Export of internal Abseil changes

--
e400e8c3dc7190b83dec57be9d858ee44e146a42 by Martijn Vels <mvels@google.com>:

Optimize Cord destructor through CordRepBtree::Destroy

This change inlines tree destruction 2 levels at a time, special casing leaf and leaf + 1 levels.

Benchmarks shows this provides up to 8% speed up

PiperOrigin-RevId: 410953099
GitOrigin-RevId: e400e8c3dc7190b83dec57be9d858ee44e146a42
Change-Id: Ic1c123fa0fa2b13cb141ee7a6be155d9e57a0466

absl/strings/internal/cord_rep_btree.cc

@@ -190,24 +190,29 @@ inline void FastUnref(R* r, Fn&& fn) {
   }
 }
 
-// Deletes a leaf node data edge. Requires `rep` to be an EXTERNAL or FLAT
-// node, or a SUBSTRING of an EXTERNAL or FLAT node.
-void DeleteLeafEdge(CordRep* rep) {
-  for (;;) {
+
+void DeleteSubstring(CordRepSubstring* substring) {
+  CordRep* rep = substring->child;
+  if (!rep->refcount.Decrement()) {
     if (rep->tag >= FLAT) {
       CordRepFlat::Delete(rep->flat());
-      return;
-    }
-    if (rep->tag == EXTERNAL) {
+    } else {
+      assert(rep->tag == EXTERNAL);
       CordRepExternal::Delete(rep->external());
-      return;
     }
-    assert(rep->tag == SUBSTRING);
-    CordRepSubstring* substring = rep->substring();
-    rep = substring->child;
-    assert(rep->tag == EXTERNAL || rep->tag >= FLAT);
-    delete substring;
-    if (rep->refcount.Decrement()) return;
+  }
+  delete substring;
+}
+
+// Deletes a leaf node data edge. Requires `IsDataEdge(rep)`.
+void DeleteLeafEdge(CordRep* rep) {
+  assert(CordRepBtree::IsDataEdge(rep));
+  if (rep->tag >= FLAT) {
+    CordRepFlat::Delete(rep->flat());
+  } else if (rep->tag == EXTERNAL) {
+    CordRepExternal::Delete(rep->external());
+  } else {
+    DeleteSubstring(rep->substring());
   }
 }
 
@@ -372,19 +377,39 @@ void CordRepBtree::Dump(const CordRep* rep, std::ostream& stream) {
   Dump(rep, absl::string_view(), false, stream);
 }
 
-void CordRepBtree::DestroyLeaf(CordRepBtree* tree, size_t begin, size_t end) {
-  for (CordRep* edge : tree->Edges(begin, end)) {
-    FastUnref(edge, DeleteLeafEdge);
+template <size_t size>
+static void DestroyTree(CordRepBtree* tree) {
+  for (CordRep* node : tree->Edges()) {
+    if (!node->refcount.Decrement()) {
+      for (CordRep* edge : node->btree()->Edges()) {
+        if (!edge->refcount.Decrement()) {
+          if (size == 1) {
+            DeleteLeafEdge(edge);
+          } else {
+            CordRepBtree::Destroy(edge->btree());
+          }
+        }
+      }
+      CordRepBtree::Delete(node->btree());
+    }
   }
-  Delete(tree);
+  CordRepBtree::Delete(tree);
 }
 
-void CordRepBtree::DestroyNonLeaf(CordRepBtree* tree, size_t begin,
-                                  size_t end) {
-  for (CordRep* edge : tree->Edges(begin, end)) {
-    FastUnref(edge->btree(), Destroy);
+void CordRepBtree::Destroy(CordRepBtree* tree) {
+  switch (tree->height()) {
+    case 0:
+      for (CordRep* edge : tree->Edges()) {
+        if (!edge->refcount.Decrement()) {
+          DeleteLeafEdge(edge);
+        }
+      }
+      return CordRepBtree::Delete(tree);
+    case 1:
+      return DestroyTree<1>(tree);
+    default:
+      return DestroyTree<2>(tree);
   }
-  Delete(tree);
 }
 
 bool CordRepBtree::IsValid(const CordRepBtree* tree, bool shallow) {

absl/strings/internal/cord_rep_btree.h

@@ -163,6 +163,9 @@ class CordRepBtree : public CordRep {
   // typically after a ref_count.Decrement() on the last reference count.
   static void Destroy(CordRepBtree* tree);
 
+  // Destruction
+  static void Delete(CordRepBtree* tree) { delete tree; }
+
   // Use CordRep::Unref() as we overload for absl::Span<CordRep* const>.
   using CordRep::Unref;
 
@@ -440,12 +443,6 @@ class CordRepBtree : public CordRep {
   // Requires `offset` < length.
   Position IndexBeyond(size_t offset) const;
 
-  // Destruction
-  static void DestroyLeaf(CordRepBtree* tree, size_t begin, size_t end);
-  static void DestroyNonLeaf(CordRepBtree* tree, size_t begin, size_t end);
-  static void DestroyTree(CordRepBtree* tree, size_t begin, size_t end);
-  static void Delete(CordRepBtree* tree) { delete tree; }
-
   // Creates a new leaf node containing as much data as possible from `data`.
   // The data is added either forwards or reversed depending on `edge_type`.
   // Callers must check the length of the returned node to determine if all data
@@ -689,19 +686,6 @@ inline CordRepBtree* CordRepBtree::New(CordRepBtree* front,
   return tree;
 }
 
-inline void CordRepBtree::DestroyTree(CordRepBtree* tree, size_t begin,
-                                      size_t end) {
-  if (tree->height() == 0) {
-    DestroyLeaf(tree, begin, end);
-  } else {
-    DestroyNonLeaf(tree, begin, end);
-  }
-}
-
-inline void CordRepBtree::Destroy(CordRepBtree* tree) {
-  DestroyTree(tree, tree->begin(), tree->end());
-}
-
 inline void CordRepBtree::Unref(absl::Span<CordRep* const> edges) {
   for (CordRep* edge : edges) {
     if (ABSL_PREDICT_FALSE(!edge->refcount.Decrement())) {