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-rw-r--r--absl/strings/internal/cord_internal.h7
-rw-r--r--absl/strings/internal/cord_rep_btree.cc2
-rw-r--r--absl/strings/internal/cord_rep_btree.h35
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;