+}
+
+TEST_P(CordTest, CordMemoryUsageSubString) {
+  static const int kLength = 2000;
+  using absl::strings_internal::CordTestAccess;
+  absl::Cord a(std::string(kLength, 'a'));
+  size_t length = a.EstimatedMemoryUsage();
+  AddExternalMemory(std::string(kLength, 'b'), &a);
+  absl::Cord b = a.Subcord(0, kLength + kLength / 2);
+  size_t delta = b.EstimatedMemoryUsage() - length;
+  EXPECT_GT(delta, kLength);
+  EXPECT_LE(delta, kLength * 1.5);
+}
+
 // Regtest for a change that had to be rolled back because it expanded out
 // of the InlineRep too soon, which was observable through MemoryUsage().
-TEST(CordMemoryUsage, InlineRep) {
+TEST_P(CordTest, CordMemoryUsageInlineRep) {
   constexpr size_t kMaxInline = 15;  // Cord::InlineRep::N
   const std::string small_string(kMaxInline, 'x');
   absl::Cord c1(small_string);
@@ -1302,7 +1350,7 @@
 }  // namespace
 
 // Regtest for 7510292 (fix a bug introduced by 7465150)
-TEST(Cord, Concat_Append) {
+TEST_P(CordTest, Concat_Append) {
   // Create a rep of type CONCAT
   absl::Cord s1("foobarbarbarbarbar");
   s1.Append("abcdefgabcdefgabcdefgabcdefgabcdefgabcdefgabcdefg");
@@ -1317,7 +1365,7 @@
   EXPECT_EQ(s2.size(), size + 1);
 }
 
-TEST(Cord, DiabolicalGrowth) {
+TEST_P(CordTest, DiabolicalGrowth) {
   // This test exercises a diabolical Append(<one char>) on a cord, making the
   // cord shared before each Append call resulting in a terribly fragmented
   // resulting cord.
@@ -1337,7 +1385,7 @@
                cord.EstimatedMemoryUsage());
 }
 
-TEST(MakeFragmentedCord, MakeFragmentedCordFromInitializerList) {
+TEST_P(CordTest, MakeFragmentedCordFromInitializerList) {
   absl::Cord fragmented =
       absl::MakeFragmentedCord({"A ", "fragmented ", "Cord"});
 
@@ -1357,7 +1405,7 @@
   ASSERT_TRUE(++chunk_it == fragmented.chunk_end());
 }
 
-TEST(MakeFragmentedCord, MakeFragmentedCordFromVector) {
+TEST_P(CordTest, MakeFragmentedCordFromVector) {
   std::vector<absl::string_view> chunks = {"A ", "fragmented ", "Cord"};
   absl::Cord fragmented = absl::MakeFragmentedCord(chunks);
 
@@ -1377,7 +1425,7 @@
   ASSERT_TRUE(++chunk_it == fragmented.chunk_end());
 }
 
-TEST(CordChunkIterator, Traits) {
+TEST_P(CordTest, CordChunkIteratorTraits) {
   static_assert(std::is_copy_constructible<absl::Cord::ChunkIterator>::value,
                 "");
   static_assert(std::is_copy_assignable<absl::Cord::ChunkIterator>::value, "");
@@ -1458,7 +1506,7 @@
   EXPECT_TRUE(post_iter == cord.chunk_end());  // NOLINT
 }
 
-TEST(CordChunkIterator, Operations) {
+TEST_P(CordTest, CordChunkIteratorOperations) {
   absl::Cord empty_cord;
   VerifyChunkIterator(empty_cord, 0);
 
@@ -1628,7 +1676,7 @@
   VerifyCharIterator(subcords);
 }
 
-TEST(Cord, StreamingOutput) {
+TEST_P(CordTest, StreamingOutput) {
   absl::Cord c =
       absl::MakeFragmentedCord({"A ", "small ", "fragmented ", "Cord", "."});
   std::stringstream output;
@@ -1636,7 +1684,7 @@
   EXPECT_EQ("A small fragmented Cord.", output.str());
 }
 
-TEST(Cord, ForEachChunk) {
+TEST_P(CordTest, ForEachChunk) {
   for (int num_elements : {1, 10, 200}) {
     SCOPED_TRACE(num_elements);
     std::vector<std::string> cord_chunks;
@@ -1654,7 +1702,7 @@
   }
 }
 
-TEST(Cord, SmallBufferAssignFromOwnData) {
+TEST_P(CordTest, SmallBufferAssignFromOwnData) {
   constexpr size_t kMaxInline = 15;
   std::string contents = "small buff cord";
   EXPECT_EQ(contents.size(), kMaxInline);
@@ -1669,7 +1717,7 @@
   }
 }
 
-TEST(Cord, Format) {
+TEST_P(CordTest, Format) {
   absl::Cord c;
   absl::Format(&c, "There were %04d little %s.", 3, "pigs");
   EXPECT_EQ(c, "There were 0003 little pigs.");
@@ -1770,7 +1818,7 @@
 };
 
