Introduce RTC_NO_UNIQUE_ADDRESS.

This macro introduces the possibility to suggest the compiler that a
data member doesn't need an address different from other non static
data members.

The usage of a macro is to maintain portability since at the moment
the attribute [[no_unique_address]] is only supported by clang
with at least -std=c++11 but it should be supported by all the
compilers starting from C++20.

Bug: webrtc:11495
Change-Id: I9f12b67b4422a2749649eaa6b004a67d5fd572d8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173331
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32246}

audio/BUILD.gn

@@ -92,6 +92,7 @@ rtc_library("audio") {
     "../rtc_base/experiments:field_trial_parser",
     "../rtc_base/synchronization:mutex",
     "../rtc_base/synchronization:sequence_checker",
+    "../rtc_base/system:no_unique_address",
     "../rtc_base/task_utils:to_queued_task",
     "../system_wrappers",
     "../system_wrappers:field_trial",

audio/channel_receive_frame_transformer_delegate.h

@@ -15,6 +15,7 @@
 
 #include "api/frame_transformer_interface.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 #include "rtc_base/thread.h"
 
@@ -61,7 +62,7 @@ class ChannelReceiveFrameTransformerDelegate : public TransformedFrameCallback {
   ~ChannelReceiveFrameTransformerDelegate() override = default;
 
  private:
-  SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequence_checker_;
   ReceiveFrameCallback receive_frame_callback_
       RTC_GUARDED_BY(sequence_checker_);
   rtc::scoped_refptr<FrameTransformerInterface> frame_transformer_

audio/voip/BUILD.gn

@@ -98,6 +98,7 @@ rtc_library("audio_egress") {
     "../../rtc_base:thread_checker",
     "../../rtc_base:timeutils",
     "../../rtc_base/synchronization:mutex",
+    "../../rtc_base/system:no_unique_address",
     "../utility:audio_frame_operations",
   ]
 }

call/BUILD.gn

@@ -222,6 +222,7 @@ rtc_library("bitrate_allocator") {
     "../rtc_base:rtc_base_approved",
     "../rtc_base:safe_minmax",
     "../rtc_base/synchronization:sequence_checker",
+    "../rtc_base/system:no_unique_address",
     "../system_wrappers",
     "../system_wrappers:field_trial",
     "../system_wrappers:metrics",
@@ -282,6 +283,7 @@ rtc_library("call") {
     "../rtc_base/experiments:field_trial_parser",
     "../rtc_base/network:sent_packet",
     "../rtc_base/synchronization:sequence_checker",
+    "../rtc_base/system:no_unique_address",
     "../rtc_base/task_utils:pending_task_safety_flag",
     "../system_wrappers",
     "../system_wrappers:field_trial",

call/bitrate_allocator.h

@@ -22,6 +22,7 @@
 #include "api/call/bitrate_allocation.h"
 #include "api/transport/network_types.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
@@ -148,7 +149,7 @@ class BitrateAllocator : public BitrateAllocatorInterface {
   // video send stream.
   static uint8_t GetTransmissionMaxBitrateMultiplier();
 
-  SequenceChecker sequenced_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequenced_checker_;
   LimitObserver* const limit_observer_ RTC_GUARDED_BY(&sequenced_checker_);
   // Stored in a list to keep track of the insertion order.
   std::vector<AllocatableTrack> allocatable_tracks_

call/call.cc

@@ -51,6 +51,7 @@
 #include "rtc_base/logging.h"
 #include "rtc_base/strings/string_builder.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_utils/pending_task_safety_flag.h"
 #include "rtc_base/thread_annotations.h"
 #include "rtc_base/time_utils.h"
@@ -532,7 +533,7 @@ class SharedModuleThread::Impl {
   }
 
  private:
-  SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequence_checker_;
   mutable int ref_count_ RTC_GUARDED_BY(sequence_checker_) = 0;
   std::unique_ptr<ProcessThread> const module_thread_;
   std::function<void()> const on_one_ref_remaining_;

call/call_factory.h

@@ -15,6 +15,7 @@
 #include "call/call.h"
 #include "call/call_config.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
@@ -27,7 +28,7 @@ class CallFactory : public CallFactoryInterface {
 
   Call* CreateCall(const CallConfig& config) override;
 
