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

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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

1 files changed, 96 insertions, 84 deletions

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);