 
-TEST(Cord, ConstinitConstructor) {
+TEST_P(CordTest, ConstinitConstructor) {
   TestConstinitConstructor(
       absl::strings_internal::MakeStringConstant(ShortView{}));
   TestConstinitConstructor(

diff --git a/absl/strings/internal/cord_internal.cc b/absl/strings/internal/cord_internal.cc
index f79cb62..1767e6f 100644
--- a/absl/strings/internal/cord_internal.cc
+++ b/absl/strings/internal/cord_internal.cc

@@ -31,6 +31,7 @@
     kCordEnableRingBufferDefault);
 ABSL_CONST_INIT std::atomic<bool> shallow_subcords_enabled(
     kCordShallowSubcordsDefault);
+ABSL_CONST_INIT std::atomic<bool> cord_btree_exhaustive_validation(false);
 
 void CordRep::Destroy(CordRep* rep) {
   assert(rep != nullptr);

diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index 371a7f9..b7f3f4c 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h

@@ -46,6 +46,12 @@
 extern std::atomic<bool> cord_ring_buffer_enabled;
 extern std::atomic<bool> shallow_subcords_enabled;
 
+// `cord_btree_exhaustive_validation` can be set to force exhaustive validation
+// in debug assertions, and code that calls `IsValid()` explicitly. By default,
+// assertions should be relatively cheap and AssertValid() can easily lead to
+// O(n^2) complexity as recursive / full tree validation is O(n).
+extern std::atomic<bool> cord_btree_exhaustive_validation;
+
 inline void enable_cord_btree(bool enable) {
   cord_btree_enabled.store(enable, std::memory_order_relaxed);
 }
@@ -210,6 +216,14 @@
   // padding space from the base class (clang and gcc do, MSVC does not, etc)
   uint8_t storage[3];
 
+  // Returns true if this instance's tag matches the requested type.
+  constexpr bool IsRing() const { return tag == RING; }
+  constexpr bool IsConcat() const { return tag == CONCAT; }
+  constexpr bool IsSubstring() const { return tag == SUBSTRING; }
+  constexpr bool IsExternal() const { return tag == EXTERNAL; }
+  constexpr bool IsFlat() const { return tag >= FLAT; }
+  constexpr bool IsBtree() const { return tag == BTREE; }
+
   inline CordRepRing* ring();
   inline const CordRepRing* ring() const;
   inline CordRepConcat* concat();
@@ -220,7 +234,6 @@
   inline const CordRepExternal* external() const;
   inline CordRepFlat* flat();
   inline const CordRepFlat* flat() const;
-
   inline CordRepBtree* btree();
   inline const CordRepBtree* btree() const;
 
@@ -271,7 +284,7 @@
   ExternalReleaserInvoker releaser_invoker;
 
   // Deletes (releases) the external rep.
-  // Requires rep != nullptr and rep->tag == EXTERNAL
+  // Requires rep != nullptr and rep->IsExternal()
   static void Delete(CordRep* rep);
 };
 
@@ -314,7 +327,7 @@
 };
 
 inline void CordRepExternal::Delete(CordRep* rep) {
-  assert(rep != nullptr && rep->tag == EXTERNAL);
+  assert(rep != nullptr && rep->IsExternal());
   auto* rep_external = static_cast<CordRepExternal*>(rep);
   assert(rep_external->releaser_invoker != nullptr);
   rep_external->releaser_invoker(rep_external);
@@ -525,32 +538,32 @@
 static_assert(sizeof(InlineData) == kMaxInline + 1, "");
 
 inline CordRepConcat* CordRep::concat() {
-  assert(tag == CONCAT);
+  assert(IsConcat());
   return static_cast<CordRepConcat*>(this);
 }
 
 inline const CordRepConcat* CordRep::concat() const {
-  assert(tag == CONCAT);
+  assert(IsConcat());
   return static_cast<const CordRepConcat*>(this);
 }
 
 inline CordRepSubstring* CordRep::substring() {
-  assert(tag == SUBSTRING);
+  assert(IsSubstring());
   return static_cast<CordRepSubstring*>(this);
 }
 
 inline const CordRepSubstring* CordRep::substring() const {
-  assert(tag == SUBSTRING);
+  assert(IsSubstring());
   return static_cast<const CordRepSubstring*>(this);
 }
 
 inline CordRepExternal* CordRep::external() {
-  assert(tag == EXTERNAL);
+  assert(IsExternal());
   return static_cast<CordRepExternal*>(this);
 }
 
 inline const CordRepExternal* CordRep::external() const {
-  assert(tag == EXTERNAL);
+  assert(IsExternal());
   return static_cast<const CordRepExternal*>(this);
 }
 

diff --git a/absl/strings/internal/cord_rep_btree.cc b/absl/strings/internal/cord_rep_btree.cc
index 6978cfd..8fe589f 100644
--- a/absl/strings/internal/cord_rep_btree.cc
+++ b/absl/strings/internal/cord_rep_btree.cc