-  SequenceChecker call_thread_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker call_thread_;
   rtc::scoped_refptr<SharedModuleThread> module_thread_
       RTC_GUARDED_BY(call_thread_);
 };

logging/BUILD.gn

@@ -268,6 +268,7 @@ if (rtc_enable_protobuf) {
       "../rtc_base:rtc_task_queue",
       "../rtc_base:safe_minmax",
       "../rtc_base/synchronization:sequence_checker",
+      "../rtc_base/system:no_unique_address",
     ]
     absl_deps = [ "//third_party/abseil-cpp/absl/types:optional" ]
   }

logging/rtc_event_log/rtc_event_log_impl.h

@@ -24,6 +24,7 @@
 #include "api/task_queue/task_queue_factory.h"
 #include "logging/rtc_event_log/encoder/rtc_event_log_encoder.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 #include "rtc_base/thread_annotations.h"
 
@@ -78,7 +79,7 @@ class RtcEventLogImpl final : public RtcEventLog {
   int64_t last_output_ms_ RTC_GUARDED_BY(*task_queue_);
   bool output_scheduled_ RTC_GUARDED_BY(*task_queue_);
 
-  SequenceChecker logging_state_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker logging_state_checker_;
   bool logging_state_started_ RTC_GUARDED_BY(logging_state_checker_);
 
   // Since we are posting tasks bound to |this|,  it is critical that the event

media/BUILD.gn

@@ -194,6 +194,7 @@ rtc_library("rtc_simulcast_encoder_adapter") {
     "../rtc_base:rtc_base_approved",
     "../rtc_base/experiments:rate_control_settings",
     "../rtc_base/synchronization:sequence_checker",
+    "../rtc_base/system:no_unique_address",
     "../rtc_base/system:rtc_export",
     "../system_wrappers",
     "../system_wrappers:field_trial",

media/engine/simulcast_encoder_adapter.h

@@ -26,6 +26,7 @@
 #include "modules/video_coding/utility/framerate_controller.h"
 #include "rtc_base/atomic_ops.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
@@ -124,7 +125,7 @@ class RTC_EXPORT SimulcastEncoderAdapter : public VideoEncoder {
   EncodedImageCallback* encoded_complete_callback_;
 
   // Used for checking the single-threaded access of the encoder interface.
-  SequenceChecker encoder_queue_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker encoder_queue_;
 
   // Store encoders in between calls to Release and InitEncode, so they don't
   // have to be recreated. Remaining encoders are destroyed by the destructor.

modules/congestion_controller/rtp/BUILD.gn

@@ -31,6 +31,7 @@ rtc_library("control_handler") {
     "../../../rtc_base:checks",
     "../../../rtc_base:safe_minmax",
     "../../../rtc_base/synchronization:sequence_checker",
+    "../../../rtc_base/system:no_unique_address",
     "../../../system_wrappers:field_trial",
     "../../pacing",
   ]

modules/congestion_controller/rtp/control_handler.h

@@ -20,6 +20,7 @@
 #include "modules/pacing/paced_sender.h"
 #include "rtc_base/constructor_magic.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 // This is used to observe the network controller state and route calls to
@@ -46,7 +47,7 @@ class CongestionControlHandler {
   const bool disable_pacer_emergency_stop_;
   int64_t pacer_expected_queue_ms_ = 0;
 
-  SequenceChecker sequenced_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequenced_checker_;
   RTC_DISALLOW_COPY_AND_ASSIGN(CongestionControlHandler);
 };
 }  // namespace webrtc

modules/rtp_rtcp/BUILD.gn

@@ -294,6 +294,7 @@ rtc_library("rtp_rtcp") {
     "../../rtc_base/experiments:field_trial_parser",
     "../../rtc_base/synchronization:mutex",
     "../../rtc_base/synchronization:sequence_checker",
+    "../../rtc_base/system:no_unique_address",
     "../../rtc_base/task_utils:pending_task_safety_flag",
     "../../rtc_base/task_utils:repeating_task",
     "../../rtc_base/task_utils:to_queued_task",

modules/rtp_rtcp/include/flexfec_receiver.h

@@ -20,6 +20,7 @@
 #include "modules/rtp_rtcp/source/forward_error_correction.h"
 #include "modules/rtp_rtcp/source/rtp_packet_received.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/thread_annotations.h"
 
 namespace webrtc {
@@ -69,7 +70,7 @@ class FlexfecReceiver {
   int64_t last_recovered_packet_ms_ RTC_GUARDED_BY(sequence_checker_);
   FecPacketCounter packet_counter_ RTC_GUARDED_BY(sequence_checker_);
 
