Add the RawPtr class, which is used to enable easily keeping a raw pointer to an object that is by default all zero bytes.

This class enables storing all such default objects in the same default zero buffer.

The motivation for this is usage in split repeated fields.

PiperOrigin-RevId: 543465667

src/google/protobuf/BUILD.bazel

@@ -343,6 +343,7 @@ cc_library(
         "map.cc",
         "message_lite.cc",
         "parse_context.cc",
+        "raw_ptr.cc",
         "repeated_field.cc",
         "repeated_ptr_field.cc",
         "wire_format_lite.cc",
@@ -371,6 +372,7 @@ cc_library(
         "metadata_lite.h",
         "parse_context.h",
         "port.h",
+        "raw_ptr.h",
         "repeated_field.h",
         "repeated_ptr_field.h",
         "serial_arena.h",

src/google/protobuf/raw_ptr.cc

@@ -0,0 +1,48 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "google/protobuf/raw_ptr.h"
+
+#include "absl/base/attributes.h"
+#include "absl/base/optimization.h"
+
+// Must be included last.
+#include "google/protobuf/port_def.inc"
+
+namespace google {
+namespace protobuf {
+namespace internal {
+
+ABSL_CONST_INIT PROTOBUF_EXPORT
+    ABSL_CACHELINE_ALIGNED const char kZeroBuffer[ABSL_CACHELINE_SIZE] = {};
+
+}  // namespace internal
+}  // namespace protobuf
+}  // namespace google

src/google/protobuf/raw_ptr.h

@@ -0,0 +1,81 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef GOOGLE_PROTOBUF_RAW_PTR_H__
+#define GOOGLE_PROTOBUF_RAW_PTR_H__
+
+#include <algorithm>
+
+#include "absl/base/optimization.h"
+
+// Must be included last.
+#include "google/protobuf/port_def.inc"
+
+namespace google {
+namespace protobuf {
+namespace internal {
+
+PROTOBUF_EXPORT ABSL_CACHELINE_ALIGNED extern const char
+    kZeroBuffer[std::max(ABSL_CACHELINE_SIZE, 64)];
+
+// This class is trivially copyable/trivially destructible and constexpr
+// constructible. The class allows for storing a raw pointer to a non-trivial
+// object in a constexpr context.
+template <typename T>
+class RawPtr {
+ public:
+  constexpr RawPtr() : RawPtr(kZeroBuffer) {
+    static_assert(sizeof(T) <= sizeof(kZeroBuffer), "");
+    static_assert(alignof(T) <= ABSL_CACHELINE_SIZE, "");
+  }
+  explicit constexpr RawPtr(const void* p) : p_(const_cast<void*>(p)) {}
+
+  bool IsDefault() const { return p_ == kZeroBuffer; }
+
+  void Set(const void* p) { p_ = const_cast<void*>(p); }
+  T* Get() const { return reinterpret_cast<T*>(p_); }
+  T* operator->() const { return Get(); }
+  T& operator*() const { return *Get(); }
+
+ private:
+  void* p_;
+};
+
+constexpr void* DefaultRawPtr() {
+  return const_cast<void*>(static_cast<const void*>(kZeroBuffer));
+}
+
+}  // namespace internal
+}  // namespace protobuf
+}  // namespace google
+
+#include "google/protobuf/port_undef.inc"
+
+#endif  // GOOGLE_PROTOBUF_RAW_PTR_H__

src/google/protobuf/raw_ptr_test.cc

@@ -0,0 +1,88 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "google/protobuf/raw_ptr.h"
+
+#include <cstdint>
+
+#include <gtest/gtest.h>
+#include "absl/base/optimization.h"
+
+namespace google {
+namespace protobuf {
+namespace internal {
+namespace {
+
+TEST(ZeroCacheline, Basic) {
+  for (int i = 0; i < ABSL_CACHELINE_SIZE; ++i) {
+    EXPECT_EQ(kZeroBuffer[i], 0) << i;
+  }
+  EXPECT_EQ(reinterpret_cast<uintptr_t>(kZeroBuffer) % ABSL_CACHELINE_SIZE, 0);
+}
+
+struct Obj {
+  int i;
+};
+
+TEST(RawPtr, Basic) {
+  RawPtr<Obj> raw;
+  EXPECT_EQ(raw->i, 0);
+  EXPECT_EQ((*raw).i, 0);
+  EXPECT_EQ(static_cast<void*>(&raw->i), kZeroBuffer);
+  EXPECT_EQ(static_cast<void*>(raw.Get()), kZeroBuffer);
+  EXPECT_TRUE(raw.IsDefault());
+
+  Obj obj = {1};
+  raw.Set(&obj);
+  EXPECT_EQ(raw->i, 1);
+  EXPECT_EQ(static_cast<void*>(raw.Get()), &obj);
+  EXPECT_FALSE(raw.IsDefault());
+}
+
+TEST(RawPtr, Constexpr) {
+  constexpr RawPtr<Obj> raw;
+  EXPECT_EQ(raw->i, 0);
+  EXPECT_EQ((*raw).i, 0);
+  EXPECT_EQ(static_cast<void*>(&raw->i), kZeroBuffer);
+  EXPECT_EQ(static_cast<void*>(raw.Get()), kZeroBuffer);
+  EXPECT_TRUE(raw.IsDefault());
+
+  static constexpr Obj obj = {1};
+  constexpr RawPtr<Obj> raw2(&obj);
+  EXPECT_EQ(raw2->i, 1);
+  EXPECT_EQ((*raw2).i, 1);
+  EXPECT_EQ(static_cast<void*>(raw2.Get()), &obj);
+  EXPECT_FALSE(raw2.IsDefault());
+}
+
+}  // namespace
+}  // namespace internal
+}  // namespace protobuf
+}  // namespace google