@@ -32,6 +32,8 @@
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
+constexpr size_t CordRepBtree::kMaxCapacity;  // NOLINT: needed for c++ < c++17
+
 namespace {
 
 using NodeStack = CordRepBtree * [CordRepBtree::kMaxDepth];
@@ -42,6 +44,10 @@
 constexpr auto kFront = CordRepBtree::kFront;
 constexpr auto kBack = CordRepBtree::kBack;
 
+inline bool exhaustive_validation() {
+  return cord_btree_exhaustive_validation.load(std::memory_order_relaxed);
+}
+
 // Implementation of the various 'Dump' functions.
 // Prints the entire tree structure or 'rep'. External callers should
 // not specify 'depth' and leave it to its default (0) value.
@@ -73,7 +79,7 @@
   // indented by two spaces per recursive depth.
   stream << std::string(depth * 2, ' ') << sharing << " (" << sptr << ") ";
 
-  if (rep->tag == BTREE) {
+  if (rep->IsBtree()) {
     const CordRepBtree* node = rep->btree();
     std::string label =
         node->height() ? absl::StrCat("Node(", node->height(), ")") : "Leaf";
@@ -357,7 +363,7 @@
   Delete(tree);
 }
 
-bool CordRepBtree::IsValid(const CordRepBtree* tree) {
+bool CordRepBtree::IsValid(const CordRepBtree* tree, bool shallow) {
 #define NODE_CHECK_VALID(x)                                           \
   if (!(x)) {                                                         \
     ABSL_RAW_LOG(ERROR, "CordRepBtree::CheckValid() FAILED: %s", #x); \
@@ -372,7 +378,7 @@
   }
 
   NODE_CHECK_VALID(tree != nullptr);
-  NODE_CHECK_EQ(tree->tag, BTREE);
+  NODE_CHECK_VALID(tree->IsBtree());
   NODE_CHECK_VALID(tree->height() <= kMaxHeight);
   NODE_CHECK_VALID(tree->begin() < tree->capacity());
   NODE_CHECK_VALID(tree->end() <= tree->capacity());
@@ -381,7 +387,7 @@
   for (CordRep* edge : tree->Edges()) {
     NODE_CHECK_VALID(edge != nullptr);
     if (tree->height() > 0) {
-      NODE_CHECK_VALID(edge->tag == BTREE);
+      NODE_CHECK_VALID(edge->IsBtree());
       NODE_CHECK_VALID(edge->btree()->height() == tree->height() - 1);
     } else {
       NODE_CHECK_VALID(IsDataEdge(edge));
@@ -389,9 +395,9 @@
     child_length += edge->length;
   }
   NODE_CHECK_EQ(child_length, tree->length);
-  if (tree->height() > 0) {
+  if ((!shallow || exhaustive_validation()) && tree->height() > 0) {
     for (CordRep* edge : tree->Edges()) {
-      if (!IsValid(edge->btree())) return false;
+      if (!IsValid(edge->btree(), shallow)) return false;
     }
   }
   return true;
@@ -402,16 +408,17 @@
 
 #ifndef NDEBUG
 
-CordRepBtree* CordRepBtree::AssertValid(CordRepBtree* tree) {
-  if (!IsValid(tree)) {
+CordRepBtree* CordRepBtree::AssertValid(CordRepBtree* tree, bool shallow) {
+  if (!IsValid(tree, shallow)) {
     Dump(tree, "CordRepBtree validation failed:", false, std::cout);
     ABSL_RAW_LOG(FATAL, "CordRepBtree::CheckValid() FAILED");
   }
   return tree;
 }
 
-const CordRepBtree* CordRepBtree::AssertValid(const CordRepBtree* tree) {
-  if (!IsValid(tree)) {
+const CordRepBtree* CordRepBtree::AssertValid(const CordRepBtree* tree,
+                                              bool shallow) {
+  if (!IsValid(tree, shallow)) {
     Dump(tree, "CordRepBtree validation failed:", false, std::cout);
     ABSL_RAW_LOG(FATAL, "CordRepBtree::CheckValid() FAILED");
   }
@@ -882,7 +889,7 @@
 }
 
 CordRepBtree* CordRepBtree::CreateSlow(CordRep* rep) {
-  if (rep->tag == BTREE) return rep->btree();
+  if (rep->IsBtree()) return rep->btree();
 
   CordRepBtree* node = nullptr;
   auto consume = [&node](CordRep* r, size_t offset, size_t length) {
@@ -898,7 +905,7 @@
 }
 
 CordRepBtree* CordRepBtree::AppendSlow(CordRepBtree* tree, CordRep* rep) {
-  if (ABSL_PREDICT_TRUE(rep->tag == BTREE)) {
+  if (ABSL_PREDICT_TRUE(rep->IsBtree())) {
     return MergeTrees(tree, rep->btree());
   }
   auto consume = [&tree](CordRep* r, size_t offset, size_t length) {
@@ -910,7 +917,7 @@
 }
 