-  SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequence_checker_;
 };
 
 }  // namespace webrtc

modules/rtp_rtcp/source/rtp_rtcp_impl2.h

@@ -37,6 +37,7 @@
 #include "rtc_base/gtest_prod_util.h"
 #include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_utils/pending_task_safety_flag.h"
 #include "rtc_base/task_utils/repeating_task.h"
 
@@ -292,7 +293,7 @@ class ModuleRtpRtcpImpl2 final : public RtpRtcpInterface,
   void PeriodicUpdate();
 
   TaskQueueBase* const worker_queue_;
-  SequenceChecker process_thread_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker process_thread_checker_;
 
   std::unique_ptr<RtpSenderContext> rtp_sender_;
 

modules/rtp_rtcp/source/rtp_sender_egress.h

@@ -30,6 +30,7 @@
 #include "rtc_base/rate_statistics.h"
 #include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_utils/pending_task_safety_flag.h"
 #include "rtc_base/task_utils/repeating_task.h"
 #include "rtc_base/thread_annotations.h"
@@ -127,7 +128,7 @@ class RtpSenderEgress {
   void PeriodicUpdate();
 
   TaskQueueBase* const worker_queue_;
-  SequenceChecker pacer_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker pacer_checker_;
   const uint32_t ssrc_;
   const absl::optional<uint32_t> rtx_ssrc_;
   const absl::optional<uint32_t> flexfec_ssrc_;

modules/video_coding/BUILD.gn

@@ -110,6 +110,7 @@ rtc_library("video_coding") {
     "../../api/video:video_bitrate_allocation",
     "../../api/video:video_bitrate_allocator_factory",
     "../../rtc_base:deprecation",
+    "../../rtc_base/system:no_unique_address",
     "../../rtc_base/task_utils:to_queued_task",
     "../../system_wrappers:field_trial",
     "../../system_wrappers:metrics",
@@ -353,6 +354,7 @@ rtc_library("video_coding_utility") {
     "../../rtc_base/synchronization:sequence_checker",
     "../../rtc_base/system:arch",
     "../../rtc_base/system:file_wrapper",
+    "../../rtc_base/system:no_unique_address",
     "../../rtc_base/task_utils:repeating_task",
     "../../rtc_base/task_utils:to_queued_task",
     "../../system_wrappers:field_trial",
@@ -701,6 +703,7 @@ if (rtc_include_tests) {
       "../../rtc_base:rtc_task_queue",
       "../../rtc_base/synchronization:mutex",
       "../../rtc_base/synchronization:sequence_checker",
+      "../../rtc_base/system:no_unique_address",
       "../../rtc_base/task_utils:to_queued_task",
       "../../test:test_support",
       "../../test:video_test_common",

modules/video_coding/codecs/test/videoprocessor.h

@@ -37,6 +37,7 @@
 #include "rtc_base/checks.h"
 #include "rtc_base/constructor_magic.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/thread_annotations.h"
 #include "rtc_base/thread_checker.h"
 #include "test/testsupport/frame_reader.h"
@@ -249,7 +250,7 @@ class VideoProcessor {
   int64_t post_encode_time_ns_ RTC_GUARDED_BY(sequence_checker_);
 
   // This class must be operated on a TaskQueue.
-  SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequence_checker_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(VideoProcessor);
 };

modules/video_coding/frame_buffer2.h

@@ -28,6 +28,7 @@
 #include "rtc_base/numerics/sequence_number_util.h"
 #include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 #include "rtc_base/task_utils/repeating_task.h"
 #include "rtc_base/thread_annotations.h"
@@ -159,8 +160,8 @@ class FrameBuffer {
   EncodedFrame* CombineAndDeleteFrames(
       const std::vector<EncodedFrame*>& frames) const;
 
-  SequenceChecker construction_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker construction_checker_;
-  SequenceChecker callback_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker callback_checker_;
 
