Export of internal Abseil changes

--
93c607726d663800b4bfa472cba043fd3f5d0e97 by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 389158822

--
55b3bb50bbc168567c6ba25d07df2c2c39e864af by Martijn Vels <mvels@google.com>:

Change CordRepRing alternative implementation to CordRepBtree alternative.

This changes makes CordRepBtree (BTREE) the alternative to CordRepConcat (CONCAT) trees, enabled through the internal / experimental 'cord_btree_enabled' latch.

PiperOrigin-RevId: 389030571

--
d6fc346143606c096bca8eb5029e4c429ac6e305 by Todd Lipcon <tlipcon@google.com>:

Fix a small typo in SequenceLock doc comment

PiperOrigin-RevId: 388972936

--
e46f9245dce8b4150e3ca2664e0cf42b75f90a83 by Martijn Vels <mvels@google.com>:

Add 'shallow' validation mode to CordRepBtree which will be the default for internal assertions.

PiperOrigin-RevId: 388753606

--
b5e74f163b490beb006f848ace67bb650433fe13 by Martijn Vels <mvels@google.com>:

Add btree statistics to CordzInfo, and reduce rounding errors

PiperOrigin-RevId: 388715878

--
105bcbf80de649937e693b29b18220f9e6841a51 by Evan Brown <ezb@google.com>:

Skip length checking when constructing absl::string_view from `const char*`.
The length check causes unnecessary code bloat.

PiperOrigin-RevId: 388271741

--
bed595158f24839efe49c65ae483f797d79fe0ae by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 387713428
GitOrigin-RevId: 93c607726d663800b4bfa472cba043fd3f5d0e97
Change-Id: I2a4840f5ffcd7f70b7d7d45cce66f23c42cf565f

15 files changed, 369 insertions, 148 deletions

diff --git a/absl/flags/internal/sequence_lock.h b/absl/flags/internal/sequence_lock.h
index 807b2a73..36318ab9 100644
--- a/absl/flags/internal/sequence_lock.h
+++ b/absl/flags/internal/sequence_lock.h
@@ -49,7 +49,7 @@ inline constexpr size_t AlignUp(size_t x, size_t align) {
 // 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/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 9720b683..f9735b4b 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel
@@ -318,6 +318,7 @@ cc_test(
         ":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 88f076a8..8ad5f9c7 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt
@@ -952,6 +952,7 @@ absl_cc_test(
     ${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 f5aa6e47..ecbd2dbd 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"
@@ -51,20 +51,20 @@ namespace absl {
 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::BTREE;
 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;
@@ -100,8 +100,8 @@ static constexpr uint64_t min_length[] = {
 
 static const int kMinLengthSize = ABSL_ARRAYSIZE(min_length);
 
-static inline bool cord_ring_enabled() {
-  return cord_internal::cord_ring_buffer_enabled.load(
+static inline bool btree_enabled() {
+  return cord_internal::cord_btree_enabled.load(
       std::memory_order_relaxed);
 }
 
@@ -218,27 +218,25 @@ static CordRepFlat* CreateFlat(const char* data, size_t length,
   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;
@@ -346,10 +344,10 @@ inline void Cord::InlineRep::remove_prefix(size_t n) {
   reduce_size(n);
 }
 
-// Returns `rep` converted into a CordRepRing.
-// Directly returns `rep` if `rep` is already a CordRepRing.
-static CordRepRing* ForceRing(CordRep* rep, size_t extra) {
-  return (rep->tag == RING) ? rep->ring() : CordRepRing::Create(rep, extra);
+// Returns `rep` converted into a CordRepBtree.
+// Directly returns `rep` if `rep` is already a CordRepBtree.
+static CordRepBtree* ForceBtree(CordRep* rep) {
+  return rep->tag == BTREE ? rep->btree() : CordRepBtree::Create(rep);
 }
 