 CordRepBtree* CordRepBtree::PrependSlow(CordRepBtree* tree, CordRep* rep) {
-  if (ABSL_PREDICT_TRUE(rep->tag == BTREE)) {
+  if (ABSL_PREDICT_TRUE(rep->IsBtree())) {
     return MergeTrees(rep->btree(), tree);
   }
   auto consume = [&tree](CordRep* r, size_t offset, size_t length) {

diff --git a/absl/strings/internal/cord_rep_btree.h b/absl/strings/internal/cord_rep_btree.h
index 7d85473..8f000ca 100644
--- a/absl/strings/internal/cord_rep_btree.h
+++ b/absl/strings/internal/cord_rep_btree.h

@@ -153,7 +153,7 @@
   };
 
   // Creates a btree from the given input. Adopts a ref of `rep`.
-  // If the input `rep` is itself a btree, i.e., `tag == BTREE`, then this
+  // If the input `rep` is itself a btree, i.e., `IsBtree()`, then this
   // function immediately returns `rep->btree()`. If the input is a valid data
   // edge (see IsDataEdge()), then a new leaf node is returned containing `rep`
   // as the sole data edge. Else, the input is assumed to be a (legacy) concat
@@ -266,10 +266,28 @@
   // holding a FLAT or EXTERNAL child rep.
   static bool IsDataEdge(const CordRep* rep);
 
-  // Diagnostics
-  static bool IsValid(const CordRepBtree* tree);
-  static CordRepBtree* AssertValid(CordRepBtree* tree);
-  static const CordRepBtree* AssertValid(const CordRepBtree* tree);
+  // Diagnostics: returns true if `tree` is valid and internally consistent.
+  // If `shallow` is false, then the provided top level node and all child nodes
+  // below it are recursively checked. If `shallow` is true, only the provided
+  // node in `tree` and the cumulative length, type and height of the direct
+  // child nodes of `tree` are checked. The value of `shallow` is ignored if the
+  // internal `cord_btree_exhaustive_validation` diagnostics variable is true,
+  // in which case the performed validations works as if `shallow` were false.
+  // This function is intended for debugging and testing purposes only.
+  static bool IsValid(const CordRepBtree* tree, bool shallow = false);
+
+  // Diagnostics: asserts that the provided tree is valid.
+  // `AssertValid()` performs a shallow validation by default. `shallow` can be
+  // set to false in which case an exhaustive validation is performed. This
+  // function is implemented in terms of calling `IsValid()` and asserting the
+  // return value to be true. See `IsValid()` for more information.
+  // This function is intended for debugging and testing purposes only.
+  static CordRepBtree* AssertValid(CordRepBtree* tree, bool shallow = true);
+  static const CordRepBtree* AssertValid(const CordRepBtree* tree,
+                                         bool shallow = true);
+
+  // Diagnostics: dump the contents of this tree to `stream`.
+  // This function is intended for debugging and testing purposes only.
   static void Dump(const CordRep* rep, std::ostream& stream);
   static void Dump(const CordRep* rep, absl::string_view label,
                    std::ostream& stream);
@@ -496,12 +514,12 @@
 };
 
 inline CordRepBtree* CordRep::btree() {
-  assert(tag == BTREE);
+  assert(IsBtree());
   return static_cast<CordRepBtree*>(this);
 }
 
 inline const CordRepBtree* CordRep::btree() const {
-  assert(tag == BTREE);
+  assert(IsBtree());
   return static_cast<const CordRepBtree*>(this);
 }
 
@@ -571,7 +589,7 @@
 
 inline CordRepBtree* CordRepBtree::New(CordRep* rep) {
   CordRepBtree* tree = new CordRepBtree;
-  int height = rep->tag == BTREE ? rep->btree()->height() + 1 : 0;
+  int height = rep->IsBtree() ? rep->btree()->height() + 1 : 0;
   tree->length = rep->length;
   tree->InitInstance(height, /*begin=*/0, /*end=*/1);
   tree->edges_[0] = rep;
@@ -834,11 +852,13 @@
 
 #ifdef NDEBUG
 
-inline CordRepBtree* CordRepBtree::AssertValid(CordRepBtree* tree) {
+inline CordRepBtree* CordRepBtree::AssertValid(CordRepBtree* tree,
+                                               bool /* shallow */) {
   return tree;
 }
 
-inline const CordRepBtree* CordRepBtree::AssertValid(const CordRepBtree* tree) {
+inline const CordRepBtree* CordRepBtree::AssertValid(const CordRepBtree* tree,
+                                                     bool /* shallow */) {
   return tree;
 }
 

diff --git a/absl/strings/internal/cord_rep_btree_test.cc b/absl/strings/internal/cord_rep_btree_test.cc
index 7a0b781..073a7d4 100644
--- a/absl/strings/internal/cord_rep_btree_test.cc
+++ b/absl/strings/internal/cord_rep_btree_test.cc

@@ -24,6 +24,7 @@
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/cleanup/cleanup.h"
 #include "absl/strings/internal/cord_internal.h"
 #include "absl/strings/internal/cord_rep_test_util.h"
 #include "absl/strings/str_cat.h"
@@ -1346,6 +1347,48 @@
   CordRep::Unref(tree);
 }
 
