diff --git a/csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs b/csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs
deleted file mode 100644
index 208ce1f..0000000
--- a/csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs
+++ /dev/null
@@ -1,17 +0,0 @@
-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2008 Google Inc.  All rights reserved.
-//
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file or at
-// https://developers.google.com/open-source/licenses/bsd
-#endregion
-
-namespace Google.Protobuf.Reflection;
-
-internal sealed partial class FeatureSetDescriptor
-{
-    // Canonical serialized form of the edition defaults, generated by embed_edition_defaults.
-    private const string DefaultsBase64 =
-        "ChMYhAciACoMCAEQAhgCIAMoATACChMY5wciACoMCAIQARgBIAIoATABChMY6AciDAgBEAEYASACKAEwASoAIOYHKOgH";
-}
diff --git a/upb/message/BUILD b/upb/message/BUILD
index 43d391d..d7b83a1 100644
--- a/upb/message/BUILD
+++ b/upb/message/BUILD
@@ -57,7 +57,6 @@
 cc_library(
     name = "internal",
     srcs = [
-        "internal/compare_unknown.c",
         "internal/extension.c",
         "internal/message.c",
         "value.h",
@@ -65,7 +64,6 @@
     hdrs = [
         "internal/accessors.h",
         "internal/array.h",
-        "internal/compare_unknown.h",
         "internal/extension.h",
         "internal/map.h",
         "internal/map_sorter.h",
@@ -110,9 +108,11 @@
     name = "compare",
     srcs = [
         "compare.c",
+        "internal/compare_unknown.c",
     ],
     hdrs = [
         "compare.h",
+        "internal/compare_unknown.h",
     ],
     copts = UPB_DEFAULT_COPTS,
     visibility = ["//visibility:public"],
@@ -121,8 +121,11 @@
         ":iterator",
         ":message",
         "//upb:base",
+        "//upb:eps_copy_input_stream",
+        "//upb:mem",
         "//upb:mini_table",
         "//upb:port",
+        "//upb:wire_reader",
         "//upb/mini_table:internal",
     ],
 )
@@ -288,10 +291,16 @@
     name = "compare_unknown_test",
     srcs = ["internal/compare_unknown_test.cc"],
     deps = [
+        ":compare",
         ":internal",
+        ":message",
+        "//upb:base",
+        "//upb:mem",
         "//upb:port",
+        "//upb:wire",
         "//upb:wire_reader",
         "//upb/base:internal",
+        "//upb/test:test_messages_proto2_upb_proto",
         "@com_google_absl//absl/types:variant",
         "@com_google_googletest//:gtest",
         "@com_google_googletest//:gtest_main",
diff --git a/upb/message/compare.c b/upb/message/compare.c
index 70ad026..49ee33e 100644
--- a/upb/message/compare.c
+++ b/upb/message/compare.c
@@ -8,6 +8,7 @@
 #include "upb/message/compare.h"
 
 #include <stddef.h>
+#include <stdint.h>
 
 #include "upb/base/descriptor_constants.h"
 #include "upb/message/accessors.h"
@@ -181,12 +182,7 @@
 
   if (!(options & kUpb_CompareOption_IncludeUnknownFields)) return true;
 
-  // Check the unknown fields.
-  size_t usize1, usize2;
-  const char* uf1 = upb_Message_GetUnknown(msg1, &usize1);
-  const char* uf2 = upb_Message_GetUnknown(msg2, &usize2);
-
   // The wire encoder enforces a maximum depth of 100 so we match that here.
-  return UPB_PRIVATE(_upb_Message_UnknownFieldsAreEqual)(
-             uf1, usize1, uf2, usize2, 100) == kUpb_UnknownCompareResult_Equal;
+  return UPB_PRIVATE(_upb_Message_UnknownFieldsAreEqual)(msg1, msg2, 100) ==
+         kUpb_UnknownCompareResult_Equal;
 }
diff --git a/upb/message/internal/compare_unknown.c b/upb/message/internal/compare_unknown.c
index a7a60f2..b2d51fa 100644
--- a/upb/message/internal/compare_unknown.c
+++ b/upb/message/internal/compare_unknown.c
@@ -7,10 +7,12 @@
 