 void Cord::InlineRep::AppendTreeToInlined(CordRep* tree,
@@ -357,8 +355,8 @@ void Cord::InlineRep::AppendTreeToInlined(CordRep* tree,
   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 +367,8 @@ void Cord::InlineRep::AppendTreeToInlined(CordRep* tree,
 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 +389,8 @@ void Cord::InlineRep::PrependTreeToInlined(CordRep* tree,
   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 +402,8 @@ void Cord::InlineRep::PrependTreeToTree(CordRep* tree,
                                         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 +425,8 @@ void Cord::InlineRep::PrependTree(CordRep* tree, MethodIdentifier method) {
 // 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->tag == BTREE && root->refcount.IsOne()) {
+    Span<char> span = root->btree()->GetAppendBuffer(max_length);
     if (!span.empty()) {
       *region = span.data();
       *size = span.size();
@@ -500,8 +498,8 @@ void Cord::InlineRep::GetAppendRegion(char** region, size_t* size,
   *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);
   }
@@ -511,8 +509,13 @@ void Cord::InlineRep::GetAppendRegion(char** region, size_t* size,
 // 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) {
+  size_t maybe_sub_size = 0;
+  if (rep->tag == SUBSTRING) {
+    maybe_sub_size = sizeof(cord_internal::CordRepSubstring);
+    rep = rep->substring()->child;
+  }
   if (rep->tag >= FLAT) {
-    *total_mem_usage += rep->flat()->AllocatedSize();
+    *total_mem_usage += maybe_sub_size + rep->flat()->AllocatedSize();
     return true;
   }
   if (rep->tag == EXTERNAL) {
@@ -520,8 +523,9 @@ static bool RepMemoryUsageLeaf(const CordRep* rep, size_t* total_mem_usage) {
     // 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 +=
-        sizeof(cord_internal::CordRepExternalImpl<intptr_t>) + rep->length;
+    *total_mem_usage += maybe_sub_size +
+                        sizeof(cord_internal::CordRepExternalImpl<intptr_t>) +
+                        rep->length;
     return true;
   }
   return false;
@@ -690,9 +694,11 @@ void Cord::InlineRep::AppendArray(absl::string_view src,
     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
@@ -769,9 +775,13 @@ inline void Cord::AppendImpl(C&& src) {
   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) {
@@ -923,8 +933,10 @@ void Cord::RemovePrefix(size_t n) {
   } else {
     auto constexpr method = CordzUpdateTracker::kRemovePrefix;
     CordzUpdateScope scope(contents_.cordz_info(), method);
-    if (tree->tag == RING) {
-      tree = CordRepRing::RemovePrefix(tree->ring(), n);
+    if (tree->tag == BTREE) {
+      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 +956,10 @@ void Cord::RemoveSuffix(size_t n) {
   } else {
     auto constexpr method = CordzUpdateTracker::kRemoveSuffix;
     CordzUpdateScope scope(contents_.cordz_info(), method);
-    if (tree->tag == RING) {
-      tree = CordRepRing::RemoveSuffix(tree->ring(), n);
+    if (tree->tag == BTREE) {
+      CordRep* old = tree;
+      tree = tree->btree()->SubTree(0, tree->length - n);
+      CordRep::Unref(old);
     } else {
       CordRep* newrep = RemoveSuffixFrom(tree, n);
       CordRep::Unref(tree);
@@ -1037,9 +1051,8 @@ Cord Cord::Subcord(size_t pos, size_t new_size) const {
     return sub_cord;
   }
 
-  if (tree->tag == RING) {
-    CordRepRing* ring = CordRep::Ref(tree)->ring();
-    tree = CordRepRing::SubRing(ring, pos, new_size);
+  if (tree->tag == BTREE) {
+    tree = tree->btree()->SubTree(pos, new_size);
   } else {
     tree = NewSubRange(tree, pos, new_size);
   }
@@ -1227,8 +1240,8 @@ bool ComputeCompareResult<bool>(int memcmp_res) {
 