   // Stores only undecoded frames.
   FrameMap frames_ RTC_GUARDED_BY(mutex_);

modules/video_coding/loss_notification_controller.h

@@ -19,6 +19,7 @@
 #include "api/array_view.h"
 #include "modules/include/module_common_types.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
@@ -102,7 +103,7 @@ class LossNotificationController {
   // (Naturally, later frames must also be assemblable to be decodable.)
   std::set<int64_t> decodable_frame_ids_ RTC_GUARDED_BY(sequence_checker_);
 
-  SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequence_checker_;
 };
 
 }  // namespace webrtc

modules/video_coding/utility/quality_scaler.h

@@ -24,6 +24,7 @@
 #include "rtc_base/ref_count.h"
 #include "rtc_base/ref_counted_object.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 
 namespace webrtc {
@@ -82,7 +83,7 @@ class QualityScaler {
   std::unique_ptr<CheckQpTask> pending_qp_task_ RTC_GUARDED_BY(&task_checker_);
   QualityScalerQpUsageHandlerInterface* const handler_
       RTC_GUARDED_BY(&task_checker_);
-  SequenceChecker task_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker task_checker_;
 
   VideoEncoder::QpThresholds thresholds_ RTC_GUARDED_BY(&task_checker_);
   const int64_t sampling_period_ms_;

p2p/BUILD.gn

@@ -100,6 +100,7 @@ rtc_library("rtc_p2p") {
     "../rtc_base:rtc_numerics",
     "../rtc_base/experiments:field_trial_parser",
     "../rtc_base/synchronization:sequence_checker",
+    "../rtc_base/system:no_unique_address",
 
     # Needed by pseudo_tcp, which should move to a separate target.
     "../rtc_base:safe_minmax",

p2p/base/dtls_transport.h

@@ -25,6 +25,7 @@
 #include "rtc_base/stream.h"
 #include "rtc_base/strings/string_builder.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/thread_checker.h"
 
 namespace rtc {
@@ -55,7 +56,7 @@ class StreamInterfaceChannel : public rtc::StreamInterface {
                           int* error) override;
 
  private:
-  webrtc::SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS webrtc::SequenceChecker sequence_checker_;
   IceTransportInternal* const ice_transport_;  // owned by DtlsTransport
   rtc::StreamState state_ RTC_GUARDED_BY(sequence_checker_);
   rtc::BufferQueue packets_ RTC_GUARDED_BY(sequence_checker_);

pc/BUILD.gn

@@ -273,6 +273,7 @@ rtc_library("peerconnection") {
     "../rtc_base/synchronization:mutex",
     "../rtc_base/synchronization:sequence_checker",
     "../rtc_base/system:file_wrapper",
+    "../rtc_base/system:no_unique_address",
     "../rtc_base/system:rtc_export",
     "../rtc_base/task_utils:pending_task_safety_flag",
     "../rtc_base/task_utils:to_queued_task",

pc/video_rtp_track_source.h

@@ -17,6 +17,7 @@
 #include "pc/video_track_source.h"
 #include "rtc_base/callback.h"
 #include "rtc_base/synchronization/mutex.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
@@ -67,7 +68,7 @@ class VideoRtpTrackSource : public VideoTrackSource {
       rtc::VideoSinkInterface<RecordableEncodedFrame>* sink) override;
 
  private:
-  SequenceChecker worker_sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker worker_sequence_checker_;
   // |broadcaster_| is needed since the decoder can only handle one sink.
   // It might be better if the decoder can handle multiple sinks and consider
   // the VideoSinkWants.

rtc_base/BUILD.gn

@@ -75,6 +75,7 @@ rtc_library("rtc_base_approved") {
     "../api:scoped_refptr",
     "synchronization:mutex",
     "system:arch",
+    "system:no_unique_address",
     "system:rtc_export",
     "system:unused",
     "third_party/base64",
@@ -479,6 +480,7 @@ rtc_source_set("rtc_operations_chain") {
     ":refcount",
     "../api:scoped_refptr",
     "synchronization:sequence_checker",
+    "system:no_unique_address",
   ]
   absl_deps = [ "//third_party/abseil-cpp/absl/types:optional" ]
 }
@@ -582,6 +584,7 @@ rtc_library("weak_ptr") {
     ":refcount",
     "../api:scoped_refptr",
     "synchronization:sequence_checker",
+    "system:no_unique_address",
   ]
 }
 