 #include "upb/message/internal/compare_unknown.h"
 
+#include <stdint.h>
 #include <stdlib.h>
 
 #include "upb/base/string_view.h"
 #include "upb/mem/alloc.h"
+#include "upb/message/message.h"
 #include "upb/wire/eps_copy_input_stream.h"
 #include "upb/wire/reader.h"
 #include "upb/wire/types.h"
@@ -47,6 +49,14 @@
   jmp_buf err;
 } upb_UnknownField_Context;
 
+typedef struct {
+  upb_UnknownField* arr_base;
+  upb_UnknownField* arr_ptr;
+  upb_UnknownField* arr_end;
+  uint32_t last_tag;
+  bool sorted;
+} upb_UnknownFields_Builder;
+
 UPB_NORETURN static void upb_UnknownFields_OutOfMemory(
     upb_UnknownField_Context* ctx) {
   ctx->status = kUpb_UnknownCompareResult_OutOfMemory;
@@ -118,14 +128,19 @@
   upb_UnknownFields_SortRecursive(fields->fields, 0, fields->size, ctx->tmp);
 }
 
-static upb_UnknownFields* upb_UnknownFields_DoBuild(
-    upb_UnknownField_Context* ctx, const char** buf) {
-  upb_UnknownField* arr_base = NULL;
-  upb_UnknownField* arr_ptr = NULL;
-  upb_UnknownField* arr_end = NULL;
+static upb_UnknownFields* upb_UnknownFields_BuildFromBuffer(
+    upb_UnknownField_Context* ctx, const char** buf);
+
+// Combines two unknown fields into one.
+static void upb_CombineUnknownFields(upb_UnknownField_Context* ctx,
+                                     upb_UnknownFields_Builder* builder,
+                                     const char** buf) {
+  upb_UnknownField* arr_base = builder->arr_base;
+  upb_UnknownField* arr_ptr = builder->arr_ptr;
+  upb_UnknownField* arr_end = builder->arr_end;
   const char* ptr = *buf;
-  uint32_t last_tag = 0;
-  bool sorted = true;
+  uint32_t last_tag = builder->last_tag;
+  bool sorted = builder->sorted;
   while (!upb_EpsCopyInputStream_IsDone(&ctx->stream, &ptr)) {
     uint32_t tag;
     ptr = upb_WireReader_ReadTag(ptr, &tag);
@@ -167,34 +182,71 @@
           ctx->status = kUpb_UnknownCompareResult_MaxDepthExceeded;
           UPB_LONGJMP(ctx->err, 1);
         }
-        field->data.group = upb_UnknownFields_DoBuild(ctx, &ptr);
+        field->data.group = upb_UnknownFields_BuildFromBuffer(ctx, &ptr);
         ctx->depth++;
         break;
       default:
         UPB_UNREACHABLE();
     }
   }
-
   *buf = ptr;
+  builder->arr_base = arr_base;
+  builder->arr_ptr = arr_ptr;
+  builder->arr_end = arr_end;
+  builder->sorted = sorted;
+  builder->last_tag = last_tag;
+}
+
+static upb_UnknownFields* upb_UnknownFields_DoBuild(
+    upb_UnknownField_Context* ctx, upb_UnknownFields_Builder* builder) {
   upb_UnknownFields* ret = upb_Arena_Malloc(ctx->arena, sizeof(*ret));
   if (!ret) upb_UnknownFields_OutOfMemory(ctx);
-  ret->fields = arr_base;
-  ret->size = arr_ptr - arr_base;
-  ret->capacity = arr_end - arr_base;
-  if (!sorted) {
+  ret->fields = builder->arr_base;
+  ret->size = builder->arr_ptr - builder->arr_base;
+  ret->capacity = builder->arr_end - builder->arr_base;
+  if (!builder->sorted) {
     upb_UnknownFields_Sort(ctx, ret);
   }
   return ret;
 }
 