 }  // 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());
@@ -1243,8 +1256,13 @@ inline absl::string_view Cord::InlineRep::FindFlatStartPiece() const {
     return absl::string_view(node->external()->base, node->length);
   }
 
-  if (node->tag == RING) {
-    return node->ring()->entry_data(node->ring()->head());
+  if (node->tag == BTREE) {
+    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.
@@ -1506,22 +1524,21 @@ Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
     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;
   }
 
@@ -1698,8 +1715,8 @@ char Cord::operator[](size_t i) const {
     if (rep->tag >= FLAT) {
       // Get the "i"th character directly from the flat array.
       return rep->flat()->Data()[offset];
-    } else if (rep->tag == RING) {
-      return rep->ring()->GetCharacter(offset);
+    } else if (rep->tag == BTREE) {
+      return rep->btree()->GetCharacter(offset);
     } else if (rep->tag == EXTERNAL) {
       // Get the "i"th character from the external array.
       return rep->external()->base[offset];
@@ -1758,8 +1775,8 @@ absl::string_view Cord::FlattenSlowPath() {
   } else if (rep->tag == EXTERNAL) {
     *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 == BTREE) {
+    return rep->btree()->IsFlat(fragment);
   } else if (rep->tag == SUBSTRING) {
     CordRep* child = rep->substring()->child;
     if (child->tag >= FLAT) {
@@ -1770,9 +1787,9 @@ absl::string_view Cord::FlattenSlowPath() {
       *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->tag == BTREE) {
+      return child->btree()->IsFlat(rep->substring()->start, rep->length,
+                                    fragment);
     }
   }
   return false;
@@ -1781,7 +1798,7 @@ absl::string_view Cord::FlattenSlowPath() {
 /* static */ void Cord::ForEachChunkAux(
     absl::cord_internal::CordRep* rep,
     absl::FunctionRef<void(absl::string_view)> callback) {
-  if (rep->tag == RING) {
+  if (rep->tag == BTREE) {
     ChunkIterator it(rep), end;
     while (it != end) {
       callback(*it);
@@ -1866,15 +1883,7 @@ static void DumpNode(CordRep* rep, bool include_data, std::ostream* os,
           *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();
@@ -1967,15 +1976,18 @@ static bool VerifyNode(CordRep* root, CordRep* start_node,
         }
         next_node = right;
       }
-    } else if (cur_node->tag == RING) {
-      total_mem_usage += CordRepRing::AllocSize(cur_node->ring()->capacity());
-      const CordRepRing* ring = cur_node->ring();
-      CordRepRing::index_type pos = ring->head(), tail = ring->tail();
-      do {
-        CordRep* node = ring->entry_child(pos);
-        assert(node->tag >= FLAT || node->tag == EXTERNAL);
-        RepMemoryUsageLeaf(node, &total_mem_usage);
-      } while ((pos = ring->advance(pos)) != tail);
+    } else if (cur_node->tag == BTREE) {
+      total_mem_usage += sizeof(CordRepBtree);
+      const CordRepBtree* node = cur_node->btree();
+      if (node->height() == 0) {
+        for (const CordRep* edge : node->Edges()) {
+          RepMemoryUsageLeaf(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);
diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index e758f1cd..ac1832f0 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 @@ class Cord {
 
    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 @@ class Cord {
     // 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 @@ class Cord {
     // 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 bool Cord::StartsWith(absl::string_view rhs) const {
 }
 
 inline void Cord::ChunkIterator::InitTree(cord_internal::CordRep* tree) {
-  if (tree->tag == cord_internal::RING) {
-    current_chunk_ = ring_reader_.Reset(tree->ring());
+  if (tree->tag == cord_internal::BTREE) {
+    current_chunk_ = btree_reader_.Init(tree->btree());
     return;
   }
 
