Use __builtin_operator_new to faciliate compiler optimizations of these allocs.

PiperOrigin-RevId: 815794039

Author: ckennelly

src/google/protobuf/message_lite.h

@@ -33,6 +33,7 @@
 #include <utility>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/log/absl_check.h"
 #include "absl/numeric/bits.h"
 #include "absl/strings/cord.h"
@@ -1445,11 +1446,19 @@ template <typename MessageLite>
 PROTOBUF_ALWAYS_INLINE MessageLite* MessageCreator::New(
     const MessageLite* prototype_for_func,
     const MessageLite* prototype_for_copy, Arena* arena) const {
-  return PlacementNew(prototype_for_func, prototype_for_copy,
-                      arena != nullptr
-                          ? arena->AllocateAligned(allocation_size_)
-                          : ::operator new(allocation_size_),
-                      arena);
+  void* mem;
+  if (arena != nullptr) {
+    mem = arena->AllocateAligned(allocation_size_);
+  } else {
+#if ABSL_HAVE_BUILTIN(__builtin_operator_new)
+    // Allows the compiler to merge or optimize away the allocation even if it
+    // would violate the observability guarantees of ::operator new.
+    mem = __builtin_operator_new(allocation_size_);
+#else
+    mem = ::operator new(allocation_size_);
+#endif
+  }
+  return PlacementNew(prototype_for_func, prototype_for_copy, mem, arena);
 }
 
 }  // namespace internal