Optimize `Cord::Swap()` for missed compiler optimization in clang.

PiperOrigin-RevId: 603342563
Change-Id: I1cd80103377f457770d5178dad8b56ae459cbd55

absl/strings/cord.h

@@ -1170,7 +1170,8 @@ inline void Cord::InlineRep::Swap(absl::Nonnull<Cord::InlineRep*> rhs) {
   if (rhs == this) {
     return;
   }
-  std::swap(data_, rhs->data_);
+  using std::swap;
+  swap(data_, rhs->data_);
 }
 
 inline absl::Nullable<const char*> Cord::InlineRep::data() const {

absl/strings/internal/cord_internal.h

@@ -520,6 +520,7 @@ class InlineData {
 
   constexpr InlineData(const InlineData& rhs) noexcept;
   InlineData& operator=(const InlineData& rhs) noexcept;
+  friend void swap(InlineData& lhs, InlineData& rhs) noexcept;
 
   friend bool operator==(const InlineData& lhs, const InlineData& rhs) {
 #ifdef ABSL_INTERNAL_CORD_HAVE_SANITIZER
@@ -770,6 +771,12 @@ class InlineData {
       char data[kMaxInline + 1];
       AsTree as_tree;
     };
+
+    // TODO(b/145829486): see swap(InlineData, InlineData) for more info.
+    inline void SwapValue(Rep rhs, Rep& refrhs) {
+      memcpy(&refrhs, this, sizeof(*this));
+      memcpy(this, &rhs, sizeof(*this));
+    }
   };
 
   // Private implementation of `Compare()`
@@ -884,6 +891,19 @@ inline void CordRep::Unref(CordRep* rep) {
   }
 }
 
+inline void swap(InlineData& lhs, InlineData& rhs) noexcept {
+  lhs.unpoison();
+  rhs.unpoison();
+  // TODO(b/145829486): `std::swap(lhs.rep_, rhs.rep_)` results in bad codegen
+  // on clang, spilling the temporary swap value on the stack. Since `Rep` is
+  // trivial, we can make clang DTRT by calling a hand-rolled `SwapValue` where
+  // we pass `rhs` both by value (register allocated) and by reference. The IR
+  // then folds and inlines correctly into an optimized swap without spill.
+  lhs.rep_.SwapValue(rhs.rep_, rhs.rep_);
+  rhs.poison();
+  lhs.poison();
+}
+
 }  // namespace cord_internal
 
 ABSL_NAMESPACE_END