 // Builds a upb_UnknownFields data structure from the binary data in buf.
+static upb_UnknownFields* upb_UnknownFields_BuildFromBuffer(
+    upb_UnknownField_Context* ctx, const char** buf) {
+  upb_UnknownFields_Builder builder = {
+      .arr_base = NULL,
+      .arr_ptr = NULL,
+      .arr_end = NULL,
+      .sorted = true,
+      .last_tag = 0,
+  };
+  const char* ptr = *buf;
+  upb_CombineUnknownFields(ctx, &builder, &ptr);
+  upb_UnknownFields* fields = upb_UnknownFields_DoBuild(ctx, &builder);
+  *buf = ptr;
+  return fields;
+}
+
+// Builds a upb_UnknownFields data structure from the unknown fields of a
+// upb_Message.
 static upb_UnknownFields* upb_UnknownFields_Build(upb_UnknownField_Context* ctx,
-                                                  const char* ptr,
-                                                  size_t size) {
-  upb_EpsCopyInputStream_Init(&ctx->stream, &ptr, size, true);
-  upb_UnknownFields* fields = upb_UnknownFields_DoBuild(ctx, &ptr);
-  UPB_ASSERT(upb_EpsCopyInputStream_IsDone(&ctx->stream, &ptr) &&
-             !upb_EpsCopyInputStream_IsError(&ctx->stream));
+                                                  const upb_Message* msg) {
+  upb_UnknownFields_Builder builder = {
+      .arr_base = NULL,
+      .arr_ptr = NULL,
+      .arr_end = NULL,
+      .sorted = true,
+      .last_tag = 0,
+  };
+  uintptr_t iter = kUpb_Message_UnknownBegin;
+  upb_StringView view;
+  while (upb_Message_NextUnknown(msg, &view, &iter)) {
+    upb_EpsCopyInputStream_Init(&ctx->stream, &view.data, view.size, true);
+    upb_CombineUnknownFields(ctx, &builder, &view.data);
+    UPB_ASSERT(upb_EpsCopyInputStream_IsDone(&ctx->stream, &view.data) &&
+               !upb_EpsCopyInputStream_IsError(&ctx->stream));
+  }
+  upb_UnknownFields* fields = upb_UnknownFields_DoBuild(ctx, &builder);
   return fields;
 }
 
@@ -235,13 +287,13 @@
 }
 
 static upb_UnknownCompareResult upb_UnknownField_DoCompare(
-    upb_UnknownField_Context* ctx, const char* buf1, size_t size1,
-    const char* buf2, size_t size2) {
+    upb_UnknownField_Context* ctx, const upb_Message* msg1,
+    const upb_Message* msg2) {
   upb_UnknownCompareResult ret;
   // First build both unknown fields into a sorted data structure (similar
   // to the UnknownFieldSet in C++).
-  upb_UnknownFields* uf1 = upb_UnknownFields_Build(ctx, buf1, size1);
-  upb_UnknownFields* uf2 = upb_UnknownFields_Build(ctx, buf2, size2);
+  upb_UnknownFields* uf1 = upb_UnknownFields_Build(ctx, msg1);
+  upb_UnknownFields* uf2 = upb_UnknownFields_Build(ctx, msg2);
 
   // Now perform the equality check on the sorted structures.
   if (upb_UnknownFields_IsEqual(uf1, uf2)) {
@@ -253,11 +305,11 @@
 }
 
 static upb_UnknownCompareResult upb_UnknownField_Compare(
-    upb_UnknownField_Context* const ctx, const char* const buf1,
-    const size_t size1, const char* const buf2, const size_t size2) {
+    upb_UnknownField_Context* const ctx, const upb_Message* msg1,
+    const upb_Message* msg2) {
   upb_UnknownCompareResult ret;
   if (UPB_SETJMP(ctx->err) == 0) {
-    ret = upb_UnknownField_DoCompare(ctx, buf1, size1, buf2, size2);
+    ret = upb_UnknownField_DoCompare(ctx, msg1, msg2);
   } else {
     ret = ctx->status;
     UPB_ASSERT(ret != kUpb_UnknownCompareResult_Equal);
@@ -269,11 +321,11 @@
 }
 
