3 files changed, 30 insertions, 14 deletions
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index fcca3a28..6022c1df 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -225,7 +225,11 @@ struct CordRep {
       : length(l), refcount(immortal), tag(EXTERNAL), storage{} {}
 
   // The following three fields have to be less than 32 bytes since
-  // that is the smallest supported flat node size.
+  // that is the smallest supported flat node size. Some code optimizations rely
+  // on the specific layout of these fields. Notably: the non-trivial field
+  // `refcount` being preceeded by `length`, and being tailed by POD data
+  // members only.
+  // # LINT.IfChange
   size_t length;
   RefcountAndFlags refcount;
   // If tag < FLAT, it represents CordRepKind and indicates the type of node.
@@ -241,6 +245,7 @@ struct CordRep {
   // allocate room for these in the derived class, as not all compilers reuse
   // padding space from the base class (clang and gcc do, MSVC does not, etc)
   uint8_t storage[3];
+  // # LINT.ThenChange(cord_rep_btree.h:copy_raw)
 
   // Returns true if this instance's tag matches the requested type.
   constexpr bool IsRing() const { return tag == RING; }
diff --git a/absl/strings/internal/cord_rep_btree.cc b/absl/strings/internal/cord_rep_btree.cc
index 7ce36128..985f0724 100644
--- a/absl/strings/internal/cord_rep_btree.cc
+++ b/absl/strings/internal/cord_rep_btree.cc
@@ -502,7 +502,7 @@ OpResult CordRepBtree::SetEdge(bool owned, CordRep* edge, size_t delta) {
     // open interval [begin, back) or [begin + 1, end) depending on `edge_type`.
     // We conveniently cover both case using a constexpr `shift` being 0 or 1
     // as `end :== back + 1`.
-    result = {CopyRaw(), kCopied};
+    result = {CopyRaw(length), kCopied};
     constexpr int shift = edge_type == kFront ? 1 : 0;
     for (CordRep* r : Edges(begin() + shift, back() + shift)) {
       CordRep::Ref(r);
diff --git a/absl/strings/internal/cord_rep_btree.h b/absl/strings/internal/cord_rep_btree.h
index eed5609e..4209e512 100644
--- a/absl/strings/internal/cord_rep_btree.h
+++ b/absl/strings/internal/cord_rep_btree.h
@@ -446,9 +446,9 @@ class CordRepBtree : public CordRep {
   template <EdgeType edge_type>
   static CordRepBtree* NewLeaf(absl::string_view data, size_t extra);
 
-  // Creates a raw copy of this Btree node, copying all properties, but
-  // without adding any references to existing edges.
-  CordRepBtree* CopyRaw() const;
+  // Creates a raw copy of this Btree node with the specified length, copying
+  // all properties, but without adding any references to existing edges.
+  CordRepBtree* CopyRaw(size_t new_length) const;
 
   // Creates a full copy of this Btree node, adding a reference on all edges.
   CordRepBtree* Copy() const;
@@ -666,15 +666,28 @@ inline void CordRepBtree::Unref(absl::Span<CordRep* const> edges) {
   }
 }
 
-inline CordRepBtree* CordRepBtree::CopyRaw() const {
-  auto* tree = static_cast<CordRepBtree*>(::operator new(sizeof(CordRepBtree)));
-  memcpy(static_cast<void*>(tree), this, sizeof(CordRepBtree));
-  new (&tree->refcount) RefcountAndFlags;
+inline CordRepBtree* CordRepBtree::CopyRaw(size_t new_length) const {
+  CordRepBtree* tree = new CordRepBtree;
+
+  // `length` and `refcount` are the first members of `CordRepBtree`.
+  // We initialize `length` using the given length, have `refcount` be set to
+  // ref = 1 through its default constructor, and copy all data beyond
+  // 'refcount' which starts with `tag` using a single memcpy: all contents
+  // except `refcount` is trivially copyable, and the compiler does not
+  // efficiently coalesce member-wise copy of these members.
+  // See https://gcc.godbolt.org/z/qY8zsca6z
+  // # LINT.IfChange(copy_raw)
+  tree->length = new_length;
+  uint8_t* dst = &tree->tag;
+  const uint8_t* src = &tag;
+  const ptrdiff_t offset = src - reinterpret_cast<const uint8_t*>(this);
+  memcpy(dst, src, sizeof(CordRepBtree) - static_cast<size_t>(offset));
   return tree;
+  // # LINT.ThenChange()
 }
 
 inline CordRepBtree* CordRepBtree::Copy() const {
-  CordRepBtree* tree = CopyRaw();
+  CordRepBtree* tree = CopyRaw(length);
   for (CordRep* rep : Edges()) CordRep::Ref(rep);
   return tree;
 }
@@ -683,8 +696,7 @@ inline CordRepBtree* CordRepBtree::CopyToEndFrom(size_t begin,
                                                  size_t new_length) const {
   assert(begin >= this->begin());
   assert(begin <= this->end());
-  CordRepBtree* tree = CopyRaw();
-  tree->length = new_length;
+  CordRepBtree* tree = CopyRaw(new_length);
   tree->set_begin(begin);
   for (CordRep* edge : tree->Edges()) CordRep::Ref(edge);
   return tree;
@@ -694,8 +706,7 @@ inline CordRepBtree* CordRepBtree::CopyBeginTo(size_t end,
                                                size_t new_length) const {
   assert(end <= capacity());
   assert(end >= this->begin());
-  CordRepBtree* tree = CopyRaw();
-  tree->length = new_length;
+  CordRepBtree* tree = CopyRaw(new_length);
   tree->set_end(end);
   for (CordRep* edge : tree->Edges()) CordRep::Ref(edge);
   return tree;