@@ -830,6 +833,7 @@ rtc_library("rtc_base") {
     "synchronization:sequence_checker",
     "system:file_wrapper",
     "system:inline",
+    "system:no_unique_address",
     "system:rtc_export",
     "task_utils:pending_task_safety_flag",
     "task_utils:to_queued_task",

rtc_base/buffer_queue.h

@@ -19,6 +19,7 @@
 #include "rtc_base/buffer.h"
 #include "rtc_base/constructor_magic.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/thread_annotations.h"
 
 namespace rtc {
@@ -55,7 +56,7 @@ class BufferQueue final {
   }
 
  private:
-  webrtc::SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS webrtc::SequenceChecker sequence_checker_;
   const size_t capacity_;
   const size_t default_size_;
   std::deque<Buffer*> queue_ RTC_GUARDED_BY(sequence_checker_);

rtc_base/net_helpers.h

@@ -23,6 +23,7 @@
 #include "rtc_base/ip_address.h"
 #include "rtc_base/socket_address.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/system/rtc_export.h"
 #include "rtc_base/task_utils/pending_task_safety_flag.h"
 #include "rtc_base/thread.h"
@@ -62,7 +63,7 @@ class RTC_EXPORT AsyncResolver : public AsyncResolverInterface {
   bool recursion_check_ =
       false;  // Protects against SignalDone calling into Destroy.
   bool destroy_called_ = false;
-  webrtc::SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS webrtc::SequenceChecker sequence_checker_;
 };
 
 // rtc namespaced wrappers for inet_ntop and inet_pton so we can avoid

rtc_base/operations_chain.h

@@ -25,6 +25,7 @@
 #include "rtc_base/ref_count.h"
 #include "rtc_base/ref_counted_object.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace rtc {
 
@@ -179,7 +180,7 @@ class OperationsChain final : public RefCountedObject<RefCountInterface> {
   std::function<void()> CreateOperationsChainCallback();
   void OnOperationComplete();
 
-  webrtc::SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS webrtc::SequenceChecker sequence_checker_;
   // FIFO-list of operations that are chained. An operation that is executing
   // remains on this list until it has completed by invoking the callback passed
   // to it.

rtc_base/system/BUILD.gn

@@ -55,6 +55,10 @@ rtc_source_set("rtc_export") {
   ]
 }
 
+rtc_source_set("no_unique_address") {
+  sources = [ "no_unique_address.h" ]
+}
+
 if (is_mac || is_ios) {
   rtc_library("cocoa_threading") {
     sources = [

rtc_base/system/no_unique_address.h

@@ -0,0 +1,35 @@
+/*
+ *  Copyright (c) 2020 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef RTC_BASE_SYSTEM_NO_UNIQUE_ADDRESS_H_
+#define RTC_BASE_SYSTEM_NO_UNIQUE_ADDRESS_H_
+
+// RTC_NO_UNIQUE_ADDRESS is a portable annotation to tell the compiler that
+// a data member need not have an address distinct from all other non-static
+// data members of its class.
+// It allows empty types to actually occupy zero bytes as class members,
+// instead of occupying at least one byte just so that they get their own
+// address. There is almost never any reason not to use it on class members
+// that could possibly be empty.
+// The macro expands to [[no_unique_address]] if the compiler supports the
+// attribute, it expands to nothing otherwise.
+// Clang should supports this attribute since C++11, while other compilers
+// should add support for it starting from C++20. Among clang compilers,
+// clang-cl doesn't support it yet and support is unclear also when the target
+// platform is iOS.
+#if (defined(__clang__) && !defined(_MSC_VER) && !defined(WEBRTC_IOS)) || \
+    __cplusplus > 201703L
+// NOLINTNEXTLINE(whitespace/braces)
+#define RTC_NO_UNIQUE_ADDRESS [[no_unique_address]]
+#else
+#define RTC_NO_UNIQUE_ADDRESS
+#endif
+
+#endif  // RTC_BASE_SYSTEM_NO_UNIQUE_ADDRESS_H_

rtc_base/task_utils/BUILD.gn

@@ -38,6 +38,7 @@ rtc_library("pending_task_safety_flag") {
     "..:thread_checker",
     "../../api:scoped_refptr",
     "../synchronization:sequence_checker",
+    "../system:no_unique_address",
   ]
 }
 

rtc_base/task_utils/pending_task_safety_flag.h

@@ -15,6 +15,7 @@
 #include "rtc_base/checks.h"
 #include "rtc_base/ref_count.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
@@ -58,7 +59,7 @@ class PendingTaskSafetyFlag : public rtc::RefCountInterface {
 
  private:
   bool alive_ = true;
-  SequenceChecker main_sequence_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker main_sequence_;
 };
 