+TEST(CordRepBtreeTest, CheckAssertValidShallowVsDeep) {
+  // Restore exhaustive validation on any exit.
+  const bool exhaustive_validation = cord_btree_exhaustive_validation.load();
+  auto cleanup = absl::MakeCleanup([exhaustive_validation] {
+    cord_btree_exhaustive_validation.store(exhaustive_validation);
+  });
+
+  // Create a tree of at least 2 levels, and mess with the original flat, which
+  // should go undetected in shallow mode as the flat is too far away, but
+  // should be detected in forced non-shallow mode.
+  CordRep* flat = MakeFlat("abc");
+  CordRepBtree* tree = CordRepBtree::Create(flat);
+  constexpr size_t max_cap = CordRepBtree::kMaxCapacity;
+  const size_t n = max_cap * max_cap * 2;
+  for (size_t i = 0; i < n; ++i) {
+    tree = CordRepBtree::Append(tree, MakeFlat("Hello world"));
+  }
+  flat->length = 100;
+
+  cord_btree_exhaustive_validation.store(false);
+  EXPECT_FALSE(CordRepBtree::IsValid(tree));
+  EXPECT_TRUE(CordRepBtree::IsValid(tree, true));
+  EXPECT_FALSE(CordRepBtree::IsValid(tree, false));
+  CordRepBtree::AssertValid(tree);
+  CordRepBtree::AssertValid(tree, true);
+#if defined(GTEST_HAS_DEATH_TEST)
+  EXPECT_DEBUG_DEATH(CordRepBtree::AssertValid(tree, false), ".*");
+#endif
+
+  cord_btree_exhaustive_validation.store(true);
+  EXPECT_FALSE(CordRepBtree::IsValid(tree));
+  EXPECT_FALSE(CordRepBtree::IsValid(tree, true));
+  EXPECT_FALSE(CordRepBtree::IsValid(tree, false));
+#if defined(GTEST_HAS_DEATH_TEST)
+  EXPECT_DEBUG_DEATH(CordRepBtree::AssertValid(tree), ".*");
+  EXPECT_DEBUG_DEATH(CordRepBtree::AssertValid(tree, true), ".*");
+#endif
+
+  flat->length = 3;
+  CordRep::Unref(tree);
+}
+
 }  // namespace
 }  // namespace cord_internal
 ABSL_NAMESPACE_END

diff --git a/absl/strings/internal/cord_rep_ring.cc b/absl/strings/internal/cord_rep_ring.cc
index 20a6fc2..db1f63f 100644
--- a/absl/strings/internal/cord_rep_ring.cc
+++ b/absl/strings/internal/cord_rep_ring.cc

@@ -40,7 +40,7 @@
 enum class Direction { kForward, kReversed };
 
 inline bool IsFlatOrExternal(CordRep* rep) {
-  return rep->tag >= FLAT || rep->tag == EXTERNAL;
+  return rep->IsFlat() || rep->IsExternal();
 }
 
 // Verifies that n + extra <= kMaxCapacity: throws std::length_error otherwise.
@@ -229,7 +229,7 @@
 }
 
 void CordRepRing::Delete(CordRepRing* rep) {
-  assert(rep != nullptr && rep->tag == RING);
+  assert(rep != nullptr && rep->IsRing());
 #if defined(__cpp_sized_deallocation)
   size_t size = AllocSize(rep->capacity_);
   rep->~CordRepRing();
@@ -360,7 +360,7 @@
   if (IsFlatOrExternal(child)) {
     return CreateFromLeaf(child, 0, length, extra);
   }
-  if (child->tag == RING) {
+  if (child->IsRing()) {
     return Mutable(child->ring(), extra);
   }
   return CreateSlow(child, extra);
@@ -433,7 +433,7 @@
 
 CordRepRing* CordRepRing::AppendSlow(CordRepRing* rep, CordRep* child) {
   Consume(child, [&rep](CordRep* child_arg, size_t offset, size_t len) {
-    if (child_arg->tag == RING) {
+    if (child_arg->IsRing()) {
       rep = AddRing<AddMode::kAppend>(rep, child_arg->ring(), offset, len);
     } else {
       rep = AppendLeaf(rep, child_arg, offset, len);
@@ -460,7 +460,7 @@
   if (IsFlatOrExternal(child)) {
     return AppendLeaf(rep, child, 0, length);
   }
-  if (child->tag == RING) {
+  if (child->IsRing()) {
     return AddRing<AddMode::kAppend>(rep, child->ring(), 0, length);
   }
   return AppendSlow(rep, child);
@@ -496,7 +496,7 @@
   if (IsFlatOrExternal(child)) {
     return PrependLeaf(rep, child, 0, length);
   }
-  if (child->tag == RING) {
+  if (child->IsRing()) {
     return AddRing<AddMode::kPrepend>(rep, child->ring(), 0, length);
   }
   return PrependSlow(rep, child);

diff --git a/absl/strings/internal/cord_rep_ring.h b/absl/strings/internal/cord_rep_ring.h
index 2082a56..44db849 100644
--- a/absl/strings/internal/cord_rep_ring.h
+++ b/absl/strings/internal/cord_rep_ring.h