@@ -1271,20 +1272,20 @@ inline Cord::ChunkIterator::ChunkIterator(const Cord* cord)
   }
 }
 
-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 @@ inline Cord::ChunkIterator& Cord::ChunkIterator::operator++() {
   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 @@ inline void Cord::ChunkIterator::AdvanceBytes(size_t n) {
   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 597378cc..0c0a8a7a 100644
--- a/absl/strings/cord_test.cc
+++ b/absl/strings/cord_test.cc
@@ -366,7 +366,7 @@ TEST(Cord, Subcord) {
 
   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;
diff --git a/absl/strings/internal/cord_internal.cc b/absl/strings/internal/cord_internal.cc
index f79cb628..1767e6fc 100644
--- a/absl/strings/internal/cord_internal.cc
+++ b/absl/strings/internal/cord_internal.cc
@@ -31,6 +31,7 @@ ABSL_CONST_INIT std::atomic<bool> cord_ring_buffer_enabled(
     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 371a7f9c..8ae644ba 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -46,6 +46,12 @@ extern std::atomic<bool> cord_btree_enabled;
 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);
 }
diff --git a/absl/strings/internal/cord_rep_btree.cc b/absl/strings/internal/cord_rep_btree.cc
index 6978cfd2..fd3a0045 100644
--- a/absl/strings/internal/cord_rep_btree.cc
+++ b/absl/strings/internal/cord_rep_btree.cc
@@ -32,6 +32,8 @@ namespace absl {
 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 @@ using CopyResult = CordRepBtree::CopyResult;
 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.
@@ -357,7 +363,7 @@ void CordRepBtree::DestroyNonLeaf(CordRepBtree* tree, size_t begin,
   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); \
@@ -389,9 +395,9 @@ bool CordRepBtree::IsValid(const CordRepBtree* tree) {
     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 @@ bool CordRepBtree::IsValid(const CordRepBtree* tree) {
 
 #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");
   }
diff --git a/absl/strings/internal/cord_rep_btree.h b/absl/strings/internal/cord_rep_btree.h
index 7d854731..56e1e4af 100644
--- a/absl/strings/internal/cord_rep_btree.h
+++ b/absl/strings/internal/cord_rep_btree.h
@@ -266,10 +266,28 @@ class CordRepBtree : public CordRep {
   // 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);
@@ -834,11 +852,13 @@ inline CordRepBtree* CordRepBtree::Prepend(CordRepBtree* tree, CordRep* rep) {
 
 #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 7a0b7811..073a7d45 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 @@ TEST(CordRepBtreeTest, AssertValid) {
   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/cordz_info.cc b/absl/strings/internal/cordz_info.cc
index a3a0b9c0..5c18bbc5 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 @@ class CordRepAnalyzer {
     // 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 @@ class CordRepAnalyzer {
   // 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 @@ class CordRepAnalyzer {
     }
   }
 
-  // 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 9f2842d9..7430d281 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 @@ inline void PrintTo(const CordzStatistics& stats, std::ostream* s) {
 
 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 @@ size_t SizeOf(const CordRepRing* rep) {
 }
 
 // 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 @@ size_t FairShare(CordRep* rep, size_t ref = 1) {
     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 @@ MATCHER_P(EqStatistics, stats, "Statistics equal expected values") {
   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 @@ TEST(CordzInfoStatisticsTest, SharedSubstringRing) {
   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 @@ TEST(CordzInfoStatisticsTest, ThreadSafety) {
                 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 e03c651e..da4c7dbb 100644
--- a/absl/strings/internal/cordz_statistics.h
+++ b/absl/strings/internal/cordz_statistics.h
@@ -40,6 +40,7 @@ struct CordzStatistics {
     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 @@ struct CordzStatistics {
 
   // 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 ec6c431f..ea760526 100644
--- a/absl/strings/string_view.h
+++ b/absl/strings/string_view.h
@@ -198,9 +198,9 @@ class string_view {
   // 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)