 // Makes using PendingTaskSafetyFlag very simple. Automatic PTSF creation

rtc_base/weak_ptr.h

@@ -18,6 +18,7 @@
 #include "rtc_base/ref_count.h"
 #include "rtc_base/ref_counted_object.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 // The implementation is borrowed from chromium except that it does not
 // implement SupportsWeakPtr.
@@ -103,7 +104,7 @@ class WeakReference {
 
     ~Flag() override;
 
-    ::webrtc::SequenceChecker checker_;
+    RTC_NO_UNIQUE_ADDRESS ::webrtc::SequenceChecker checker_;
     bool is_valid_;
   };
 

rtc_tools/network_tester/BUILD.gn

@@ -50,6 +50,7 @@ if (rtc_enable_protobuf) {
       "../../rtc_base:rtc_task_queue",
       "../../rtc_base/synchronization:mutex",
       "../../rtc_base/synchronization:sequence_checker",
+      "../../rtc_base/system:no_unique_address",
       "../../rtc_base/third_party/sigslot",
     ]
     absl_deps = [ "//third_party/abseil-cpp/absl/types:optional" ]

rtc_tools/network_tester/packet_sender.h

@@ -18,6 +18,7 @@
 #include "rtc_base/constructor_magic.h"
 #include "rtc_base/ignore_wundef.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 
 #ifdef WEBRTC_NETWORK_TESTER_PROTO
@@ -49,7 +50,7 @@ class PacketSender {
   void UpdateTestSetting(size_t packet_size, int64_t send_interval_ms);
 
  private:
-  SequenceChecker worker_queue_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker worker_queue_checker_;
   size_t packet_size_ RTC_GUARDED_BY(worker_queue_checker_);
   int64_t send_interval_ms_ RTC_GUARDED_BY(worker_queue_checker_);
   int64_t sequence_number_ RTC_GUARDED_BY(worker_queue_checker_);

video/BUILD.gn

@@ -130,6 +130,7 @@ rtc_library("video") {
     "../rtc_base/experiments:rate_control_settings",
     "../rtc_base/synchronization:mutex",
     "../rtc_base/synchronization:sequence_checker",
+    "../rtc_base/system:no_unique_address",
     "../rtc_base/system:thread_registry",
     "../rtc_base/task_utils:pending_task_safety_flag",
     "../rtc_base/task_utils:repeating_task",
@@ -255,6 +256,7 @@ rtc_library("video_stream_encoder_impl") {
     "../rtc_base/experiments:rate_control_settings",
     "../rtc_base/synchronization:mutex",
     "../rtc_base/synchronization:sequence_checker",
+    "../rtc_base/system:no_unique_address",
     "../rtc_base/task_utils:pending_task_safety_flag",
     "../rtc_base/task_utils:repeating_task",
     "../system_wrappers",

video/adaptation/BUILD.gn

@@ -53,6 +53,7 @@ rtc_library("video_adaptation") {
     "../../rtc_base/experiments:quality_scaler_settings",
     "../../rtc_base/synchronization:mutex",
     "../../rtc_base/synchronization:sequence_checker",
+    "../../rtc_base/system:no_unique_address",
     "../../rtc_base/task_utils:repeating_task",
     "../../rtc_base/task_utils:to_queued_task",
     "../../system_wrappers:field_trial",

video/adaptation/balanced_constraint.h

@@ -18,6 +18,7 @@
 #include "call/adaptation/degradation_preference_provider.h"
 #include "rtc_base/experiments/balanced_degradation_settings.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
@@ -38,7 +39,7 @@ class BalancedConstraint : public AdaptationConstraint {
       const VideoSourceRestrictions& restrictions_after) const override;
 
  private:
-  SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequence_checker_;
   absl::optional<uint32_t> encoder_target_bitrate_bps_
       RTC_GUARDED_BY(&sequence_checker_);
   const BalancedDegradationSettings balanced_settings_;