@@ -570,12 +570,12 @@
 
 // Now that CordRepRing is defined, we can define CordRep's helper casts:
 inline CordRepRing* CordRep::ring() {
-  assert(tag == RING);
+  assert(IsRing());
   return static_cast<CordRepRing*>(this);
 }
 
 inline const CordRepRing* CordRep::ring() const {
-  assert(tag == RING);
+  assert(IsRing());
   return static_cast<const CordRepRing*>(this);
 }
 

diff --git a/absl/strings/internal/cordz_info.cc b/absl/strings/internal/cordz_info.cc
index a3a0b9c..5c18bbc 100644
--- a/absl/strings/internal/cordz_info.cc
+++ b/absl/strings/internal/cordz_info.cc

@@ -19,6 +19,7 @@
 #include "absl/container/inlined_vector.h"
 #include "absl/debugging/stacktrace.h"
 #include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
 #include "absl/strings/internal/cord_rep_ring.h"
 #include "absl/strings/internal/cordz_handle.h"
 #include "absl/strings/internal/cordz_statistics.h"
@@ -83,19 +84,23 @@
     // Process all top level linear nodes (substrings and flats).
     repref = CountLinearReps(repref, memory_usage_);
 
-    // We should have have either a concat or ring node node if not null.
     if (repref.rep != nullptr) {
-      assert(repref.rep->tag == RING || repref.rep->tag == CONCAT);
       if (repref.rep->tag == RING) {
         AnalyzeRing(repref);
+      } else if (repref.rep->tag == BTREE) {
+        AnalyzeBtree(repref);
       } else if (repref.rep->tag == CONCAT) {
         AnalyzeConcat(repref);
+      } else {
+        // We should have either a concat, btree, or ring node if not null.
+        assert(false);
       }
     }
 
     // Adds values to output
     statistics_.estimated_memory_usage += memory_usage_.total;
-    statistics_.estimated_fair_share_memory_usage += memory_usage_.fair_share;
+    statistics_.estimated_fair_share_memory_usage +=
+        static_cast<size_t>(memory_usage_.fair_share);
   }
 
  private:
@@ -117,13 +122,13 @@
   // Memory usage values
   struct MemoryUsage {
     size_t total = 0;
-    size_t fair_share = 0;
+    double fair_share = 0.0;
 
     // Adds 'size` memory usage to this class, with a cumulative (recursive)
     // reference count of `refcount`
     void Add(size_t size, size_t refcount) {
       total += size;
-      fair_share += size / refcount;
+      fair_share += static_cast<double>(size) / refcount;
     }
   };
 
@@ -215,28 +220,32 @@
     }
   }
 
-  // Counts the provided ring buffer child into `child_usage`.
-  void CountRingChild(const CordRep* child, MemoryUsage& child_usage) {
-    RepRef rep{child, static_cast<size_t>(child->refcount.Get())};
-    rep = CountLinearReps(rep, child_usage);
-    assert(rep.rep == nullptr);
-  }
-
-  // Analyzes the provided ring. As ring buffers can have many child nodes, the
-  // effect of rounding errors can become non trivial, so we compute the totals
-  // first at the ring level, and then divide the fair share of the total
-  // including children fair share totals.
+  // Analyzes the provided ring.
   void AnalyzeRing(RepRef rep) {
     statistics_.node_count++;
     statistics_.node_counts.ring++;
-    MemoryUsage ring_usage;
     const CordRepRing* ring = rep.rep->ring();
-    ring_usage.Add(CordRepRing::AllocSize(ring->capacity()), 1);
+    memory_usage_.Add(CordRepRing::AllocSize(ring->capacity()), rep.refcount);
     ring->ForEach([&](CordRepRing::index_type pos) {
-      CountRingChild(ring->entry_child(pos), ring_usage);
+      CountLinearReps(rep.Child(ring->entry_child(pos)), memory_usage_);
     });
-    memory_usage_.total += ring_usage.total;
-    memory_usage_.fair_share += ring_usage.fair_share / rep.refcount;
+  }
+
+  // Analyzes the provided btree.
+  void AnalyzeBtree(RepRef rep) {
+    statistics_.node_count++;
+    statistics_.node_counts.btree++;
+    memory_usage_.Add(sizeof(CordRepBtree), rep.refcount);
+    const CordRepBtree* tree = rep.rep->btree();
+    if (tree->height() > 0) {
+      for (CordRep* edge : tree->Edges()) {
+        AnalyzeBtree(rep.Child(edge));
+      }
+    } else {
+      for (CordRep* edge : tree->Edges()) {
+        CountLinearReps(rep.Child(edge), memory_usage_);
+      }
+    }
   }
 
   CordzStatistics& statistics_;

diff --git a/absl/strings/internal/cordz_info_statistics_test.cc b/absl/strings/internal/cordz_info_statistics_test.cc
index 9f2842d..7430d28 100644
--- a/absl/strings/internal/cordz_info_statistics_test.cc
+++ b/absl/strings/internal/cordz_info_statistics_test.cc