 upb_UnknownCompareResult UPB_PRIVATE(_upb_Message_UnknownFieldsAreEqual)(
-    const char* buf1, size_t size1, const char* buf2, size_t size2,
-    int max_depth) {
-  if (size1 == 0 && size2 == 0) return kUpb_UnknownCompareResult_Equal;
-  if (size1 == 0 || size2 == 0) return kUpb_UnknownCompareResult_NotEqual;
-  if (memcmp(buf1, buf2, size1) == 0) return kUpb_UnknownCompareResult_Equal;
+    const upb_Message* msg1, const upb_Message* msg2, int max_depth) {
+  bool msg1_empty = !upb_Message_HasUnknown(msg1);
+  bool msg2_empty = !upb_Message_HasUnknown(msg2);
+  if (msg1_empty && msg2_empty) return kUpb_UnknownCompareResult_Equal;
+  if (msg1_empty || msg2_empty) return kUpb_UnknownCompareResult_NotEqual;
 
   upb_UnknownField_Context ctx = {
       .arena = upb_Arena_New(),
@@ -285,5 +337,5 @@
 
   if (!ctx.arena) return kUpb_UnknownCompareResult_OutOfMemory;
 
-  return upb_UnknownField_Compare(&ctx, buf1, size1, buf2, size2);
+  return upb_UnknownField_Compare(&ctx, msg1, msg2);
 }
diff --git a/upb/message/internal/compare_unknown.h b/upb/message/internal/compare_unknown.h
index ba67554..ad655f7 100644
--- a/upb/message/internal/compare_unknown.h
+++ b/upb/message/internal/compare_unknown.h
@@ -11,6 +11,7 @@
 #include <stddef.h>
 
 // Must be last.
+#include "upb/message/message.h"
 #include "upb/port/def.inc"
 
 #ifdef __cplusplus
@@ -37,8 +38,7 @@
 } upb_UnknownCompareResult;
 
 upb_UnknownCompareResult UPB_PRIVATE(_upb_Message_UnknownFieldsAreEqual)(
-    const char* buf1, size_t size1, const char* buf2, size_t size2,
-    int max_depth);
+    const upb_Message* msg1, const upb_Message* msg2, int max_depth);
 
 #ifdef __cplusplus
 } /* extern "C" */
diff --git a/upb/message/internal/compare_unknown_test.cc b/upb/message/internal/compare_unknown_test.cc
index 03cd2d0..d655f66 100644
--- a/upb/message/internal/compare_unknown_test.cc
+++ b/upb/message/internal/compare_unknown_test.cc
@@ -15,7 +15,10 @@
 
 #include <gtest/gtest.h>
 #include "absl/types/variant.h"
+#include "google/protobuf/test_messages_proto2.upb.h"
 #include "upb/base/internal/endian.h"
+#include "upb/base/upcast.h"
+#include "upb/mem/arena.hpp"
 #include "upb/wire/types.h"
 
 // Must be last.
@@ -103,10 +106,21 @@
 upb_UnknownCompareResult CompareUnknownWithMaxDepth(UnknownFields uf1,
                                                     UnknownFields uf2,
                                                     int max_depth) {
+  upb::Arena arena1;
+  upb::Arena arena2;
+  protobuf_test_messages_proto2_TestAllTypesProto2* msg1 =
+      protobuf_test_messages_proto2_TestAllTypesProto2_new(arena1.ptr());
+  protobuf_test_messages_proto2_TestAllTypesProto2* msg2 =
+      protobuf_test_messages_proto2_TestAllTypesProto2_new(arena2.ptr());
+  // Add the unknown fields to the messages.
   std::string buf1 = ToBinaryPayload(uf1);
   std::string buf2 = ToBinaryPayload(uf2);
+  UPB_PRIVATE(_upb_Message_AddUnknown)(UPB_UPCAST(msg1), buf1.data(),
+                                       buf1.size(), arena1.ptr());
+  UPB_PRIVATE(_upb_Message_AddUnknown)(UPB_UPCAST(msg2), buf2.data(),
+                                       buf2.size(), arena2.ptr());
   return UPB_PRIVATE(_upb_Message_UnknownFieldsAreEqual)(
-      buf1.data(), buf1.size(), buf2.data(), buf2.size(), max_depth);
+      UPB_UPCAST(msg1), UPB_UPCAST(msg2), max_depth);
 }
 
 upb_UnknownCompareResult CompareUnknown(UnknownFields uf1, UnknownFields uf2) {