video/adaptation/bitrate_constraint.h

@@ -19,6 +19,7 @@
 #include "call/adaptation/video_source_restrictions.h"
 #include "call/adaptation/video_stream_input_state.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
@@ -40,7 +41,7 @@ class BitrateConstraint : public AdaptationConstraint {
       const VideoSourceRestrictions& restrictions_after) const override;
 
  private:
-  SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequence_checker_;
   absl::optional<EncoderSettings> encoder_settings_
       RTC_GUARDED_BY(&sequence_checker_);
   absl::optional<uint32_t> encoder_target_bitrate_bps_

video/adaptation/overuse_frame_detector.h

@@ -21,6 +21,7 @@
 #include "rtc_base/experiments/field_trial_parser.h"
 #include "rtc_base/numerics/exp_filter.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_utils/repeating_task.h"
 #include "rtc_base/thread_annotations.h"
 
@@ -134,7 +135,7 @@ class OveruseFrameDetector {
   static std::unique_ptr<ProcessingUsage> CreateProcessingUsage(
       const CpuOveruseOptions& options);
 
-  SequenceChecker task_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker task_checker_;
   // Owned by the task queue from where StartCheckForOveruse is called.
   RepeatingTaskHandle check_overuse_task_ RTC_GUARDED_BY(task_checker_);
 

video/call_stats2.h

@@ -19,6 +19,7 @@
 #include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "rtc_base/constructor_magic.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 #include "rtc_base/task_utils/pending_task_safety_flag.h"
 #include "rtc_base/task_utils/repeating_task.h"
@@ -132,8 +133,8 @@ class CallStats {
   // for the observers_ list, which makes the most common case lock free.
   std::list<CallStatsObserver*> observers_;
 
-  SequenceChecker construction_thread_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker construction_thread_checker_;
-  SequenceChecker process_thread_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker process_thread_checker_;
   TaskQueueBase* const task_queue_;
 
   // Used to signal destruction to potentially pending tasks.

video/receive_statistics_proxy2.h

@@ -28,6 +28,7 @@
 #include "rtc_base/rate_statistics.h"
 #include "rtc_base/rate_tracker.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_utils/pending_task_safety_flag.h"
 #include "rtc_base/thread_annotations.h"
 #include "rtc_base/thread_checker.h"
@@ -213,9 +214,9 @@ class ReceiveStatisticsProxy : public VCMReceiveStatisticsCallback,
 
   ScopedTaskSafety task_safety_;
 
-  SequenceChecker decode_queue_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker decode_queue_;
   rtc::ThreadChecker main_thread_;
-  SequenceChecker incoming_render_queue_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker incoming_render_queue_;
 };
 
 }  // namespace internal

video/rtp_streams_synchronizer2.h

@@ -14,6 +14,7 @@
 #include <memory>
 
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 #include "rtc_base/task_utils/repeating_task.h"
 #include "video/stream_synchronization.h"
@@ -54,7 +55,7 @@ class RtpStreamsSynchronizer {
   // we might be running on an rtc::Thread implementation of TaskQueue, which
   // does not consistently set itself as the active TaskQueue.
   // Instead, we rely on a SequenceChecker for now.
-  SequenceChecker main_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker main_checker_;
 
   Syncable* const syncable_video_;
 

video/rtp_video_stream_receiver.h

@@ -46,6 +46,7 @@
 #include "rtc_base/numerics/sequence_number_util.h"
 #include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/thread_annotations.h"
 #include "rtc_base/thread_checker.h"
 #include "video/buffered_frame_decryptor.h"
@@ -323,7 +324,7 @@ class RtpVideoStreamReceiver : public LossNotificationSender,
   ReceiveStatistics* const rtp_receive_statistics_;
   std::unique_ptr<UlpfecReceiver> ulpfec_receiver_;
 
-  SequenceChecker worker_task_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker worker_task_checker_;
   bool receiving_ RTC_GUARDED_BY(worker_task_checker_);
   int64_t last_packet_log_ms_ RTC_GUARDED_BY(worker_task_checker_);
 

video/rtp_video_stream_receiver2.h

@@ -43,6 +43,7 @@
 #include "rtc_base/experiments/field_trial_parser.h"
 #include "rtc_base/numerics/sequence_number_util.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/thread_annotations.h"
 #include "video/buffered_frame_decryptor.h"
 #include "video/rtp_video_stream_receiver_frame_transformer_delegate.h"
@@ -233,7 +234,7 @@ class RtpVideoStreamReceiver2 : public LossNotificationSender,
       bool decodability_flag;
     };
 