@@ -21,6 +21,7 @@
 #include "absl/base/config.h"
 #include "absl/strings/cord.h"
 #include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cord_rep_btree.h"
 #include "absl/strings/internal/cord_rep_flat.h"
 #include "absl/strings/internal/cord_rep_ring.h"
 #include "absl/strings/internal/cordz_info.h"
@@ -42,6 +43,8 @@
 
 namespace {
 
+using ::testing::Ge;
+
 // Creates a flat of the specified allocated size
 CordRepFlat* Flat(size_t size) {
   // Round up to a tag size, as we are going to poke an exact tag size back into
@@ -134,8 +137,8 @@
 }
 
 // Computes fair share memory used in a naive 'we dare to recurse' way.
-size_t FairShare(CordRep* rep, size_t ref = 1) {
-  size_t self = 0, children = 0;
+double FairShareImpl(CordRep* rep, size_t ref) {
+  double self = 0.0, children = 0.0;
   ref *= rep->refcount.Get();
   if (rep->tag >= FLAT) {
     self = SizeOf(rep->flat());
@@ -143,22 +146,32 @@
     self = SizeOf(rep->external());
   } else if (rep->tag == SUBSTRING) {
     self = SizeOf(rep->substring());
-    children = FairShare(rep->substring()->child, ref);
+    children = FairShareImpl(rep->substring()->child, ref);
+  } else if (rep->tag == BTREE) {
+    self = SizeOf(rep->btree());
+    for (CordRep*edge : rep->btree()->Edges()) {
+      children += FairShareImpl(edge, ref);
+    }
   } else if (rep->tag == RING) {
     self = SizeOf(rep->ring());
     rep->ring()->ForEach([&](CordRepRing::index_type i) {
-      self += FairShare(rep->ring()->entry_child(i));
+      self += FairShareImpl(rep->ring()->entry_child(i), 1);
     });
   } else if (rep->tag == CONCAT) {
     self = SizeOf(rep->concat());
-    children = FairShare(rep->concat()->left, ref) +
-               FairShare(rep->concat()->right, ref);
+    children = FairShareImpl(rep->concat()->left, ref) +
+               FairShareImpl(rep->concat()->right, ref);
   } else {
     assert(false);
   }
   return self / ref + children;
 }
 
+// Returns the fair share memory size from `ShareFhareImpl()` as a size_t.
+size_t FairShare(CordRep* rep, size_t ref = 1) {
+  return static_cast<size_t>(FairShareImpl(rep, ref));
+}
+
 // Samples the cord and returns CordzInfo::GetStatistics()
 CordzStatistics SampleCord(CordRep* rep) {
   InlineData cord(rep);
@@ -191,6 +204,7 @@
   STATS_MATCHER_EXPECT_EQ(node_counts.concat);
   STATS_MATCHER_EXPECT_EQ(node_counts.substring);
   STATS_MATCHER_EXPECT_EQ(node_counts.ring);
+  STATS_MATCHER_EXPECT_EQ(node_counts.btree);
   STATS_MATCHER_EXPECT_EQ(estimated_memory_usage);
   STATS_MATCHER_EXPECT_EQ(estimated_fair_share_memory_usage);
 
@@ -424,6 +438,103 @@
   EXPECT_THAT(SampleCord(substring), EqStatistics(expected));
 }
 