-    SequenceChecker worker_task_checker_;
+    RTC_NO_UNIQUE_ADDRESS SequenceChecker worker_task_checker_;
     KeyFrameRequestSender* const key_frame_request_sender_;
     NackSender* const nack_sender_;
     LossNotificationSender* const loss_notification_sender_;
@@ -286,7 +287,7 @@ class RtpVideoStreamReceiver2 : public LossNotificationSender,
   ReceiveStatistics* const rtp_receive_statistics_;
   std::unique_ptr<UlpfecReceiver> ulpfec_receiver_;
 
-  SequenceChecker worker_task_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker worker_task_checker_;
   bool receiving_ RTC_GUARDED_BY(worker_task_checker_);
   int64_t last_packet_log_ms_ RTC_GUARDED_BY(worker_task_checker_);
 

video/rtp_video_stream_receiver_frame_transformer_delegate.h

@@ -16,6 +16,7 @@
 #include "api/frame_transformer_interface.h"
 #include "modules/video_coding/frame_object.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/thread.h"
 
 namespace webrtc {
@@ -61,7 +62,7 @@ class RtpVideoStreamReceiverFrameTransformerDelegate
   ~RtpVideoStreamReceiverFrameTransformerDelegate() override = default;
 
  private:
-  SequenceChecker network_sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker network_sequence_checker_;
   RtpVideoFrameReceiver* receiver_ RTC_GUARDED_BY(network_sequence_checker_);
   rtc::scoped_refptr<FrameTransformerInterface> frame_transformer_
       RTC_GUARDED_BY(network_sequence_checker_);

video/video_receive_stream.h

@@ -25,6 +25,7 @@
 #include "modules/video_coding/video_receiver2.h"
 #include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 #include "system_wrappers/include/clock.h"
 #include "video/receive_statistics_proxy.h"
@@ -150,9 +151,9 @@ class VideoReceiveStream : public webrtc::VideoReceiveStream,
 
   void UpdateHistograms();
 
-  SequenceChecker worker_sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker worker_sequence_checker_;
-  SequenceChecker module_process_sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker module_process_sequence_checker_;
-  SequenceChecker network_sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker network_sequence_checker_;
 
   TaskQueueFactory* const task_queue_factory_;
 

video/video_receive_stream2.h

@@ -25,6 +25,7 @@
 #include "modules/video_coding/frame_buffer2.h"
 #include "modules/video_coding/video_receiver2.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 #include "rtc_base/task_utils/pending_task_safety_flag.h"
 #include "system_wrappers/include/clock.h"
@@ -177,8 +178,8 @@ class VideoReceiveStream2 : public webrtc::VideoReceiveStream,
 
   void UpdateHistograms();
 
-  SequenceChecker worker_sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker worker_sequence_checker_;
-  SequenceChecker module_process_sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker module_process_sequence_checker_;
 
   TaskQueueFactory* const task_queue_factory_;
 

video/video_source_sink_controller.h

@@ -19,6 +19,7 @@
 #include "api/video/video_source_interface.h"
 #include "call/adaptation/video_source_restrictions.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 
 namespace webrtc {
 
@@ -62,7 +63,7 @@ class VideoSourceSinkController {
   // Used to ensure that this class is called on threads/sequences that it and
   // downstream implementations were designed for.
   // In practice, this represent's libjingle's worker thread.
-  SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequence_checker_;
 
   rtc::VideoSinkInterface<VideoFrame>* const sink_;
   rtc::VideoSourceInterface<VideoFrame>* source_

video/video_stream_encoder.cc

@@ -41,6 +41,7 @@
 #include "rtc_base/logging.h"
 #include "rtc_base/strings/string_builder.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/thread_annotations.h"
 #include "rtc_base/trace_event.h"
 #include "system_wrappers/include/field_trial.h"
@@ -274,7 +275,7 @@ class VideoStreamEncoder::DegradationPreferenceManager
     }
   }
 
-  SequenceChecker sequence_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker sequence_checker_;
   DegradationPreference degradation_preference_
       RTC_GUARDED_BY(&sequence_checker_);
   bool is_screenshare_ RTC_GUARDED_BY(&sequence_checker_);