+TEST(CordzInfoStatisticsTest, BtreeLeaf) {
+  ASSERT_THAT(CordRepBtree::kMaxCapacity, Ge(3));
+  RefHelper ref;
+  auto* flat1 = Flat(2000);
+  auto* flat2 = Flat(200);
+  auto* substr = Substring(flat2);
+  auto* external = External(3000);
+
+  CordRepBtree* tree = CordRepBtree::Create(flat1);
+  tree = CordRepBtree::Append(tree, substr);
+  tree = CordRepBtree::Append(tree, external);
+  size_t flat3_count = CordRepBtree::kMaxCapacity - 3;
+  size_t flat3_size = 0;
+  for (size_t i = 0; i < flat3_count; ++i) {
+    auto* flat3 = Flat(70);
+    flat3_size += SizeOf(flat3);
+    tree = CordRepBtree::Append(tree, flat3);
+  }
+  ref.NeedsUnref(tree);
+
+  CordzStatistics expected;
+  expected.size = tree->length;
+  expected.estimated_memory_usage = SizeOf(tree) + SizeOf(flat1) +
+                                    SizeOf(flat2) + SizeOf(substr) +
+                                    flat3_size + SizeOf(external);
+  expected.estimated_fair_share_memory_usage = expected.estimated_memory_usage;
+  expected.node_count = 1 + 3 + 1 + flat3_count;
+  expected.node_counts.flat = 2 + flat3_count;
+  expected.node_counts.flat_128 = flat3_count;
+  expected.node_counts.flat_256 = 1;
+  expected.node_counts.external = 1;
+  expected.node_counts.substring = 1;
+  expected.node_counts.btree = 1;
+
+  EXPECT_THAT(SampleCord(tree), EqStatistics(expected));
+}
+
+TEST(CordzInfoStatisticsTest, BtreeNodeShared) {
+  RefHelper ref;
+  static constexpr int leaf_count = 3;
+  const size_t flat3_count = CordRepBtree::kMaxCapacity - 3;
+  ASSERT_THAT(flat3_count, Ge(0));
+
+  CordRepBtree* tree = nullptr;
+  size_t mem_size = 0;
+  for (int i = 0; i < leaf_count; ++i) {
+    auto* flat1 = ref.Ref(Flat(2000), 9);
+    mem_size += SizeOf(flat1);
+    if (i == 0) {
+      tree = CordRepBtree::Create(flat1);
+    } else {
+      tree = CordRepBtree::Append(tree, flat1);
+    }
+
+    auto* flat2 = Flat(200);
+    auto* substr = Substring(flat2);
+    mem_size += SizeOf(flat2) + SizeOf(substr);
+    tree = CordRepBtree::Append(tree, substr);
+
+    auto* external = External(30);
+    mem_size += SizeOf(external);
+    tree = CordRepBtree::Append(tree, external);
+
+    for (size_t i = 0; i < flat3_count; ++i) {
+      auto* flat3 = Flat(70);
+      mem_size += SizeOf(flat3);
+      tree = CordRepBtree::Append(tree, flat3);
+    }
+
+    if (i == 0) {
+      mem_size += SizeOf(tree);
+    } else {
+      mem_size += SizeOf(tree->Edges().back()->btree());
+    }
+  }
+  ref.NeedsUnref(tree);
+
+  // Ref count: 2 for top (add 1), 5 for leaf 0 (add 4).
+  ref.Ref(tree, 1);
+  ref.Ref(tree->Edges().front(), 4);
+
+  CordzStatistics expected;
+  expected.size = tree->length;
+  expected.estimated_memory_usage = SizeOf(tree) + mem_size;
+  expected.estimated_fair_share_memory_usage = FairShare(tree);
+
+  expected.node_count = 1 + leaf_count * (1 + 3 + 1 + flat3_count);
+  expected.node_counts.flat = leaf_count * (2 + flat3_count);
+  expected.node_counts.flat_128 = leaf_count * flat3_count;
+  expected.node_counts.flat_256 = leaf_count;
+  expected.node_counts.external = leaf_count;
+  expected.node_counts.substring = leaf_count;
+  expected.node_counts.btree = 1 + leaf_count;
+
+  EXPECT_THAT(SampleCord(tree), EqStatistics(expected));
+}
+
 TEST(CordzInfoStatisticsTest, ThreadSafety) {
   Notification stop;
   static constexpr int kNumThreads = 8;
@@ -471,9 +582,15 @@
                 CordRep::Unref(cord.as_tree());
                 cord.set_inline_size(0);
               } else {
-                // 50/50 Ring or Flat coin toss
+                // Coin toss to 25% ring, 25% btree, and 50% flat.
                 CordRep* rep = Flat(256);
-                rep = (coin_toss(gen) != 0) ? CordRepRing::Create(rep) : rep;
+                if (coin_toss(gen) != 0) {
+                  if (coin_toss(gen) != 0) {
+                    rep = CordRepRing::Create(rep);
+                  } else {
+                    rep = CordRepBtree::Create(rep);
+                  }
+                }
                 cord.make_tree(rep);
 
                 // 50/50 sample

diff --git a/absl/strings/internal/cordz_statistics.h b/absl/strings/internal/cordz_statistics.h
index e03c651..da4c7db 100644
--- a/absl/strings/internal/cordz_statistics.h
+++ b/absl/strings/internal/cordz_statistics.h

@@ -40,6 +40,7 @@
     size_t substring = 0;  // #substring reps
     size_t concat = 0;     // #concat reps
     size_t ring = 0;       // #ring buffer reps
+    size_t btree = 0;      // #btree reps
   };
 
   // The size of the cord in bytes. This matches the result of Cord::size().
@@ -61,6 +62,8 @@
 
   // The total number of nodes referenced by this cord.
   // For ring buffer Cords, this includes the 'ring buffer' node.
+  // For btree Cords, this includes all 'CordRepBtree' tree nodes as well as all
+  // the substring, flat and external nodes referenced by the tree.
   // A value of 0 implies the property has not been recorded.
   int64_t node_count = 0;
 

diff --git a/absl/strings/string_view.h b/absl/strings/string_view.h
index ec6c431..ea76052 100644
--- a/absl/strings/string_view.h
+++ b/absl/strings/string_view.h

@@ -198,9 +198,9 @@
   // Implicit constructor of a `string_view` from NUL-terminated `str`. When
   // accepting possibly null strings, use `absl::NullSafeStringView(str)`
   // instead (see below).
+  // The length check is skipped since it is unnecessary and causes code bloat.
   constexpr string_view(const char* str)  // NOLINT(runtime/explicit)
-      : ptr_(str),
-        length_(str ? CheckLengthInternal(StrlenInternal(str)) : 0) {}
+      : ptr_(str), length_(str ? StrlenInternal(str) : 0) {}
 
   // Implicit constructor of a `string_view` from a `const char*` and length.
   constexpr string_view(const char* data, size_type len)